FINAL REPORT to Lee County and the City of Bonita Springs 

Drift Rhodophyte Blooms Emerge in Lee County, FL: 

Evidence of Escalating Coastal Eutrophication

by 

Brian E. Lapointe 

and 

Bradley J. Bedford

Harbor Branch Oceanographic Institution, Inc. 

Division of Marine Science 

5600 US 1 North 

Ft. Pierce, FL 34946

March, 2006 

Key Words: Nitrogen, phosphorus, macroalgae, eutrophication, southwest Florida 

Running Head: Macroalgal blooms on Florida’s southwest coast 

Corresponding author: 772-465-2400 ext 276; 772-468-0757 (fax); lapointe@hboi.edu
1

 

Executive Summary 

Macroalgal blooms have increased globally in recent decades as a result of increased 

nutrient enrichment and eutrophication of coastal waters. In Lee County, FL, this problem

reached a critical stage in 2003 and 2004 when massive drift rhodophyte blooms washed ashore 

between Sanibel Island and Bonita Springs, making beaches unsuitable for recreation and 

requiring an expensive removal program. To better understand the ecology of these blooms, 

water quality and macroalgae sampling was conducted in early August 2004 prior to hurricane 

Charley and again in late October following several months of heavy discharges from the 

Caloosahatchee River. During both samplings, water and macroalgae were collected along a 

gradient extending from the Caloosahatchee River to natural and artificial reefs some 26 km

from shore. 

Concentrations of dissolved nutrient pools (DIN= NH4

+ + NO3− + NO2

− , TDN, TDP) were 

generally high throughout the study area with significantly enriched (~ 10-fold) concentrations in 

the Caloosahatchee River. The mean DIN concentrations increased from the Ortona Lock (< 18 

μM) to the Franklin Lock (23-28 μM) in the Caloosahatchee River during both samplings, 

indicating significant enrichment within the basin. The mean concentrations of NH4

+

 and SRP at 

the coastal reefs increased six-fold (≤ 0.20 to 1.31 μM) and three-fold (0.30 to 0.92 μM), 

respectively, between August and October following increased freshwater discharges. The mean 

C:N ratios of macroalgae at the reefs were relatively low and similar between August and 

October (13.9 vs. 13.5). The mean C:P and N:P ratios were also low and indicative of Nlimitation, and decreased significantly (386 to 242 and 27.4 to 17.5, respectively) between 

August and October. The δ15N values of macroalgae increased from the Ortona Lock (+ 8-9 ‰) 

to the Franklin Lock (+ 12-15 ‰) during both samplings and were within the range reported for 
2

sewage nitrogen; these values decreased with increasing distance from shore to ~ + 3.0 ‰ at the 

most offshore reef site. Macroalgae (Gracilaria, Hypnea, Botryocladia, Eucheuma, Sargassum) 

collected in July 2004 from Bonita Springs Beach and Sanibel Island had mean δ15N values > + 

6.0 ‰, similar to values measured in macroalgae from shallow inshore reefs and within the range 

of sewage nitrogen. However, mean δ15N values of coastal macroalgae decreased from August 

(+ 5.84 ‰) to October (+ 3.89 ‰) with increased discharges from the Caloosahatchee River, 

suggesting an increased contribution of N from rainfall and agricultural sources with lower δ15N 

values (< + 3 ‰) in the wet season. These results suggest that improved management of 

freshwater releases from Lake Okeechobee, combined with nutrient removal strategies for 

sewage within the Caloosahatchee River drainage basin, could help mitigate the development of 

these macroalgal HABs in the future. 
3

1. Introduction 

Point-source and non-point source enrichment of nitrogen (N) and phosphorus (P) is now 

recognized as the most serious pollution problem facing coastal waters worldwide (GESAMP, 

1990; NRC, 2000; Howarth et al., 2000). In the United States, scientists and policymakers 

recognize that a wide range of problems plaguing nearshore waters can be tied, directly or 

indirectly, to nutrient over-enrichment (Pew Oceans Commission, 2003; U.S. Commission on 

Ocean Policy, 2004). Nutrient pollution is the common thread that links an array of problems 

including eutrophication, harmful algal blooms (HABs), bio-invasions, fish kills, shellfish 

poisonings, loss of seagrass and kelp beds, coral reef die-off, emerging marine diseases, and 

marine mammal and seabird deaths (Howarth et al., 2000; Lapointe et al., 2004; Lapointe et al., 

2005). 

The development of macroalgal HABs is a predictable ecological response to increased 

nutrient loading in shallow bays, estuaries, and coastal waters (Morand and Briand, 1996; 

Lapointe et al., 1994; Valiela et al., 1997). Unlike toxic phytoplankton HABs such as red tides, 

macroalgal HABs lack direct chemical toxicity but typically have a broader range of ecological 

impacts. The effects of macroalgal blooms are largely indirect, multi-faceted, and of longer 

duration than toxic phytoplankton HABs. Excessive biomass of macroalgae can cause hypoxia, 

anoxia, and die-off of seagrasses and other benthic biota (Lapointe et al., 1994; Valiela et al., 

1997), thereby reducing habitat for desirable and economically important fisheries. In 

oligotrophic subtropical and tropical waters, nutrient-enriched macroalgal blooms can alter food 

web dynamics by increasing the abundance of grazers in seagrass (McGlathery, 1995) and coral 

reef ecosystems (Lapointe and Thacker, 2002). Increasingly, macroalgal blooms foul beaches 
4

and shorelines important to local tourist economies and require ever more expensive biomass 

removal programs (Morand and Briand, 1996; Lapointe and Thacker, 2002). 

Land-based nutrient discharges to bays and coastal waters along southwest Florida have 

long been linked to the development of macroalgae and phytoplankton blooms. Sewage-driven 

eutrophication in Tampa Bay during the 1960’s, 1970’s, and 1980’s led to drift macroalgal 

blooms that included the rhodophyte Gracilaria and chlorophyte Ulva (Humm, 1973; Guist and 

Humm, 1976). In Hillsboro Bay, a subdivision of Tampa Bay, drift macroalgal HABs with 

biomass levels > 600 g dry wt/m2 developed in the early 1980’s, which included the rhodophytes 

Gracilaria, Spyridia, Hypnea, and Agardhiella, and the chlorophytes Ulva and Caulerpa (Avery, 

1997). Following considerable seagrass loss in Hillsboro Bay by the late 1970’s, N removal 

from the local wastewater treatment plant was initiated in 1979, and by 1994 drift macroalgae 

had decreased by > 90% while seagrass cover increased from 0.2 ha in 1986 to over 28 ha in 

1995 (Avery, 1997). In coastal waters of southwest Florida, Ketchum and Keen (1947) 

correlated red tide blooms off Sarasota, FL, with unusually high P concentrations in the water 

column and suggested “the excessive nutrient content may be the result of terrigenous 

contamination or fertilization of the waters.” Slobodkin (1953) reported that the red tide 

outbreaks off southwest Florida may be intiated by the development of a stratified water mass 

characterized by reduced salinity and elevated nutrients resulting from discharges of the 

combined Charlotte Harbor – Caloosahatchee River drainage basins. 

Recent increases in nutrient loading associated with expanding urbanization of the 

watershed and discharges from the Peace and Caloosahatchee rivers could be linked to emerging 

blooms of benthic macroalgae in estuaries and coastal waters of Lee County, FL (Fig. 1). 

McPherson and Miller (1990) noted that projected increases in nitrogen loadings from the Peace 
5

River basin would favor undesirable increases in phytoplankton and benthic algae in the 

Charlotte Harbor estuarine system. An analysis of monitoring data for the Caloosahatchee River 

indicated that water quality in the downstream estuary changes as a function of total discharge 

and water source (river basin, Lake Okeechobee; Doering and Chamberlain, 1999). While these 

assessments were limited to the estuarine regions of the Peace and Caloosahatchee rivers, the 

combined flows and nutrient loads associated with these discharges have the potential to impact 

coastal waters for considerable distances distances from shore (> 50 km; Yang et al., 1999). 

Accordingly, increased land-based nutrient pollution could be influencing coastal waters off

southwest Florida and may help explain the unprecedented accumulation of drift rhodophytes 

that fouled coastal beaches between Sanibel Island and Bonita Springs in 2003/2004 (Fig 2 A, B, 

E). The excessive biomass of these rhodophytes caused odor problems and greatly diminished 

the use of the beaches by tourists. Concerns of the local governments of Lee County and the 

City of Bonita Springs prompted the present investigation in order to determine if these 

macroalgal HABs could be linked to increasing land-based nutrient pollution as in Tampa Bay 

and other parts of the world (Morand and Briand, 1996; Valiela et al., 1997; NRC, 2000). 

Several approaches can be used to assess the spatial extent and degree of land-based 

nutrient enrichment in coastal waters of Lee County. One traditional method involves 

measurements of salinity and concentrations of dissolved inorganic nitrogen (DIN = 

) and soluble reactive phosphorus (SRP) in water samples along inshore to 

offshore gradients. If nutrient concentrations of lower-salinity inshore waters are higher than 

that of higher-salinity offshore waters, then a land-based source of nutrients is indicated 

(Ketchum, 1967). A more specific approach involves the measurement of stable nitrogen isotope 

ratios in macroalgae that can be used to “fingerprint” the source of N when the various N sources

NH4

+ + NO3− + NO2−
6

are known (Heaton, 1986; Owens, 1987). Attached macroalgae have a distinct advantage over 

free-floating phytoplankton as nutrient indicators because they provide a long-term integration of 

the aqueous N signal for a particular location (Lapointe et al., 2004). Enrichment of δ

15Nin 

aquatic systems can result from N transformations that occur prior to, during, or following the 

treatment and discharge of sewage. Volatilization of ammonia and isotopic fractionation by 

microbes during nitrification and denitrification produce residual DIN with elevated δ

15N values 

of + 6 ‰ to + 22 ‰ (Heaton, 1986; Lindau et al., 1989). This range includes secondarily treated 

discharges from sewage outfalls (Hoch et al., 1995; Table 1) as well as shallow (< 10 m)

groundwaters contaminated by septic tanks in south Florida (Lapointe and Krupa, 1995 a, b; 

Table 1). In comparison, N derived from rainfall, fertilizers and organic peat associated with 

stormwater runoff from agricultural areas have δ

15N values ranging from -3 ‰ to + 3 ‰ 

(Heaton, 1986; Paerl and Fogel, 1994; Table 1) and can therefore be effectively discriminated 

from the higher sewage N signature. 

