LOXAHATCHEE RIVER SEAGRASS MONITORING & MAPPING REPORT

DRAFT

TASK 4: FINAL REPORT

In Partial Fulfillment of Agreement No. 4600001907

For the Period

October 2009 through September 2010

Respectfully Submitted by:

Bud Howard, Lorene Bachman, Jerry Metz, Albrey Arrington, Ph.D.

Loxahatchee River District

January 2011

   
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Executive Summary

Seagrasses are a valued ecosystem component of the Loxahatchee River estuary.    Scientists and

managers use seagrass condition and distribution to assess the health and condition of the estuary.  

Further, these data will be used to assess restoration success following modified freshwater inflows

resulting from the Comprehensive Everglades Restoration Project (CERP 2001) and the Restoration Plan

for the Northwest Fork of the Loxahatchee River (SFWMD 2006).

Between October 2009 and September 2010, Loxahatchee River District’s Wildpine Ecological

Laboratory staff monitored seagrass every‐other‐month (bi‐monthly) at four sites located within the

Loxahatchee River Estuary and at a reference site in the southern Indian River Lagoon. Monitoring was

conducted to characterize spatial and temporal variation in seagrass percent cover and canopy height

within the Loxahatchee River estuary. Here we use data collected over the past 12 months to evaluate

seagrass species composition across the upstream to downstream gradient. Similarly, we characterize

spatial and temporal variability among our sampling sites.

In a separate seagrass project during the summer of 2010, we quantified seagrass occurrence and

density using in situ observations based on a random stratified design that locates sampling points

throughout the Loxahatchee River estuary.  Results from this project provide landscape‐scale, species‐

specific assessment of seagrasses in the estuary.    The 2010 mapping project complements a similar

mapping project conducted in 2007.

These ongoing seagrass monitoring projects in the Loxahatchee River Estuary have provided interesting

insights into the ecology and dynamics of seagrasses.   During the period October 2009 – September

2010 seagrass in the Loxahatchee River Estuary appeared to be relatively healthy, though percent cover

values for some species and monitoring sites remain below those observed prior to the September 2004

hurricanes. By comparing seagrass conditions in the Loxahatchee River against those of the reference

site and across the upstream‐downstream gradient in the Loxahatchee River we were better able to

understand the impacts freshwater discharges had on seagrass conditions. The 2010 mapping project

provides unprecedented detail on the distributions of seagrasses throughout the estuary.  Comparisons

of the 2010 and 2007 mapping data help to illustrate, and eventually quantify, the extent and variability

in the seagrass distributions throughout the estuary.    With the exception of Paddle Grass, the

distributions of Johnson’s Seagrass, Shoal Grass, Manatee Grass, and Turtle grass were generally similar

in 2010 to the observations in 2007.    The distributions of Paddle Grass showed marked changes

between the 2007 and 2010 surveys.

   
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Introduction

As a valued ecosystem component, seagrasses will be used to assess restoration success following

modified freshwater inflows resulting from the Comprehensive Everglades Restoration Project (CERP

2001) and the Restoration Plan for the Northwest Fork of the Loxahatchee River (SFWMD 2006).

Seagrasses have been identified as a valued ecosystem component because they fulfill key ecological

functions in estuaries.    For example, they provide food and refuge from predation for numerous

economically and ecologically important species (Zieman 1982; Zeiman et al. 1989; Holmquist et al.

1989; Montague and Ley 1993).    Seagrasses also are a critically important component of estuarine

productivity (Short et al.1993; Fourqurean et al. 2001).  Furthermore, seagrasses have been identified as

a biological indicator of water quality and ecosystem health (Montague and Ley 1993; Provancha and

Scheidt 2000; Lirman and Cropper 2003; Ridler et al. 2006), which suggests that tracking changes in

seagrass occurrence and abundance may provide insights into the ecological health of the broader

estuary.    

Between October 2009 and September 2010, Loxahatchee River District’s Wildpine Ecological

Laboratory staff monitored seagrass every‐other‐month (bi‐monthly) at four sites located within the

Loxahatchee River Estuary and at a reference site in the southern Indian River Lagoon. Monitoring was

conducted to characterize spatial and temporal variation in seagrass percent cover and canopy height

within the Loxahatchee River estuary.  

In a separate seagrass project, during the summer of 2010, we quantified seagrass occurrence and

density using in situ observations based on a random stratified design that locates sampling points

throughout the Loxahatchee River estuary.  Results from this project provide landscape‐scale, species‐

specific assessment of seagrasses in the estuary.  This project complements a similar mapping project

conducted in 2007.

Here we use data collected over the past 12 months to evaluate seagrass species composition across the

upstream to downstream gradient. Similarly, we characterize spatial and temporal variability among our

sampling sites. With the landscape‐scale mapping project, we evaluate seagrass distributions

throughout the estuary, with generalized comparisons to the results obtained in 2007.   

