Oyster monitoring in the northern estuaries on the 

Southeast coast of Florida 

2008 Annual Report 

Melanie L. Parker 

Stephen P. Geiger 

Florida Fish and Wildlife Research Institute 

100 Eighth Avenue SE 

St. Petersburg, Florida 33701 

June 25, 2009 

Contract Number: 4600001084 File Code: F2724-08-A1 
2

 

TABLE OF CONTENTS 

Executive Summary ........................................................................................................3

Introduction.................................................................................................................5 

Methods......................................................................................................................8 

Results.................................................................................................................................................12

St. Lucie Estuary.............................................................................................................................................12 

SLE Figures.............................................................................................................16 

Loxahatchee River.............................................................................................................................32 

 LOX Figures....................................................................................................35 

Lake Worth Lagoon........................................................................................................................................50 

 LWL Figures....................................................................................................53 

Discussion.....................................................................................................................................................60 

References....................................................................................................................................................67 
3

Executive Summary 

 In this report we present the 2008 observations by the Molluscan Fisheries research group at 

the Florida Fish and Wildlife Research Institute (FWRI) on oysters in southeast Florida. The main 

objective of this project is to continue a long-term monitoring program intended to document the response 

of oysters, Crassostrea virginica, in south Florida to CERP activities. FWRI monitored changes in oyster 

ecology at six sites in three estuaries in southeast Florida: the St. Lucie estuary (3 sites), the Loxahatchee 

River (2 sites) and Lake Worth Lagoon (1 site). Four aspects of oyster ecology were monitored: spatial and 

size distribution patterns of adult oysters, distribution and frequency patterns of the oyster diseases 

Perkinsus marinus (dermo) and Haplosporidium nelsoni (MSX), reproduction and recruitment, and 

juvenile oyster growth. Water quality parameters including salinity, temperature and chlorophyll a

concentrations were also monitored at each study site. 

Salinities in the St. Lucie estuary (SLE) were near the optimal range (15-20 ppt) for oyster 

survival and recruitment success during the first six months of the year, decreased rapidly in July leading to 

the subsequent oyster die-off at all sites in the fall and recovered quickly enough to to allow successful 

settlement of the last recruits of the year. Salinities in the Loxahatchee River (LOX) were near the optimal 

range (15-20 ppt) for oyster survival and recruitment success in early months of the year, but throughout 

the rest of the year salinities were erratic and often either too high (increased predation and disease) or too 

low. Salinity was highly variable in the Lake Worth Lagoon (LWL), but was generally higher than 

salinities recorded in the other estuaries. 

Oysters were abundant at all nine stations in SLE during the spring 2008 survey but declined 

dramatically by the fall survey and were limited to only a scattering of live oysters in central estuarine 

reefs. Oyster densities at both the LOX and LW sites were high and increased from spring to fall 2008. 

Dermo infection was consistently present in oysters from the LOX and LWL study sites. Oysters from the 

St. Lucie-Central study site also had a consistent presence of the parasite until salinities substantially 

declined in late summer. However, infection intensity levels were low throughout the year in oysters 

sampled from all sites. Reproductive development increased steadily through spring in the SLE and LOX 

study sites Both estuaries exhibited a spring and fall peak in recruitment. In contrast, gametogenesis was 

delayed and increased more rapidly in the LWL study site. Recruitment in LWL was much higher and had 
4

a single peak which lasted for several months. Juvenile growth rates were high in both the SLE and the 

LOX. Observed juvenile growth rate was low in Lake Worth, but may have been biased by the continual 

recruitment of small oysters to the population.
5

Introduction

 The Comprehensive Everglades Restoration Program (CERP) has been implemented as a means 

of reinitiating, to the greatest degree possible, natural freshwater flow to coastal waters on both coasts of 

south Florida. The Monitoring and Assessment Program (MAP) component of CERP is designed to 

provide a diverse approach to documenting and describing the impacts of changed freshwater flow to the 

flora and fauna inhabiting inland landscapes and coastal waters. Because of their wide distribution, 

historical context, and essential habitat value, the eastern oyster (Crassostrea virginica) is included as a 

target species for monitoring. Oyster beds provide important habitat for numerous organisms and are 

indicators of a healthy estuary. Changes in the spatial extent and health of oyster beds in southeast Florida 

estuaries are key performance measures that will help assess the success of CERP. 

