Scientific Investigations Report 2010-5071

U.S. Department of the Interior

U.S. Geological Survey

PREPARED IN COOPERATION WITH THE

Puerto Rico Environmental Quality Board for the Conservation Trust of Puerto Rico

Selected Hydrologic, Water-Quality, 

Biological, and Sedimentation 

Characteristics of Laguna Grande, 

Fajardo, Puerto Rico, 

March 2007-February 2009
Cover:

Photograph taken from Las Cabezas de San Juan Nature Reserve Lighthouse., of the Laguna Grande, 

Fajardo Puerto Rico.

(Photograph by Luis R. Soler-López, U.S. Geological Survey.)
Selected Hydrologic, Water-Quality, 

Biological, and Sedimentation 

Characteristics of Laguna Grande, Fajardo, 

Puerto Rico, March 2007-February 2009

By Luis R. Soler-López and Carlos R. Santos 

Prepared in cooperation with the Puerto Rico Environmental Quality Board for the 

Conservation Trust of Puerto Rico

Scientific Investigations Report 2010–5071

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Soler-López, L.R., Santos, C.R., 2010, Selected Hydrologic, Water-Quality, Biological, and Sedimentation 

Characteristics of Laguna Grande, Fajardo, Puerto Rico, March 2007-February 2009: U.S. Geological Survey 

Scientific Investigations Report 2010-5071, 51 p.
v

Contents

Abstract ............................................................................................................................................1

Introduction ......................................................................................................................................3

Purpose and Scope ...................................................................................................................3

Description of Study Area ........................................................................................................5

Methods of Investigation..................................................................................................................5

Bathymetric Survey...................................................................................................................5

Surface-Water and Rainfall Data Collection............................................................................5

Water-Quality Sampling and Analysis......................................................................................7

Determination of Biological Characteristics.............................................................................7

Sediment Core Collection .........................................................................................................9

Bathymetry......................................................................................................................................10

Selected Hydrologic Characteristics...............................................................................................10

Rainfall and Runoff.................................................................................................................10

Seawater Exchange ................................................................................................................15

Water Quality..................................................................................................................................15

Field-measured properties and constituents..........................................................................15

Temperature....................................................................................................................16

Turbidity..........................................................................................................................16

pH....................................................................................................................................16

Dissolved Oxygen ...........................................................................................................17

Specific Conductance .....................................................................................................17

Salinity............................................................................................................................17

Water Transparency........................................................................................................18

Other Chemical Constituents..................................................................................................19

Nitrogen and Phosphorus...............................................................................................19

Chlorophyll a...................................................................................................................20

Biological Characteristics...............................................................................................................20

Primary Productivity................................................................................................................20

Phytoplankton Net Primary Productivity, Plankton Respiration, and Plankton 

Gross Productivity...............................................................................................21

Community Net Primary Productivity, Respiration, and Gross Productivity...................21

Phytoplankton Biomass...........................................................................................................25

Fecal Indicator Bacteria ..........................................................................................................27

Sediment Accretion.........................................................................................................................27

Summary and Conclusions..............................................................................................................29

References Cited.............................................................................................................................31
vi

Tables

1. Sample collection and preservation for selected analytes measured at

 Laguna Grande, Fajardo, Puerto Rico. ......................................................................6

2. Laguna Grande sampling stations and applicable determinations..................................9

3. Comparison between the long-term mean monthly rainfall at the 

 National Weather Service Fajardo rainfall station no. 663657, and 

 monthly rainfall at the University of Puerto Rico rainfall station at 

 Laguna Grande during 2007-2008. ..........................................................................13

4. Turbidity at stations 6 and 8 from March 2007 to June 2008........................................16

5. Total nitrogen species concentration by month at stations 6 and 8 from 

 March 2007 to June 2008. .......................................................................................18

6. Total phosphorus concentration by month at stations 6 and 8 from 

 March 2007 to June 2008.........................................................................................19

7. Chlorophyll-a concentration at stations 6 and 8, from March 2007 to 

 June 2008 .................................................................................................................21

8. Primary productivity results at Laguna Grande station 6 obtained by the 

 light and dark bottle method, from March 2007 to June 2008. ...............................22

9. Primary productivity results for the March 29, 2007 and November 15, 2007

 diel studies at Laguna Grande, Fajardo, Puerto Rico. .............................................25

10. Phytoplankton biomass at stations 6 and 8 from March 2007 to June 2008. ..............27

Figures

1. Location of Laguna Grande in eastern Puerto Rico. .........................................................2

2. Aerial photograph showing Laguna Grande main water body and 

 inlet/outlet channel. ...................................................................................................4

3. Aerial photograph showing Laguna Grande water quality, biological, and 

 sediment-core station locations. ...............................................................................8

4. Map showing bathymetry of Laguna Grande main water body, Fajardo, 

 Puerto Rico................................................................................................................11

5. Diagrams showing typical lagoon formation sequence. ................................................12

6. Graph showing typical tidal cycles in Laguna Grande and the nearshore 

 Atlantic Ocean during the periods of A, lowest and B, highest tides 

 of the year. ...............................................................................................................14

7. Diagram showing primary productivity categorization for Laguna Grande, 

 Fajardo, Puerto Rico..................................................................................................23

8. Graph showing A, dissolved oxygen concentration, and B, rate of change curves for

 March 29, 2007, at station 6 in Laguna Grande, Fajardo, Puerto Rico. ...................24

9. Graphs showing gross productivity rates and phytoplankton biomass at 

 A, stations 6, and B, station 8 in Laguna Grande, Fajardo, Puerto Rico. .................26

10. Graphs showing activity of A, lead-210, and B, cesium-137 compared to depth in

 the Laguna Grande sediment core............................................................................28
vii

Appendix 

1. In situ determination of pH, temperature, specific conductance, 

 dissolved oxygen, salinity, and Secchi disk depth for water-quality

 sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, 

 Puerto Rico................................................................................................................33

2. March 29, 2007, diel study physical properties.............................................................41

3. November 15, 2007, diel study physical properties. ......................................................46

4. Fecal bacteria counts from March 2007 to June 2008...................................................51
viii

Conversion Factors, Datum, Acronyms, and Translations

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

hectare (ha) 2.471 acre

square kilometer (km2) 247.1 acre

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 264.2 gallon (gal) 

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Flow rate

cubic meter per day (m3/d) 35.31 cubic foot per day (ft3/d) 

cubic meter per day (m3/d) 264.2 gallon per day (gal/d) 

cubic meter per year (m3/yr) 35.31 cubic foot per year (ft3/yr) 

Deposition rate

centimeter per year (cm/yr) 0.3937 inch per year (in/yr)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram per day (kg/d) 2.205 pound avoirdupois per day 

(lb/d)

metric ton per year (t/yr) 1.102 ton per year (ton/yr) 

Radioactivity

miliBecquerel per gram (mBq/g) 27,027 picocurie per gram(pCi/g) 
ix

Datum:

Horizontal Datum – Puerto Rico Datum, 1940 Adjustment

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929) – a geodetic datum derived from general adjustments of the first-order level 

nets of both the United States and Canada, formerly called “Sea Level Datum of 1929.” 

Acronyms used in this report:

ADCP Acoustic Doppler Current Profiler

BLASS Bathymetric/land survey system

CTPR Conservation Trust of Puerto Rico

GIS Geographic information system

GPS Global positioning system

NOAA National Oceanic and Atmospheric Administration

NWQL National Water Quality Laboratory

PRDNER Puerto Rico Department of Natural and Environmental Resources

PREQB Puerto Rico Environmental Quality Board

TIN Triangulated irregular network

USGS U.S. Geological Survey

Translations:

Spanish English

Lago Lake (in Puerto Rico, also reservoir)

Río River

Municipio Municipality
Abstract 

Laguna Grande is a 50-hectare lagoon in the municipio 

of Fajardo, located in the northeasternmost part of Puerto 

Rico. Hydrologic, water-quality, and biological data were 

collected in the lagoon between March 2007 and February 

2009 to establish baseline conditions and determine the health 

of Laguna Grande on the basis of preestablished standards. In 

addition, a core of bottom material was obtained at one site 

within the lagoon to establish sediment depositional rates.

Water-quality properties measured onsite (temperature, 

pH, dissolved oxygen, specific conductance, and water 

transparency) varied temporally rather than areally. All 

physical properties were in compliance with current regulatory 

standards established for Puerto Rico. Nutrient concentrations 

were very low and in compliance with current regulatory 

standards (less than 5.0 and 1.0 milligrams per liter for total 

nitrogen and total phosphorus, respectively). The average 

total nitrogen concentration was 0.28 milligram per liter, 

and the average total phosphorus concentration was 0.02 

milligram per liter. Chlorophyll a was the predominant 

form of photosynthetic pigment in the water. The average 

chlorophyll-a concentration was 6.2 micrograms per liter. 

Bottom sediment accumulation rates were determined 

in sediment cores by modeling the downcore activities of 

lead-210 and cesium-137. Results indicated a sediment 

depositional rate of about 0.44 centimeter per year. At this rate 

of sediment accretion, the lagoon may become a marshland in 

about 700 to 900 years.

About 86 percent of the community primary productivity 

in Laguna Grande was generated by periphyton, primarily 

algal mats and seagrasses, and the remaining 14 percent was 

generated by phytoplankton in the water column. Based on 

the diel studies the total average net community productivity 

equaled 5.7 grams of oxygen per cubic meter per day (2.1 

grams of carbon per cubic meter per day). Most of this 

productivity was ascribed to periphyton and macrophytes, 

which produced 4.9 grams of oxygen per cubic meter per day 

(1.8 grams of carbon per cubic meter per day). Phytoplankton, 

the plant and algal component of plankton, produced about 0.8 

gram of oxygen per cubic meter per day (0.3 gram of carbon 

per cubic meter per day).

The total diel community respiration rate was 23.4 

grams of oxygen per cubic meter per day. The respiration 

rate ascribed to plankton, which consists of all free floating 

and swimming organisms in the water column, composed 

10 percent of this rate (2.9 grams of oxygen per cubic meter 

per day); respiration by all other organisms composed the 

remaining 90 percent (20.5 grams of oxygen per cubic meter 

per day). Plankton gross productivity was 3.7 grams of oxygen 

per cubic meter per day, equivalent to about 13 percent of the 

average gross productivity for the entire community (29.1 

grams of oxygen per cubic meter per day). 

The average phytoplankton biomass values in Laguna 

Grande ranged from 6.0 to 13.6 milligrams per liter. During 

the study, Laguna Grande contained a phytoplankton standing 

crop of approximately 5.8 metric tons. Phytoplankton 

community had a turnover (renewal) rate of about 153 times 

per year, or roughly about once every 2.5 days. 

Fecal indicator bacteria concentrations ranged from 

160 to 60,000 colonies per 100 milliliters. Concentrations 

generally were greatest in areas near residential and 

commercial establishments, and frequently exceeded current 

regulatory standards established for Puerto Rico. 

Selected Hydrologic, Water-Quality, Biological, and 

Sedimentation Characteristics of Laguna Grande, Fajardo, 

Puerto Rico, March 2007-February 2009

By Luis R. Soler-López and Carlos R. Santos
2 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 1. Location of Laguna Grande in eastern Puerto Rico.
Introduction 3

Introduction 

Laguna Grande is a 50-ha saltwater lagoon located in 

the municipio of Fajardo, in the northeasternmost part of 

Puerto Rico (fig. 1). The lagoon is part of the Las Cabezas de 

San Juan Nature Reserve established in 1986 to protect the 

natural beauty and ecological integrity of 178 ha of coastal and 

land resources, and to provide a natural setting for education 

and scientific research. Although the reserve is owned by 

the Conservation Trust of Puerto Rico (CTPR; Fideicomiso 

de Conservación de Puerto Rico), the lagoon is under the 

jurisdiction of the Puerto Rico Department of Natural and 

Environmental Resources (PRDNER). 

The lagoon is connected to the Atlantic Ocean by a 

1.5-km channel (fig. 2).To preserve the aquatic ecosystem at 

Laguna Grande, the U.S. Forest Service has identified areas 

of study in need of additional research and data, including 

lagoon hydrology, morphology, and water quality (Weaver 

and others, 1999). Of particular importance is the biological 

productivity of Laguna Grande, and thus, its ability to support 

higher trophic-level species. Biological productivity is a major 

indicator of the health of the aquatic ecosystem. The type and 

degree of the biological productivity provides an indication of 

the predominant aquatic communities that control the first link 

of the lagoon’s food chain. 

Although the Las Cabezas de San Juan Nature Reserve 

was designated as such in 1986, the water quality and 

ecological conditions of Laguna Grande have been continously 

threatened by expanding urban development within its 

watershed since the mid-1960s. Runoff entering the reserve 

from the Laguna Grande basin is suspected to contain sewage 

and other contaminants from nearby substandard housing 

units and commercial establishments that lack proper sewage 

infrastructure. During periods of high rainfall, runoff may 

transport sewage and high nutrient loads into the lagoon, 

which can be deleterious to its ecological health and impair its 

homeostatic capacity. In addition, flood tides may contribute 

pollutants through the channel that are derived from local and 

distant sources, such as boat marinas and the Río Fajardo, 

respectively. 

In addition to fecal contamination and high nutrient loads, 

the lagoon is threatened by decreased water transparency and 

biological change. The lagoon is one of the few remaining 

habitats of its kind in Puerto Rico that supports bioluminescent 

dinoflagellates. Of major concern is the conservation of water 

quality in the lagoon, which if altered, could negatively affect 

these dinoflagellates. Other similar coastal resources on the 

island have lost their bioluminescent characteristic, arguably 

due to a lack of baseline data necessary to establish a resource 

management plan.

Water-quality standards for surface waters in Puerto 

Rico have been established by the Puerto Rico Environmental 

Quality Board (PREQB) on the basis of the designated or 

intended use, including fishing, swimming, or source for 

public supply (Puerto Rico Environmental Quality Board, 

2003). The PREQB has no established standards for Laguna 

Grande, however, because baseline data describing its ambient 

conditions are lacking.

To address the aforementioned issues, the U.S. 

Geological Survey (USGS) in cooperation with the PREQB, 

initiated a study in (2007) to determine selected hydrologic, 

water-quality, biological and sedimentation characteristics 

of Laguna Grande. Information from this study will be 

used by the CTPR to establish baseline conditions and to 

monitor changes in the ecological health of the lagoon. The 

establishment of baseline conditions will enable the PREQB 

to establish water-quality criteria for this water body, a 

requirement prior to including the lagoon and offshore waters 

to a distance of 500 m under the SA Classification. This 

categorization is applicable to “bioluminescent lagoons and 

bays and other estuarine and coastal waters of exceptional 

water quality or significant ecological or recreational value.” 

The possible classification of Laguna Grande under SA 

Class waters would need to be designated by the PREQB by 

enacting a resolution (Puerto Rico Environmental Quality 

Board, 2003).

Purpose and Scope

The purpose of this report is to document baseline data 

of the physical, chemical, biological, and bacteriological 

conditions of Laguna Grande to define its present 

characteristics. The study included Laguna Grande’s main 

water body, the lagoon’s canal, and other adjacent areas with 

surface and groundwater drainage to the lagoon. The lagoon 

and drainage area are contained within about 2.3 km2. 

Data collection included 12 sampling trips conducted at 

approximately 1-month intervals to obtain physical, chemical, 

biological, and bacteriological data to define seasonal 

variability over a 13-month period. The study also included the 

determination of (1) community productivity rates through two 

diel data collection efforts; (2) diurnal variation of selected 

water-quality characteristics; (3) surface-water exchange 

between the lagoon and ocean through the outlet channel; and 

(4) sediment deposition rates and the expected lifespan of the 

lagoon. This is the first study to characterize the surface-water 

hydrology and sedimentation of Laguna Grande. Lagoon flora, 

fauna, and soil types have been identified and described by 

Weaver and others (1999) and Boccheciamp (1977). 
4 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 2. Aerial photograph showing Laguna Grande main water body and inlet/outlet channel.
Methods of Investigation 5

Description of Study Area 

The study area includes Laguna Grande, the lagoon 

channel, and other adjacent areas with surface-water and 

groundwater drainage into the lagoon (fig. 2). The Laguna 

Grande drainage area was estimated to encompass about 2.3 

km2. The 50-ha saltwater lagoon has a mean depth of about 3 

m, and a maximum depth of about 5 m in distinct areas located 

at the southwestern flank of the lagoon, which is connected 

to the Atlantic Ocean by the shallow Laguna Grande Channel 

(fig. 2). Laguna Grande contains tidal swamps that are 

underwater most of the year, and tidal flats that are slightly 

above mean sea level and only affected by saltwater during 

high tides. No perennial streams discharge into the lagoon; 

consequently, it only receives freshwater inflow during 

intense rainfall/runoff events. The climate is predominantly 

dry and windy, with an annual long-term average rainfall 

of about 1,605 mm (National Oceanic and Atmospheric 

Administration, 1948-1996). 

The drainage area is composed largely of volcanic 

breccias and tuffaceous sandstone and siltstone, which are 

primarily of marine origin. Basin sediments also include 

alluvium, sand, and calcium carbonate in the form of 

limestone (Weaver and others, 1999).