Study Area

Overview

The Loxahatchee River estuary encompasses approximately 400 ha and drains a watershed of

approximately 700 km2 located in northeastern Palm Beach County and southeastern Martin County,

Florida, USA.  Freshwater discharges into the estuary from the North Fork, the Northwest Fork, and the

Southwest Fork of the Loxahatchee River.  The hydrology of the basin has been substantially altered by

flood control efforts since the 1950s.   Historically (pre‐1950), most surface water runoff reaching the

estuary originated in the Loxahatchee and Hungryland Sloughs and flowed gradually to the Northwest

Fork. In the 1930s   

Lainhart Dam, a small fixed‐weir dam, was constructed in the Northwest Fork at river mile 14.5 to

reduce “over” drainage of upstream reaches of the Northwest Fork during the dry season.  In 1958 a
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major canal (C‐18) and flood control structure (S‐46) were constructed to divert flows from the

Northwest Fork to the Southwest Fork, which increased the intensity and decreased the duration of

storm‐related discharge to the estuary.    Furthermore, since 1947 the eastern link to the ocean, the

Jupiter Inlet, has been kept permanently open through ongoing dredging projects. This has contributed

to increased saltwater intrusion into the primarily freshwater Northwest Fork. Ongoing restoration

efforts seek to increase base flows into the Northwest Fork while not compromising the ecological

integrity of downstream reaches (i.e., estuary) nor impairing valued ecosystem components of the

estuary such as oysters and seagrasses (SFWMD 2006).   

Figure 1. Loxahatchee River watershed and associated features.  

Bi‐Monthly Seagrass Monitoring Sites

Four seagrass beds in the central embayment of the Loxahatchee River Estuary and a reference seagrass

bed in the southern Indian River Lagoon were selected as sample sites based on three primary factors:

proximity to the river forks flowing into the estuary, seagrass cover, and seagrass persistence. This

project was established following a four year fixed‐transect based seagrass monitoring project in the

Loxahatchee River which included four of the five sites assessed in the present study. At each sample

site the seagrass bed to be monitored was delineated with a GPS to allow random sampling within the

defined seagrass patch over time (Figure 2). The North Bay (NB) seagrass bed was 5.29 acres (2.14 ha)
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and was located at river mile 1.5 in the northern most embayment area approximately 500 meters west

of the railroad bridge. This site was the most downstream site so therefore experienced the most stable,

marine‐like conditions among the four Loxahatchee River sampling sites. North Bay was characterized as

a shallow cove seldom more than one meter deep and is protected from the main boating channel by a

sandbar located to the south and running the length of the seagrass bed. The shore line was residential

and mostly composed of red mangroves with occasional seawalls. Six species of seagrass have been

found within this bed.  Manatee grass, shoal grass, Johnson’s grass, turtle grass, paddle grass, and star

grass were present during previous studies.  

The Sand Bar (SB) seagrass bed was 8.71 acres (3.52 ha) and was located at river mile 1.8 in the central

portion of the central embayment. Therefore, the site was directly influenced by water flowing in from

the inlet and downstream from both the Northwest, North, and Southwest Forks of the Loxahatchee

River. The area was a shallow sand bar adjacent to and south of the main channel in the central

embayment. Shallowest portions of the sand bar were intertidal and received considerable foot traffic

as people recreated on the sand bar. The site is separated from the shore line by a marginal channel.

Five species of seagrass have been found within this area.  Johnson’s grass, shoal grass, paddle grass,

manatee grass, and turtle grass were found at this site during previous studies.

The Pennock Point (PP) seagrass bed was 2.96 acres (1.20 ha) and was located at river mile 2.6 at the far

western edge of the central embayment    along the eastern shore of the peninsula created by the

Northwest and Southwest forks (Figure 1). This site was directly influenced by freshwater flowing

downstream from both the Northwest and Southwest Forks of the Loxahatchee River. The shoreline at

this location was predominantly undeveloped residential lined by a continuous seawall with some

scattered red mangroves. The seagrass bed at this site is shallow and extends from the sandy shore out

fifty meters to a typical depth of 1.5 m. Shoal grass and Johnson’s grass were the only two seagrass

species recorded at this location during previous studies.  

The Northwest Fork (NWF) seagrass bed was 0.97 acres (0.49 ha), and was located at river mile 3.3 in

the Northwest Fork of the Loxahatchee River. This site was the most upstream site, and therefore was

most affected by freshwater flowing downstream from the Northwest Fork of the Loxahatchee River.

The site was characterized as a slight embayment along the western shore with the shoreline being

residential and mostly lined by red mangroves with occasional seawalls. The seagrass bed at this site

was approximately 175 meters wide and extended 25 meters out from the sandy shore to a typical

depth of 1.4 m. Johnson’s grass and shoal grass have been found at this site during earlier assessments

(e.g., 2007 mapping), though it was not sampled as part of the 2003‐2007 transect‐based study.  

The reference (HS) seagrass bed was 10.79 acres (4.37 ha), and was located in the southern Indian River

Lagoon. This site was an excellent reference site because it was located 8 km north of the Jupiter Inlet

and was not affected by freshwaters discharged from the Loxahatchee River watershed; thus, the

reference site experienced more stable, marine‐like salinity conditions than the Loxahatchee River

Estuary. The site was located on the western bank of the Indian River Lagoon adjacent to the sandy

shore of a bird sanctuary.  Unlike the Sand Bar site, this site received very little public use.  The seagrass

bed at this site extended approximately 165 meters off shore, and the deep edge of the bed exceeded

1.5 m in depth. During the previous study manatee grass, shoal grass, turtle grass, Johnson’s grass, and

paddle grass were found at this site.
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Figure 2. Seagrasses were sampled every other month in the Loxahatchee River at four locations

(Northwest Fork, North Bay, Sand Bar, and Pennock Point) and at a reference location in the Indian

River Lagoon (Hobe Sound). The red polygons represent the actual size and shape of the seagrass bed

monitored at each location.  
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Landscape‐Scale Seagrass Mapping Area

Sample points were sited from just inside the mouth of the inlet to the upstream extent of seagrass in

each of the three river forks (North, Northwest, and Southwest) but do not include the Indian River

Lagoon (north of the Loxahatchee River) or the southern Intracoastal Waterway (south of the

Loxahatchee River). The greatest density of sampling points occurs in the central portion of the estuary

known to support the greatest diversity of seagrass (Loxahatchee River District, unpublished data). We

did not collect samples in areas known to be too deep to support seagrass growth, or in the navigation

channels for safety considerations.