We hypothesized that, if land-based anthropogenic N sources such as sewage was the 

primary DIN source supporting macroalgal HABs in Lee County’s coastal waters, then the 

highest δ

15N values would occur in macroalgae from the Caloosahatchee River, beaches, and 

shallow coastal reefs most influenced by land-based discharges. Macroalgae that rely on natural 

nitrogen fixation have low δ

15N values of ~ 0 ‰ (France et al., 1998; Table 1) in contrast to 

those using sewage N, which become increasingly enriched in δ

15N with increasing sewage N 

contributions over a range from + 3 ‰ to + 16 ‰ (Lapointe, 1997; Costanzo et al., 2001). 

Globally, many case studies have used δ

15N as a tool to discriminate between natural and 

anthropogenic N sources supporting macroalgal growth (Lapointe, 1997; France et al., 1998; 

McClelland and Valiela, 1998; Costanzo et al., 2001; Wayland and Hobson, 2001; Umezawa et 
7

al., 2002; Gartner et al., 2002; Barile, 2004; Savage and Elmgren, 2004; Lapointe et al., 2004). 

Several studies have successfully utilized δ

15N values in macroalgae and reef corals to assess the 

spatial extent of land-based N enrichment along gradients into the coastal ocean, on scales from

several kilometers (Umezawa et al., 2002; Lapointe et al., 2004; Lapointe et al., 2005b), to nearly 

forty kilometers across the Great Barrier Reef lagoon (Sammarco et al., 1999). 

We also predicted that “wet versus dry” seasonality could significantly affect the degree 

and relative importance of various sources of land-based N enrichment. In the Florida Keys, 

local N loadings from sewage were relatively constant compared to the large non-point source 

agricultural N loads that were transported into coastal waters during wet years when large water 

releases from the Everglades occurred (Lapointe et al., 2004). We initiated our study in Lee 

County in early August 2004, prior to landfall of three hurricanes that struck Florida in August 

and September, 2004. Because historically significant amounts of rainfall resulted from the 

overlapping paths of hurricanes Charley, Frances, and Jeanne in the Kissimmee River drainage 

basin north of Lake Okeechobee (South Florida Water Management District, rainfall data), we 

had an opportunity to test this hypothesis by re-sampling in late October of 2004. 

2. Materials and methods 

2.1. Sample collection and analysis

Samples of the water column and attached macroalgae and/or seagrasses were collected 

August 8-10 (dry season) and October 28-29 (wet season), 2004, along an onshore to offshore 

gradient extending from the Caloosahatchee River into coastal waters of Lee County, FL (Fig. 

1). Nine fixed stations sampled in both August and October included the Ortona and Franklin 

locks on the Caloosahatchee River; seagrass beds in San Carlos Bay and S. Charlotte Harbor; a 
8

shallow, natural, nearshore reef off Boca Grande, known locally as the 17th Street Reef (4 m

depth); the Belton-Johnson Reef (10 m), an artificial reef; Blanda’s Reef (14 m), an artificial 

reef; ARC Reef (19 m,), an artificial reef; and North Deep Ledge (20 m), a natural ledge and 

farthest station from shore (26 km; Fig. 1). In August, two additional stations (Peace and 

Imperial rivers) were sampled for water column nutrients, and macroalgal tissue was collected 

for δ15N analysis from beaches at Bonita Springs, Fort Myers, Sanibel, and Captiva. In addition, 

dried rhodophytes collected from stranded material on Bonita Beach in late July, 2004 (Table 2;

Fig. 2B) were analyzed for δ15N. 

At each of 11 sites in August and 9 sites in October, replicate samples (n = 2) of nearbottom water were collected into clean, 250 ml HDPE bottles and held on ice in a cooler until 

processing. In the lab, 100 ml sample aliquots were filtered via syringe through 0.45 μm

Whatman GF/F filters into clean, 150 ml HDPE bottles and frozen. The samples were 

subsequently analyzed for NH -N, 4

+ NO3

− + NO2− -N and PO4

3−

-P (SRP) at the Nutrient Analytical 

Services Laboratory, Chesapeake Biological Laboratory, Center for Environmental and Estuarine 

Studies, University of Maryland, Solomons, MD (NASL). A Technicon Auto-Analyzer II was 

used for determination of nitrate (NO3

−

) and soluble reactive phosphate (SRP), and a Technicon 

TRAACS 800 was used for analysis of ammonium (NH4

+ ) and nitrite (NO ). Detection limits 

were 0.20 μM for , 0.01 μM for NO

2

−

NH4

+

3

− + NO2− , 0.01 μM for NO2−

, and 0.02 μM for SRP 

(D’Elia et al., 1997). We used the f-ratio (NO3 / (NO -

3

- + NH4

+ )) to gauge the relative importance 

of versus as a DIN source (McCarthy et al. 1975; Harrison et al., 1987) to macroalgae 

at the study sites. Samples collected in October were also analyzed for total dissolved nitrogen 

(TDN) and total dissolved phosphorus (TDP) at NASL, using a Technicon Auto-Analyzer II with 

detection limits of 1.43 μM for TDN, and 0.03μM for TDP (D’Elia et al., 1997), and were used 

NO3

- NH4+
9

to calculate dissolved organic nitrogen (DON = TDN – DIN) and dissolved organic phosphorus 

(DOP = TDP – SRP). 

Samples of macroalgae and/or seagrasses were collected at the nine stations in August 

and October by SCUBA or snorkeling (Table 2). Rhodophytes were collected at the five coastal 

reef sites (17th St. Reef, Culvert Reef, Blanda’s Reef, ARC Reef, and N. Deep Ledge); the 

seagrass Thalassia testudinum was collected at S. Charlotte Harbor and San Carlos Bay; and the 

chlorophyte Cladophora sp. and the cyanophyte Lyngbya sp. were collected at Ortona and 

Franklin locks (Table 2). Field samples were stored in plastic, zipper-lock, storage bags and held 

on ice in a cooler until processing. In the lab, composite samples (thalli from 5-8 different plants 

of each species) of macroalgae were sorted, cleaned of visible epiphytes and sediments, 

identified (Dawes, 1974; Littler and Littler, 2000), and rinsed briefly (3-5 sec) in deionized water 

to remove salt and debris. The cleaned, composite samples were dried in a Fisher Scientific 

IsotempTM oven at 60  ̊

C for 48 h and then ground to a fine powder using a mortar and pestle. 

Samples of the dried, powdered macroalgae were stored in plastic screwtop vials and placed in a 

dessicator until analysis for C:N:P contents (molar ratios) at NASL. Percent C and N were 

measured on an Exeter Analytical, Inc (EAI) CE-440 Elemental Analyzer and percent P was 

measured following the methodology of Asplia et. al. (1976) using a Technicon Autoanalyzer II 

with a IBM compatible Labtronics Inc. DP500 software data collection system (D’Elia et al., 

1997). All samples were also analyzed for δ15N (n = 2 analytical replicates per sample) with a 

Carlo-Erba N/A 1500 Elemental Analyzer and a VG Isomass mass spectrometer using Dumas 

combustion, at Isotope Services, Inc., Los Alamos, NM. The standard used for stable nitrogen 

isotope analysis was N2 in air. δ15N values (‰) were calculated using 

R ; where R = Sample [( /RStandard )−1 *103

] 15N/14N. 
10

2.2. Statistical analysis

Data were tested for normality using the Shapiro-Wilk test (W statistic), and for 

homoscedasticity using Levene’s test of equality of error variances. Normally distributed 

datasets were compared using the Generalized Linear Model (GLM, Type III sum of squares) 

procedure in SPSS 11.0 for Mac. Data not normally distributed were compared using either the 

Kruskal-Wallis H test (three or more groups), or the Mann-Whitney U test (two groups). Post 

hoc comparisons were made using Tukey’s HSD test. 

Sites where water samples were collected in August only (Imperial and Peace rivers) 

were not included in overall (August and October) water column nutrient statistics. However, 

data analyzed to compare seasonal differences (August vs. October) included all sites sampled. 

In order to assess water column charcteristics in the vicinity of the coastal macroalgal blooms, a 

subset of coastal water samples (San Carlos Bay, S. Charlotte Harbor, 17th St. Reef, BeltonJohnson Reef, Blanda’s Reef, ARC Reef, and N. Deep Ledge) from August and October were 

tested for significant effects of season. For all analyses, differences were considered significant 

at p ≤ 0.05. 

3. Results 

3.1. Taxonomic Composition of the Macroalgal Blooms

The drift macroalgal community that washed ashore on Bonita Springs beach in July and 

August 2004 was dominated by rhodophytes (Table 2). Species identified from the beach 

collections included Botryocladia occidentalis, Eucheuma isiforme var. denudatum, Gracilaria 

cervicornis, Gracilaria tikvahiae, Agardhiella subulata, and Hypnea musciformis. In August, 
11

the pelagic phaeophyte Sargassum fluitans was also collected from Ft. Myers Beach, Sanibel 

Island, and Captiva Island. 