The lagoon is surrounded by a variety physiographic 

areas, including a headland dominated by a secondary, semievergreen forest; rocky and sandy shorelines covered by a 

secondary woodland; tidal flats; mangrove swamps; and 

brackish-water lagoons (Weaver and others, 1999). About 

50 percent (1.15 km2) of the Laguna Grande drainage area 

consists of mangroves and wetlands

The lagoon currently is used primarily for recreational 

and educational purposes. The Las Cabezas de San Juan 

Nature Reserve receives about 40,000 visitors every year 

through the CTPR-organized tours (Elizabeth Padilla, 

Conservation Trust of Puerto Rico, written commun., 2009), 

and at least 30,000 visitors per year by non-CTPR organized 

kayak tours, through the Laguna Grande inlet channel, based 

on observations made as part of this study. Prior to this study, 

no water and sediment properties have been described in the 

literature.

Methods of Investigation

Data for this study were collected at Laguna Grande 

between March 2007 and February 2009. Hydrologic 

properties, water-chemistry data, sediment deposition rates, 

and selected biological characteristics of Laguna Grande were 

determined from field measurements and sampling following 

standard USGS methods. Sample volumes, sample containers, 

and preservation methods used for individual chemical and 

biological, samples are listed in table 1.

Bathymetric Survey

A bathymetric survey of the Laguna Grande was 

conducted on August 21, 2008, to contour the lagoon bottom 

and calculate the water volume of the lagoon at mean sea 

level. A bathymetric/land survey system (BLASS) interfaced 

to a depth sounder was used to conduct the survey. The system 

consists of a Novatel Global Positioning System (GPS) 

and a digital depth sounder (Intelligent Depth, Specialty 

Devices Inc.). Geographic location and water depth data were 

simultaneously collected and stored in a portable computer. 

The data were then transferred into the USGS GIS for 

processing and analysis.

After the data reduction process, the lagoon bottom was 

contoured and converted into a triangulated irregular network 

(TIN) to calculate the volume. The TIN represents the lagoon 

bottom surface as thousands of adjoining triangles with x, 

y, and z coordinates assigned to all vertices (Environmental 

Systems Research Institute, Inc., 1992).

Surface-Water and Rainfall Data Collection

Water exchange between the Atlantic Ocean and Laguna 

Grande during low and high tides were determined through a 

series of discharge measurements at the lagoon inlet channel 

using USGS standard wading and hydroacoustic methods 

(Laenen, 1985). 

Two tidal stations were installed, one at the lagoon 

and the other on the seaward side near the inlet channel, to 

measure tidal fluctuations in the lagoon and Atlantic Ocean, 

respectively, and to characterize daily and annual tidal cycles. 

Both stations were referenced to the mean sea level datum and 

operated continuously from June 5, 2007, to June 16, 2008, 

with data collected at 15-minute intervals using a data logger. 

In addition, the tidal stations were also operated selectively 

during the two tidal exchange cycles (October 21-22, 2008, 

and February 17-18, 2009) to monitor stage at Laguna Grande 

and the Atlantic Ocean near the inlet during the tidal cycle 

discharge measurements. Based on the previously collected 

yearly tidal fluctuation data analysis, it was determined that 

the highest tidal cycle occurred on October 26, 2007, and the 

lowest tidal cycle occurred on February 4, 2008. 

To characterize the maximum and minimum flood 

and ebb flows, the discharge measurements were therefore 

conducted during October 21-22, 2008, and February 17-18, 

2009, to coincide with the typical periods of highest and 

lowest tides for the year. Discharge measurements were 

made at hourly intervals starting on October 21st at 8:30 

a.m. and ending on October 22nd at 8:30 a.m. An acoustic 

Doppler current profiler (ADCP) was employed on both 

days to measure water flow into (flood flow) and out of (ebb 

flow) the lagoon during a full tidal cycle period of 25 hours. 

During February 17-18, 2009, flood and ebb tide discharge 

measurements were made using a SonTek Flow Tracker ® 

from 11:15 a.m. on the 17th to 11:15 a.m. on the 18th.
6 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R. Table 1. Sample collection and preservation for selected analytes measured at Laguna Grande, Fajardo, Puerto Rico. [cm, centimeter; m, meter; mL, milliliter; mm, millimeter; L, liter; °C, degrees Celsius; H2SO4, sulfuric acid; N, normality; μm, micron] Sample matrix Analyte Sample volume Sample container Method of sample preservation Water Total nutrients (nitrogen and phosphorus) Two 125-mL aliquots Brown and white polyethylene bottles Brown bottle: filtered through 0.45-μm , chilled at 4°C White bottle: field rinsed, added 1 mL H2SO4 4.5 N, chilled at 4°C Water Chlorophyll a 1 L White polyethylene bottle, field- rinsed 47 -mm glass fiber filter, recorded volume filtered. Placed in vial and wrapped in aluminum foil, frozen Water Fecal coliform 125 mL Sterilized, narrow-mouth bottle Chilled at 4°C, processed within 6 hours of collection Water Phytoplankton biomass 1 L White polyethylene bottle, field- rinsed 47 -mm glass fiber filter, recorded volume filtered. Placed in vial and wrapped in aluminum foil, frozen Water Primary productivity 350 mL Clear and dark glass dissolved- oxygen bottle Initial: added 1 pillow of pre-measured manganeous sulfate reagent, 1 pillow of pre-measured alkaline io- dide reagent, and 1 pillow of pre-measured sulfamic acid reagent: chilled at 4°C Light: added 1 pillow of pre-measured manganeous sulfate reagent, 1 pillow of pre-measured alkaline io- dide reagent, and 1 pillow of pre-measured sulfamic acid reagent. Dark: added 1 pillow of pre-measured manganeous sulfate reagent, 1 pillow of pre-measured alkaline io- dide reagent, and 1 pillow of pre-measured sulfamic acid reagent. Sediment Age dating by 137Cs, and 210Pb 1-m-long by 5.1-cm-diameter core Clear polyethylene wide mouth jar with caps. Collected and capped at both ends and stored vertically 
Methods of Investigation 7

Rainfall data in the drainage area were collected at a 

meteorological station about 700 m northeast of the center of 

the lagoon as part of the University of Puerto Rico Long-Term 

Ecological Research Program (fig. 2). The site is operated and 

maintained by University of Puerto Rico, Río Piedras Campus.

Water-Quality Sampling and Analysis

Water-quality data were collected and analyzed to 

determine selected physical and chemical characteristics, 

as well as fecal coliform indicator bacteria concentrations. 

Selected physical characteristics of the water in Laguna 

Grande were measured at eight sites using a YSI multiparameter water-quality instrument (fig. 3). Meaurements 

included temperature, turbidity, pH, dissolved oxygen, specific 

conductance, salinity, and water transparency. At two sites, 

samples were collected for chemical analysis of nutrients and 

chlorophyll-a concentrations (fig. 3). Table 2 lists the physical 

and chemical, and biological determinations made at each 

station. Data collected from March 2007 to June 2008 are 

listed in appendix 1.

A multiparameter water-quality probe (YSI 650®) 

was used to measure in-situ temperature, pH, dissolved 

oxygen, specific conductance, and salinity derived from 

specific conductance on an approximately 1-month basis 

(during morning hours) from March 2007 to June 2008. 

Water transparency was also measured during these visits 

by lowering a 20-cm-diameter Secchi disk into the water 

and recording the depth at which it disappeared from view. 

The upper surface of the Secchi disk was divided into four 

quadrants with alternate black and white areas. 

Nitrogen and phosphorus compounds and chlorophyll 

a were sampled on a nearly monthly basis at stations 6 and 

8, located in the southern and northern portions of Laguna 

Grande, respectively (fig. 3), from March 2007 to June 2008. 

These stations were positioned to represent areas of nutrient 

input from steep-slope areas within the Laguna Grande 

drainage basin. Nutrient water samples were collected, 

filtered onsite as needed, chilled, and sent to the USGS 

National Water Quality Laboratory (NWQL) in Denver, 

Colorado, for analysis (table 1). The nitrogen species analyzed 

were organic nitrogen, ammonium, nitrate, and nitrite; 

total nitrogen is the sum of all of these species. Phosphorus 

was analyzed for total concentrations only, with no species 

determinations. Chlorophyll-a samples were collected in 

labeled bottles, filtered onsite, and packed in dry ice, and 

sent to the NWQL for analysis (table 1). Nutrients results are 

reported in milligrams per liter and chlorophyll a results are 

reported in micrograms per liter. 

Determination of Biological Characteristics

Primary productivity, phytoplankton biomass, and 

fecal indicator bacteria concentrations for Laguna Grande 

were determined using standard USGS procedures (Clesceri 

and others, 1998). In estimating primary productivity rates, 

respiration and gross productivity rates were also determined. 

The most straightforward and inexpensive method for 

estimating primary productivity in water bodies is the light 

(translucent) and dark (non-translucent) bottle method, which 

can be used to determine net changes in the concentration of 

dissolved oxygen for a given volume of water within a given 

time interval (Clesceri and others, 1998). This technique 

was used for the analysis of net primary productivity by 

phytoplankton in the water column. In addition, two diel 

(24-hour) studies were conducted to determine the aquatic 

net community primary productivity, which includes 

phytoplankton, periphyton, and macrophyte productivity. The 

productivity associated only with periphyton and macrophytes 

can therefore be calculated by subtracting phytoplankton 

net primary productivity from community net primary 

productivity. 

At roughly monthly intervals, productivity measurements 

were made at stations 6 and 8 by sampling lagoon water 

in light and dark dissolved oxygen bottles (table 1). These 

sampling sites correspond to those where nutrient samples 

were collected (fig. 3). Incubation was typically initiated 

before 6:00 a.m. and terminated at about 10:00 a.m. The 

early initial time was selected to avoid high oxygen saturation 

levels at the time of sample collection so that an increase in 

dissolved oxygen concentration could be observed at the end 

of the incubation period. The initial concentration of dissolved 

oxygen prior to incubation was determined using the Winkler 

titration method (Clesceri and others, 1998). At both sites, an 

aluminum rack was deployed that contained two contiguous 

light bottles and two contiguous dark bottles (Clesceri and 

others, 1998). The samples of lagoon water were incubated at 

the depths from which they were collected for at least 4 hours. 

After incubation, the bottles were removed and their dissolved 

oxygen concentration was determined by titration. The initial 

dissolved oxygen concentration was expected to decrease in 

the dark bottle by respiration, and was expected to increase in 

the light bottle by photosynthesis. The oxygen production and 

respiration results, therefore, can be expressed as:

 Gross productivity (Pg) = OL - OD;

 Net productivity (Pn) = OL - OI; and

 Respiration (R) = OI – OD,

where, 

OL is the dissolved oxygen concentration at the 

end of the incubation period in the light 

bottle; 

OD is the dissolved oxygen concentration at the 

end of the incubation period in the dark 

bottle; 

and 

OI is the initial dissolved oxygen concentration 

prior to incubation.
8 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 3. Aerial photograph showing Laguna Grande water quality, biological, and sediment-core station locations. 
Methods of Investigation 9

Table 2. Laguna Grande sampling stations and applicable determinations.

[Monthly field determinations included measurements of temperature, turbidity, pH, dissolved oxygen, specific conductance, salinity, and water transparency 

(determined by Secchi disk). USGS, U.S. Geological Survey].

Station name USGS 

identification number Determinations

Laguna Grande no. 1 50069660 Field determinations, fecal indicator bacteria

Laguna Grande no. 2 50069670 Field determinations

Laguna Grande no. 3 50069650 Field determinations, fecal indicator bacteria

Laguna Grande no. 4 50069630 Field determinations 

Laguna Grande no. 5 50069640 Field determinations, fecal indicator bacteria

Laguna Grande no. 6 50069620 Field determinations, turbidity, nutrients, primary productivity 1, 2, phytoplankton biomass, chlorophyll-a

Laguna Grande no. 7 50069610 Field determinations, primary productivity2

, fecal indicator bacteria, 

sediment cores for age dating

Laguna Grande no. 8 50069600 Field determinations, turbidity, nutrients, primary productivity1, 2, phytoplankton biomass, chlorophyll a

Laguna Grande no. 9 50069615 Primary productivity2

Laguna Grande no. 10 50069605 Primary productivity2

1 Light and dark bottle technique.

2 Diel study.

As noted earlier, the diel studies were conducted at 

Laguna Grande to estimate the community net primary 

productivity, the respiration rate, and the gross productivity 

rate. These studies involved measuring physical and chemical 

water properties, such as temperature, pH, and dissolved 

oxygen, all of which are affected by metabolic processes in 

a body of water during one complete diurnal cycle. Specific 

conductance and salinity were also measured because they 

control the concentration of dissolved oxygen in water. The 

collected data can be used to define the hourly variation in 

dissolved oxygen and its saturation levels to calculate the net 

oxygen production and respiration rates (Odum, 1956).

The diel studies were conducted on March 29, 2007, and 

November 15, 2008, near the respective annual periods of 

lowest and highest tides for the eastern coast of Puerto Rico. 

Data collection began at sunrise and continued until after 

sunset. Measurements were made at stations 6-10 (fig. 3); 

stations 9 and 10 were established solely for these studies. 

 Phytoplankton biomass concentration was measured at 

stations 6 and 8 by collecting samples in designated bottles, 

filtering the water onsite, and shipping overnight to the NWQL 

for analysis. Phytoplankton biomass was determined using 

the ash-weight/dry-weight ratio method (Clesceri and others, 

1998). The samples were collected as sequential replicates 

within the water column at the same locations where samples 

were obtained for the light and dark bottle incubation. The 

water samples were filtered through a glass-fiber (borosilicate) 

filter, refrigerated, and sent to the NWQL for analysis (table 

1). At the laboratory, these filters were dried at 105 °C 

overnight, cooled to room temperature, and weighed; the 

samples were then redried at 60 °C for several hours and 

reweighed. The difference in weight was reported as the ashfree dry phytoplankton biomass, in milligrams per liter.

Water samples were collected for fecal coliform bacteria 

analysis using the “hand-dip” method, which is applicable 

to streams and standing waters (Clesceri and others, 1998). 

Samples were collected at stations 1, 3, 5, and 7, for fecal 

indicator bacteria concentrations (fig. 3). At each station, two 

or three sterilized borosilicate 100-mL bottles were immersed 

in the water to a depth of about 15 cm, leaving a small air 

pocket to maintain oxygenation and to allow subsequent 

mixing before analysis (table 1). Samples were passed through 

a 0.65-μm-pore-size filter using the microfiltration method 

(Clesceri and others, 1998) within 6 hours of collection. Fecal 

coliform bacteria were cultured in m-FC agar and incubated 

at 44.5 °C for 20 to 24 hours; colonies were counted using a 

magnifying glass (Clesceri and others, 1998). The results were 

reported as colonies of bacteria per 100 milliliters of sample. 

Sediment Core Collection

Sediment cores were collected at one site in Laguna 

Grande to determine sediment deposition rates. It was 

determined that the center of the lagoon was a representative 

location to collect the cores, based on the lagoon-bottom 

morphology and the TIN generated from the bathymetric 

survey of Laguna Grande. Ten 1-m-long by 5.1-cm-diameter 

cores were collected using a WILDCO stainless steel hand 

corer with transparent polyethylene removable liner and a 

3-m extension handle to reach the lagoon bottom. The corer 

was pushed through the bottom material until refusal—the 
10 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

point at which the corer cannot be driven further into bottom 

sediments. Once retrieved, the liner was removed from the 

stainless steel outer corer, excess water was removed, and the 

core was capped on the bottom and top (table 1). 

The cores were stored vertically until dry enough to be 

extruded horizontally without losing their integrity. Because 

the laboratory analysis requires at least 200 g of material, the 

cores were sectioned at 2-cm intervals from top to bottom 

(each section weighing about 50 g), composited into a single 

sample per every 2-cm layer, and placed in a labeled, widemouthed polypropylene jar. All composited samples were 

sent to a USGS NWQL-contracted laboratory (Eberline 

Laboratories in California, USA) for radioisotope analysis. 

Bathymetry

The water volume of Laguna Grande was calculated to 

be 661,582 m3 at mean sea level datum. The lagoon bottom 

had little relief, with a mean water depth of about 3 m and 

a maximum depth of 5 m below mean sea level in the 

southwestern part (fig. 4). Most of the lagoon depths ranged 

from 1 to 4 m, with the shallower areas located along the 

eastern part of the lagoon. The Laguna Grande flooded area 

encompassed about 0.3 km2 (as calculated using GIS), which 

is about 13 percent of the total drainage area. 

Based on this analysis and the resulting bathymetric map, 

it can be inferred that Laguna Grande was an open bay with its 

mouth or entrance facing the western flank thousands of years 

ago, which was gradually closed by wave action and sand 

deposition (fig. 5). This probably explains why the eastern 

portion of the lagoon is shallower than the western portion, 

where the mouth of Laguna Grande was originally located.

Selected Hydrologic Characteristics

Freshwater inflow to Laguna Grande can be characterized 

as sporadic and relatively minor compared to the amount of 

seawater inflow to the lagoon. Seawater exchange between 

Laguna Grande and the Atlantic Ocean is substantially greater 

during the annual period of highest tides compared to the 

period of lowest tides. 

Rainfall and Runoff

The long-term distribution of monthly rainfall at Laguna 

Grande was considered similar to that measured at National 

Oceanic Atmospheric Administration (NOAA) observation 

station 663657, in Fajardo, Puerto Rico, with a historical 

period of record from 1948 to 1996 (table 3; National Oceanic 

and Atmospheric Administration, 1948-1996). The long-term 

mean annual rainfall in the lagoon drainage area was estimated 

to be at about 1,605 mm.

To compare rainfall at Laguna Grande during the 

period of study from March 2007 to February 2008 with 

long-term mean rainfall, data from the University of Puerto 

Rico meteorological station were used (fig. 2, table 3). 