Materials and Methods

Patch‐scale (Bi‐Monthly) Monitoring Methods

Seagrass beds were monitored bi‐monthly for the period October 2009 through September 2010. During

each sampling event we assessed percent occurrence for each seagrass species encountered. Divers

quantified percent cover of all seagrasses at each site by haphazardly deploying a 1 m2 quadrat

approximately 30 times within each seagrass sampling site.  Each 1 m2 quadrat was subdivided into 25

equilateral 20 cm x 20 cm cells. Seagrass percent cover was assessed by quantifying the number of cells

(out of 25) in which each seagrass species occurred. In addition, the number of bare (no seagrass) cells

was quantified for each quadrat sampled. These count data (i.e., how many cells had manatee grass)

were converted to percent cover data by dividing the number of cells in which the seagrass species

occurred by the total number of cells (25). For example, a quadrat in which manatee grass was present

in 12 of 25 cells, shoal grass was present in 16 of 25 cells, and 3 cells were bare (i.e., no seagrass

present) was recorded as having the following percent cover values: manatee grass 48%, shoal grass

64%, and total seagrass cover 88%. Seagrass canopy height was quantified in each quadrat assessed for

seagrass percent cover by measuring the average blade length (distance from substrate to blade tip) of

shoots of the dominant canopy‐forming seagrass species in each quadrat. In order to facilitate spatial

analyses, the position of each quadrat was georeferenced using a mapping‐grade GPS (Trimble

Navigation Inc. GeoXT or GeoXH).  

Concurrent with seagrass data collection, physical and chemical water quality parameters were

evaluated and freshwater discharge into the system was recorded.  Throughout the study temperature,

conductivity, salinity, dissolved oxygen, pH, turbidity, chlorophyll a, and Photosynthetic Available

Radiation (PAR) were recorded bi‐monthly at the time of seagrass sampling in the channel adjacent to

each seagrass site and at a site in the middle of each seagrass bed.  Furthermore, data from RiverKeeper

water quality stations 40 and 42 (central embayment), 60 (Northwest Fork), and 25 (Hobe Sound)

provide monthly or bi‐monthly assessments of nutrients in the vicinity of the seagrass sampling sites

(see Project RiverKeeper reports at www.loxahatcheeriver.org/reports.php). Salinity, temperature, and

depth were recorded every 15 min at the NB and PP sites using a Hydrolab Minisonde 4a positioned at

seagrass canopy height (~ 25 cm off the bottom). Water quality samples were processed following

Standard Methods by the Loxahatchee River District’s Wildpine Laboratory which is certified under the

National Environmental Laboratory Accreditation Program. Photosynthetically active radiation (PAR)

was assessed by taking multiple replicates of PAR using 3 LI‐COR spherical sensors (4 π) simultaneously

located at 20 cm, 50 cm, and 100 cm below the water surface.  Data were recorded on a LI‐COR LI‐1400

data logger.   Light attenuation coefficient (Kd) was calculated as the slope of natural log transformed

PAR values regressed against depth.  Following Kemp et al. (2004), the percent of light passing through
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the water column to seagrasses (PLW) was calculated as PLW = 100 exp [(‐Kd)(Z)], where Kd is the light

attenuation coefficient and Z is the depth of seagrass growth.  

Landscape‐scale Mapping Methods

Because of depth and limited water clarity typical of the Loxahatchee River estuary, mapping work was

performed while snorkeling. Weighted buoys were randomly deployed from a power boat in a manner

of even distribution and generally with no two buoys closer than 10 m and up to roughly 100 m in areas

with uniform conditions. Each buoy marked a sample point. We then quantifed seagrass presence or

absence by deploying a collapsible 9m2 quadrat at each sample point. The 3 m x 3 m quadrat, dubbed

“Quadzilla”, is made using four 3 m long (3/4”) PVC pipes connected with a 3/8” nylon line running

through each side and tied at one corner. Each 3 m side has its middle 1 m length painted black, which

allows the deployed quadrat to be divided into nine 1 m2 cells. A string network defining each of the 9

square meters is attached. One leg is marked with 10 cm segments for measuring water and muck

depth.

Figure 4. "Quadzilla", 9m2 sampling quad.

Because the quad is held together by nylon rope running through the PVC segments and not by fixed

elbows at the corners, field crews are able to fold the large quadrat up into one 3m long bundle of PVC

poles and carry it while snorkeling between sampling sites.  