The rhodophytes from the beach strandings were found to be abundant on the natural and 

artificial reefs in coastal waters off Lee County (Table 2). In both August and October, 

Botryocladia occidentalis was collected from all five reef sites. Other rhodophytes collected 

from the reefs included Agardhiella subulata, Eucheuma isiforme var. denudatum, Gracilaria 

cervicornis, Gracilaria tikvahiae, Hypnea musciformis, and Rhodymenia divaricata. 

3.2. Dissolved nutrient concentrations, f-ratios, DIN/SRP ratios, and salinity

Overall NH concentrations varied significantly with location (F = 89.882, p < 0.001, 

GLM), season (p < 0.001, Mann-Whitney) and the location-season interaction (F = 50.164, p < 

0.001, GLM), averaging 1.74 ± 3.66 μM (n = 22) in August and 2.48 ± 2.59 μM (n = 18) in 

October. Concentrations were significantly higher in the rivers, with maximum values of 12.70 

μM at the Imperial River (August) and 8.30 μM at Franklin Lock (October). In August, values 

≤ 0.21 μM (detection limit) were measured at all bay and coastal reef sites whereas in October 

values were > 0.55 μM at all sites (Fig. 3). At the five coastal reef sites, mean 

concentrations increased from ≤ 0.21 μM (n = 14) in August to 1.31 ± 1.08 μM (n = 14) in 

October (Table 3) with a highly significant effect of season (F = 715.765, p < 0.001, GLM). 

4

+

NH4

+

 concentrations varied significantly with location (F = 1644.205, p < 0.001, GLM) 

and location-season interaction (F = 354.582, p < 0.001, GLM), averaging 5.62 ± 9.29 μM (n = 

22) in August and 3.92 ± 5.70 μM (n = 18) in October. High concentrations occurred in the 

rivers, with maximum values at the Franklin Lock in both August (26.15 μM ) and October 

(14.35 μM ); minimum values occurred at the bay and coastal sites, with minimums of 0.49 μM 

at S. Charlotte Harbor and Belton-Johnson Reef (August) and 0.27 μM at 17

NO3

−

th St. Reef 
12

(October). At the coastal reef sites, the mean NO3

−

 concentrations were similar in August (0.91 ± 

0.72 μM, n = 14) and October (0.97 ± 0.69 μM, n = 14; Table 3). 

Overall, DIN concentrations varied significantly with location (F = 169.055, p < 0.001, 

GLM), season (p = 0.034, Mann-Whitney), and location-season interaction (F = 133.093, p < 

0.001, GLM), averaging 7.36 ± 10.61 μM (n = 22) in August and decreasing to 6.40 ± 8.09 μM 

(n = 18) in October. The highest DIN concentrations occurred in the rivers, with the maximum 

value at the Franklin Lock in August (27.70 μM) and October (22.65 μM); the lowest levels 

occurred at the bay and coastal reef sites, with minimum values of 0.69 μM at S. Charlotte 

Harbor and Belton-Johnson Reef in August and 0.83 μM at 17th St. Reef in October (Fig. 3). On 

the coastal reefs, mean DIN concentrations increased significantly (F = 66.322, p < 0.001, GLM) 

from August (1.11 ± 0.72 μM, n = 14) to October (2.29 ± 1.57 μM, n = 14; Table 3). 

Overall, the f-ratio varied with location (F = 11.822, p < 0.001, GLM), season (p < 0.001, 

Mann-Whitney), and location-season interaction (F = 7.702, p < 0.001, GLM) with a higher 

mean value in August (0.75 ± 0.19, n = 22) compared to October (0.48 ± 0.17, n = 18). In 

August, the f-ratios were statistically similar among sites (maximum of 0.94 at Franklin Lock) 

except the Imperial River (p ≤ 0.015, THSD) where the minimum (0.27) occurred. In October, 

the maximum values occurred in the rivers (Ortona, 0.75) and the minimum at the offshore ARC 

Reef (0.32, Fig. 3). The f-ratios for the coastal reef sites averaged 0.77 ± 0.11 (n = 14) in August 

and decreased significantly (F = 168.32, p = 0.006, GLM) to 0.41 ± 0.14 (n = 14) in October 

(Table 3). 

 SRP concentrations varied with location (F = 195.032, p < 0.001, GLM), season (p = 

0.011, Mann-Whitney), and the location-season interaction (F = 119.383, p <0.001, GLM), 

averaging 2.01 ± 4.96 μM (n = 22) in August and 1.04 ± 1.08 μM (n = 18) in October. In 
13

August, the maximum concentrations occurred in the Peace River (16.80 μM), which was 

significantly (p < 0.001, THSD) higher than all other sites, and the minimum concentrations 

(0.14 μM) occurred at San Carlos Bay (Fig. 4). In October, the maximum SRP concentrations 

(3.74 μM) occurred at S. Charlotte Harbor downstream of the Peace River (which was not 

sampled) with lower values at the coastal reefs (minimum 0.19 μM at N. Deep Ledge) (Fig. 4). 

Mean SRP concentrations at the coastal reefs were significantly (F = 289.115, p < 0.001, GLM) 

lower in August (0.30 ± 0.18 μM, n = 14) than in October (0.92 ± 1.20 μM, n = 14; Table 3). 

The DIN/SRP ratio varied significantly with location (F = 25.510, p < 0.001, GLM), 

averaging 11.25 ± 19.51 (n = 22) in August and 6.92 ± 4.95 (n = 18) in October. The highest 

DIN/SRP ratio in August was in the Imperial River (68.30) compared to the Franklin Lock 

(14.86) in October. The lowest DIN/SRP ratios in August (1.32) and October (0.29) both 

occurred at S. Charlotte Harbor (Fig. 4). At the coastal reef sites, the DIN/SRP ratios were 

similar in August (4.77 ± 3.10, n = 14) and October (4.88 ± 3.41, n = 14; Table 3). 

Total dissolved N and P were measured only during the October sampling, when both 

TDN (F = 430.949, p < 0.001, GLM) and TDP (F = 437.560, p < 0.001, GLM) varied with 

location. TDN averaged 35.8 ± 30.8 μM and concentrations in the Caloosahatchee River (70.9 

μM at Franklin Lock) were significantly (p < 0.001, THSD) higher than all other sites; the bays 

were significantly (p ≤ 0.016, THSD) higher than coastal reef sites, and the minimum value 

(11.07 μM) occurred at N. Deep Ledge (Fig. 5). TDP averaged 1.57 ± 1.42 μM and 

concentrations were significantly elevated in the rivers and S. Charlotte Harbor (p < 0.001, 

THSD) compared to all other sites, with the highest value at S. Charlotte Harbor (5.16 μM) and 

the lowest at N. Deep Ledge (0.37 μM, Fig. 5). 
14

In October, DON (F = 255.172, p < 0.001, GLM) and DOP (F = 30.976, p < 0.001, 

GLM) varied significantly with location. DON dominated the TDN pool (averaged 29.4 ± 23.1 

μM) with the highest concentrations in the rivers (70.92 μM maximum at Franklin Lock), which 

were significantly (p < 0.001, THSD) higher than all other sites. Lower values occurred in the 

bays, which were significantly (p < 0.001, THSD) higher than the coastal reef sites where the 

minimum of 9.69 μM occurred at N. Deep Ledge (Fig. 5). In contrast, DOP comprised a 

relatively minor portion of the TDP pool, averaging 0.53 ± 0.36 μM. The highest DOP 

concentrations were in S. Charlotte Harbor (1.42 μM), which was was the only site significantly 

(p < 0.001, THSD) different than the other sites; the lowest value (0.18 μM) occurred at N. Deep 

Ledge (Fig. 5). 

 Salinities ranged from 0.3 ‰ at the Ortona and Franklin locks in both August and 

October to 37.3 ‰ offshore at N. Deep Ledge and 37.0 ‰ at ARC Reef in August. During both 

samplings, a lower salinity surface layer (buoyant plume) occurred at our most offshore stations, 

especially ARC Reef. In August, the surface layer had a salinity of 32 ‰ compared to a nearbottom salinity of 37 ‰. Stratification was also observed in October at ARC Reef, where a 

lower-salinity (34.8 ‰), highly colored surface layer extended to a depth of ~ 12 m over the 

clearer, higher salinity (36.0 ‰) bottom layer (12-19 m depth). 

3.3. C:N:P analysis of macroalgae

Overall, there were no significant effects of location or season on C:N ratios of 

macroalgae and seagrasses, which averaged 13.2 ± 3.0 (n = 20) in August and 12.9 ± 3.5 (n = 15) 

in October. The lowest C:N ratios occurred at Franklin Lock in August (9.0) and October (8.7); 
15

the highest ratios were recorded at the coastal reef sites N. Deep Ledge in August (36.6) and 

Belton-Johnson Reef in October (15.3) (Fig. 6). 

C:P ratios varied significantly with season (F = 4.268, p = 0.053, GLM), averaging 343 ± 

197 (n = 20) in August and 236 ± 132 (n = 15) in October. The lowest C:P ratios were 170 at 

Franklin Lock in August and 139 at N. Deep Ledge in October when a buoyant darkwater 

surface layer was evident 26 km from the coast (Fig. 6). At the coastal reef sites, seasonal 

differences were significant (F = 6.141, p = 0.024, GLM), with the August mean (386 ± 198, n = 

16) higher than October (17.5 ± 6.7, n = 13) (Table 4). 