Unfortunately, many months of rainfall data were missing for 

this site, making comparisons between entire years impossible. 

Nonetheless, a comparison was made between the two sites 

for months with common data (May- August, December); 

long-term rainfall totals for these months range from 120 to 

188 mm. The comparison revealed that 2007 was a year with 

below-normal rainfall during these months (table 3); equaling 

about 44 percent of the corresponding long-term monthly 

means for the area. 

As noted earlier, the Laguna Grande drainage area has 

no ephemeral or perennial streams. Runoff in the basin is 

considered moderate, and occurs predominantly in the form 

of infrequent, intense rainfall. Freshwater inflow to Laguna 

Grande is estimated to be 184,575 m3/yr, or about 28 percent 

of the water volume in the lagoon, based on: (1) the long-term 

annual rainfall average of 1,605 mm for the drainage area, 

(2) the rainfall/runoff ratio of about 5 percent (Giusti and 

López, 1967), and (3) the 2.3 km2 extent of the drainage area. 

This freshwater inflow, however, typically occurs only during 

infrequent rainfall events with sufficient intensity to produce 

runoff, and is not considered to represent an effective waterrenewal mechanism for Laguna Grande. Such events, however, 

are of importance in causing a rapid change in water-quality 

conditions, as was documented during a November 2007 

sampling run in this study (app. 1).
Selected Hydrologic Characteristics 11

Figure 4. Map showing bathymetry of Laguna Grande main water body, Fajardo, Puerto Rico.
12 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 5. Diagrams showing typical lagoon formation sequence.
Selected Hydrologic Characteristics 13

Table 3. Comparison between the long-term mean monthly rainfall at the National Weather Service Fajardo rainfall station no. 663657, and monthly rainfall at the University of Puerto Rico rainfall station at Laguna Grande during 2007-2008. [All values are shown in millimeters; ND, no data; NA, not applicable]

Data type Month Annual rainfall

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Long term (1948-1996) mean 89.15 68.83 75.69 98.30 188.72 126.24 134.11 157.48 166.37 197.89 182.12 120.39 1,604.772007-2008 rainfall ND ND ND ND 91.40 78.09 61.72 49.51 NND NND NND 37.78 NNADifference - 34.84 - 48.15 - 72.39 -107.97 -82.61 NA
14 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 6. Graph showing typical tidal cycles in Laguna Grande and the nearshore Atlantic Ocean during the 

periods of A, lowest and B, highest tides of the year. 
Water Quality 15

Seawater Exchange

Seawater exchange between Laguna Grande and the 

Atlantic Ocean occurs through the Laguna Grande Channel, 

which is about 1.5 km long, and 5 m wide and 1 m deep, on 

average (fig. 2). Flow to (flood) and from (ebb) the lagoon is 

controlled by oceanic tidal fluctuations, the flooded area of 

the lagoon, and the resistance to flow that the lagoon-water 

content exerts over the inflowing water.

The 25-hour tidal measurements made during periods 

of low and high tides for the year (February and October, 

respectively), indicate that stage at Laguna Grande peaked 

about 3.5 to 4 hours after the corresponding oceanic tidal 

peak (fig. 6). This time period may equal the travel time that 

a contaminant entering the lagoon through the channel would 

take to affect Laguna Grande. During February, the lagoon 

tidal cycle peak amplitude was about 12 cm and the sea tidal 

cycle was about 30 cm; during October, the lagoon tidal cycle 

peak amplitude was about 40 cm and the sea tidal cycle peak 

amplitude was 60 cm (fig. 6).

The maximum water level in Laguna Grande was 18 cm 

lower than that of the ocean, on average, during the October 

and February measurements (fig. 6). The corresponding low 

and high tides for the lagoon and ocean differed by about 30 

cm on both occasions (fig. 6), which indicates unrestricted 

seawater exchange between the Atlantic Ocean and Laguna 

Grande during the annual periods of lowest and highest tides.

The flushing rate of a lagoon can be determined by 

calculating its flood (inflow) and ebb (outflow) discharges. 

High flushing rates are generally associated with good water 

quality, and healthy lagoon conditions unless the oceanwater quality is substantially impaired. Lagoon water is 

less susceptible to the effects of point-source contamination 

originating within the shoreline perimeter due to the relatively 

rapid dilution with seawater or purging during ebb flows. 

However, sudden changes in nearshore seawater quality, such 

as from a fuel or oil spill from nearby marinas or docks, can 

rapidly affect water quality in a nearby lagoon depending upon 

the proximity of the lagoon to the contamination source and 

the degree of connection between the lagoon and ocean.

During the October 21-22 tidal cycle, total outflow 

during the ebb tide was 94,254 m3 and total inflow during 

flood tide was 110,019 m3 (a 14 percent difference) indicating 

that 15,765 m3 of water remained inside the lagoon or its 

surrounding wetland zone. During flood tides, seawater seeps 

inland through mangrove stand roots, as observed during the 

study. Thus, much of the 15,765 m3 of seawater unaccounted 

for by ebb flow may have been lost to evapotranspiration 

within the extensive mangrove stands that border the lagoon 

and its inlet channel. Based on a water volume of 661,582 

m3, Laguna Grande flushed about 110,000 m3/d during the 

October 21-22 tidal cycle, equivalent to 16.6 percent of its 

entire water volume. Based on this finding, it is estimated that 

the lagoon flushes its entire water volume every 6 days during 

the period of annual high tides. 

During the February 17-18 tidal cycle, total outflow 

during the ebb tide was 62,398 m3 and the total inflow during 

the flood tide was 60,971 m3. The difference between the flood 

and ebb flows was only 2.3 percent, which can be considered 

within the discharge measurements process accuracy. It can 

therefore be concluded that an equal amount of water enters 

and exits the lagoon during the annual period of lowest tides, 

with insufficient water available to seep inland during this 

period. Based on a water volume of 661,582 m3, Laguna 

Grande flushed about 61,684 m3/d, or 9.3 percent of its 

entire water volume during the February 17-18 tidal cycle. 

Therefore, based on this finding, it is estimated that Laguna 

Grande flushes its entire water volume every 10.7 days during 

the period of annual low tides. 

Assuming that the average net tidal flux between the 

annual high and low tides represents the mean annual tidal 

flux for the lagoon, Laguna Grande flushes 85,852 m3/d of 

water, or 13 percent of its entire water volume during every 

tidal cycle. This flux correponds to a flushing rate of once 

every 7.7 days, or 47 times per year.

Water Quality

Although Laguna Grande currently has no specific 

water-quality classification (July 2009), the data collected 

during this study were analyzed and the water quality of the 

lagoon characterized using comparable values established for 

Class SB water. Laguna Grande is believed to satisfy all of the 

criteria for Class SB waters, which are classified as coastal and 

estuarine waters intended for use for primary and secondary 

contact and for the propagation and preservation of desirable 

species, including those that are threatened or endangered 

(Puerto Rico Environmental Quality Board, 2003).

Field-measured properties and constituents

As noted earlier, the physical characteristics determined 

for water at Laguna Grande were temperature, turbidity, pH, 

dissolved oxygen, specific conductance, salinity, and water 

transparency. Although data are presented for all sites in 

appendix 1, the water-quality characteristics of stations 1 and 

2 are not discussed in the following analysis because station 1 

is at the inlet mouth and thus affected by ebb and flood tides 

that can modify water-quality properties rapidly, and station 

2 is the offshore station established at a navigation marker to 

represent nearshore conditions in the Atlantic Ocean, which 

can vary substantially from those inside Laguna Grande 

(fig. 3). Station 3 is located halfway along the Laguna Grande 

Channel and adjacent to housing units that may affect local 

water quality. Station 5 is at another area adjacent to housing 

units, and the other stations (4, 6, 7, and 8) are within the main 

water body of Laguna Grande.
16 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Temperature

Temperature exerts a major influence on the biological 

activity and growth of aquatic organisms. To a point, the 

higher the water temperature, the greater the biological 

activity. Temperature is also important because of its influence 

on water chemistry. The rate of chemical reactions generally 

increases at higher temperature, which in turn affects 

biological activity.

Water temperature varied seasonally, but was areally and 

vertically uniform within the main water body of the lagoon 

during each sampling (stations 4, 6, 7, and 8, fig. 3, app. 1). 

Vertical stratification was not determined at stations 3 and 

5 outside the main water body, because the water was too 

shallow to collect two measurements in the water column.

All temperature measurements at Laguna Grande 

were within the PREQB standards for SB waters, which 

establish that water temperature should not increase above 

32.2 °C other than for natural causes. The top water column 

temperature ranged from 24.3 °C on January 29, 2008, at 

station 5 to 30.4 °C on September 11, 2007, at stations 6 

and 8 (app. 1). The bottom water temperature ranged from 

25.5 °C on March 4, 2008, at station 7 to 31.3 °C on May 

31, 2007, at station 6. The observed temperature ranges can 

be divided into two general categories. During the winter 

months (November 2007 to March 2008) the top water column 

temperature ranged from to 24.3 °C to 29.5 °C. During spring 

and summer months (March 2007 to October 2007), the 

temperature range was warmer, ranging from 28.8 °C to 30.4 

°C. The bottom water-column temperature during spring and 

summer ranged from 29.7 °C to 31.3 °C.

Turbidity

Turbidity caused by suspended colloidal particles is a 

measure of water clarity. The amount of total suspended solids 

in water is inversely related to clarity and directly related to 

the measured turbidity. High concentrations of particulate 

matter can modify light penetration, and can lead to rapid 

siltation of shallow lakes and bays, effectively smothering 

benthic habitats.

Turbidity was measured at stations 6 and 8 to account for 

runoff from parts of the Laguna Grande drainage area where 

the topography is relatively steep and sediment input to the 

lagoon is likely to originate. All observed turbidity values 

were well within the PREQB standard maximum of 10 NTU. 

Turbidity at station 6 ranged from 2.2 to 5.2 NTU, measured 

in October 2007 and May 2008, respectively. At station 8, 

turbidity ranged from 1.2 to 5.4 NTU, with extremes occurring 

during October 2007 and March 2008 respectively (table 4). 

Turbidity values generally were greater at station 6 than 

station 8 during most of the study, which suggests that the 

southern part of the Laguna Grande drainage area contributes 

most of the colloidal particulate matter to the lagoon. It 

cannot be concluded that the turbidity is generated solely by 

pH

The pH is a measure of the activity of hydrogen ions. The 

pH of water affects the solubility and biological availability 

of chemical constituents, such as nutrients. In addition, pH 

reflects biological processes because photosynthesis consumes 

dissolved carbon dioxide (CO2). Removal of CO2 reduces the 

acidity of the water, thereby increasing water pH. In contrast, 

respiration of organic matter produces CO2, which dissolves in 

water to produce carbonic acid, thus lowering water pH.

The pH values varied seasonally near the top of the 

water column, but were areally uniform within the lagoon 

(excluding stations 1 and 2), and ranged from 7.1 to 7.9; 

seawater pH typically ranges from 7.3 to 8.5. Top and bottom 

pH values were similar, with no vertical stratification observed 

during the entire study except on March 28, 2007 at stations 

4 and 6, where a marked vertical stratification of one standard 

unit was observed (app. 1). The lowest value (about 7.1) was 

obtained on January 29, 2008, and the highest value (about 

7.9) was obtained on March 4, 2008 (app. 1). The mean top 

and bottom water-column pH values ranged from 7.6 to 7.7. 

The pH values in Laguna Grande did not show the wide 

diurnal variations expected in eutrophic waters due to high 

primary productivity. Rather, the diurnal variation in pH 

in Laguna Grande was consistent with mesotrophic water 

bodies. Mesotrophic water bodies have an intermediate level 

of productivity, with a rough balance of productivity and 

Table 4. Turbidity at stations 6 and 8 from March 2007 to June 

2008.

[Values are in nephelometric turbidity units]

 Sampling date Station 6 Station 8

Mar. 28, 2007 2.25 1.42

May 31, 2007 3.87 1.87

June 27, 2007 3.44 3.76

July 24, 2007 4.08 2.93

Sep. 11, 2007 3.02 2.85

Oct. 16, 2007 2.16 1.16

Nov. 14, 2007 2.47 2.23

Jan. 29, 2008 2.83 3.87

Mar. 4, 2008 3.35 5.37

Apr. 1, 2008 3.95 4.41

May 13, 2008 5.21 4.45

June 17, 2008 2.46 4.51

Average 3.26 3.24

runoff-derived resuspension of sediments within the basin. 

Development is greater in the southern part of the drainage 

area than the northern part and could potentially increase 

sediment loads and turbidity beyond current levels. 
Water Quality 17

decomposition. Diurnal variation in pH can be expected where 

primary or community biological productivity is substantial. 

During daylight hours, when primary productivity is high, 

dissolved carbon dioxide is captured during photosynthesis, 

increasing pH. During nighttime hours, cellular respiration 

releases carbon dioxide into the waters, decreasing pH. The 

diurnal variation in pH in Laguna Grande was within the 

PREQB standard of 7.3 to 8.5 units.

Dissolved Oxygen

The absolute concentration of dissolved oxygen in an 

aquatic system is controlled primarily by physical variables, 

such as water temperature, barometric pressure, salinity, as 

well as by photosynthetic activity and community respiration. 

In the presence of high photosynthetic activity, concentrations 

of oxygen increase monotonically from morning to late 

afternoon. These increases in dissolved oxygen concentration 

can be used to estimate the extent of photosynthetic 

activity. Monthly dissolved oxygen concentration 

measurements were used to evaluate water quality in terms 

of pre-established standards and to help establish longer-term 

trends. 

The dissolved oxygen concentrations in the lagoon and 

channel were generally closer to or slightly lower than, the 

PREQB standard of 5.0 mg/L; however, these conditions are 

considered typical for a mesotrophic lagoon. The monthly 

variation in dissolved oxygen concentrations at Laguna 

Grande was lowest at stations 6, 7, and 8, near the center of 

the lagoon. Dissolved oxygen concentrations in top samples 

ranged from 3.3 to 6.1 mg/L (at station 6), which corresponds 

to an oxygen saturation range of about 52 to 89 percent. 

Bottom water column dissolved oxygen concentrations ranged 

from 0.1 to 4.8 mg/L at stations 8 and 7, respectively, which 

corresponds to an oxygen saturation range of 2 to 80 percent. 

Monthly variation in dissolved oxygen concentration was 

greatest at stations 3 and 5, located halfway along the Laguna 

Grande Channel and in the small embayment southwest of 

the main water body, respectively (fig. 3). Stations 3 and 5 are 

the closest to developed areas, and may reflect anthropogenic 

influence. The dissolved oxygen data indicate that this is the 

most impaired area of Laguna Grande, which is consistent 

with the fecal coliform data presented later in this report. 

Dissolved oxygen concentrations ranged from 1.8 to 4.8 mg/L 

at station 3, and from 0.1 to 2.9 mg/L at station 5. Oxygen 

saturation values ranged from 32 to 72 percent at station 

3, and from 1 to 44 percent at station 5. Dissolved oxygen 

concentrations were markedly lower at station 5 than at the 

other stations in the lagoon during all sampling trips. 

Supersaturation of dissolved oxygen was not observed 

during monthly samplings, which is consistent with 

mesotrophic rather than eutrophic conditions. All monthly 

samplings occurred between 6:00 a.m. and 9:00 a.m., however, 

and morning dissolved oxygen values are not good trophiclevel indicators because sunlight intensity is low during this 

period. Supersaturation was not documented during the diel 

study on March 29, 2007, but was recorded in the afternoon 

hours during the November 15, 2007, diel study at all stations 

used for that effort (stations 6-10). This condition was 

probably a consequence of nutrient-rich runoff that must have 

occurred days prior to the diel study, as can be inferred from 

the specific conductance and salinity data presented next in 

this report.

Specific Conductance

Specific conductance is a measure of the capacity of 

water (or other substance) to conduct an electrical current. 

Specific conductance is a function of the types and quantities 

of dissolved constituents, and can be used to approximate 

dissolved-solids concentrations in water. The concentration of 

dissolved solids, expressed in milligrams per liter, is typically 

55 to 75 percent of the specific conductance in microsiemens 

per centimeter at 25 °C (Díaz and others, 2004).

Specific conductance ranged from 11,840 to 60,240 

μS/cm near the top of the water column in Laguna Grande; 

both extremes were measured at station 5. Values of specific 

conductance varied seasonally, but were areally uniform 

within the lagoon. Specific conductance was lowest in 

November 2007, coinciding with high rainfall, and highest in 

October 2007. 

Specific conductance was vertically stratified at stations 

6, 7, and 8, and was about 16 percent less at the top of 

the water column than the bottom (45,000 and 54,000 μS/

cm, respectively). As noted earlier, temperature was also 

vertically stratified at these sites, ranging from about 27 

°C at the top of the water column to 30 °C at the bottom. 

The vertical stratification of specific conductance and 

temperature at stations 6, 7 and 8, as well as the low measured 

specific conductance at station 5, indicate a recent rainfallrunoff event. Specific conductance standards have not been 

established by the PREQB.

Salinity

Water salinity refers to the accumulation of total 

dissolved salts. Typical salinity values for marine and coastal 

lagoons with good seawater exchange range from 35 to 40 ppt. 

Salinity is linearly correlated with specific conductance by the 

relation described in Clesceri and others (1998). This relation 

was used to determine salinity indirectly at Laguna Grande 

using specific conductance measurements.