Within each sample (9 m2 quadrat) a diver quantified water depth, identified predominant substrate

type (much, sand, rock/shell, detritus), quantified muck depth if present, and documented seagrass

presence  and percent cover. Seagrass presence was assessed within the 9 m2 quadrat in the following

manner. First, bare substrate was scored as the total number of 1 m2 cells in which no seagrass was

present (0 to 9). Next, each seagrass species was scored according to the number of 1 m2 cells in which it

was present (0 to 9). We conservatively defined seagrass presence as the occurrence of at least one

seagrass shoot. Each sample, therefore, received a score between 0 and 9 for bare substrate and a score
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of 0 to 9 for each of the seven seagrass species known to occur in the estuary. Seagrass density was

somewhat more subjectively assessed. Within each quadrat a diver visually estimated seagrass density,

in cells where seagrass was present, using the following categories: (1) low density (< 5% coverage); (2)

moderate density (> 5% but < 90% coverage); and (3) high density (≥ 90% coverage) and illustrated in

Figures 5‐7.  

   
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Figure 5.  Example of low density seagrass (score = 1), characterized by very sparse conditions equivalent

to < 5% seagrass cover.

   

Figure 6.  Example of moderate density seagrass (score = 2). Seagrass cover was > 5% but coverage, but

< 90%.

   

Figure 7. Example of high density seagrass (score = 3), characterized by dense, continuous seagrass

cover (≥ 90% ).   Photos on this page by Jerry Metz
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After all of the data was recorded on the field sheet, the sheet was clipped to a buoy marking the

sample location and the diver moved to the next sampling site.

Figure 8. Field team of two divers and one GPS operator.

   

GPS Data Collection

Each sampling site was mapped with a mapping‐grade Trimble GPS systems (GeoXT or GeoXH) running

Trimble’s TerraSync data collection software. The hand‐held GPS was used to record the spatial location

of each sample point as well as pertinent data (i.e., seagrass percent occurrence, substrate type, etc)

while in the field.    Each day the GPS was downloaded and the positional data post‐processed to

maximize accuracy. The data entered into the GPS while in the field is checked by cross‐referencing data

from field data sheets.  

GIS Data Analysis

For illustration purposes we created maps depicting each sample point colorized by a corresponding

quadrat score for all seagrasses observed, each species, seagrass density scores, and the soft/muck

sediments.  For seagrasses we symbolized each sample point according to the following quadrat score:

1) no part of the 9m2 quadrat had seagrass  (0 of the 9 1m2 cells); 2) one to five of the nine 1m2 had

seagrasses (1‐5/9 cells) was categorized and labeled as ‘Patchy Seagrass’; and 3) six to nine of the nine

1m2 had seagrasses (6‐9/9 cells) was categorized and labeled as ‘Continuous Seagrass’.  We used this

same categorization for each of the species‐specific maps. The soft sediments/muck data was

symbolized as sample points having ‘no muck’ or sandy sediments, samples with less than 10 cm muck

depth, and samples having greater than 10 cm muck depth.
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We have performed extensive testing to identify an effective interpolation approach that accurately

represents the seagrass coverage for the entire survey area from the point data collected in the field.  

While we continue to refine our data processing approach, we have found the following combination of

analysis methods and software settings within the GIS software provides the most accurate and

representative maps.  However, our work continues to identify the best approach to create the coverage

maps.    In particular, we have determined that the boundaries the user creates for the GIS data

processing is critical for obtaining accurate results.

We have found the Inverse Distance Weighted (IDW) interpolation method available through ESRI’s

ArcGIS Spatial and 3D Analyst Extensions to be the most effective analysis tool to create the seagrass

coverages that best represent the total area occupied by each seagrass species.  We use this tool is to

assign expected percent cover values for each seagrass species in each grid cell based on interpolation

of our actual observations. IDW estimates cell values by averaging values of field‐based observations

(samples) in the neighborhood of each cell being estimated. The closer a point is to the center of the cell

being estimated, the more influence, or weight, it has in the interpolation process. We perform IDW

using six neighborhood points and a weight of four. Points are assessed using a variable search radius.

The entire estuary is converted into a grid of 3 m x 3 m cells. The estuary grid used for interpolation is

constrained using a polyline‐based mask to limit the interpolation to areas that potentially supported

seagrass. This polyline limit is critical element of the processing because it provides the boundaries, or

limits, of the interpolation.  As such, it is important to use all data available to define the locations of the

polyline boundary.  To define these boundaries we symbolized the seagrass sample points as presence

or absence, considered bathymetry, and used aerial imagery to exclude shoreline, mangrove islands,

and boat docks.  Because light attenuation increases in an upstream manner, shallower depth contours

are used in the three forks than in the central embayment. While the manual creation of the polyline

boundary is somewhat subjective, it is critical to prevent interpolation beyond logical boundaries and to

obtain the most accurate coverage maps.  We continue to refine this methodology and the boundary

layers so we can make more thorough comparisons between the 2007 and 2010 mapping events, where

sufficient overlapping data exists.

After the interpolated grid is computed, we use the Reclassify function to assign an integer to the

species scores.  This step facilitates visual interpretation of output from the IDW interpolation process

into categories.  For example, interpolated seagrass scores of 5.0 to 5.99 is assigned 5 indicating that 5

of the 9 cells of the quad contained seagrass.    We commonly established the following categories:

Continuous Seagrass (defined as cells with interpolated seagrass cover ≥56%; or seagrass scores of 5 to

9), Patchy Seagrass (interpolated seagrass cover  ≥11% and <56%; or score of 1‐4), and No Seagrass

(interpolated seagrass cover <11%, or score of 0).   