There was significant (F = 4.868, p = 0.040, GLM) seasonal variation in N:P ratios, with 

a higher mean in August (25.4 ± 11.0, n = 20) than October (18.1 ± 6.4, n = 15). The lowest N:P 

ratios occurred at Ortona Lock (15.8 in August) and S. Charlotte Harbor (12.8 in October); the 

highest ratios were at N. Deep Ledge (42.4, August) and Belton-Johnson Reef (24.3, October) 

(Fig. 6). On the coastal reefs, N:P ratios decreased significantly (F = 8.979, p = 0.008, GLM), 

from 27.4 ± 11.4 (n = 16) in August, to 17.5 ± 6.7 (n = 13) in October (Table 4). 

3.4. Stable nitrogen isotopes in macroalgae

Overall, δ15N values in macroalgae and seagrasses varied significantly with location (F = 

50.118, p < 0.001, GLM), season (F = 12.746, p = 0.001, GLM) and the location-season 

interaction (F = 5.280, p < 0.001, GLM), with a higher mean in August (+ 6.2 ± 2.3 ‰, n = 46) 

than October (+ 4.7 ± 3.1 ‰, n = 34) and values generally decreasing with increasing distance 

from shore. In both August and October, δ15N values at Franklin Lock were significantly (p ≤

0.001, THSD) higher than all other stations except Ortona Lock, with values of + 11.8 ‰ and + 

15.6 ‰, respectively (Fig. 7A). Blanda’s Reef had the lowest values: + 3.2 ‰ in August and + 
16

3.0 ‰ in October (Fig. 7A). At the coastal reef sites δ15N in macroalgae decreased significantly, 

from + 5.84 ± 1.37 ‰ (n = 32) in August to + 3.89 ± 0.96 ‰ (n = 26) in October (Table 4). 

The δ15N values of macroalgae (phaeophytes and rhodophytes, Fig. 7B) collected from 

Lee County beaches in August 2004 did not vary significantly with location (p = 0.264, KruskalWallis) and values ranged from a high of + 7.1 ‰ at Fort Myers Beach to a low of + 5.7 ‰ at 

Bonita Springs Beach, with an overall mean of + 6.1 ± 1.3 ‰ (n = 14). Samples of the 

rhodophyte Hypnea musciformis (Fig. 7B) collected from drift accumulations on Bonita Springs 

Beach were enriched (+ 8.0 ± 0.55) compared to samples of the same species collected at 17th St. 

Reef (+ 7.19 ± 0.15) and N. Deep Ledge (+ 5.12 ± 0.08). 

4. Discussion 

Results of this study support the hypothesis that the drift rhodophyte blooms that have 

developed in Lee County’s coastal waters in recent years are linked to increasing land-based 

nutrient enrichment, especially N. Although similar drift macroalgal blooms have developed in 

shallow seagrass meadows in Tampa Bay (Humm, 1973; Guist and Humm, 1976 ; Avery, 1997), 

the Indian River Lagoon (Virnstein and Carbonara, 1985), coral reefs off southeast Florida 

(Lapointe et al. 2005 a,b), and the Florida Keys (Lapointe et al., 1994; Lapointe et al., 2004), the 

blooms of these particular rhodophytes in Lee County are the first to be reported for shallow 

coastal waters along barrier beaches of southwest Florida (Fig. 2 E). Our results documented 

that significant N and P enrichment extended at least 26 km from shore in fall of 2004, 

supporting previous suggestions that river discharges can cause large-scale enrichment of coastal 

waters off southwest Florida (Slobodkin, 1953; Yang et al., 1999). Considering that these 

macroalgal HAB phenomena are symptomatic of cultural eutrophication in shallow, coastal 
17

waters of south Florida (Humm, 1973; Lapointe and Ryther, 1979), the large scale and economic 

impact of these blooms demand serious concern by water and resource managers for protection 

of this popular tourist destination. 

4.1 Taxonomic composition and ecology of blooms

 The drift rhodophyte community that washed ashore on Bonita Springs Beach in July 

2004, prior to hurricane Charley, was dominated by the genera Gracilara, Hypnea, and 

Agardhiella. Humm (1973) noted that rhodophytes dominated (51%) the benthic macroalgal 

communities in the eastern Gulf of Mexico, compared chlorophytes (31%) and phaeophytes 

(18%). We found a diverse rhodophyte community to be common on natural and artificial reefs 

throughout the study area prior to hurricane Charley, indicating that “seed” populations for these 

blooms are present in local waters. 

The abundance of macroalgae in the eastern Gulf of Mexico is limited by the scarcity of 

suitable rocky substrata (Humm, 1973). Most of the intertidal zone and coastal waters are 

unconsolidated sediments that will not support the establishment of most macroalgae. Where 

limestone reef outcrops or other suitable benthic substrata are available, the rhodophytes begin 

growing as attached plants and can become the dominant biotic cover of inshore reefs and 

suitable soft bottom communities when nutrient availability is adequate (see Fig. 2C). Under 

nutrient enriched conditions, some of these rhodophytes are capable of very rapid growth and 

bloom formation. For example, Gracilaria tikvahiae, which was abundant in the Bonita Springs 

Beach bloom and coastal waters in Lee County, can double its biomass in < 3 days when 

irradiance, temperature, and nutrients are not limiting (Lapointe et al., 1984). As the 

rhodophytes grow, waves and currents detach larger plants that continue to grow as unattached 
18

(drift) populations. Irradiance is a critical factor regulating growth rate of G. tikvahiae, which 

can utilize nearly full, natural irradiance levels (Lapointe et al., 1984). The irradiance we 

measured on the bottom at the offshore reefs in 10-20 m water depths was < 30 μmol photons m2

 sec-1, very low values that would result in severe light-limitation for G. tikvahiae at 20-30 oC 

(Lapointe et al., 1984). Hence, the reflective sand bottom in shallow, nearshore coastal waters of 

Lee County would provide the higher irradiance levels ( > 400 μmol photons m-2 sec-1) needed to 

generate these blooms (Fig. 2E). 

 The tissue C:N data show that the rhodophytes in coastal waters of Lee County are 

generally enriched in N (a low C:N ratio) compared to macroalgae on reefs in southeast Florida 

and the wider Caribbean (Fig. 8). The mean C:N ratio of the rhodophytes was ~ 13.7, a value 

that can support maximum growth rates in Gracilaria tikvahiae. For example, growth rates as 

high as 0.37 doublings d-1 (a biomass doubling every 2.5 days) were observed in G. foliifera var. 

angustissima (= G. tikvahiae) growing under full, natural irradiance at a C:N ratio of 13.4; lower 

growth rates of 0.2 doublings d-1 occurred when these plants were grown under lower irradiance 

and a lower C:N ratio of 8.56 (Lapointe, 1981). Under such light-limited conditions, G.

tikvahiae increases its characteristic protein pigment, phycoerythrin, which decreases the C:N 

ratio and provides a N storage pool that can support growth when N availability decreases 

(Lapointe, 1981; Lapointe and Ryther, 1979). Because of this physiological profile, G. tikvahiae 

and other rhodophytes are well adapted to assimilate and store pulses of DIN in coastal waters 

associated with stormwater runoff and discharges from the DIN-enriched waters of the 

Caloosahatchee and Peace rivers. 

 The low C:P and N:P ratios of the rhodophytes in Lee County further indicate a high 

degree of P enrichment compared to macroalgae from southeast Florida and the wider Caribbean 
19

region (Fig. 8). Although relatively high C:P (622) and N:P (42.4) ratios occurred in August at 

the most offshore station (N. Deep Ledge), these ratios decreased significantly with increased 

runoff following hurricanes Charley, Frances, and Jeanne, and were generally low, averaging 

314 and 22.5, respectively, in the coastal waters during the study. These C:P and N:P ratios are 

much lower than the means of 976 and 43.4 reported for carbonate-rich waters of the Caribbean 

where macroalgal growth is strongly limited by P (Lapointe et al., 1992). In experimental field 

studies in the Lower Florida Keys in 1983, G. tikvahiae experienced severe P limitation when 

very high C:P (892-2,816) and N:P (73-250) ratios developed in ambient seawater of Pine 

Channel (Lapointe, 1987). The low SRP concentrations (< 0.1 μM) in the Florida Keys 

obviously preclude G. tikvahiae from forming blooms in that location, in contrast to Lee County 

where much higher SRP concentrations (0.30-0.92 μM) support low C:P and N:P ratios and 

result in N limitation of growth. The low mean C:P (314) and N:P (22.5) ratios in rhodophytes 

from Lee County are indicative of N rather than P limitation of growth (Lapointe et al. 1992). 

The phaeophyte Sargassum fluitans also washed ashore on Lee County beaches in 

summer 2004. Populations of pelagic Sargassum (S. natans, S. fluitans) have been present for 

centuries and are continually transported by winds and surface currents between the Caribbean 

Sea, the Gulf of Mexico and the western North Atlantic Ocean. During this large-scale 

circulation, these pelagic oceanic populations frequently encounter relatively nutrient enriched 

neritic waters where the C:N, C:P, and N:P ratios decrease while productivity and growth rate 

increase. For example, the C:N, C:P, and N:P ratios of Sargassum natans decreased significantly 

from mean values of 49.4, 877, and 18.1 in the Sargasso Sea, to 27.9, 347, and 10.2 in neritic 

waters off the southeastern U.S. coastline (Lapointe, 1995). Such nutrient enrichment in coastal 

waters, coupled with strong onshore winds, can bring excessive biomass of Sargassum ashore 
20

and cause a variety of problems. Along the Florida panhandle and Texas coastlines, strandings 

of pelagic Sargassum have become a major problem for beach tourism in recent years (Lapointe, 

1995). At the nuclear power plant in Crystal River, FL, a massive influx of Sargassum in 1990 

caused a blockage in the cooling system, forcing a system shutdown (Rogers, 1991). 