Salinity in Laguna Grande varied seasonally but not 

areally, and vertical stratification was minimal except during 

November 2007, when the vertical stratification of salinity 

reflected that of specific conductance. Salinity was highest in 

October 2007 and June 2008 at station 5 (40.4 and 39.9 ppt, 

respectively) and lowest in May and November 2007, also at 

station 5 (12.4 and 6.8 ppt, respectively). Salinity in the lagoon 

was generally comparable to that of coastal seawater (about 35 

ppt); a salinity standard has not been established by PREQB.
18 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Water Transparency

Water transparency is a physical water property that 

has ecological and recreational importance. Specifically, 

water transparency affects sunlight penetration, and thus, 

photosynthetic activity, and is an aesthetically valuable 

characteristic. Water transparency and light penetration can 

be reduced by natural factors, such as inorganic particulate 

matter, plankton biomass, and humic and fulvic acids 

produced from decomposing organic matter that can color the 

water.

As noted earlier, the water transparency of Laguna 

Grande was measured using a Secchi disk, which provides a 

means for determining the limit of visibility based on contrast 

between four quadrants with alternate black and white areas 

on the disk. Water transparency is roughly twice the Secchi 

disk depth of dissappearance, because light must travel twice 

through a water column before it is seen. Biologists consider 

the depth of the euphotic zone to be roughly three times the 

measured transparency; the eupthotic zone is defined as the 

upper layer of a water body into which enough light penetrates 

to permit the growth of green plants and algae.

Water transparency of Laguna Grande was generally 

high compared to the lagoon mean water depth of about 3 

m, with transparencies ranging between 76 and 201 cm, and 

a mean depth of 90 cm. Thus, the euphotic zone extended 

to the lagoon bottom at stations 1, 3, 4, and 5. However, 

light penetration near the middle of the lagoon (stations 6, 

Table 5. Total nitrogen species concentration by month at stations 6 and 8 from March 2007 to June 2008. 

[N, nitrogen; NH4

+

, ammonium; NO3

-

, nitrate nitrogen; NO2

-

, nitrite nitrogen; all concentrations as N, in milligrams per liter].

Sampling date Total 

organic N Total NH4

+ Total NO3- Total NO2- Total N

Station 6

Mar. 28, 2007 0.115 0.076 0.00137 0.00963 0.202

May 31, 2007 0.274 0.050 0.00149 0.01051 0.336

June 27, 2007 0.233 0.027 0.00104 0.00796 0.269

July 24, 2007 0.290 0.071 0.00036 0.01664 0.378

Sep. 11, 2007 0.232 0.051 0.00150 0.00550 0.290

Oct. 16, 2007 0.164 0.055 0.00059 0.00741 0.227

Nov. 14, 2007 0.247 0.035 0.00422 0.05378 0.340

Jan. 29, 2008 0.158 0.086 0.00220 0.01180 0.258

Mar. 4, 2008 0.195 0.057 0.00004 0.01596 0.268

Apr. 1, 2008 0.145 0.055 0.00000 0.01300 0.213

May 13, 2008 0.293 0.046 0.00000 0.01100 0.350

June 17, 2008 0.166 0.059 0.00027 0.00873 0.234

Average 0.209 0.056 0.00100 0.01400 0.280

Station 8

Mar. 28, 2007 0.167 0.067 0.00079 0.00721 0.242

May 31, 2007 0.218 0.040 0.00087 0.00513 0.264

June 27, 2007 0.253 0.024 0.00045 0.01055 0.288

July 24, 2007 0.270 0.063 0.00000 0.01951 0.353

Sep.11, 2007 0.229 0.049 0.00225 0.00575 0.286

Oct. 16, 2007 0.076 0.059 0.00050 0.00750 0.143

Nov. 14, 2007 0.242 0.034 0.00500 0.06600 0.347

Jan. 29, 2008 0.177 0.095 0.00216 0.01184 0.286

Mar. 4, 2008 0.185 0.058 0.00000 0.01800 0.261

Apr. 1, 2008 0.179 0.055 0.00000 0.01100 0.245

May 13, 2008 0.218 0.050 0.00114 0.01386 0.283

June 17, 2008 0.238 0.060 0.01079 0.00021 0.309

Average 0.204 0.055 0.00200 0.01500 0.276
Water Quality 19

7, and 8) did not reach the bottom in about one third of the 

measurements (app. 1, values less than 100 cm), which could 

inhibit plant growth on the Laguna Grande bottom substrate. 

Water transparency varied temporally and areally, and 

was greatest in October 2007 at station 6, when the lagoon 

bottom was visible (201 cm). Transparency was lowest on 

four occasions at station 5, when visibility was only 15 cm; 

however, these low transparencies would not prevent light 

from reaching the bottom because water is only about 30 cm 

deep at this station. The PREQB has no established standards 

for water transparency. 

Other Chemical Constituents

Water chemisty determinations for Laguna Grande 

included nutrient concentrations (nitrogen and phosphorus), 

and chlorophyll-a concentrations. Nutrients are indicators 

of trophic levels, and chlorophyll a is an indicator of 

photosynthetic activity as well as trophic level. 

Nitrogen and Phosphorus

Nutrients are elements and compounds essential to 

sustain plant and animal life (Hutchinson, 1973). Nitrogen 

(N) and phosphorus (P) compounds are considered to be 

the most important in aquatic ecosystems because they 

are often limiting factors. Waters with ambient nutrient 

concentrations can become enriched through organic matter 

decay, autotrophic or heterotrophic processes, and by runoff. 

Human and animal activities, as well as poor soil conservation 

practices, can also increase nutrient inputs to waters with 

background concentrations of these constituents. Several 

chemical species of nitrogen, such as ammonium, nitrites, 

nitrates, and orthophosphorus, are the primary anthropogenic 

contaminants related directly to eutrophication. Excessive 

concentrations of these constituents can lead to increased 

densities of planktonic and benthic algae, and substantial daily 

variation in dissolved oxygen concentrations and pH (Webb 

and Gómez-Gómez, 1997).

Total nitrogen concentrations in Laguna Grande 

indicate that total nitrogen appears to be distributed uniformly 

in Laguna Grande, and were substantially below the PREQB 

standard of 5 mg/L. Concentrations at station 6 ranged from 

0.20 to 0.38 mg/L, with a mean concentration of 0.28 mg/L 

(table 5). At station 8, total nitrogen concentrations ranged 

from 0.14 to 0.35 mg/L, with a mean value of 0.28 mg/L.

Most of the nitrogen measured in Laguna Grande was in 

the form of organic nitrogen, with an average concentration 

of 0.209 mg/L, and composing roughly 74 percent of the total 

nitrogen as N present (table 5). The next most predominant 

nitrogen species was ammonium, although it existed in very 

low concentrations at both stations. The average ammonium 

concentration was 0.056 mg/L, or 20 percent of the total 

nitrogen as N present. Of lesser abundance were nitrite and 

nitrate, with mean concentrations of 0.014 and 0.002 mg/L N, 

respectively, and composing 5 and roughly 1 percent of the 

total nitrogen as N present. 

The high proportion of total N accounted for by organic 

nitrogen and ammonium, compared to that available as nitrate 

and nitrite, is typical of mesotrophic and eutrophic water 

bodies because of the rapid uptake of the reduced states of 

nitrogen by photosynthetic activity. In the absence of regular 

stream inflow to the lagoon and with the relatively shortduration runoff episodes during the study period, the sources 

of nitrogen measured were probably internal (decomposition 

and metabolism), with infrequent, external input from 

surrounding wetlands. The relatively high observed rates of 

phytoplankton production (discussed later) support this.

Total phosphorus concentrations in Laguna Grande 

ranged from 0.01 to 0.11 mg/L as P, well below the PREQB 

standard of 1 mg/L. The highest concentration was measured 

in October 2007, and the mean concentration for both stations 

was 0.02 mg/L (table 6). 

The low phosphorus concentrations suggest that available 

P uptake occurs rapidly through photosynthetic activity by 

benthic and planktonic flora, and that phosphorus may limit 

primary production. The total phosphorus concentrations 

measured at Laguna Grande were substantially lower than 

those measured in other coastal lagoons in Puerto Rico, 

where concentrations are greater than 1 mg/L. For example, 

in Laguna de Las Salinas in Ponce, Puerto Rico, mean total 

phosphorus concentration was 1.86 mg/L (Soler-López and 

others, 2005). In addition, mean concentrations in Laguna San 

José and Laguna de Piñones (both in the San Juan Bay estuary 

system) ranged between 1.0 and 2.5 mg/L, respectively (U.S. 

Geological Survey, Caribbean Water Science Center, unpub. 

data, 2000-2005). 

Table 6. Total phosphorus concentration by month at stations 6 

and 8 from March 2007 to June 2008.

 [P, phosphorus; all concentrations as total P, in milligrams per liter]

Sampling date Station 6 Station 8

Mar. 28, 2007 0.0299 0.0248

May 31, 2007 0.0268 0.0173

June 27, 2007 0.0211 0.0205

July 24, 2007 0.0088 0.0019

Sep.11, 2007 0.0197 0.0156

Oct. 16, 2007 0.0115 0.0379

Nov. 14, 2007 0.1064 0.0210

Jan. 29, 2008 0.0060 0.0125

Mar. 4, 2008 0.0168 0.0220

Apr. 1, 2008 0.0183 0.0169

May 13, 2008 0.0213 0.0199

June 17, 2008 0.0288 0.0226

Average 0.0260 0.0190
20 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Redfield and others (1963) found that the atomic ratio 

of carbon (C), nitrogen (N), and phosphorus (P) are related 

generally by a C:N:P ratio of 106:16:1 in planktonic tissue, 

which is known as the Redfield’s ratio. This ratio can be 

simplified to the total nitrogen to total phosphorus ratio 

(TN/TP), which can be used to determine the potential for 

phytoplankton and plant growth in ambient waters. According 

to Vollenweider (1968), TN/TP ratios exceeding a value 

of 15 indicate that phosphorus is the limiting factor for 

phytoplankton and aquatic plant growth. The TN/TP mean 

ratio in Laguna Grande was 12.5, indicating that nitrogen 

could become a limiting factor in primary productivity 

in Laguna Grande if the ratio were to decrease further. 

Phosphorus is also a limiting factor in primary productivity 

at Laguna Grande, because primary production could not 

occur in its absence. Phosphorus may be a co-limiting nutrient 

because the N/P ratio is close to 15.

Chlorophyll a

Chlorophyll a is the principal photosynthetic 

pigment in vascular plants and algae, converting sunlight into 

the chemical energy needed to fix CO2 into carbohydrates. 

The pigment was sampled at stations 6 and 8, to provide a 

reasonable estimate of algal biomass in Laguna Grande 

(fig. 3).

Chlorophyll-a data collected during this study indicate 

that Laguna Grande is mesotrophic. Concentrations were 

fairly consistent between stations 6 and 8 in Laguna Grande, 

averaging 6.02 and 6.39 μg/L, respectively. Concentrations 

ranged from from 2.19 to 13.01 μg/L for both sites. The 

highest concentrations were measured in September and 

October 2007 and May 2008, and lowest were measured in 

January 2008 (table 7). The mean chlorophyll-a concentration 

of 6.2 μg/L for Laguna Grande was well below the 

concentrations of other coastal lagoons such as Laguna de Las 

Salinas, where mean concentrations were 14 μg/L (SolerLópez and others, 2005). Concentrations ranged between 14 

and 23 μg/L in Laguna San José, and in Laguna de Piñones 

concentrations were about 26 μg/L (U.S. Geological Survey, 

Caribbean Water Science Center, unpub. data, 2000-2005). 

Biological Characteristics

Primary productivity, phytoplankton biomass, and fecal 

indicator bacteria concentrations were determined at Laguna 

Grande to characterize biotic conditions within its aquatic 

ecosystem. Laguna Grande appears to be mesotrophic, 

with relatively high primary productivity, and constitutes 

an important food supply for primarily aquatic organisms. 

Most of the biomass standing crop, however, was found to 

be generated by benthic organisms in the form of periphyton 

and macrophytes. Fecal indicator bacteria were present in 

relatively high concentrations near developed areas compared 

to concentrations at the center of Laguna Grande. 

Primary Productivity

The primary productivity of an ecosystem is defined 

as the rate at which new organic matter is created by 

photosynthesis (or chemosynthesis), and is expressed as the 

production of organic matter per unit of volume (or integrated 

under a surface area) per unit of time. The principal primary 

producers in aquatic ecosystems include phytoplankton, 

periphyton, and macrophytes. Of these, phytoplankton often 

synthesizes the most organic matter in aquatic systems, 

although periphyton and macrophytes add substantial 

amounts. The primary productivity is summarized by the 

universal equation:

Light

6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2 

Pigment 

In an estuarine system, primary productivity supplies 

organic matter that supports aquatic and terrestrial food chains. 

Three processes control the net organic matter created:

1. Net productivity (Pn)—The net production of 

organic matter, or stored energy minus the losses per 

selected time interval. This process involves photosynthetic organisms only.

2. Respiration (R)—The amount of oxygen consumed 

by metabolic processes. This process involves all 

community organisms.

3. Gross productivity (Pg)—The observed change in 

biomass, plus all predatory and non-predatory losses 

per selected time interval. This process involves all 

community organisms and is the sum of net productivity plus respiration.

Natural systems are affected by many factors that cause 

rapid, frequent, and variable changes in productivity, such that 

only average rates can be determined in a typical environment. 

The annual time period, therefore, is probably the most 

meaningful interval in comparative quantitative analyses of 

material and energy fluxes at population, community, and 

ecosystem levels (Wetzel, 2001). 
Biological Characteristics 21

Table 7. Chlorophyll-a concentration at stations 6 and 8, from 

March 2007 to June 2008. 

[Del, sample damaged; all concentrations are in micrograms per liter].

Sampling date Station 6 Station 8

Mar. 28, 2007 Del Del

May 31, 2007 Del Del

June 27, 2007 8.82 6.34

July 24, 2007 6.93 9.70

Sep. 11, 2007 10.02 7.25

Oct. 16, 2007 3.36 13.01

Nov. 14, 2007 3.61 3.52

Jan. 29, 2008 3.76 2.19

Mar. 4, 2008 3.59 4.79

Apr. 1, 2008 5.26 3.74

May 13, 2008 10.07 6.04

June 17, 2008 4.73 7.33

Average 6.02 6.39

Phytoplankton Net Primary Productivity, Plankton 

Respiration, and Plankton Gross Productivity

As described earlier, light and dark bottle primary 

productivity measurements were made to determine the 

net primary productivity by phytoplankton, and plankton 

respiration and gross productivity. Table 8 summarizes the 

primary productivity results obtained using the light and 

dark bottle method at stations 6 and 8. The dissolved oxygen 

concentrations shown in table 8 represent the average of two 

titrations for each of the two light bottles and the average 

of two titrations for each of the two dark bottles. Therefore, 

each individual OL, OI, and OD value presented in table 

8 is the average of four separate titrations. Although some 

initial dissolved oxygen values were at or near saturation (or 

supersaturated), these were not excluded from the calculations 

because oxygen concentrations continued to increase during 

incubation, indicating net primary productivity.

On the basis of dissolved oxygen production and 

consumption as a measure of photosynthetic and respiration 

rates, results of the light and dark bottle primary productivity 

studies indicate that the Laguna Grande plankton community 

is very productive. The phytoplankton yielded an annual 

average net productivity rate of 0.10 mgO2/L-hr, and the 

plankton yielded an annual average respiration rate of 0.12 

mgO2/L-hr (table 8). The average annual plankton gross 

productivity rate was 0.22 mgO2/L-hr. Multiplying the average 

annual net productivity rate by the number of hours for which 

net primary productivity occurred (typically 8 hours based on 

the diel study results) yields a daily net productivity of 0.8 

mgO2/L. This value is equivalent to a daily net productivity of 

about 0.8 gO2/m3 for phytoplankton in the uppermost meter in 

the lagoon (fig. 7), which is considered to be representative of 

the entire water column based on water transparency values. 

The same procedure was used to calculate daily respiration, 

which was 2.9 gO2/m3. In this case, however, the hourly 

respiration rate was multiplied by 24 because respiration can 

be assumed to remain constant throughout a 24-hour period. 

The average daily gross productivity for plankton was 

3.7 gO2/m3 (fig. 7). 

Multiplying the daily net productivity by 0.375 provides 

an estimate of the net organic carbon produced (Fox, 2000). 

Therefore, phytoplankton in the the Laguna Grande water 

column had an estimated daily net organic carbon production 

of about 0.3 g/m3. In comparison, upwelling coastal zones, 

where nutrient-rich waters move upward by currents and 

wind, are considered to be very productive with a primary 

productivity rate in the range of 0.25 to 0.5 gC/m3-d (Lecture 

Notes in Biological Oceanography, 1999). Thus, productivity 

in Laguna Grande overlaps the lower range of coastal 

upwelling zones. In addition, the Laguna Grande daily net 

productivity rate was about half of the rate of 0.5 gC/m3 

calculated for Laguna de Las Salinas (Soler-López and others, 

2005).

The productivity of the upper meter layer of the water 

column at Laguna Grande is representative of the entire water 

column, considering its shallow depth and substantial windinduced mixing. Therefore, based on the estimated water 

volume of the Laguna Grande of about 662,000 m3, the lagoon 

phytoplankton community produced an estimated average 

of about 199 kgC/d, or about 73 tC/yr. This organic carbon 

production is about 14 percent of the total organic carbon 

production in the lagoon, which is about 507 tC/yr. 