Results & Discussion

Patch‐scale Monitoring  

The primary purpose of this study was to generate a more comprehensive understanding of spatial and

temporal dynamics of seagrass percent occurrence and canopy height (i.e., measures of seagrass health)

within the Loxahatchee River Estuary prior to restoration (augmentation) of freshwater flows to the

Northwest Fork of the Loxahatchee River. As such, we assessed seagrass percent cover and canopy

height in 870 individual quadrat samples among our five sampling areas during the period October 2009
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through September 2010. Seagrasses appeared healthy in the Loxahatchee River, though there are

notable differences in seagrass percent occurrence and canopy height among sampling sites. Figure 9

shows percent occurrence of the three dominant seagrass species across the five sampling sites. These

data span from June, 2003 through December, 2010, and provide an amazing opportunity to understand

temporal dynamics within seagrass beds. It is important to remember that in October 2004 several

major storms adversely impacted seagrasses in the Loxahatchee River Estuary (Ridler et al. 2006). Data

collected since then document the recovery trajectory that has occurred at each site by each species.  

Data prior to October 2007 was transect‐based, percent cover data collected at each of these sites and

is described in previous monitoring reports available at www.loxahatcheeriver.org/reports.php.  

Previous comparisons of the transect and random‐quadrat methods at these sites have demonstrated

similar results.  Therefore we have combined the data sets in Figure 9.

Today the seagrass bed at North Bay is composed predominantly of shoal grass, manatee grass, and

Johnson’s grass. Prior to the storms of 2004 this seagrass bed was dominated by manatee grass. It is

important to recognize the lack of recovery by manatee grass six years following the disturbance. While

manatee grass appears to be mounting a slow and steady recovery, the present occurrence of manatee

grass remains about 50% of its pre‐disturbance occurrence. Shoal grass appears to have filled the space

vacated by the loss of manatee grass, and occurred in approximately 70% of North Bay samples since

August, 2010. Johnson’s grass showed significant increases in occurrence the two years following the

disturbance, but had declined to near pre‐disturbance occurrence levels (i.e., ~20%) in 2010. Maximum

canopy height within North Bay was generally around 30 cm, with healthy manatee grass forming the

tallest canopy. This site had the most stable salinity regime of the Loxahatchee River Estuary sites, which

may explain why manatee grass was most abundant among the Loxahatchee River sites.

Shoal grass and Johnson’s grass within the Sand Bar seagrass bed appear to have made a full recovery.

While manatee grass occurred in 30‐40% of Sand Bar samples prior to the 2004 disturbance, manatee

grass was found in isolated, small patches occupying less than 10% of samples in 2010. We are

somewhat surprised by the lack of recovery of manatee grass at the Sand Bar site, though we are

encouraged by the prevalence of shoal grass and Johnson’s grass at this site. Johnson’s grass showed

pronounced seasonality at this site with annual peaks typically occurring in April, though peak

abundance in 2008 occurred in February. In general, seagrass canopy height averages around 10 cm at

the Sand Bar site except for patches that contain manatee grass where canopy height peaks around 20

cm.  

The Pennock Point seagrass bed has never had manatee grass, and appears to have fully recovered

following the 2004 disturbance. In 2010 shoal grass occupied roughly 60% of the seagrass patch and

Johnson’s grass occupied 30% to 70% of the seagrass patch. Johnson’s seagrass has exhibited a

pronounced increase in occurrence especially since 2008. Johnson’s seagrass appears to demonstrate a

slightly different seasonal pattern of abundance at Pennock Point relative to the Sand Bar site. Seagrass

canopy height at Pennock Point is typically less than 10 cm, with the dominant canopy species being

shoal grass. We suggest manatee grass was absent from this site due to the wide fluctuations observed

for salinity conditions at this site.

We began sampling the Northwest Fork seagrass bed in October, 2007, so pre‐disturbance comparisons

cannot be made for this site. In April, 2010 Johnson’s grass, the dominant species, occupied over 70% of

the patch, while shoal grass consistently occupied 10%‐20% of the patch. Canopy height was similar to

Pennock Point with shoal grass the dominant canopy species and mean canopy height less than 10 cm.

Because of freshwater flowing down the Northwest Fork of the Loxahatchee River, this site had the
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highest average water color, lowest light penetration, and lowest average salinity condition, and most

variable salinity conditions among the seagrass sites sampled.

The reference seagrass bed in Hobe Sound continued to exhibit relatively stable seagrass conditions.

Manatee grass occupied nearly 80% of the bed throughout the year. Shoal grass occupied around 40%

of the bed for most of the year, and Johnson’s grass generally occupied less than 5% of the seagrass bed.

Canopy height at Hobe Sound was generally 30 cm – 40 cm with manatee grass as the dominant canopy

species. It appears that the relatively stable salinity conditions at this site, due to the lack of freshwater

inflows, have allowed manatee grass, a canopy‐forming species sensitive to salinity fluctuations, to

uniformly dominate this site through time.  