4.2. Land-based sources of nutrient enrichment

Multiple lines of evidence support the hypothesis that discharges from the 

Caloosahatchee River and other land-based sources can provide nutrients to blooms of red drift 

macroalgae (and phytoplankton) for considerable distances from shore. The evidence includes 

significant enrichment of DIN, SRP, TDN, and TDP in fresh waters of the Caloosahatchee and 

Peace Rivers relative to coastal waters of the study area. Following increased discharges from

these rivers after hurricanes Charley, Frances, and Jeanne, we observed a six-fold increase in 

 (≤ 0.20 vs. 1.31 μM) and a three-fold increase in SRP (0.30 vs. 0.92 μM) to ~ 26 km from

shore. 

NH4

+

The increasing importance of NH4

+

 relative to NO3

−

 with increasing land-based runoff to 

Lee County’s coastal waters is apparent from the significant decrease in the f-ratio between our 

dry (August) and wet (October) samplings. Although the f-ratio was historically used by 

oceanographers to gauge the relative importance of upwelled NO3

−

 to phytoplankton growth 

(Harrison et al., 1987), our previous research in the Florida Keys (Lapointe et al., 2004) and in 

Lee County during this study demonstrate the utility of this ratio in assessing land-based 

discharges of NH . Concentrations of 4

+ NO3

−

 in Lee County’s coastal waters were statistically 

similar (~ 0.9 μM) in August and October, compared to the six-fold increase (0.20 vs. 1.31 μM) 

in NH concentrations that resulted in the decreased 4

+ f-ratio. This NH4

+

concentration is high for 
21

coastal waters and two-fold greater than the concentration needed to support maximum growth 

rates of Karenia brevis (Steidinger et al., 1998), as well as the rhodophytes Neoagardhiella 

bayleii and Gracilaria tikvahiae (DeBoer et al., 1978). Considering that is the preferred N 

source for growth of K. brevis (Steidinger et al., 1998) and the rhodophytes N. bayleii and G. 

tikvahiae (DeBoer et al., 1978), it is not surprising that blooms of these species can follow 

seasonal increases in land-based runoff and NH

NH4

+

4

+

enrichment of coastal waters following the 

onset of the wet season in Lee County. 

Although considerable nutrient loadings from the Caloosahatchee River discharges are 

linked to water releases from Lake Okeechobee, a comparison of the δ15N values of macroalgae 

along the Caloosahatchee River provide evidence of significant N enrichment from within the 

basin itself. The algal tissue δ15N values increased westward from the Ortona (+ 8-9 ‰) to 

Franklin locks (+16 ‰) along the Caloosahatchee River, indicating significant localized N 

enrichment. This δ15N enrichment correlated with increased DIN concentrations between these 

two structures, a phenomenon that is also apparent in the nutrient monitoring data collected by 

the South Florida Water Management District (SFWMD, DBHYDRO). The δ15N values at the 

Franklin Lock are at the high end of the sewage nitrogen range (Heaton, 1986; Lapointe, 1997; 

Costanzo et al., 2001) and suggest significant sewage N enrichment of the Caloosahatchee River 

from the surrounding basin. Doering and Chamberlain (1999) analyzed the importance of source 

(Lake Okeechobee vs. Caloosahatchee River basin) to the quality of freshwater discharges to 

Caloosahatchee estuary and found that nutrient concentrations (except ammonia) and color in the 

estuary were higher when the basin rather than Lake Okeechobee was the source. Downstream

of the Franklin Lock, ~ 20 million gallons per day (MGD) of sewage effluent receiving advanced 

wastewater treatment (AWT; DIN and SRP concentrations of ~ 214 μM and 32 μM, 
22

respectively) is discharged directly into the Caloosahatchee estuary upstream of San Carlos Bay 

(Florida Department of Environmental Protection, personal communication). 

The δ15N values in macroalgae from Lee County’s beaches and coastal reefs indicate that 

land-based N-enrichment affects the water column for considerable distances from shore. 

Compared to the Caloosahatchee River, lower δ15N values of ~ + 6 ‰ occurred in drift 

macroalgae on Lee County’s beaches, values within the range reported for macroalgae enriched 

with sewage N (Costanzo et al., 2001; Lapointe et al., 2004). The δ15N values of macroalgae 

decreased with increasing distance from shore, but remained at or above + 3 ‰ (the low end of 

the sewage range) at N. Deep Ledge, the most offshore station some 26 km offshore Sanibel 

Island. Although these high δ15N values in macroalgae of the Caloosahatchee River and 

downstream receiving waters suggest the importance of surface water transport of wastewater N, 

submarine groundwater discharge in the study area could be of the same magnitude as the river 

discharges (Miller et al., 1990) and may therefore be an important route for the transport of 

sewage N from septic tanks and/or injection wells to coastal waters. The significant N 

enrichment of the water column to at least 26 km from shore would be available to support 

growth of not only macroalgae, but phytoplankton as well. Blooms of the red tide dinoflagellate, 

Karenia brevis, develop along this coastline at similar distances from shore (Tester and 

Steidinger, 1997). 

The present study cannot resolve the relative importance of specific sourcs of 

sewage N enrichment, although multiple sources on the southwest Florida watershed could 

contribute. Historically, the Caloosahatchee River was not connected to Lake Okeechobee and 

canal projects were developed in the early 1880’s to lower the water level in Lake Okeechobee. 
23

This resulted in “new” nutrient sources from Lake Okeechobee, which receives drainage from 

the Kissimmee River basin, the Everglades Agricultural Area, and the St. Lucie drainage basin 

to the east when that water is backpumped into Lake Okeechobee (Steinman et al., 2002). 

Accordingly, runoff from dairy and cattle farms in the Kissimmee basin, domestic treated 

sewage that is discharged into surface waters or re-used in agricultural areas, and septic tank 

discharges from throughout this region could all contribute to the sewage δ15N signature we 

observed at both the Ortona and Franklin locks on the Caloosahatchee River. 

However, the significant reduction in δ15N values of the coastal macroalgae from August 

(+ 5.84 ‰) to October (+ 3.89 ‰) may reflect an increased contribution of N from rainfall and 

agricultural N sources following the high Lake Okeechobee discharges and the 2004 hurricanes 

(Fig. 9). The δ15N values of N in rainfall, organic peat, and fertilizers used on sugarcane and 

citrus farms in south Florida have δ15N values in the range -3 to + 3 ‰ (Table 1). Hence, periods 

of peak discharge from Lake Okeechobee, such as those between August and October 2004, 

would result in increased N (especially ammonia) contributions from these sources that would 

lower the δ15N signature of macroalgae in downstream coastal waters. In the Lower Florida 

Keys, elevated δ15N values of macroalgae reflect sewage N sources from local sources in the 

Keys during drought periods when little agricultural runoff of N from the Everglades occurs. In 

wet years, however, increased stormwater runoff from agricultural areas lowers the δ15N values 

of macroalgae to values similar to the source signatures of fertilizers and peat (Lapointe et al., 

2004). 

 The three-fold increase in SRP concentrations between August and September throughout 

the study area also indicate widespread P enrichment from land-based runoff. The SRP 

concentrations were highest in the Peace River (~ 17 μM) and downstream waters of S. Charlotte 
24

Harbor (3.7 μM) during our study, supporting previous conclusions of the importance of this 

river as a P source (McPherson and Miller, 1990). SRP concentrations in rivers along the 

southwest coast of Florida are substantially higher than in most North American rivers (Lovejoy 

et al., 1990; Flannery et al., 1991) and correlate with the natural phosphatic rock formations in 

this region (Kaufman, 1969). The greatest contribution of SRP to these waters is a direct result 

of anthropogenic pollution (Odum, 1953), particularly the phosphate mining industry (Task 

Group Report, 1967). These large P burdens make the Alafia, Peace, and Fenholloway rivers the 

greatest P carriers in all of Florida (LaRock and Bittacker, 1973). Runoff from the Peace River 

following Hurricane Charley would contribute to our observed increase in mean SRP 

concentrations in coastal waters from 0.30 μM in early August to 0.92 μM in October, 

respectively. These represent very high SRP concentrations (which dominated the TDP pool) as 

only ~ 0.1 μM is required to satisfy growth demands of macroalgae (Lapointe, 1997) and the red 

tide dinoflagellate, Karenia brevis (Wilson and Ray, 1958). 

4.3. Understanding and managing macroalgal blooms in Lee County’s coastal waters

The development of macroalgal blooms in coastal waters of southern Lee County and 

accumulation on adjacent beaches is dependant on local hydrodynamic and meteorological 

factors, including antecedent events. Discharges from the Caloosahatchee River deliver not only 

nutrients from Lake Okeechobee and basin sources, but also estuarine nutrient loads associated 

with municipal sewage outfalls in Ft. Myers that are discharged into the estuary. Although the 

initial beach accumulation of red drift macroalgae in August 2003 followed major discharges (> 

12,000 cfs) and nutrient loading from the Caloosahatchee River (Fig. 9), the massive biomass 

involved would require a considerable time period for development prior to stranding. In 

addition to nutrients, light is a critical factor in the development of benthic macroalgal HABs and 
25

increased light attenuation follows major freshwater discharges as a result of the high color 

(dissolved humic compounds) content of the fresh water and increased chlorophyll a that 

develops downstream of the discharges (Doering and Chamberlain, 1999). Considering the 

importance of light to the growth of these rhodophytes, the development of benthic macroalgal 

HABs might be favored during periods of low or moderate flows from the Caloosahatchee River, 

which occurred during and following the 2000/2001 drought (Fig. 9) in South Florida (Abtew et 

al., 2002). On Florida’s east coast, invasive blooms of Codium isthmocladum first developed 

during the drought years of 1989-1990 with subsequent blooms of Caulerpa brachypus forma 

parvifolia occurring during the 2000/2001 drought (Lapointe et al., 2005). Periods following 

peak flows and nutrient loads from the Caloosahatchee River and other land-based sources 

would favor phytoplankton rather than macroalgal blooms (Valiela et al., 1997), and the blooms 

of rhodophytes that occurred in 2003 and early 2004 have not re-emerged with the increased 

discharges following the 2004 hurricanes (Fig. 9). However, an intense Karenia brevis bloom 

persisted in southwest Florida throughout 2005, which led to a 2,100 square mile hypoxic dead 

zone and the death of 79 manatees (Rothschild, 2005). 