Community Net Primary Productivity, Respiration, 

and Gross Productivity

As noted earlier, diel studies were conducted in 

Laguna Grande to estimate the community net primary 

productivity, the respiration rate, and the gross productivity 

rate. Appendixes 2 and 3 summarize the dissolved oxygen 

concentrations for diel studies on March 29, 2007, and 

November 15, 2007, at stations 6-10, along with temperature, 

pH, specific conductance, and salinity data.

Dissolved oxygen data were corrected for diffusion, and 

the net rate of change in dissolved oxygen was calculated. 

Values above zero indicate a positive rate of change and 

represent community net primary productivity; values below 

zero indicate a negative rate and represent community 

respiration. The net productivity and respiration were 

calculated by mathematical integration over the water column 

depth. Figure 8A shows the March 29, 2007, dissolved oxygen 

data collected at station 6. Dissolved oxygen concentration 

and saturation increased steadily on March 29, 2007, after 

about 7:00 a.m., reached a maximum from about 2:00 to 5:00 

p.m., and subsequently decreased as respiration became more 

evident (fig. 8B). 
22 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R. Table 8. Primary productivity results at Laguna Grande station 6 obtained by the light and dark bottle method, from March 2007 to June 2008. [All dissolved oxygen values are in milligrams per liter; all productivity and respiration rates are in milligrams of oxygen per cubic meter per hour. DO sat., dissolved oxygen saturation, in percent; OI, initial oxygen; OL, light bottle oxygen; OD, dark bottle oxygen; GP, gross productivity rate; NP, net productivity rate; R, respiration rate; IT, incubation time, in hours; --, Values in bold are at or near saturation] Month DO sat. OI OL OD GP NP R IT GP rate NP rate R rate Station 6 Mar. 28, 2007 80 5.20 5.20 4.72 0.48 0.00 0.48 4.00 0.120 0.000 0.120 May 31, 2007 84 5.40 5.60 4.90 0.70 0.20 0.50 5.50 0.127 0.036 0.091 June 27, 2007 96 5.90 5.90 5.40 0.50 0.00 0.50 4.50 0.111 0.000 0.111 July 24, 2007 82 5.00 5.45 4.70 0.75 0.45 0.30 4.50 0.167 0.100 0.067 Sep. 11, 2007 77 4.60 4.65 3.95 0.70 0.05 0.65 3.83 0.183 0.013 0.170 Oct. 16, 2007 83 5.20 5.40 4.80 0.60 0.20 0.40 4.50 0.133 0.044 0.089 Nov. 14, 2007 83 5.60 5.80 5.20 0.60 0.20 0.40 4.17 0.144 0.048 0.096 Jan. 29, 2008 69 4.60 5.10 3.60 1.50 0.50 1.00 4.83 0.311 0.104 0.207 Mar. 4, 2008 98 6.40 7.65 6.20 1.45 1.25 0.20 4.00 0.363 0.313 0.050 Apr. 1, 2008 86 5.60 6.00 3.80 2.20 0.40 1.80 4.00 0.550 0.100 0.450 May 13, 2008 49 3.00 5.20 3.00 2.20 2.20 0.00 4.00 0.550 0.550 0.000 June 17, 2008 91 5.60 5.60 4.90 0.70 0.00 0.70 4.00 0.175 0.000 0.175 Average 5.18 5.63 4.60 1.03 0.45 0.58 0.244 0.109 0.135 Station 8 Mar. 28, 2007 80 5.20 5.62 5.21 0.41 0.40 0.01 4.00 0.103 0.100 0.002 May 31, 2007 93 6.10 6.30 5.50 0.80 0.20 0.60 5.50 0.145 0.036 0.109 June 27, 2007 108 6.60 6.90 6.00 0.90 0.30 0.60 3.58 0.251 0.084 0.168 July 24, 2007 90 5.50 5.50 5.08 0.42 0.00 0.42 4.50 0.093 0.000 0.093 Sep. 11, 2007 82 5.00 5.10 5.00 0.10 0.10 0.00 3.83 0.026 0.026 0.000 Oct. 16, 2007 86 5.40 5.60 5.00 0.60 0.20 0.40 4.42 0.136 0.045 0.090 Nov. 14, 2007 79 5.40 5.80 5.10 0.70 0.40 0.30 4.50 0.156 0.089 0.067

Jan. 29, 2008 69 4.60 5.40 3.60 1.80 0.80 1.00 5.00 0.360 0.160 0.200Mar. 4, 2008 101 6.60 7.40 6.15 1.25 0.80 0.45 4.00 0.313 0.200 0.113Apr. 1, 2008 92 5.90 6.00 5.00 1.00 0.10 0.90 4.00 0.250 0.025 0.225May 13, 2008 64 4.00 4.95 4.00 0.95 0.95 0.00 4.00 0.238 0.238 0.000June 17, 2008 92 5.60 5.60 4.40 1.20 0.00 1.20 4.00 0.300 0.000 0.300Average 5.49 5.85 5.00 0.84 0.35 0.49 0.198 0.084 0.114
Biological Characteristics 23

The average community net primary productivity for 

the March 29, 2007, study was 2.3 gO2/m3-d, the average 

community respiration was 11.6 gO2/m3-d, and the average 

community gross productivity was 13.9 gO2/m3-d (table 9). 

Thus, respiration exceeded net primary productivity by a 

factor of five, on average. The average community net primary 

productivity for the November 15, 2007, study was 9.1 gO2/

m3-d (3.4 gC/m3-d), the average community respiration was 

35.3 gO2/m3-d, and the average gross community productivity 

was 44.4 gO2/m3-d (table 9). Respiration exceeded net primary 

productivity by a factor of four, on average. 

Averaging the results for the March 29 and November 

15, 2007, diel studies yielded a community net primary 

productivity of 5.7 gO2/m3-d (2.1 gC/m3-d), a community 

respiration of 23.4 gO2/m3-d, and a community gross 

productivity of 29.1 gO2/m3-d (fig. 7). Average net primary 

productivity was 3.9 times higher in November than in March 

(9.1 and 2.3 gO2/m3-d, respectively), and average respiration 

was three times as high (35.3 and 11.6 gO2/m3-d, respectively); 

as a result, average gross primary productivity was also higher 

in November than in March. Net primary productivity in 

November could have been enhanced by nutrient input from 

rainfall and runoff, because top conductivity and salinity 

values in the water column were about 13 percent lower than 

corresponding bottom values (app. 2). This explanation is 

consistent with the higher nitrogen concentrations measured in 

November 2007 compared to March 2007. 

The results of light and dark bottle and diel studies 

indicate that periphyton and macrophyte photosynthesis 

were the predominant form of net primary productivity in 

Laguna Grande. Periphyton and macrophytes produced the 

greatest amount of dissolved oxygen, 4.9 gO2/m3-d (1.8 gC/

m3-d) (fig. 7). Approximately 0.8 gO2/m3-d (0.3 gC/m3-d) 

was estimated to be produced by the phytoplankton. The 

average overall community net primary productivity, which 

includes phytoplankton, periphyton, and macrophytes, was 

5.7 gO2/m3-d (2.1 gC/m3-d) (fig. 7). Thus, the percentage of 

net primary productivity in Laguna Grande accounted for by 

phytoplankton was 14 percent, and the percentage accounted 

for by periphyton and macrophytes is 86 percent. 

Figure 7. Diagram showing primary productivity categorization for Laguna Grande, Fajardo, Puerto Rico. All 

values are in grams of oxygen per cubic meter per day.
24 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 8. Graph showing A, dissolved oxygen concentration, and B, rate of change curves for 

March 29, 2007, at station 6 in Laguna Grande, Fajardo, Puerto Rico.
Biological Characteristics 25

Table 9. Primary productivity results for the March 29, 2007 and 

November 15, 2007diel studies at Laguna Grande, Fajardo,

Puerto Rico. 

[All values are in grams of oxygen per cubic meter per day. Productivity calculated for a 24-hour period]

Station 

number

Net 

productivity Respiration Gross productivity

March 29, 20071

 6 1.47 5.76 7.23

 7 1.87 6.72 8.59

 8 5.13 21.36 26.49

 9 1.49 11.33 12.82

 10 1.62 12.96 14.58

Average 2.32 11.63 13.94

November 15, 20072

 6 4.01 2.40 6.41

 7 3.93 9.60 13.53

 8 22.94 62.40 85.34

 9 0.27 0.96 1.23

 10 14.42 101.04 115.46

Average 9.11 35.28 44.39

1 Study commenced at 6:00 a.m. and ended at 7:23 p.m.

2 Study commenced at 5:30 a.m. and ended at 6:46 p.m.

The net organic carbon production of Laguna Grande is 

about 507 tC/yr. This value was obtained by multiplying the 

daily oveall net primary productivity of 2.1 gC/m3-d by 365 to 

obtain a yearly value of 766.5 gC/m3-yr. The yearly value was 

multiplied by the lagoon water volume of 661,582 m3 to obtain 

507,102,603 gC/yr, or roughly 507 tC/yr. 

The respiration rate ascribed to the water column 

(plankton plus bacteria) averaged about 2.9 gO2/m3, whereas 

the respiration rate by all organisms present in Laguna 

Grande, which include plankton, benthos, bacteria, fish, 

and invertebrates, averaged 23.4 gO2/m3-d. Therefore, the 

respiration rate for benthos, bacteria, fish, and invertebrates 

alone was 20.5 gO2/m3-d (fig. 7). About 88 percent of the 

respiration in Laguna Grande was a function of benthic and 

non-phytoplankton nektonic organisms (including bacteria).

The gross productivity for water column organisms 

(plankton) was 3.7 gO2/m3; the gross productivity for 

all organisms, which include plankton, periphyton and 

macrophytes, fish, and invertebrates averaged 29.1 gO2/m3-d. 

This finding indicates that at the time of the diel studies, the 

gross productivity for all organisms excluding plankton was 

25.4 gO2/m3-d. This value is an estimate at the time of data 

collection, however, and only represents a snapshot of the 

community dynamics in Laguna Grande, which are complex 

because higher motile organisms migrate into and out of the 

lagoon. As with respiration, about 88 percent of the gross 

productivity in Laguna Grande occurs through the metabolism 

of benthic and higher organisms. 

The highest community net and gross productivity 

occurred at station 10, located along the western littoral 

zone, during the November 2007 sampling. The high 

productivity observed at this site indicates that the algal mats 

and seagrasses on the western littoral zone of the lagoon are 

quite prolific. The second highest community net and gross 

productivity occurred at station 8 near the northern portion of 

Laguna Grande also during the November 2007 sampling. On 

the basis of the community productivity results, it is presumed 

that algal mats and seagrasses could also be present on the 

lagoon bottom at station 8.

Phytoplankton Biomass

Phytoplankton biomass is defined as the mass or weight 

of photosynthetic organisms present in the water column per 

unit of area or volume, at a given instant in time (Wetzel, 

2001), and is synonymous with phytoplankton standing crop. 

As noted earlier, phytoplankton biomass was determined 

in Laguna Grande at stations 6 and 8 from March 2007 to 

June 2008, where light and dark bottle primary productivity 

measurements were made (fig. 3). The average phytoplankton 

biomass at the two stations in Laguna Grande ranged from 6.0 

mg/L in October and November 2007 to 13.6 mg/L in January 

2008 (table 10). 

The average phytoplankton biomass between March 

2007 and June 2008 was 8.8 mg/L, which is equal to 8.8 g/

m3. Assuming that samples from the top of the water column 

are representative of the entire water column, and an estimated 

water volume of 662,000 m3, Laguna Grande contained a 

phytoplankton standing crop of approximately 5.8 t. 

The ratio of gross productivity to biomass (P/B) is 

a weighted mean value of biomass growth rates for all 

individuals in a population (Benke, 1993); the ratio can be 

used to estimate the turnover rate of organisms, yielding a 

general index of the rate of energy flow relative to biomass. 

The P/B ratio generally decreases as the trophic level increases 

(Benke, 1993).

Based on the average plankton daily gross productivity 

of 3.7 gO2/m3, the annual plankton gross productivity was 

1,350.5 gO2/m3. Using the average annual phytoplankton 

biomass of 8.8 g/m3, the P/B ratio (1,350.5/8.8) is about 153, 

which indicates that the phytoplankton community has a 

turnover rate of about 153 times per year, or about once about 

every 2.4 days. Phytoplankton gross productivity generally 

correlates with phytoplankton biomass for most months at 

both sites (fig. 9), with the exception of July 2007 and January 

2008 at station 6, and July 2007 and May 2008 at station 8. 

The observed correlations indicate that biomass determination, 

which is a simple and relatively inexpensive method, could 

probably be used to estimate gross plankton productivity.
26 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 9. Graphs showing gross productivity rates and phytoplankton biomass at A, stations 6, and B, 

station 8 in Laguna Grande, Fajardo, Puerto Rico.
Sediment Accretion 27

Fecal Indicator Bacteria

Microorganisms such as fecal coliform bacteria can be 

used to indicate the presence of human and animal waste. 

Because coliform bacteria cannot live for prolonged periods 

outside a host organism, a positive test result indicates recent 

fecal contamination or inappropriate sewer processing. 

Fecal indicator bacteria were sampled and analyzed on 

a roughly monthly basis in Laguna Grande at stations 1, 3, 5, 

and 7 (fig. 3). Stations 1, 3, and 5 are presumed to be affected 

by human activity and sewer discharge. Station 7, located in 

the middle of the lagoon was sampled to document possible 

sewage dispersion by wind and water currents.

Potential fecal contamination sources to the lagoon 

include: (1) river discharge that flows by way of longshore 

ocean currents into the lagoon through the inlet channel during 

high tide; (2) overflow of a sewer main from the area of Las 

Croabas south of the lagoon during periods of storm runoff; 

(3) possible illegal raw sewage discharge from housing units 

along the western flank of the Laguna Grande Channel; and 

(4) discharge from unsewered housing units at the end of the 

southwestern channel in an area known as Soroco. 

The PREQB fecal coliform standard for Class SB waters 

specifies that the geometric mean concentration of at least five 

samples collected sequentially should not exceed 200 col/100 

mL, and not more than 20 percent of the samples should 

exceed 400 col/100 mL (Puerto Rico Environmental Quality 

Board, 2003). The regulations do not specify a time period 

during which the sequential samples must be obtained. 

Fecal coliform concentrations often exceeded the 200 

col/100 mL PREQB standard (app. 4), mostly at stations 1, 

3 and 5, located adjacent to developed/populated areas with 

possible illegal sewage discharges. In contrast, this limit was 

exceeded only twice at station 7 on the 12 sampling dates. 

The highest concentrations were observed at station 5 

near the Soroco area, where the fecal bacteria concentrations 

exceeded 200 col/100mL on all but two of the sampling dates. 

Among samples that exceeded the standard, counts ranged 

from 750 col/100mL, to 60,000 col/100mL, the latter value 

was observed twice during the study (May 31, 2007, and 

November 14, 2007). The coliform bacteria count was also 

high on January 29, 2008, at 39,000 col/100mL. 

Fecal bacteria counts exceeded 200 col/100mL in half 

of the samples at station 3, halfway from the channel mouth 

to the lagoon and near a developed area; concentrations at 

this site ranged from 450 to 8,270 col/100mL. At the channel 

entrance (station 1), fecal bacteria colony counts exceeded 200 

col/100mL five times, and ranged from 240 to 860 col/100mL. 

At station 7, fecal bacteria counts exceeded the standard twice, 

with 240 col/100mL measured on May 31, 2007, and 970 

col/100mL measured on November 14, 2008.

Fecal bacteria colony counts indicate that station 5 

at Soroco appears to have the largest fecal coliform input 

followed by station 3 halfway along the Laguna Grande 

channel, and station 1 at the channel entrance.

Sediment Accretion

Coastal lagoons lose depth and water volume as they 

age, primarily by (1) sedimentation from erosion and runoff 

transported within the drainage basin, (2) deposition from 

marine coastal currents, and to a minor extent, (3) atmospheric 

deposition. As a consequence, coastal lagoons have relatively 

short life spans that are on the order of thousands of years. To 

document this process and for future monitoring of sediment 

deposition in Laguna Grande, 10 sediment cores were 

collected on October 20, 2008, at station 7 in the center of the 

lagoon (fig. 3) and analyzed for radioisotopes. 

The individual core samples from station 7 were analyzed 

for 137Cs and 210Pb radioisotopes. The 137Cs radioisotope is 

primarily a byproduct of atmospheric nuclear-bomb testing 

and has a half-life of about 30 years. This radioisotope does 

not occur naturally, and partitions from the atmosphere to fine 

sediment particles that are then transported to water bodies 

by runoff. Consequently, sediment layers deposited shortly 

after periods of major nuclear testing will have relatively high 

radioisotope activity. The year of first occurrence (1952), 

along with the year of maximum above-ground testing (1963), 

are used for dating. The activity peaks in terrestrial sediment 

may lag a year or two behind the test dates because of the 

travel time in the atmosphere prior to deposition in bottom 

sediments. Therefore, the year of first ocurrence is assigned as 

the year 1954 instead of 1952.

Table 10. Phytoplankton biomass at stations 6 and 8 from March 

2007 to June 2008. 