   
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Manatee Grass Shoal Grass Johnson's Grass

<‐‐‐ TransectMethod ‐ Percent Cover Patch‐quadratMethod ‐ Percent Occurrence ‐‐>

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Figure 9. Percent cover (Pre October 2007) and percent occurrence (Post October 2007) data suggest strong differences in seagrass health among

Loxahatchee River monitoring sites across the upstream‐downstream gradient. Error bars represent ± 1 Standard Error.
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Landscape‐scale Monitoring  

In 2010, the LRD lab staff mapped species‐specific seagrass occurrence at 1,667 samples located

throughout the Loxahatchee River estuary. Fifty‐nine percent of samples included at least one seagrass

species while 41% of samples had no seagrass present. Johnson’s grass, a Federally listed species as

threatened under the Endangered Species Act, was the most frequently encountered seagrass and

occurred in 46% of samples. Shoal grass was the second most encountered species occupying 39% of

samples. Paddle grass, manatee grass, and turtle grass were infrequently found, and occupied 4%, 3%,

and 2% of samples, respectively.  

From a spatial perspective, seagrasses extended to approximately 5,800 m upstream into the North

Fork, 6,500 m into the Northwest Fork, and 5,400 m into the Southwest Fork, as measured from the

mouth Jupiter Inlet (Appendix Figure A1). These limits are likely determined by salinity variability, water

clarity, and water depth (bathymetry).  Figure A2 presents the seagrass data with the 2008 bathymetry

data and helps to illustrate the relationship between seagrass presence and water depth, relative to the

distance to the inlet.  Figure A3 presents the interpolated seagrass coverage from the point data.  Again,

we continue to refine these figures to ensure optimum accuracy by manipulating the interpolation

boundary limits and settings.    Final figures will be included in a forthcoming paper that we intend to

submit for publication.  Appendix B contains comparative figures from the 2007 mapping project.   

In general, seagrass in the Loxahatchee River estuary are found fringing the shoreline in downstream

segments of the system (i.e., downstream of river mile 4), or in the shallow portions of the central

embayment. In general, the distribution of seagrasses within the Loxahatchee River estuary is assumed

to be driven by salinity variability (Ridler et al. 2006) and water clarity (Hall et. Al. 1991, Onuf 1991).

LRD’s 2010 Datasonde Monitoring Report (www.loxhatcheeriver.org/reports.php) describes the

variation in salinity characteristics at several locations in the river and provides insight into the

conditions influencing the various seagrass distributions. Undoubtedly water depth and sediment type

are factors that influence seagrass distributions.  The data presented in Table 1 show a strong negative

relationship between seagrass occurrence and water depth and muck depth.    Appendix Figure A9

illustrates the locations of samples where muck sediments were encountered.

Table 1. Water depth and muck depth appear to limit landscape‐scale occurrence of seagrass in the

Loxahatchee River estuary. Data based on 1,667 samples collected in 2010.

Seagrass

Score

Seagrass

Description

# of

Samples

% of

Samples

Water Depth

(cm)

Muck Depth

(cm)

0 Absent 689 41% 158 9.5

1‐5 Patchy 181 11% 136 1.3

6‐9 Continuous 797 48% 82 0.6

We provide species‐specific maps illustrating the samples that contained Johnson’s Seagrass (Halophila

johnsonii), Shoal Grass (Halodule wrightii), Paddle Grass (Halophila decipies), Manatee Grass

(Syringodium filiforme), and Turtle Grass (Thalassia testudinum) in Appendix Figures A4 through A8.  

Johnson’s Seagrass and Shoal Grass were the most widely distributed seagrass throughout the estuary,

and both species extending the furthest upstream (Figures A4 and A5).    The distributions of these

seagrasses were generally similar to the 2007 observations (Figures B3 and B4).    The distribution of

Paddle Grass showed the most pronounced changes between 2007 and 2010 (Figures A6 and B5).  In
16

2007 Paddle Grass was present throughout much of the southwest and far southeast portions of the

central embayment, but in 2010 Paddle Grass far less common in these areas.  Curiously, the occurrence

of Paddle Grass in the North Fork appeared greater in 2010 than 2007.    Variations in rainfall, the

subsequent freshwater flows into the river, and the resulting salinity variability were likely factors

influencing the distribution of Paddle Grass between the drought year of 2007, and the extended wet

season of 2010 (see LRD’s 2010 Datasonde Report).  The distributions of Manatee Grass were generally

similar between 2007 and 2010 with occurrence limited to central portions of the central embayment

(Figures A7 and B6).  Lastly, the distributions of Turtle Grass, the rarest species in the Loxahatchee, were

largely limited to east portion of the central embayment with isolated occurrences scattered throughout

the central embayment (Figure A8 and B7).  The scattered occurrences of Turtle Grass in the central part

of the central embayment observed in 2007 were far less common in 2010.

Conclusions

Ongoing seagrass monitoring in the Loxahatchee River Estuary has provided interesting insights into the

ecology and dynamics of seagrasses. During the period October 2009 – September 2010 seagrass in the

Loxahatchee River Estuary appeared to be relatively healthy, though percent cover values for some

species and monitoring sites remain below those observed prior to the October 2004 hurricanes.   By

comparing seagrass conditions in the Loxahatchee River against those of the reference site and across

the upstream‐downstream gradient in the Loxahatchee River we were better able to understand the

impacts freshwater discharges had on seagrass conditions.  