Increasing urbanization of southwest Florida, combined with pulsed water releases from 

the Caloosahatchee River, could make protected bays and shallow, nearshore waters of southern 

Lee County more prone to the development of rhodophyte blooms. The inshore bays (Estero 

Bay, San Carlos Bay) and shallow coastal waters off southern Lee County are directly impacted 

by the Caloosahatchee River plume. Although the shallow waters between Ft. Myers Beach and 

Bonita Springs are dominated by soft bottom sediments not conducive to growth of attached 

rhodophytes, surveys of this area indicated extensive populations of Gracilaria tikvahiae, 

Agardhiella subulata, and Hypnea sp. growing attached to the tubes of the polychaete worm 
26

Chaetopterus cf. variopedatus (Fig. 2D). This large, tubiculous, suspension feeding polychaete 

plays an important role in nutrient cycling and benthic-pelagic coupling in lower Chesapeake 

Bay (Thompson and Schaffner, 2001) and could provide not only suitable substrate, but also 

recycled nutrients that may enhance the growth of rhodophytes attached to their tubes. Strong 

winds, currents, and tides eventually detach the macroalgae from the benthos and deposit the 

biomass on beaches (Fig. 2 A,B, and E), where odor and aesthetic problems diminish the use of 

the beaches by residents and tourists alike. 

 Excessive biomass of macroalgae can have major impacts on coastal economies through 

loss of tourism and increased costs associated with beach cleanup programs. Along Maui’s 

Kihei coast, Hawaii, USA, over $20 million (U.S.) a year in tourism revenues and property 

values have been lost as a result of macroalgal blooms 

(www.hawaii.edu/ssri/hcri/ev/kihei_coast.htm). In Maui County, some $250,000 (U.S.) is spent 

annually by condominium owners to remove excessive seaweed biomass from the beaches. In 

the Peel Inlet, Australia, removal of seaweeds cost $160,000 (U.S.) annually for 13,000 m3

 of 

macroalgae (Atkins et al., 1993). In France, the cost exceeded 3.6 million francs for 90,000 m3 

of “green tides” removed from the Brittany coastline in 1992 (CEVA, 1993). In Lee County, 

seaweed removal programs for the beaches were historically nominal but increased dramatically, 

to $260,503 for the fiscal year 2003/2004, with the onset of the red drift macroalgal blooms 

(U.S.; Lee County Visitor and Convention Bureau; Fig. 10). Following the termination of these 

blooms by hurricane Charley, beach cleanup costs decreased in 2005 (Fig. 10). 

Because the physical harvesting of macroalgae is costly and often insufficient to control 

macroalgal blooms, water quality restoration is usually necessary (Morand and Briand, 1996). 

The recent emergence of the red drift macroalgal HABs in Lee County suggests that nutrient 
27

loading to these inshore waters is increasing. Data generated from ongoing and new water 

quality monitoring programs could be used to develop a conceptual model for the influence of 

various land-based nutrient sources, including discharges from the Peace and Caloosahatchee 

rivers, on the development of phytoplankton and macroalgal HABs. Such a model could help 

visualize how biological, chemical, physical, and meteorological factors interact to produce these 

blooms. For example, biomass of Ulva and Gracilaria in the Venice Lagoon, Italy, has become 

so large that it can regulate seasonal nutrient cycles through growth and decomposition, thereby 

controlling the seasonal development of phytoplankton blooms (Sfriso et al., 1992). In Lee 

County, nutrient uptake by extensive blooms of rhodophytes could play an increasing role in 

regulating nutrient cycles and seasonal phytoplankton blooms, including blooms of Karenia 

brevis. Studies in the Bachimen Sea, China, showed that Gracilaria tenuistipitata depressed red 

tide blooms (Tang et al., 2003), suggesting that nutrient competition and/or allelopathy by 

rhodophytes may provide a means to mitigate red tides. 
28

Acknowledgements 

We are grateful to the many people who contributed to this research project. Eve 

Haverfield (Turtle Time, Inc.) collected drift macroalgae, Michael Wynne (University of 

Michigan, Ann Arbor) assisted with taxonomic identification, and the Sierra Club (Calusa 

Chapter) provided logistical support for the field work. Roland Ottolini, Wayne Daltry, Steve 

Boutelle, and Chris Koepfer provided technical information and boat support. Lee County 

Commissioner Ray Judah, Lee County Tourist Development Council Director Tamara Pigott, 

and Bonita Springs Mayor Gary Price are especially thanked for their financial support of this 

research. Comments of Charles Yentsch, Larry Brand, Scott Hurley, and two anonymous 

reviewers improved this manuscript. This project was funded by Lee County and the City of 

Bonita Springs, FL. This is contribution number 1619 of the Harbor Branch Oceanographic 

Institution, Inc. 
29

References 

Abtew, W. , Huebner, R. S., Sunderland, S. 2002. Part I. Hydrological analysis of the 2000-

2001 drought in South Florida. Technical Report EMA-405, South Florida Water 

Management District, West Palm Beach, FL. 

Asplia, I., Agemian, H., Chau, A.S.Y., 1976. A semi-automated method for the determination of 

inorganic, organic, and total phosphate in sediments. Analyst 101, 187-197. 

Atkins, R.P., Deeley, D.M., McAlpine, K.W., 1993. Managing the aquatic environment. Fert. 

Res. 36, 171-175. 

Avery, W., 1997. Distribution and abundance of macroalgae and seagrass in Hillsborough Bay, 

Florida, from 1986 to 1995. In: Treat, S.F. (ed.), Proceedings, Tampa Bay Area Scientific 

Information Symposium 3. Text, Tampa, FL. 151-165. 

Barile, P.J., 2004. Evidence of anthropogenic nitrogen enrichment of the littoral waters of east 

central Florida. J. Coastal Res. 20(4), 1237-1245. 

CEVA, 1993. Marée vertes: Les traitements curatifs. Document du CEVA. CEVA (Centre 

d’Etude et de Valorisation des Algues), 22610, France. 

Costanzo, S.D., O’Donohue, M.J., Dennison, W.C., Loneragan, N.R., Thomas, M., 2001. A new 

approach for detecting and mapping sewage impacts. Mar. Poll. Bull. 42, 149-156. 

Dawes, C.J., 1974. Marine Algae of the West Coast of Florida. University of Miami Press, 

Miami, FL. 

DeBoer, J.A., Guigli, H.J., Isreal, T.L., D’Elia, C.F., 1978. Nutritional studies of two red algae: 

1. Growth rate as a function of nitrogen source and concentration. J. Phycol. 14, 266-272. 
30

D’Elia, C.F., Connor, E.E., Kaumeyer, N.L., Keefe, C.W., Wood, K.V., Zimmerman, C.F., 1997. 

Nutrient Analytical Services: Standard operating procedures. Technical Report Series No. 

158-97, Chesapeake Biological Laboratory, Solomons, MD. 

Doering, P. H., Chamberlain, R. H., 1999. Water quality and source of freshwater discharge to 

the Caloosahatchee estuary, Florida. J. American Water Resources Association 35 (4),793-

806. 

Flannery, M.S., Downing, H.C., Jr., McGarry, G.A., Walters, M.O. 1991. Increased nutrient 

loading and baseflow supplementation in the Little Manatee watershed. Proc. Tampa Bay 

Area Scientific Information Symposium 2, 369-396. 

France, R., Holmquist, J., Chandler, M., Cattaneo, A., 1998. δ

15N evidence for nitrogen fixation 

Associated with macroalgae from a seagrass-mangrove-coral reef ecosystem. Mar. Ecol. 

Prog. Ser. 167, 297-299. 

Gartner, A., Lavery, P., Smit, A.J., 2002. Use of δ

15N signatures of different functional forms of 

macroalgae and filter feeders to reveal temporal and spatial patterns in sewage dispersal. 

Mar. Ecol. Prog. Ser. 235, 63-73. 

GESAMP, 1990. The state of the marine environment. Joint Group of Experts on the Scientific 

Aspects of Marine Pollution Rep. Stud. 39. UNEP. 

Guist, G.G., Humm, H.J., 1976. Effects of sewage effluents on the growth of Ulva lactuca. 

Florida Sci. 39, 267-271. 

Harrison, W.G., Platt, T., Lewis, M.R., 1987. The f-ratio and its relationship to ambient nitrogen 

concentration in coastal waters. J. Plankton Res. 9, 235-248. 

Heaton, T.H.E., 1986. Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: 

A review. Chem. Geol. (Isotope Geoscience Section) 59, 87-102. 
31

Hoch, M.P., Cifuentes, L.A., Coffin, R., 1995. Assessing geochemical and biological fate for 

point source loads of sewage-derived nitrogen and organic carbon in coastal waters of 

southern Florida. Final Report to the USEPA. 

Howarth, R., Anderson, D., Cloern, J., Elfring, C., Hopkinson, C., Lapointe, B., Malone, T., 

Marcus, N., McGlathery, K., Sharpley A., Walker, D., 2000. Nutrient pollution of 

coastal rivers, bays, and seas. Issues in Ecology 7, 1-15. 

Humm, H. J. 1973. Benthic algae of the eastern Gulf of Mexico. In: Jones, J.I. et al. (eds.), A 

Summary of Knowledge of the Eastern Gulf of Mexico, 1973. State University System 

of Florida Institute of Oceanography, St. Petersburg, FL. IIIB: 1-15. 