[All concentrations are in milligrams per liter]

Sampling date Station 6 Station 8

Mar. 28, 2007 6.4 6.2

May 31, 2007 8.6 8.0

June 27, 2007 9.0 8.6

July 24, 2007 11.4 10.0

Sep. 11, 2007 8.0 7.8

Oct. 16, 2007 6.0 6.0

Nov. 14, 2007 6.0 6.0

Jan. 29, 2008 13.6 9.2

Mar. 4, 2008 7.4 10.2

Apr. 1, 2008 10.3 10.2

May 13, 2008 10.6 12.8

June 17, 2008 8.4 10.0

Average 8.8 8.8
28 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Figure 10. Graphs showing activity of A, lead-210, and B, cesium-137 compared to depth in the Laguna Grande sediment core.
Summary and Conclusions 29

In contrast to the rapid change in fluxes of 137Cs, the 210Pb 

method depends on the constant flux of radon gas from the 

Earth’s surface (Krishnaswami and others, 1971). 210Pb has a 

half-life of 22.3 years, and is a member of the uranium-238 

(

238U) decay series. Natural 238U decays to radon-222 (222Rn), 

a natural gas in the atmosphere that has a half-life of 3.8 days, 

and eventually decays to 210Pb, which precipitates as a solid 

adsorbed to particles. If the sedimentation rate of a water body 

and the 210Pb fluxes are constant, the age of the sediment can 

be determined by assuming the excess 210Pb activity decreases 

with depth (downcore) by a factor of two every 22.3 years. 

The excess activity is the total activity minus the activity 

supported by the in-situ decay of radium-226 (226Ra). 

To use this 210Pb-based sediment deposition rates 

method, the radioactivity profile must indicate monotonically 

decreasing activity downward from the top of the core. Both 

137Cs and 210Pb were used and an average rate was estimated 

from the data, because the 210Pb radioactivity profile exhibited 

monotonically decreasing activity (fig. 10).

The sediment accretion rate determined using 210Pb 

was calculated by using the top of the core activity of 1.25 

× 10-4 mBq/g, and locating the layer of sediment that had 

half or nearly half of the top-core activity, which in this case 

corresponded to the 10-cm layer. Because the radioactivity of 

210Pb decreases by a factor of two every 22.3 years, the 10-cm 

layer was estimated to be deposited during and/or near the 

year 1986 (2008 - 22.3 = 1985.7). Therefore, an accumulation 

of 10 cm divided by 22.3 years yields a sediment deposition 

rate of about 0.45 cm/yr. 

The first detectable 137Cs activities were found in the 

24-cm layer, which was assigned a year of 1954. Dividing 24 

cm by 54 years (2008-1954), the depositional rate using 137Cs 

as a marker is 0.44 cm/yr, which is in agreement with the rate 

derived from 210Pb data. 

Based on the radioactivity decay for both isotopes, the 

average and most probable long-term sediment depositional 

rate in Laguna Grande is approximately 0.44 cm/yr. This 

sediment accretion rate is in close agreement with other 

accretion rates estimated for other coastal lagoons in Puerto 

Rico with similar hydrologic characteristics, such as Laguna 

Los Corozos (0.49 cm/yr), and Laguna Torrecilla (0.37 cm/yr), 

located within the San Juan Bay estuary system (Webb and 

Gómez-Gómez, 1997).

Assuming a long-term sediment accretion rate of about 

0.44 cm/yr and a mean water depth of about 3-4 m for Laguna 

Grande, the lagoon is estimated to have a remaining lifespan 

on the order of 700 to 900 years before it becomes marshland. 

This estimate, however, does not consider possible Laguna 

Grande basin modifications that could increase sediment 

deposition rates, or any potential future water-level change that 

could alter the lagoon bathymetry. 

Summary and Conclusions

Laguna Grande is a unique ecosystem on the eastern 

coast of Puerto Rico with ecological and recreational value, 

and is one of the few remaining coastal lagoons that supports 

bioluminescent dinoflagellates. Development and human 

activities, including unregulated ecotourism, are having 

deleterious effects on the lagoon. Unregulated recreational 

use of Laguna Grande may already be affecting its ecosystem. 

Planned construction projects threaten to alter the basin by 

modifying runoff into the lagoon (and thus, its hydrology), 

and accelerate sedimentation in the lagoon. To address the 

aforementioned issues, the U.S. Geological Survey (USGS) 

in cooperation with the Puerto Rico Environmental Quality 

Board (PREQB) for the Conservation Trust of Puerto Rico, 

initiated a study in 2007 to determine selected hydrologic, 

water-quality, biological and sedimentation characteristics of 

Laguna Grande.

Historical rainfall data indicate that rainfall was about 44 

percent below average for months with available data during 

the period of study. Because extreme rainfall did not occur 

in the basin during the period of study, the effect of such an 

event on the lagoon hydrology and water quality was not 

documented. Freshwater inflow to Laguna Grande is estimated 

to be 184,575 m3/yr, or about 28 percent of the water volume 

in the lagoon. Seawater exchange between Laguna Grande 

and the Atlantic Ocean occurs through the 1.5-km Laguna 

Grande Channel and is driven by tidal fluctuations. During 

normal wind and tide conditions, the lagoon daily flushing 

rate is estimated to be 13 percent of its water volume, which 

corresponds to a flushing rate of once every 7.7 days or about 

47 times per year.

A bathymetric survey indicated that Laguna Grande has a 

water volume of approximately 662,000 m3. At the time of this 

study, the maximum water depth was about 5 m and the mean 

water depth was about 3 m.

The physical and chemical properties of water in Laguna 

Grande were found to be in compliance with the PREQB 

standards, except in areas influenced by human development. 

The lagoon is a generally homogeneous water body, 

exhibiting temporal rather than spatial variation, with some 

vertical stratification within the main water body. The water 

temperature ranged from about 26 to 31 °C, with areas of 

higher temperatures near developed areas and shallow waters. 

The pH of the water ranged from 7.1 to 7.9 and did not 

exceed PREQB standards. Dissolved oxygen concentrations 

varied areally and temporally, with vertical stratification in 

the water column of the lagoon main water body. Dissolved 

oxygen concentrations varied most at stations along the 

eastern periphery of the lagoon where sea grasses are abundant 

and varied less in the center of the lagoon. Dissolved oxygen 

concentrations were lowest adjacent to an area locally referred 

to as Soroco, where fecal coliform concentrations exceeded 

PREQB standards in most samples.
30 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Water transparency was consistently high, with all 

readings greater than 70 cm. Because the lagoon has no 

perennial streams, sediment resuspension, high algal 

counts (“self shading”), as well as runoff from the basin, 

are presumed to be the principal factors affecting water 

transparency and light penetration. Specific conductance and 

salinity varied temporally but not areally, and were directly 

associated with coastal tides and rainfall runoff. 

Total nitrogen and total phosphorus concentrations were 

below the PREQB standards of 5 and 1 mg/L, respectively. 

Total nitrogen concentrations ranged from 0.14 to 0.38 mg/L, 

with a mean of about 0.28 mg/L. Of the nitrogen species 

present in Laguna Grande, organic nitrogen composed about 

75 percent of the total nitrogen, ammonium composed 20 

percent, and nitrate and nitrite collectively composed the 

remaining 5 percent. Total phosphorus concentrations ranged 

from 0.01 to 0.11, with a mean of 0.02 mg/L. The average 

concentration of chlorophyll a ranged from about 2.2 to 13 

μg/L, with a mean concentration of 6.2 μg/L. 

Light and dark bottle primary productivity measurements 

were made at about monthly intervals between March 2007 

and June 2008. The average phytoplankton net primary 

productivity rate was about 0.8 gO2/m3-d (0.3 gC/m3-d), the 

plankton daily respiration rate was about 2.9 gO2/m3, and 

the mean daily plankton gross productivity was 3.7 gO2/m3. 

Diel primary productivity studies were conducted on March 

29, 2007, and November 15, 2008, and the net community 

primary productivity averaged 5.7 gO2/m3-d (2.1 gC/m3-d). 

The mean community respiration rate was 23.4 gO2/m3-d, and 

the mean community gross productivity was 29.1 gO2/m3-d. 

Periphyton and macrophytes were responsible for about 86 

percent of the total primary productivity, and yielded 4.9 gO2/

m3-d (1.8 gC/m3-d). Primary productivity results indicate that 

Laguna Grande is mesotrophic, with relatively high primary 

productivity, and constitutes an important food supply for 

primarily aquatic organisms. Most of the biomass standing 

crop, however, is generated by benthic organisms (periphyton 

and macrophytes), indicating that most primary production 

occurs at the sediment/water interface rather than in the water 

column.

The average phytoplankton biomass values in Laguna 

Grande ranged from 6.0 to 13.6 mg/L. During the study, 

Laguna Grande is estimated to have contained a phytoplankton 

standing crop of about 5.8 t. Phytoplankton community had a 

turnover rate of about 153 times per year, equivalent to about 

once every 2.4 days. 

Fecal indicator bacteria samples were collected at 

about monthly intervals during the study at three locations. 

Fecal coliform bacteria counts ranged from 160 to 60,000 

col/100mL, and often exceeded the 200 col/100mL PREQB 

standard, particularly in areas near housing units and human 

activity. Fecal bacteria counts in the Socoro area exceeded 

the 200 col/100mL standard on all but two samplings, but 

never exceeded the standard near the center of the lagoon. 

The distribution of fecal coliform bacteria indicate that water 

quality is impaired near developed areas in the southern part of 

Laguna Grande.

Sediment cores collected at the middle of Laguna Grande 

indicated that sedimentation in the lagoon occurs at a rate of 

about 0.44 cm/yr. This process will convert Laguna Grande 

into a marshland in about 700 to 900 years if not altered by 

human and/or natural causes. 

The data obtained from this study will provide reserve 

managers with a holistic perspective of the local hydrology 

and its relation to water-quality conditions in Laguna Grande. 

Specifically, information from this study will be used by the 

CTPR to establish baseline conditions and to monitor changes 

in the ecological health of the lagoon. In addition, the data 

provide stakeholders with a broad understanding of natural 

processes that can be used effectively to monitor and manage 

the Las Cabezas de San Juan Natural Reserve watershed 

and coastal surrounding areas. Protection of the ecological 

conditions will promote habitat utilization by the indigenous 

fauna that attract thousands of visitors to the reserve annually. 

Continued monitoring of Laguna Grande may be 

necessary to further document the effects of development in 

nearby areas. This information will allow timely and informed 

decisionmaking by resource managers, and help prevent 

further and potentially irreversible impairment to Laguna 

Grande.
References Cited 31

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Benke, A.C., 1993, Concepts and patterns of invertebrate 

production in running waters: Verhandlungen Internationale 

Vereinigung für Theoretische und Angewandte Limnologie, 

v. 25, p. 15-38.

Boccheciamp, Rafael, A., 1977, Soil Survey of the Humacao 

area of eastern Puerto Rico, San Juan, PR: U.S. Department 

of Agriculture, Soil Conservation Service and University of 

Puerto Rico College of Agricultural Sciences, 103 p.

Clesceri, L.S., Greenberg, A.E., and Eaton, A.E., eds., 

1998, Standard Methods for the examination of water and 

wastewater (20th ed.): Washington D.C., American Public 

Health Association, 1,220 p.

Díaz, P.L., Aquino, Zaida, Figueroa-Alamo, Carlos, García, 

René, and Sánchez, A.V., 2004, Water resources data Puerto 

Rico and the U.S. Virgin Islands, Water Year 2002: U.S. 

Geological Survey Water-Data Report PR-02-1, p. 274-311.

Environmental Systems Research Institute, Inc., 1992, Surface 

modeling with TIN, Surface analysis and display: Redlands, 

California, Environmental Systems Research Institute, Inc., 

p. 4-1, 6-1.

Fox, R., 2000, Phytoplankton primary productivity: 

Laboratory exercises for biology 306: Accessed October 

2004 at http://www.lander.edu/RSFOX/306prod-BLab.html.

Giusti, E.V., and López, M.A., 1967, Climate and streamflow 

of Puerto Rico: Caribbean Journal of Science, v. 7, nos. 3-4, 

p. 87-93.

Hutchinson, G.E., 1973, Eutrophication—The scientific 

background of a contemporary problem: American 

Scientist, v. 61, p. 269-279.

Krishnaswami, S., Lal, D., Martin, J.M., and Maybeck, 

M., 1971, Geochronology of lake sediments: Earth and 

Planetary Sciences Letters, v. 11, p. 407-414.

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Investigations book 3, chap. A17.

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to biological oceanography, primary productivity: Chap. 

13-16, accessed October 2004 at http://www.duedall.fit.edu/

ocn1010eng/13apr.htm.

National Oceanic and Atmospheric Administration, 1948-1996 

Southeast Regional Climate Center, climate information, 

Historical climate summaries for Puerto Rico and the U.S. 

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state.sc.us/cgi-bin/sercc/cliMAIN.pl?pr7295.

Odum, H.T., 1956, Primary production in flowing waters: 

Limnology and Oceanography, v. 1, no. 2, p. 102-117.

Puerto Rico Environmental Quality Board, 2003, Junta 

de Calidad Ambiental de Puerto Rico, reglamentos de 

estándares de calidad de agua de Puerto Rico: 82 p. 

(Amended March 2003.)

Redfield, A.C., Ketchum, B.H., and Richards, F.A., 1963, The 

influence of organisms on the composition of sea-water, 

chap. 2 of The sea—Ideas and observations of progress in 

the study of the seas: M.N. Hill, ed.: New York, Wiley & 

Sons, v. 2, p. 26-77.

Soler-López, Luis R., Gómez-Gómez, Fernando, and 

Rodríguez-Martínez, Jesús, 2005, Hydrologic, waterquality, and biological assessment of Laguna de Las Salinas, 

Ponce, Puerto Rico, January 2003-September 2004: U.S. 

Geological Survey Scientific Investigations Report 2005-

5154, 50 p.

Vollenweinder, R.V., 1968, The scientific basis of lake 

and stream eutrophication, with particular reference to 

phosphorus and nitrogen as eutrophication factors: Paris, 

Organization of Economic Cooperation and Development, 

Technical Report DAS/CSI/68:27: p. 1-182.

Weaver, P.L., Ramírez, J.L., and Coll-Rivera, J.L., 1999, Las 

Cabezas de San Juan Nature Reserve (El Faro): U.S. Forest 

Service, International Institute of Tropical Forestry, General 

Technical Report IITF-5, 58 p. 

Webb, R.M.T., and Gómez-Gómez, Fernando, 1997, Synoptic 

survey of water quality and bottom sediments, San Juan Bay 

estuary system, Puerto Rico, December 1994-July 1995: 

U.S. Geological Survey Water-Resources Investigations 

Report 97-4144, 69 p.

Wetzel, R.G., 2001, Limnology: Lake and River Ecosystems 

(3d ed.): Academic Press, p. 129-150.
32 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

APPENDIX
APPENDIX 33

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 1 (50069660)

Mar. 28, 2007 9:35 a.m. Top 7.6 26.0 53,700 2.9 35.4

Bottom 7.6 26.1 53,700 2.9 35.4 61

May 31, 2007 7:40 a.m. Top 7.6 29.1 56,010 4.2 37.1

Bottom 7.6 29.2 56,000 4.2 37.1 76

June 27, 2007 7:25 a.m. Top 7.6 29.3 55,200 1.7 36.5

Bottom 7.6 29.4 55,300 1.7 36.6 86

July 24, 2007 9:15 a.m. Top 7.5 29.7 55,300 0.5 36.6

Bottom 7.5 29.7 55,300 0.6 36.6 46

Sept. 11, 2007 7:10 a.m. Top 7.9 29.1 56,010 4.2 37.1

Bottom 7.9 29.2 56,000 4.2 37.1 112

Oct. 16, 2007 7:05 a.m. Top 7.7 29.0 56,860 3.7 37.7

Bottom 7.6 29.0 56,860 3.6 37.7 61

Nov. 14, 2007 7:15 a.m. Top 7.6 27.4 46,650 4.1 30.2

Bottom 7.6 27.4 46,650 4.2 30.2 76

Jan. 29, 2008 8:40 a.m. Top 7.6 25.4 53,220 4.5 35.1

Bottom 7.6 25.4 53,220 4.2 35.1 76

Mar. 4, 2008 7:25 a.m. Top 8.1 25.1 57,250 4.9 38.1

Bottom 8.1 25.1 57,250 5.0 38.1 76

Apr. 1, 2008 7:35 a.m. Top 8.0 25.5 56,330 5.1 37.4

Bottom 8.0 25.5 56,310 5.0 37.4 91

May 13, 2008 6:45 a.m. Top 7.6 27.3 55,080 1.4 36.4

Bottom 7.5 27.3 55,100 1.4 36.5 51

June 17, 2008 7:00 a.m. Top 7.9 28.5 55,970 2.9 37.1

Bottom 7.9 28.6 55,950 2.9 37.1 74
34 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 2 (50069670)