The 2010 mapping project provides unprecedented detail on the distributions of seagrasses throughout

the estuary.  Comparisons of the 2010 and 2007 mapping data help to illustrate and eventually quantify

the extent and variability in the seagrass distributions throughout the estuary.   

The methods employed in the present study are synonymous with methods being employed throughout

the other estuaries in South Florida as part of CERP Monitoring and Assessment Program. Therefore, we

expect these results, when combined with results from other systems, will provide a comprehensive

understanding of seagrass dynamics across all of the South Florida estuaries.   

Recommendations for future work:

1. Continue the seagrass monitoring program to assess long‐ and short‐term trends in

water quality in the Loxahatchee River.  In particular, we need to better understand

differences in salinity envelopes among seagrass patches, so we can determine

thresholds that constrain assemblage membership (i.e., what conditions lead to loss of

manatee grass). Such data will provide essential information for adaptive management

of future restoration activities.

2. We need to better understand how various physical factors interact across the

landscape to constrain the occurrence of seagrass. For example, water quality changes

significantly across the upstream to downstream gradient, and these changes likely

interact to define habitat patches potentially suitable for seagrass.  
17

Literature Cited

Catchpole, W. R. and C. J. Wheeler. 1992. Estimating plant biomass: a review of techniques.

Australian Journal of Ecology 17:121 – 131.

CERP (Comprehensive Everglades Restoration Plan). 2001. Baseline Report for the Comprehensive

Everglades Restoration Plan. South Florida Water Management District, West Palm Beach, Florida.  

Duarte, C. M. and H. Kirkman. 2001. Methods for the measurement of seagrass Abundance and depth

distribution. In “Global Seagrass Research Methods” (F.T. Short and R.G. Coles, ed) Elsevier Science,

B.V.

Fourqurean, J. W., A. Willsie, C. D. Rose and L. M. Rutten. 2001.  Spatial and temporal pattern in seagrass

community composition and productivity in south Florida. Marine Biology 138: 341‐354.

Gilbert, S. and K. B. Clark. 1981. Seasonal variation in standing crop of the seagrass  

Syringodium filiforme and associated macrophytes in the northern Indian River, Florida.  

Estuaries 4: 223‐225.

Hall, M.O. , D.A. Tomasko, and F.X. Courtney.  1991.  Responses of Thalassia testudinum to in situ light

reduction.  PP 77‐86 in Kenworth, W.J. and D. Haunert (eds.), The capability of water quality criteria

standards and monitoring programs to protect seagrasses from deteriorating water transparency.  

Results and recommendations of a workshop held January 30, 1991 in West Palm Beach, FL.  

Submitted to NOAA Coast. Ocean Prog. Est. Habitat Stud. 151p.

Holmquist, J. G., G. V. N. Powell and S. M. Sogard. 1989. Decapod and stomatopod assemblages on a

system of seagrass‐covered mud banks in Florida Bay. Marine Biology 100:473‐483.

Kemp, W. M., R. Batiuk, R. Bartleson, P. Bergstrom, V. Carter, C. Gallegos, W. Hunley, L. Karrh, E. W.

Koch, J. M. Landwehr, K. A. More, L. Murray, M. Naylor, N. B. Rybicki, J. C. Stevenson and D. J.

Wilcox. 2004. Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: Water

quality, light regime, and Physical‐chemical factors. Estuaries 27:363‐377.

Lirman, D. and W. P. Cropper Jr. 2003.  The influence of Salinity on Seagrass Growth, Survivorship and

Distribution within Biscayne Bay, Florida: Field, Experimental and Modeling Studies.  Estuaries

26:131‐141.

Montague, C. L. and J. A. Ley. 1993. A possible effect of salinity fluctuation on abundance of benthic

vegetation and associated fauna in Northeastern Florida Bay.  Estuaries 16:703‐717.

Onuf, C.P. 1991.  Light requirements of Halodule wrightii, Syringodium filiforme, and Halophila

engelmanni in a heterogeneous and variable environment inferred from long‐term monitoring. Pp

87‐97 in Kenworth, W.J. and D. Haunert (eds.), The capability of water quality criteria standards and

monitoring programs to protect seagrasses from deteriorating water transparency.  Results and

recommendations of a workshop held January 30, 1991 in West Palm Beach, FL.  Submitted to NOAA

Coast. Ocean Prog. Est. Habitat Stud. 151p.
18

Provancha, J. A. and D. M. Scheidt. 2000. Long‐term trends in seagrass beds in the Mosquito Lagoon and

Northern Banana River, Florida, 177‐193. In S. A. Bortone (ed.), Seagrasses: Monitoring, Ecology,

Physiology and Management. CRC Press, Boca Raton, FL.

Ridler, M. S., R. C. Dent and D. A. Arrington. 2006. Effects of two hurricanes on Syringodium filiforme,

manatee grass, within the Loxahatchee River Estuary, Southeast Florida. Estuaries and Coasts

29:1019‐1025.

Ridler, M. S., R. C. Dent and L. R. Bachman. 2003. Viability and variability of seagrass communities in the

Loxahatchee Estuary and associated reach of the Indian River Lagoon 1998, 2000, 2002. Loxahatchee

River District, Unpublished Report.

Short, F. T., J. Montgomery, C. F. Zimmermann and C. A. Short. 1993.  Production and Nutrient Dynamics

of Syringodium filiforme Kutz. Seagrass bed in Indian River Lagoon, Florida. Estuaries 16:323‐334.  