Kaufman, M.I. 1969. Generalized distribution and concentration of orthophosphate in Florida 

 streams. U.S. Geological Survey Map Series 33. The Eastern Gulf of Mexico 1973, The 

 State University System of Florida Institute of Oceanography IIC-46. 

Ketchum, B.H., 1967. Phytoplankton nutrients in estuaries. In: Lauf, G.H., (ed.) Estuaries. 

 American Association for the Advancement of Science, Washington, D.C. 329-335. 

Ketchum, B.H. and Keen, J. 1947. Unusual phosphorus concentrations in the Florida “Red Tide” 

sea water. J. Mar. Res. 7(1), 17-21. 

Lapointe, B.E., 1981. The effects of light and nitrogen on growth, pigment content, and 

 biochemical composition of Gracilaria foliifera var. angustissima (Gigartinales, 

Rhodophyta). J. Phycol. 17, 90-95. 

Lapointe, B.E., 1987. Phosphorus and nitrogen-limited photosynthesis and growth of Gracilaria 

tikvahiae (Rhodophyceae) in the Florida Keys: An experimental field study. Mar. Biol. 

93, 561-568. 
32

Lapointe, B.E., 1995. A comparison of nutrient-limited productivity in Sargassum natans from 

neritic vs. oceanic waters of the western North Atlantic ocean. Limnol. Oceanogr. 40(3), 

625-633. 

Lapointe, B.E., 1997. Nutrient thresholds for bottom-up control of macroalgal blooms on coral 

reefs in Jamaica and southeast Florida. Limnol. Oceanog. 42(5-2), 1119-1131. 

Lapointe, B. E., Krupa, S., 1995a. Jupiter Creek septic tank water quality investigation. Final 

Report to the Loxahatchee River Environmental Control District. Jupiter, FL. 

Lapointe, B. E., Krupa, S., 1995b. Tequesta Peninsula septic tank/water quality investigation. 

Final Report to the Loxahatchee River Environmental Control District. Jupiter, FL. 

Lapointe, B.E., Thacker, K., 2002. Community-based water quality and coral reef monitoring in 

the Negril Marine Park, Jamaica: Land-based nutrient inputs and their ecological 

consequences. In: Porter, J.W., Porter, K.G. [Eds.], The Everglades, Florida Bay, and 

Coral Reefs of the Florida Keys: An Ecosystem Sourcebook. CRC Press, Boca Raton, 

FL. pp. 939-963 

Lapointe, B.E., Ryther, J.H., 1979. The effects of nitrogen and seawater flow rate on the growth 

and biochemical composition of Gracilaria foliifera var. angustissima in mass outdoor 

cultures. Bot. Mar. XXII, 529-537. 

Lapointe, B.E., Barile, P.J., Matzie, W.R., 2004. Anthropogenic nutrient enrichment of seagrass 

and coral reef communities in the Lower Florida Keys: Discrimination of local versus 

regional nitrogen sources. J. Exp. Mar. Biol. Ecol. 308(1), 23-58. 

Lapointe, B.E., Littler, D.S., Littler, M.M., 1992. Nutrient availability to marine macroalgae in 

siliciclastic versus carbonate-rich coastal waters. Estuaries 15(1), 75-82. 

Lapointe, B.E., Tenore, K.R., Dawes, C.J., 1984. Interactions between light and temperature on 
33

the physiological ecology of Gracilaria tikvahiae (Gigartinales: Rhodophyta): I. Growth, 

photosynthesis and respiration. Mar. Biol. 80, 161-170. 

Lapointe, B.E., Tomasko, D.A., Matzie, W.R., 1994. Eutrophication and trophic state 

classification of seagrass communities in the Florida Keys. Bull. Mar. Sci. 54, 696-717. 

Lapointe, B. E.,Barile, P. J., Littler, M. M., Littler, D. S., Gasque, C., 2005a. Macroalgal blooms 

on southeast Florida coral reefs: I. Nutrient stoichiometry of the invasive green alga 

Codium isthmocladum in the wider Caribbean indicates nutrient enrichment. Harmful 

Algae 4, 1092-1105. 

Lapointe, B.E., Barile, P.J., Littler, M.M., Littler, D.S., 2005b. Macroalgal blooms on southeast 

Florida coral reefs: II. Cross-shelf discrimination of nitrogen sources indicates 

widespread assimilation of sewage nitrogen. Harmful Algae, 4, 1106-1122. 

LaRock, P., Bittaker, H.L., 1973. Chemical data of the estuarine and nearshore environments in 

the Eastern Gulf of Mexico. The State University System of Florida Institute of 

Oceanography IIC-8. 

Lindau, C.W., Delaune, R.D., Patrick, W.H. Jr., Lambremont, E.N., 1989. Assessment of stable 

isotopes in fingerprinting surface water inorganic nitrogen sources. Water Air Soil Poll. 

48, 489-496. 

Littler, D.S., Littler, M.M., 2000. Caribbean Reef Plants. Offshore Graphics, Washington, D. C. 

Lovejoy, S.B., Dunkelberg, B., Lee, J.G. 1990. Sources and quantities of nutrients entering the 

Gulf of Mexico. The environmental status of Gulf of Mexico. Gulf of Mexico Program 

41-43. 
34

McCarthy, J. J., Taylor, W. R., Taft, J. L. 1975. The dynamics of nitrogen and phosphorus 

cycling in the open waters of Chesapeake Bay. In: Church, T. M. (editor), Marine 

Chemistry of the Coastal Environment, 664-681. ACS Symposium Series, # 18. 

McClelland, J.W., Valiela, I., 1998. Changes in food web structure under the influence of 

increased anthropogenic nitrogen inputs to estuaries. Mar. Ecol. Prog. Ser. 168, 259-271. 

McGlathery, K. J., 1995. Nutrient and grazing influences on a subtropical seagrass community. 

Mar. Ecol. Prog. Ser. 122, 239 – 252.

McPherson, B. F.,Miller, R.L. 1990. Nutrient distribution and variability in the Charlotte Harbor 

estuarine system, Florida. Water Research Bull. 26:67-81. 

Miller, R.L., Kraemer, T.F., McPherson, B.F., 1990. Radium and radon in Charlotte Harbor 

estuary, Florida. Estuar. Coast. Shelf S. 31, 439-457. 

Morand, P., Briand, X., 1996. Excessive growth of macroalgae: A symptom of environmental 

disturbance. Bot. Mar. 39, 491-516. 

National Research Council, 2000. Clean Coastal Waters: Understanding and Reducing the 

Effects of Nutrient Pollution. Ocean Studies Board, Water Science and Technology 

Board. 

Odum, H.T., 1953. Dissolved phosphorus in Florida waters. Florida Geological Survey Report 

of Investigation 9 (Pt. 1), 1-40. 

Owens, N. P. J., 1987. Natural variations in 15N in the marine environment. Adv. Mar. Biol. 24, 

389-451. 

Paerl, H. W., Fogel, M. L., 1994. Isotopic characterization of atmospheric nitrogen inputs as 

sources of enhanced primary production in coastal Atlantic Ocean waters. Mar. Biol. 

119(4), 635-645. 
35

Pew Oceans Commission, 2003. America’s Living Oceans: Charting a Course for Sea Change. 

A Report to the Nation. May 2003. Pew Oceans Commission, Arlington, VA. 

Rothschild, W. 2005. Red tide kills 79 manatees off southwest Florida in 2005. Sarasota Herald 

Tribune, December 10. 

Rogers, D. K. 1991. Pollution is behind invasion of seaweed. St. Petersburg Times, St. 

Petersburg, FL. August 14. 

Sammarco, P.W., Risk, M.J., Schwarcz, H.P., Heikoop, J.M., 1999. Cross-continental shelf 

trends in coral δ

15N on the Great Barrier Reef: Further consideration of the reef nutrient 

paradox. Mar. Ecol. Prog. Ser. 180, 131-138. 

Savage, C., Elmgren, R., 2004. Macroalgal δ15N values trace decrease in sewage influence. 

Ecol. Appl. 14(2), 517-526 

Sfriso, A., Pavoni, B., Marcomini, A., Orio, A.A., 1992. Macroalgae, nutrient cycles, and 

pollutants in the Lagoon of Venice. Estuaries 15, 517-528. 

Sigman, D.M., Altabet, M.A., McCorkle, D.C., Francois, R., Fisher, G., 2000. The δ15N of 

nitrate in the southern ocean: Nitrogen cycling and circulation in the ocean interior. J. 

Geophys. Res. C 105, 19599-19614. 

Slobodkin, L. B. 1953. A possible initial condition for red tides on the west coast of Florida. J. 

Mar. Res. 12 (1),148-155. 

Steidinger, K.A., Vargo, G.A., Tester, P.A., Tomas, C.R. 1998. Bloom dynamics and 

physiology of Gymnodinium breve with emphasis on the Gulf of Mexico. In: Anderson, 

D.A., Cembella, A.D., Hallegraeff, G.M., (eds.), Physiological Ecology of Harmful Algal 

Blooms. Springer-Verlag, New York, NY. pp. 133-154. 
36

Steinman, A.D., Havens, K.E., Carrick, H.J., VanZee, R., 2002. The past, present, and future 

hydrology and ecology of Lake Okeechobee and its watersheds. In: Porter, J.W. and K.G. 

Porter, The Everglades, Florida Bay, and Coral Reefs of the Florida Keys: An Ecosystem

Sourcebook. CRC Press, Boca Raton, FL. pp. 19-37. 

Tang, X. K., Yuan, D. X., and Lin, S. B. 2003. Depression and effect of red tide on water quality 

index by Gracilaria tenuistipitata. Mar. Env. Sci. 22(2), 24-27. 