Mar. 28, 2007 Top -- -- -- -- -- --

Bottom -- -- -- -- -- --

May 31, 2007 7:25 a.m. Top 7.8 28.5 53,200 3.1 35.0

Bottom 8.0 28.6 54,000 4.7 35.6 244

June 27, 2007 7:17 a.m. Top 8.0 28.9 54,100 4.1 35.7

Bottom 8.0 28.9 54,100 4.0 35.7 91

July 24, 2007 – 7:00 a.m. Top 8.0 28.8 55,000 3.8 36.3

Bottom 8.0 28.8 55,000 4.1 36.3 137

Sept. 11, 2007 7:00 a.m. Top 8.1 29.6 55,460 5.5 36.7

Bottom 8.1 29.6 55,500 5.5 36.7 213

Oct. 16, 2007 7:00 a.m. Top 7.9 29.0 55,820 5.0 36.9

Bottom 7.9 29.0 55,840 4.9 37.0 213

Nov. 14, 2007 7:05 a.m. Top 7.8 27.2 50,330 4.5 32.8

Bottom 7.9 27.6 53,860 4.3 35.5 94

Jan. 29, 2008 8:30 a.m. Top 7.8 25.7 55,810 4.5 37.0

Bottom 8.0 26.1 56,250 5.5 37.3 221

Mar. 4, 2008 7:14 a.m. Top 8.1 24.9 57,270 5.0 38.2

Bottom 8.0 24.4 57,310 4.7 38.2 91

Apr. 1, 2008 7:25 a.m. Top 8.1 25.7 56,370 5.5 37.5

Bottom 8.1 25.7 56,390 5.4 37.5 89

May 13, 2008 6:35 a.m. Top 7.9 27.4 54,650 3.5 36.1

Bottom 7.9 27.2 54,660 2.8 36.1 203

June 17, 2008 6:55 a.m. Top 8.0 29.0 55,900 3.9 37.0

Bottom 8.0 29.0 55,930 3.6 37.0 183

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
APPENDIX 35

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 3 (50069650)

Mar. 28, 2007 9:25 a.m. Top 7.7 26.2 53,800 3.0 35.5

Bottom -- -- -- -- -- 7

May 31, 2007 7:25 a.m. Top 7.6 29.8 51,600 2.7 34.0

Bottom -- -- -- -- -- 61

June 27, 2007 7:17 a.m. Top 7.6 29.6 55,400 2.0 36.6

Bottom -- -- -- -- -- 112

July 24, 2007 7:25 a.m. Top 7.7 29.9 56,200 2.9 37.2

Bottom -- -- -- -- -- 8

Sept. 11, 2007 7:20 a.m. Top 7.8 29.2 56,090 3.4 37.1

Bottom -- -- -- -- -- 107

Oct. 16, 2007 7:15 a.m. Top 7.6 29.0 56,870 4.2 37.7

Bottom -- -- -- -- -- 76

Nov. 14, 2007 7:25 a.m. Top 7.6 27.7 47,310 4.5 30.7

Bottom -- -- -- -- -- 53

Jan. 29, 2008 8:45 a.m. Top 7.6 25.6 53,140 4.8 35.1

Bottom -- -- -- -- -- 71

Mar. 4, 2008 7:33 a.m. Top 8.0 24.9 57,210 4.5 38.1

Bottom -- -- -- -- -- 81

Apr. 1, 2008 7:40 a.m. Top 7.9 25.4 56,500 4.1 37.6

Bottom -- -- -- -- -- 61

May 13, 2008 6:55 a.m. Top 7.6 27.6 54,840 1.8 36.3

Bottom -- -- -- -- -- 61

June 17, 2008 7:10 a.m. Top 7.7 29.6 57,230 2.0 38.0

Bottom -- -- -- -- -- 51

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
36 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 4 (50069630)

Mar. 28, 2007 9:08 a.m. Top 7.8 26.7 55,200 3.7 36.5

Bottom 6.8 27.4 54,800 3.4 36.4 157

May 31, 2007 8:05 a.m. Top 7.7 30.2 53,700 3.5 35.3

Bottom 7.7 30.6 54,300 3.7 36.0 94

June 27, 2007 8:03 a.m. Top 7.8 29.8 55,400 4.4 36.6

Bottom 7.8 29.8 55,400 3.6 36.6 76

July 24, 2007 7:35 a.m. Top 7.9 30.0 56,300 4.8 37.3

Bottom 7.9 30.0 56,300 5.0 37.3 91

Sept. 11, 2007 7:30 a.m. Top 7.8 30.3 56,900 4.8 30.3

Bottom 7.8 30.3 56,880 4.0 30.3 91

Oct. 16, 2007 7:25 a.m. Top 7.7 29.1 56,780 4.9 37.7

Bottom 7.7 29.1 56,740 4.6 37.6 91

Nov. 14, 2007 7:30 a.m. Top 7.7 27.5 46,995 4.7 30.6

Bottom 7.6 28.4 48,950 4.4 31.9 107

Jan. 29, 2008 8:55 a.m. Top 7.6 26.0 53,580 5.3 35.4

Bottom 7.6 26.0 53,580 5.2 35.4 76

Mar. 4, 2008 7:42 a.m. Top 7.9 25.5 57,320 4.2 38.2

Bottom 7.9 25.3 57,270 4.0 38.1 122

Apr. 1, 2008 7:50 a.m. Top 7.9 26.1 57,220 4.5 38.1

Bottom 7.9 26.3 57,310 4.6 38.1 107

May 13, 2008 7:00 a.m. Top 7.8 29.1 55,480 3.1 36.7

Bottom 7.7 28.9 55,420 2.6 36.6 152

June 17, 2008 7:20 a.m. Top 7.7 29.7 57,110 3.2 37.9

Bottom 7.7 29.7 57,210 2.1 38.0 76

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
APPENDIX 37

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 5 (50069640)

Mar. 28, 2007 8:50 a.m. Top 7.5 27.0 49,000 2.3 29.5

Bottom -- -- -- -- -- 46

May 31, 2007 8:15 a.m. Top 7.3 26.9 20,600 0.1 12.4

Bottom -- -- -- -- -- 15

June 27, 2007 7:53 a.m. Top 7.3 26.9 58,800 0.2 39.2

Bottom -- -- -- -- -- 30

July 24, 2007 7:50 a.m. Top 7.4 27.1 50,100 0.1 32.7

Bottom -- -- -- -- -- 20

Sept. 11, 2007 7:40 a.m. Top 7.6 29.1 57,380 2.0 38.1

Bottom -- -- -- -- -- 46

Oct. 16, 2007 7:35 a.m. Top 7.3 26.5 60,240 0.2 40.4

Bottom -- -- -- -- -- 46

Nov. 14, 2007 7:45 a.m. Top 7.4 24.7 11,840 0.8 6.8

Bottom -- -- -- -- -- 15

Jan. 29, 2008 9:05 a.m. Top 7.4 24.3 47,770 2.5 31.1

Bottom -- -- -- -- -- 38

Mar. 4, 2008 7:53 a.m. Top 7.7 25.0 56,600 2.9 37.6

Bottom -- -- -- -- -- 23

Apr. 1, 2008 8:00 a.m. Top 7.7 25.5 58,530 2.7 39.1

Bottom -- -- -- -- -- 15

May 13, 2008 7:05 a.m. Top 7.6 27.7 57,090 1.3 37.9

Bottom -- -- -- -- -- 20

June 17, 2008 7:30 a.m. Top 7.3 26.9 59,870 0.3 39.9

Bottom -- -- -- -- -- 15

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
38 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 6 (50069620)

Mar. 28, 2007 6:50 a.m. Top 7.8 26.7 55,300 4.3 36.6

Bottom 6.8 27.0 55,800 4.0 37.0 152

May 31, 2007 6:30 a.m. Top 7.7 29.4 52,400 4.5 34.1

Bottom 7.7 31.3 55,300 4.1 36.6 170

June 27, 2007 6:00 a.m. Top 7.7 29.7 55,600 4.2 36.7

Bottom 7.7 29.7 55,600 4.0 36.7 91

July 24, 2007 6:10 a.m. Top 7.8 30.2 56,200 4.9 37.2

Bottom 7.9 30.2 56,300 3.5 37.2 76

Sept. 11, 2007 6:10 a.m. Top 7.7 30.4 56,850 5.0 37.7

Bottom 7.7 30.2 56,810 1.5 37.7 170

Oct. 16, 2007 6:10 a.m. Top 7.4 29.1 56,740 4.9 37.6

Bottom 7.6 29.2 56770 4.5 37.6 201

Nov. 14, 2007 6:05 a.m. Top 7.6 26.9 45,130 6.1 29.2

Bottom 7.4 29.5 53,800 1.0 35.4 122

Jan. 29, 2008 8:05 a.m. Top 7.6 26.0 53,510 5.3 35.3

Bottom 7.7 26.1 53,840 5.0 35.4 122

Mar. 4, 2008 6:25 a.m. Top 7.7 25.6 57,430 5.3 38.2

Bottom 7.6 25.7 57,360 0.3 38.2 91

Apr. 1, 2008 6:30 a.m. Top 7.8 26.4 57,410 5.6 38.2

Bottom 7.9 26.4 57,500 2.8 38.3 122

May 13, 2008 5:55 a.m. Top 7.7 29.3 55,500 3.3 36.7

Bottom 7.3 29.4 48,210 0.1 31.4 122

June 17, 2008 6:05 a.m. Top 7.4 29.7 56,900 4.2 37.7

Bottom 7.6 29.7 56,830 0.6 37.7 107

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
APPENDIX 39

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 7 (50069610)

Mar. 28, 2007 

8:30 a.m. Top 7.8 26.7 55,500 4.6 36.7

Bottom 7.8 27.1 56,000 3.7 37.1 183

May 31, 2007

8:30 a.m. Top 7.7 30.3 53,700 4.2 35.4

Bottom 7.8 31.0 54,300 3.3 35.8 183

June 27, 2007

8:13 a.m. Top 7.8 29.7 55,400 4.3 36.6

Bottom 7.8 29.7 55,400 4.0 36.6 91

July 24, 2007

8:00 a.m. Top 7.9 29.9 56,300 4.8 37.3

Bottom 7.8 30.0 56,300 4.8 37.3 91

Sept. 11, 2007 7:50 a.m. Top 7.8 30.3 56,930 5.1 37.7

Bottom 7.8 30.3 56,850 2.4 37.7 152

Oct. 16, 2007 6:35 a.m. Top 7.7 29.2 56,480 4.9 37.4

Bottom 7.6 29.3 56,860 3.8 37.7 107

Nov. 14, 2007 6:35 a.m. Top 7.7 26.5 44,410 5.9 28.7

Bottom 7.6 29.5 53,800 2.8 29.5 91

Jan. 29, 2008 8:20 a.m. Top 7.7 25.8 53,370 5.7 35.2

Bottom 7.6 26.5 54,700 4.0 36.2 107

Mar. 4, 2008 6:55 a.m. Top 7.9 25.6 57,440 5.2 38.3

Bottom 7.9 25.5 57,430 4.8 38.3 107

Apr. 1, 2008 7:05 a.m. Top 7.9 26.5 57,640 5.2 38.4

Bottom 7.9 26.5 57,630 5.2 38.4 119

May 13, 2008 6:20 a.m. Top 7.8 29.1 55,400 3.6 36.6

Bottom 7.6 29.1 55,220 0.3 36.6 157

June 17, 2008 6:30 a.m. Top 7.7 29.9 57,030 4.0 37.8

Bottom 7.7 30.0 57,160 1.2 37.9 91

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
40 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Sample collection Sample 

position

Constituent

Date Time pH T SC DO SAL SD

Station 8 (50069600)

Mar. 28, 2007 8:00 a.m. Top 7.8 26.4 55,100 4.7 36.4

Bottom 7.9 27.1 56,000 4.1 37.2 180

May 31, 2007 8:50 a.m. Top 7.7 28.2 49,000 4.3 31.4

Bottom 7.8 31.2 54,600 3.8 36.0 168

June 27, 2007 5:45 a.m. Top 7.8 29.8 55,500 4.2 36.7

Bottom 7.8 29.8 55,500 4.1 36.7 76

July 24, 2007 6:30 a.m. Top 7.9 29.9 56,300 5.1 37.3

Bottom 7.8 29.9 56,200 0.1 37.2 91

Sept. 11, 2007 6:35 a.m. Top 7.8 30.4 56,750 5.3 37.6

Bottom 7.8 30.3 56,780 4.5 37.6 122

Oct. 16, 2007 6:25 a.m. Top 7.7 29.1 56,760 5.3 37.6

Bottom 7.6 29.2 56,820 3.6 37.7 132

Nov. 14, 2007 6:25 a.m. Top 7.7 26.3 44,910 5.9 29.0

Bottom 7.4 29.5 53,810 0.8 35.4 91

Jan. 29, 2008 7:30 a.m. Top 7.1 26.0 53,510 5.6 35.3

Bottom 7.3 26.5 54,590 1.8 36.1 122

Mar. 4, 2008 6:40 a.m. Top 7.9 25.6 57,430 5.2 38.3

Bottom 7.9 25.6 57,420 4.7 38.2 76

Apr. 1, 2008 6:55 a.m. Top 7.9 26.6 57,620 5.4 38.4

Bottom 7.9 26.7 57,460 0.5 38.3 102

May 13, 2008 6:10 a.m. Top 7.7 29.2 55,520 4.5 36.7

Bottom 7.7 29.2 55,530 0.8 36.7 107

June 17, 2008 6:15 a.m. Top 7.7 29.8 56,380 4.0 37.3

Bottom 7.5 29.9 54,130 0.4 35.6 76

Appendix 1. In situ determination of pH, temperature, specific conductance, dissolved oxygen, salinity, and Secchi disk depth 

for water-quality sites sampled from March 2007 to June 2008 at Laguna Grande, Fajardo, Puerto Rico.—Continued

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

T, temperature, in degrees Celsius; SC, specific conductance, in microsiemens per centimeter; DO, dissolved oxygen, in milligrams per liter; 

SAL, salinity in parts per thousand; SD, Secchi disk depth in centimeters; -- not collected]
APPENDIX 41

Appendix 2. March 29, 2007, diel study physical properties.

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows. Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]

TIME Depth pH Temp SC DO SAL

Station 6

6:46 a.m. Top 7.8 27.0 54,500 4.0 36.1

Bottom 7.8 25.5 55,300 3.8 36.6

7:40 a.m. Top 7.8 26.8 54,300 4.1 35.8

Bottom 7.8 27.2 54,700 3.8 36.2

8:27 a.m. Top 7.8 26.9 54,200 4.2 35.8

Bottom 7.8 27.4 55,100 4.1 36.4

9:27 a.m. Top 7.8 27.1 54,100 3.9 35.8

Bottom 7.8 27.4 54,200 4.1 36.3

10:24 a.m. Top 7.8 27.5 54,200 5.0 35.7

Bottom 7.9 27.2 54,200 4.3 35.7

11:26 a.m. Top 7.8 27.8 54,300 5.1 35.8

Bottom 7.8 27.6 54,300 4.8 35.8

12:30 p.m. Top 7.8 28.0 54,200 5.0 35.7

Bottom 7.8 27.9 54,200 4.1 35.8

1:40 p.m. Top 7.8 28.1 54,100 5.2 35.7

Bottom 7.8 28.0 54,100 4.1 35.7

2:22 p.m. Top 7.8 28.1 54,100 5.4 35.7

Bottom 7.8 28.1 54,200 4.3 35.8

3:21 p.m. Top 7.8 28.2 54,400 5.3 35.9

Bottom 7.8 28.2 54,400 4.4 35.9

4:25 p.m. Top 7.9 28.0 54,100 5.4 35.7

Bottom 7.9 28.0 54,200 4.2 35.8

5:24 p.m. Top 7.9 28.2 54,100 5.2 35.7

Bottom 7.9 28.2 54,400 4.1 35.9

6:28 p.m. Top 7.8 27.9 53,700 4.9 35.3

Bottom 7.9 28.1 54,200 3.9 35.7

7:23 p.m. Top 7.8 27.9 54,000 4.7 35.5

Bottom 7.8 28.0 55,400 3.2 36.6
42 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

TIME Depth pH Temp SC DO SAL

Station 7

6:36 a.m. Top 7.9 26.8 54,500 4.4 36.0

Bottom 7.9 26.8 54,500 3.6 36.0

7:31 a.m. Top 7.8 26.9 54,500 4.5 36.0

Bottom 7.8 26.9 54,600 4.1 36.1

8:20 a.m. Top 7.8 26.9 54,500 5.2 36.0

Bottom 7.8 27.0 54,600 4.3 36.1

9:19 a.m. Top 7.8 27.0 54,300 4.7 35.9

Bottom 7.9 27.1 54,700 3.6 36.2

10:18 a.m. Top 7.8 27.4 54,100 4.7 35.6

Bottom 7.9 27.1 54,300 4.3 35.8

11:20 a.m. Top 7.8 27.5 54,100 5.2 35.7

Bottom 7.8 27.4 54,600 4.5 36.1

12:23 p.m. Top 7.8 27.8 54,300 5.0 35.9

Bottom 7.8 27.7 54,300 4.4 35.9

1:35 p.m. Top 7.8 27.9 54,300 5.2 35.8

Bottom 7.8 27.8 54,300 4.5 35.8

2:17 p.m. Top 7.9 28.2 54,500 5.5 36.0

Bottom 7.8 28.0 54,500 4.6 36.0

3:15 p.m. Top 7.8 27.9 54,500 5.4 36.0

Bottom 7.8 27.9 54,500 4.6 36.0

4:19 p.m. Top 7.9 27.9 54,600 5.4 36.0

Bottom 7.9 27.9 54,600 4.8 36.0

5:14 p.m. Top 7.9 28.2 54,400 5.3 35.9

Bottom 7.9 28.0 54,500 4.6 36.0

6:20 p.m. Top 7.9 28.0 53,800 5.2 35.6

Bottom 7.9 28.2 54,400 4.2 35.9

7:17 p.m. Top 7.8 27.8 53,600 4.5 35.3

Bottom 7.9 27.9 54,700 4.0 36.2

Appendix 2. March 29, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows. Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]
APPENDIX 43

Appendix 2. March 29, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows. Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]