SFWMD. 2006. Restoration Plan for the Northwest Fork of the Loxahatchee River. South Florida Water

Management District, West Palm Beach, Florida.   

Zieman, J. C. 1982. The ecology of the seagrasses of south Florida: A community profile. Unites States

Fish and Wildlife Service, Office of Biological Services, Washington D.C. FWS/OBS‐82/25. 158 pp.

Zieman, J. C., J. W. Fourqurean and R. L. Iverson. 1989. Distribution, abundance and productivity of

seagrasses and macroalgae in Florida Bay. Bulletin of Marine Science 44:292‐311.  
19

Appendix A

Plots from 2010 Seagrass Mapping Project   
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

All Seagrass Species

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
-8

-3 -6

-7

-5

-4

-2 -4.5

-9

-1

-10

0

-4.5

-9

-5

-1

-5

-7

-4

-4.5

-4 -4.5

-4.5

-3

-4

-9

-6

-1

-4.5

-4

-4

-4.5

-4

-4.5

-3

-3

-6

-7

-7

-4.5

-5

-4.5

-6

-5

-5

-8

-4

-4

-5

-4

-1

-9

-1

-4.5

-4.5

-5

-1

-5

-6

-4.5

-3

-3

-4

-1

-3

-3

-4

-4.5

-3

-4

-5

-4.5

-3

-4

-3

-4.5

-3

-4

-3

-3

-4

-4.5

-6

-5

-7

-4.5

-5

-4

-3

-6

-3

-4.5

-7

-5

-4.5

-5

-9

-4.5

-5

-4

-3

-4

-1

-4.5

-1

-1

-3

-4

-2

-4.5

-4.5

-4

-6

-4.5

-3

-4.5

-4.5

-3

-4.5

-3

-2

-6

-4.5

-2

-9

-10

-1

-3

-4

-5

-9

-4.5

-3

-3

-4.5

-4

-3

-4.5

-5

-4

-2

-4.5

-4

-5

-6

-5

-5

-4

-4.5

-4.5

-5

-2

-4

-5

-3

-4.5

-4

-3

-4

-4.5

-4

-1

-4

-5

-4.5

-4.5

-10

-2

-4.5

-6

-3

-4

-4.5

-2

-4

-3

-6

-1

-3

-5

-2

-9

-4

-4.5

-4

-3

-5

-2

-4

-4.5

-5

-3

-6

-4.5

-7

-3

-4

-5

-2

-5

-1

-5

-4

-4.5

-6

-5

-5

-4

-4.5

-4

-3

-3

-3

-4

-5

-4.5

-4

-2

-3

-4

-4.5

-4

-5

-4.5

-6

-4

-4

-5

-5

-3

-4.5

-9

-4

-4

-4.5

-4

-2

-3

-3

-3

-4

-4

-7

-5

-5

-4

-4.5

-4

-2

-2

-5

-5

-4

-5 -4.5

-2

-10

-1

-2

-1

-9

-5

-2

-4.5

-4

-5 -4.5

-2

-4.5

-4

-6

-2

-3

-3

-1

-4

-5

-5

-5

-5

-8

-4

-4

-2

-1

-3

-3

-3

-3

-4

-4.5

-5

-3

-7

-4

-5

-4

-1

± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

All Seagrass Species with 2008 Bathymetry

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass

Elevation (NGVD)

-10.0

-9.9 - -9.0

-8.9 - -8.0

-7.9 - -7.0

-6.9 - -6.0

-5.9 - -5.0

-4.9 - -4.0

-3.9 - -3.0

-2.9 - -2.0

-1.9 - -1.0

-0.9 - 0.0

2008 Bathymetry Data from Jupiter Inlet District
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Interpolated Seagrass Coverage - All Species

~ Subject To Change ~

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Continuous

Patchy

No Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Johnson's Seagrass (Halophila johnsonii) 

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Shoal Grass (Halodule wrightii) 

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Paddle Grass (Halophila decipiens) 

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Manatee Grass (Syringodium filiforme)

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Turtle Grass (Thalassia testudinum)

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2010 Loxahatchee River Seagrass Mapping

Soft/Muck Sediment Depths

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Muck Depth

No Muck

< 10 cm

10 - 140 cm
20

Appendix B

Plots from 2007 Seagrass Mapping Project
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

All Seagrass Species

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

Interpolated Seagrass Coverage - All Species

~ Subject To Change ~

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Continuous

Patchy

No Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Johnson's Seagrass (Halophila johnsonii) 

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Shoal Grass (Halodule wrightii) 

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Paddle Grass (Halophila decipiens) 

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Manatee Grass (Syringodium filiforme)

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass
± Date: January 2011

S:\Data\LRD\Lab\Seagrass\2010 Spatial Study\2010 SG General Letter.mxd

2500 Jupiter Park Drive

Jupiter, FL 33458

(561) 747-5700

www.loxahatcheeriver.org

Aerial 2009

2007 Loxahatchee River Seagrass Mapping

Species-Specific Seagrass Distribution

Turtle Grass (Thalassia testudinum)

0 500 1,000

Meters

Loxahatchee River District

Drawn By: BH

Revised: 

Quadrat Score

(0/9) No Seagrass

(1 - 5/9) Patchy Seagrass

(6 - 9/9) Continuous Seagrass