Task Group Report, 1967. Sources of nitrogen and phosphorus in water supplies. J. AWWA 59, 

344-366. 

Tester, P. A. and Steidinger, K. A. 1997. Gymnodinium breve red tide blooms:Initiation, 

transport, and consequences of surface circulation. Limnol. Oceanogr. 42(5, part 2),1039-

1051. 

Thompson, M. L., Schaffner, L. C., 2001. Population biology and secondary production of the 

suspension feeding polychaete Chaetopterus cf. variopedatus: Implications for benthicpelagic coupling in lower Chesapeake Bay. Limnol. Oceanogr. 46(8):1899-1907. 

Umezawa, Y., Miyajima T., Yamamuro M., Kayanne H., Koike, I., 2002. Fine-scale mapping of 

land-derived nitrogen in coral reefs by δ

15N in macroalgae. Limnol. Oceanogr. 47(5), 

1405-1416. 

U.S. Commission on Ocean Policy, 2004. An Ocean Blueprint for the 21st Century. Final 

Report of the U.S. Commission on Ocean Policy. Washington, D.C. 

Valiela, I., McClelland, J., Hauxwell, J., Behr, P.J., Hersh, D., Foreman, K., 1997. Macroalgal 

blooms in shallow estuaries: Controls and ecophysiological and ecosystem

consequences. Limnol. Oceanogr. 42, 1105-1118. 
37

Virnstein, R.W., Carbonara, P.A., 1985. Seasonal abundance and distribution of drift algae and 

seagrasses in the mid-Indian River lagoon, Florida. Aquat. Bot. 23, 67-82. 

Wayland, M., Hobson, K.A., 2001. Stable carbon, nitrogen, and sulfur isotope ratios 

in riparian food webs on rivers receiving sewage and pulp-mill effluents. Canad. J. 

Zool., 79, 5-15. 

Wilson, W. B., Ray, S. 1958. Nutrition of red tide organisms. U. S. Fish. Wild. Serv. Gulf Fish 

Invest. Ann. Rep., pp. 62-65. 

Yang, H., Weisberg, R. H., Niiler, P. P., Sturges, W., and W. Johnson. 1999. Lagrangian 

circulation and forbidden zone on the west Florida shelf. Cont. Shelf. Res. 19, 1221-

1245. 
38

Figure Legend 

Fig. 1. A map of the Lee County, FL area showing station locations for the 2004 study: 

Ortona Lock (ORT) and Franklin Lock (FRK) on the Caloosahatchee River, Peace River 

(PCR), Imperial River (IMP), San Carlos Bay (SCB), S. Charlotte Harbor (SCH), 17th Street 

Reef (NSL), N. Deep Ledge (NDL), ARC Reef (ARC), Blanda’s Reef (BLD), and BeltonJohnson Culvert Reef (CLV). 

Fig. 2. Red drift macroalgae in coastal waters of Lee County, FL: A.) rhodophytes, Bonita 

Springs Beach, January, 2004; B.) rhodophytes, Bonita Springs Beach, July, 2004; C.) 

rhodophytes, 17th Street Reef, August, 2004; D.) worm tube with attached rhodophyte, 

Bonita Springs Beach, August, 2005; E.) rhodophytes in shallow water along beaches in 

southern Lee County. 

Fig. 3. Mean concentrations (μM, ± 1 SD, n = 2) of water column dissolved inorganic 

nitrogen (DIN), ammonium (NH4

+

), nitrate (NO3

−

) and f-ratios (NO3

−

:DIN) in August and 

October 2004 at sampling stations in Lee County, FL. 

Fig. 4. Mean water column SRP concentrations (μM, ± 1 SD, n = 2) and DIN:SRP ratios in 

August and October 2004 at sampling stations in Lee County, FL. 

Fig. 5. Mean water column TDN and TDP concentrations (μM, ± 1 SD, n = 2), with relative 

contributions of organic and inorganic forms, in August and October at sampling stations in 

Lee County, FL. 
39

Fig. 6. Mean tissue C:N, C:P, and N:P molar ratios (± 1 SD, n = 1-8) in macroalgae collected 

August and October 2004 at sampling stations in Lee County, FL. 

Fig. 7. A.) Mean tissue δ15N (‰, ± 1 SD, n = 2-8) of macroalgae collected in August and 

October 2004 at sampling stations in Lee County, FL and B.) from Bonita Springs Beach in 

July and August 2004, including a comparison among beach and reef samples of the 

rhodophyte Hypnea musciformis.

Fig. 8. A comparison of mean tissue C:N, C:P, and N:P molar ratios (± 1 SD, n = 3-6) in 

macroalgae from Lee County, FL during this study with macroalgae from southeast Florida 

(Lapointe et al. 2005), Jamaica and the Bahamas (Lapointe et al. 1992). 

Fig. 9. Monthly mean flow rates (cubic feet per second, cfs) at Franklin Locks on the 

Caloosahatchee River, FL in 2003-2004. Arrows indicate beach drift algae events. 

Fig. 10. Beach clean-up costs in Lee County, FL, 1997-2005 ($ U.S.).
40

Fig. 1 
41

Fig. 2 
42

 August October

 Rivers Bays Coastal Reefs 

12.7

0

2

4

6

8

10

23 26

0

5

10

15

20

26 28 23

0

5

10

15

20

0.0

0.2

0.4

0.6

0.8

1.0

 NH4

+

 (μM) 

 NO3

-

 (μM) 

 DIN

 (μM) 

 f 

Ratio 

Fig. 3 
43

16.8

0

1

2

3

4

SRP

(μM)

68.3

0

5

10

15

r

r

k

k

y

r

)

)

)

)

)

DIN

SRP

 August October 

 Rivers Bays Coastal Reefs 

Fig. 4 
44

68.6 70.9

0

10

20

30

40

50

TDN

(μM)

 Rivers Bays Coastal Reefs 

 DON DIN

0

1

2

3

4

5

6

TDP

(μM)

 DOP SRP 

Fig. 5 
45

 August October

 Rivers Bays Coastal Reefs 

0

5

10

15

20

C:N

622

0

100

200

300

400

500

C:P

42.4

0

5

10

15

20

25

30

N:P

Fig. 6 
46

A. August October

 Rivers Bays Coastal Reefs 

0

3

6

9

12

15

 δ15N (‰) 

0

3

6

9

B. 

Phaeophyta Hypnea musciformis

 Rhodophyta 

 δ15N 

 (‰) 

Fig. 7 
47

0

10

20

30

40

C:N 

0

500

1000

1500

2000

C:P 

0

10

20

30

40

50

60

Lee

County

Palm Bch.

County

Jamaica Bahamas

N:P 

Fig. 8 
48

0

4,000

8,000

12,000

16,000

J FMAMJ JASOND J FMAMJ JASOND J FMAMJ JASOND J FMAMJ J ASOND J FMAMJ JASOND

2000 2001 2002 2003 2004

Flow

(cfs)

Mean Min Max

 Aug. Nov.

 Jan. July 

Fig. 9 
49

$0

$50,000

$100,000

$150,000

$200,000

$250,000

97-98 98-99 99-00 00-01 01-02 02-03 03-04 04-05

Fiscal Year (Oct. 1 - Sept. 30)

Cost

(U.S. $)

Fig. 10 
50
51

Table 2. Stations, depths, and species collected for analysis from study sites in Lee County, FL, USA in

 July (J), August (A), and October (O) of 2004.

 Site, Depth

 Species

Rhodopyta

ORT

<1 m

FRK

<1 m

SCB

1 m

SCH

1 m

NSL

3 m

CLV

10 m

BLD

14 m

ARC

19 m

NDL

20 m

BSB

Bch.

FMB

Bch.

SAN

Bch.

CAP

Bch.

- - - - - - --- - - - -

 O J

A O A O A O A O A O J

var. denudatum O O A O J

A O A A A A J

A O

J

 A A

A O A O A

Agardhiella subulata

Botryocladia occidentalis

Euchuma isiforme

 

Gracilaria cervicornis

Gracilaria mammalaris

Gracilaria tikvahiae

Hypnea musciformis

Rhodymenia divaricata

Phaeophyta - - - - - - --- - - - -

Sargassum fluitans A A A

Chlorophyta - - - - - - --- - - - -

Cladophora sp. A O A O

Cyanophyta

Lyngbya sp. A A

Angiosperma - - - - - - --- - - - -

Thalassia testudinum A O A O
52

Table 3. Water column nutrient concentrations at study sites in Lee County, FL, USA in August and

 October of 2004. Values represent means ± 1 S.D.

n

August 14 0.20 ± 0.00 0.91 ± 0.72 1.11 ± 0.72 0.77 ± 0.11 0.30 ± 0.18 6.88 ± 2.35

October 14 1.31 ± 1.08 0.97 ± 0.69 2.29 ± 1.57 0.41 ± 0.14 0.92 ± 1.20 4.96 ± 0.99

Ammonium

(μM)

Nitrate+Nitrite

(μM)

DIN

(μM)

f -Ratio

(NO3+2) / DIN

SRP

(μM)

DIN / SRP

Ratio
53

Table 4. Tissue C:N, C:P, and N:P molar ratios and ¶15N in macroalgae from coastal study sites in Lee

 County, FL, USA in August and October 2004. Values represent means ± 1 S.D.

August 13.93 ± 2.78 (n=16) 386.3 ± 198.0 (n=16) 27.43 ± 11.40 (n=16) 5.84 ± 1.37 (n=32)

October 13.54 ± 3.43 (n=13) 242.0 ± 142.1 (n=13) 17.51 ± 6.74 (n=13) 3.89 ± 0.96 (n=26)

C:N Ratio C:P Ratio N:P Ratio ¶15N (ä)