TIME Depth pH Temp SC DO SAL

Station 8

6:10 a.m. Top 7.9 26.7 54,200 4.9 35.8

Bottom 7.9 27.2 55,000 2.4 36.3

7:12 a.m. Top 7.8 26.5 53,800 4.7 35.5

Bottom 7.8 26.7 54,300 4.0 35.9

8:05 a.m. Top 7.8 26.6 53,500 4.7 35.5

Bottom 7.8 26.8 54,300 4.5 35.9

9:03 a.m. Top 7.8 26.7 53,400 4.8 35.2

Bottom 7.9 26.8 54,300 3.7 35.9

10:03 a.m. Top 7.8 27.0 53,400 5.1 35.4

Bottom 7.8 27.1 54,400 3.8 36.0

11:06 a.m. Top 7.8 27.3 54,200 5.2 35.8

Bottom 7.8 27.4 54,700 3.9 36.2

12:07 p.m. Top 7.8 27.6 54,400 5.3 36.0

Bottom 7.8 27.5 54,800 4.3 36.2

1:02 p.m. Top 7.8 27.7 54,600 5.1 36.1

Bottom 7.8 27.7 54,600 4.6 36.1

2:03 p.m. Top 7.8 27.9 54,500 5.2 36.0

Bottom 7.8 27.9 54,600 4.6 36.0

3:02 p.m. Top 7.8 27.8 54,600 5.3 36.0

Bottom 7.8 27.8 54,700 4.6 36.2

4:04 p.m. Top 7.9 27.9 54,700 5.6 36.1

Bottom 7.9 27.9 54,800 4.5 36.2

5:02 p.m. Top 7.9 28.0 54,700 5.5 36.1

Bottom 7.9 27.7 54,900 3.8 36.3

6:04 p.m. Top 7.8 27.9 54,400 5.0 35.9

Bottom 7.9 27.9 54,800 3.8 36.2

7:03 p.m. Top 7.8 27.7 53,400 5.2 35.2

Bottom 7.9 28.0 54,800 1.5 36.3
44 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

TIME Depth pH Temp SC DO SAL

Station 9

6:00 a.m. Top 7.8 26.7 54,200 4.6 35.8

Bottom 7.9 26.8 54,400 4.2 35.9

7:00 a.m. Top 7.9 26.9 54,400 4.5 35.9

Bottom 7.8 27.2 54,700 4.5 36.2

8:00 a.m. Top 7.9 26.6 54,100 4.5 35.7

Bottom 7.8 26.7 54,200 4.6 35.8

9:00 a.m. Top 7.8 26.8 54,100 4.6 35.7

Bottom 7.8 26.7 54,100 4.5 35.7

10:00 a.m. Top 7.9 27.1 53,700 4.7 35.4

Bottom 7.8 26.9 54,000 4.2 35.7

11:00 a.m. Top 7.8 27.5 52,000 4.8 34.2

Bottom 7.8 27.6 54,600 4.3 36.1

12:00 p.m. Top 7.7 27.7 52,500 4.5 34.7

Bottom 7.8 27.8 54,600 4.3 36.1

1:00 p.m. Top 7.6 27.8 52,700 4.2 34.7

Bottom 7.6 27.8 53,800 4.6 35.5

2:00 p.m. Top 7.7 27.9 53,800 4.4 35.1

Bottom 7.2 28.1 54,500 3.6 35.9

3:00 p.m. Top 7.7 27.9 53,600 4.0 35.3

Bottom 7.6 28.1 54,400 4.0 35.9

4:00 p.m. Top 7.7 27.7 53,100 4.5 34.9

Bottom 7.6 28.3 54,700 4.3 36.1

5:00 p.m. Top 7.8 27.9 53,900 4.2 35.6

Bottom 7.5 28.2 54,600 3.1 36.1

6:00 p.m. Top 7.8 28.1 54,400 4.0 35.9

Bottom 7.7 28.2 54,700 3.0 36.2

7:00 p.m. Top 7.7 28.0 54,600 3.9 36.0

Bottom 7.74 28.1 54,800 2.6 36.2

Appendix 2. March 29, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows. Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]
APPENDIX 45

Appendix 2. March 29, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows. Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]

TIME Depth pH Temp SC DO SAL

Station 10

6:25 a.m. Top 7.8 26.4 53,400 3.4 35.2

Bottom 7.8 27.1 54,700 3.4 36.1

7:22 a.m. Top 7.7 26.6 53,500 3.1 35.1

Bottom 7.8 27.1 54,400 3.4 35.9

8:13 a.m. Top 7.7 26.9 53,800 3.3 35.5

Bottom 7.8 27.3 54,800 4.6 36.2

9:13 a.m. Top 7.7 27.3 54,400 3.5 36.0

Bottom 7.8 27.2 54,600 4.9 36.1

10:11 a.m. Top 7.8 27.6 54,500 4.7 36.0

Bottom 7.9 27.5 54,500 5.0 36.0

11:13 a.m. Top 7.8 28.2 54,400 4.9 35.9

Bottom 7.8 27.6 54,500 5.2 36.0

12:16 p.m. Top 7.8 28.4 54,600 5.2 36.1

Bottom 7.8 27.7 54,700 5.2 36.1

1:09 p.m. Top 8.0 28.9 54,600 6.5 36.0

Bottom 7.9 28.1 54,800 6.8 36.2

2:10 p.m. Top 7.8 28.3 54,500 5.2 36.0

Bottom 7.9 28.3 54,700 5.4 36.1

3:09 p.m. Top 7.9 28.1 54,600 5.6 36.0

Bottom 7.9 28.2 54,600 5.7 36.0

4:13 p.m. Top 7.9 28.3 54,600 6.4 36.1

Bottom 7.9 28.3 54,700 6.2 36.1

5:08 p.m. Top 7.9 28.0 54,700 5.7 36.1

Bottom 7.9 28.2 54,800 4.8 36.2

6:13 p.m. Top 7.9 28.2 54,400 5.7 35.9

Bottom 7.9 28.1 54,700 4.5 36.1

7:10 p.m. Top 7.9 28.1 54,500 6.6 36.1

Bottom 7.9 28.0 54,900 5.2 36.3
46 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Appendix 3. November 15, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows; Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]

TIME Depth pH Temp SC DO SAL

Station 6

5:30 a.m. Top 7.7 26.8 47,320 6.6 30.7

Bottom 7.4 29.6 54,340 1.7 35.8

6:30 a.m. Top 7.8 26.7 47,250 6.4 30.7

Bottom 7.6 29.7 54,330 1.7 35.8

7:30 a.m. Top 7.8 26.6 47,310 6.5 30.8

Bottom 7.6 29.7 54,250 1.8 35.7

8:30 a.m. Top 7.8 26.8 46,910 6.4 30.4

Bottom 7.5 29.7 54,290 2.9 35.8

9:30 a.m. Top 7.8 27.2 47,110 6.6 30.6

Bottom 7.8 29.7 54,320 2.9 35.8

10:25 a.m. Top 7.8 27.5 47,370 6.8 30.8

Bottom 7.5 29.7 54,280 2.8 35.8

11:25 a.m. Top 7.8 27.5 47,380 7.1 30.8

Bottom 7.5 29.7 54,280 2.7 35.8

12:25 p.m. Top 7.8 27.9 47,600 7.3 30.9

Bottom 7.5 29.7 54,270 2.8 35.8

1:25 p.m. Top 7.9 28.1 47,600 7.4 30.9

Bottom 7.5 29.8 54,200 2.8 35.7

2:25 p.m. Top 7.9 28.3 47,660 7.9 30.9

Bottom 7.5 29.8 54,240 2.8 35.7

3:25 p.m. Top 7.9 28.5 47,700 8.2 31.0

Bottom 7.5 29.8 54,200 2.9 35.7

4:25 p.m. Top 7.9 28.5 47,710 8.2 31.0

Bottom 7.5 29.9 54,140 2.4 35.7

5:25 p.m. Top 7.9 28.4 47,840 8.2 31.1

Bottom 7.5 29.8 53,330 1.8 35.1

6:25 p.m. Top 7.9 28.1 47,890 8.0 31.1

Bottom 7.5 29.8 54,130 1.7 35.7
APPENDIX 47

TIME Depth pH Temp SC DO SAL

Station 7

5:49 a.m. Top 7.8 26.9 47,260 6.7 30.7

Bottom 7.5 29.6 54,350 0.7 35.8

6:34 a.m. Top 7.8 26.8 47,280 6.7 30.7

Bottom 7.5 29.6 54,370 0.8 35.8

7:35 a.m. Top 7.8 26.8 47,340 6.8 30.8

Bottom 7.5 29.6 54,380 2.8 35.8

8:32 a.m. Top 7.8 26.9 47,430 7.0 30.8

Bottom 7.5 29.6 54,330 2.9 35.8

9:32 a.m. Top 7.9 27.1 47,500 7.4 30.9

Bottom 7.5 29.6 54,260 3.7 35.7

10:30 a.m. Top 7.9 27.3 47,350 7.4 30.8

Bottom 7.5 29.6 54,290 3.8 35.8

11:28 a.m. Top 7.9 27.4 47,430 7.5 30.7

Bottom 7.5 29.6 54,300 2.6 35.8

12:27 p.m. Top 7.8 27.7 47,450 7.8 30.8

Bottom 7.5 29.7 54,260 3.8 35.8

1:28 p.m. Top 7.9 27.8 47,160 8.2 30.6

Bottom 7.5 29.7 54,230 3.8 35.7

2:26 p.m. Top 7.9 28.0 47,230 8.5 30.6

Bottom 7.5 29.7 54,240 3.8 35.7

3:25 p.m. Top 8.0 28.1 47,170 8.6 30.6

Bottom 7.5 29.7 54,210 3.0 35.7

4:30 p.m. Top 8.0 28.1 47,290 8.8 30.7

Bottom 7.5 29.7 54,190 3.0 35.7

5:27 p.m. Top 8.0 28.0 47,390 8.4 30.8

Bottom 7.4 29.7 53,110 1.9 34.9

6:30 p.m. Top 8.0 27.8 47,170 8.3 30.6

Bottom 7.5 29.7 54,150 1.7 35.7

Appendix 3. November 15, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows; Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]
48 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

TIME Depth pH Temp SC DO SAL

Station 8

5:55 a.m. Top 7.8 26.7 46,330 6.6 30.0

Bottom 7.5 29.7 54,310 0.9 35.8

6:39 a.m. Top 7.8 26.5 45,950 6.6 29.7

Bottom 7.4 29.8 54,240 1.8 35.7

7:42 a.m. Top 7.8 26.7 46,900 6.8 30.4

Bottom 7.5 29.8 54,160 2.0 35.7

8:34 a.m. Top 7.8 26.7 46,270 6.8 30.2

Bottom 7.4 29.7 54,240 2.6 35.7

9:47 a.m. Top 7.8 26.9 46,700 7.0 30.3

Bottom 7.6 29.7 54,180 3.4 35.7

10:35 a.m. Top 7.9 27.0 46,470 7.5 30.1

Bottom 7.4 29.7 54,230 3.5 35.7

11:34 a.m. Top 7.9 27.1 46,380 7.5 30.0

Bottom 7.4 29.8 54,180 3.2 35.7

12:33 p.m. Top 7.9 27.4 46,700 8.0 30.3

Bottom 7.5 29.8 54,160 4.1 35.7

1:38 p.m. Top 7.9 27.7 46,490 8.4 30.1

Bottom 7.5 29.8 54,150 4.2 35.7

2:34 p.m. Top 8.0 27.9 46,660 8.6 30.2

Bottom 7.5 29.8 54,210 7.6 35.7

3:29 p.m. Top 7.8 28.0 46,360 8.8 30.0

Bottom 7.5 29.8 54,160 7.5 35.7

4:38 p.m. Top 8.0 28.1 46,720 9.4 30.3

Bottom 7.5 29.8 54,180 3.2 35.7

5:36 p.m. Top 8.0 28.1 47,07 8.9 30.5

Bottom 7.5 29.8 54,090 1.6 35.6

6:34 p.m. Top 8.0 27.5 45,130 8.4 29.2

Bottom 7.5 29.7 54,130 1.1 35.7

Appendix 3. November 15, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows; Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]
APPENDIX 49

TIME Depth pH Temp SC DO SAL

Station 9

6:01 a.m. Top 7.7 27.3 47,340 5.9 30.7

Bottom 7.5 29.3 51,480 2.6 33.7

6:46 a.m. Top 7.8 27.2 47,340 5.8 30.7

Bottom 7.6 28.7 50,210 4.2 32.7

7:48 a.m. Top 7.7 27.2 47,460 6.1 30.8

Bottom 7.5 29.2 51,730 3.6 33.9

8:39 a.m. Top 7.8 27.2 46,900 6.7 30.4

Bottom 7.6 29.1 51,580 3.9 33.8

9:52 a.m. Top 7.8 27.3 47,250 6.5 30.7

Bottom 7.6 29.0 51,610 3.6 33.8

10:46 a.m. Top 7.8 27.4 47,090 7.2 30.5

Bottom 7.6 28.6 49,460 4.7 32.3

11:43 a.m. Top 7.8 27.6 47,710 6.8 31.0

Bottom 7.6 28.8 50,540 4.4 33.0

12:40 p.m. Top 7.8 27.8 47,380 7.3 30.7

Bottom 7.8 27.9 48,490 6.8 31.6

1:51 p.m. Top 7.8 28.0 47,800 7.0 31.1

Bottom 7.6 29.3 51,280 5.3 33.6

2:42 p.m. Top 7.9 28.2 47,300 8.3 30.7

Bottom 7.7 29.0 50,160 5.7 32.8

3:36 p.m. Top 7.9 28.0 47,370 8.3 30.7

Bottom 7.6 29.5 51,740 4.1 33.9

4:45 p.m. Top 7.8 28.1 47,530 7.9 30.9

Bottom 7.7 29.0 50,250 5.1 32.8

5:41 p.m. Top 7.9 28.0 47,230 8.0 30.6

Bottom 7.7 29.3 51,020 4.1 33.4

6:40 p.m. Top 7.9 27.9 47,120 7.8 30.6

Bottom 7.6 29.3 51,510 3.3 33.8

Appendix 3. November 15, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows; Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]
50 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

TIME Depth pH Temp SC DO SAL

Station 10

6:10 a.m. Top 7.6 25.7 39,900 3.9 25.4

Bottom 7.7 26.4 46,220 5.0 28.3

6:53 a.m. Top 7.6 25.5 39,910 4.0 25.4

Bottom 7.6 27.1 46,270 2.8 29.8

7:55 a.m. Top 7.6 25.7 41,170 5.1 26.4

Bottom 7.6 25.8 41,010 5.2 26.2

8:45 a.m. Top 7.9 26.2 42,510 7.4 27.3

Bottom 7.9 26.2 42,750 7.5 27.4

9:57 a.m. Top 7.8 26.4 41,320 7.7 26.4

Bottom 7.9 26.5 41,440 8.0 26.5

10:58 a.m. Top 7.9 26.6 40,900 8.1 26.1

Bottom 7.9 26.6 40,190 8.1 26.1

11:49 a.m. Top 8.0 26.9 42,200 9.7 26.9

Bottom 8.0 26.9 42,110 9.6 27.0

12:49 p.m. Top 8.1 27.4 42,150 10.4 27.0

Bottom 8.1 27.4 42,320 10.1 27.1

1:58 p.m. Top 8.3 28.1 43,300 12.4 27.8

Bottom 8.2 28.0 43,830 11.9 28.2

2:53 p.m. Top 8.3 28.3 43,230 12.4 27.7

Bottom 8.2 28.3 44,000 11.9 28.3

3:48 p.m. Top 8.2 28.3 43,540 12.7 28.0

Bottom 8.3 28.3 43,620 12.6 28.0

4:51 p.m. Top 8.2 28.1 43,530 12.6 28.0

Bottom 8.2 28.2 43,520 11.5 28.0

5:47 p.m. Top 8.2 28.0 42,780 13.1 27.5

Bottom 8.1 28.0 44,030 9.8 28.3

6:46 p.m. Top 8.1 27.5 42,580 10.1 27.3

Bottom 8.0 28.0 44,800 5.8 29.1

Appendix 3. November 15, 2007, diel study physical properties.—Continued

 

[Top values were obtained at about 15 centimeters in depth and bottom values were obtained about 10 centimeters from the lagoon bottom. 

Station identification number (STA-ID) is shown on spanner rows; Temp, temperature; SC, specific conductance; DO, dissolved oxygen; SAL, 

salinity]
APPENDIX 51

Appendix 4. Fecal bacteria counts from March 2007 to June 2008.[All values shown are colonies per 100 milliliters]

Station 

number Sample collection date

Mar. 28, 

2007

May 31, 

2007

June 27, 

2007

July 24, 

2007

Sept. 11, 

2007

Oct. 16, 

2007

Nov. 14, 

2007

Jan. 29, 

2008

Mar. 4, 

2008

Apr. 1, 

2008

May 13, 

2008

June 17, 

2008

1 240 570 45 64 9 2 860 270 40 720 32 603 280 8,270 10 900 34 20 810 600 200 450 33 105 600 60,000 2,800 7,000 160 750 60,000 39,000 350 160 1,900 2,0007 97 240 100 9 4 2 970 8 4 0 5 2
52 Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, P.R.

Selected Hydrologic, Water-Quality, Biological, and Sedimentation Characteristics of Laguna Grande, Fajardo, P.R., March 2007-February 2009 —Scientific Investigations Report 2010-5071