The eastern oyster occupies estuarine and nearshore habitats throughout the eastern and Gulf of 

Mexico coasts of the United States. This animal supported a subsistence fishery even before European 

colonization of the United States (MacKenzie et al., 1997), and throughout recent history has provided an 

important economic and cultural resource to coastal inhabitants. In addition to its direct economic benefits, 

the oyster also provides essential habitat for many other estuarine inhabitants (Bahr and Lanier, 1981). The 

eastern oyster is one of the most culturally, economically, and ecologically important residents of U.S. 

coastal waters. 

 In Florida, oysters occur along both the Atlantic and Gulf of Mexico coasts in almost all estuarine 

and nearshore waters. Along the Atlantic coast, oysters are generally confined within bays and lagoons 

such as Lake Worth Lagoon or the Indian River. These areas are expected to experience changes in water 

quality, quantities and timing due to the implementation of the C-44 and Indian River Lagoon – South 

CERP components. Those waters, and other coastal waters on the southeast coast of the state, have 

experienced altered patterns of water delivery and quality as a result of water management practices related 

to the St. John’s River and Kissimmee River basins, Lake Okeechobee, and the Everglades. In particular, 

the redirection of freshwater out of those inland basins and into the coastal waters mentioned above has 

altered both the timing and the range of salinity variation in those coastal waters. Alterations in freshwater 

flow have reduced or eliminated many oyster reef areas and have impacted both the timing and extent of 

oyster reproduction (Berrigan et al., 1991). The diverse community associated with the oyster reefs has 
6

been impacted to an equivalent or greater degree. Monitoring in the St. Lucie estuary, the Loxahatchee 

River, and Lake Worth Lagoon began in 2003 to establish pre-restoration conditions to assess and evaluate 

any changes related to the implementation of the previously mentioned restoration components. 

 The main objective of this project is to continue a long-term monitoring program intended to 

document the response of oysters, Crassostrea virginica, in south Florida to CERP activities. The 

Molluscan Fisheries research group at the Florida Fish and Wildlife Research Institute (FWRI) in 

conjunction with Dr. Aswani Volety at Florida Gulf Coast University (FGCU) monitored changes in oyster 

distribution and abundance at a variety of sites on the Atlantic and Gulf of Mexico coasts of south Florida. 

Six sites in three estuaries in southeast Florida were monitored by FWRI (Figure 1): the St. Lucie estuary 

(3 sites), the Loxahatchee River (2 sites) and Lake Worth Lagoon (1 site). The Caloosahatchee River and 

nearby Ten Thousand Islands were monitored by FGCU. The data collected by FWRI and FGCU use 

methods that, to the greatest degree possible, ensure the resultant data from these contemporaneous projects 

are statistically and conceptually comparable. Four aspects of oyster ecology are monitored: spatial and 

size distribution patterns of adult oysters, distribution and frequency patterns of the oyster diseases 

Perkinsus marinus (dermo) and Haplosporidium nelsoni (MSX), reproduction and recruitment, and 

juvenile oyster growth. In this report we present the 2008 observations by FWRI on oysters in southeast 

Florida. 

 
7

Figure 1. CERP oyster monitoring study sites and stations on the east coast of Florida. 
8

Methods

Study Sites 

Oyster sampling occurred on oyster reefs within three estuarine ecosystems on the southeast coast 

of Florida that included the St. Lucie estuary, the Loxahatchee River, and Lake Worth Lagoon. Within the 

St. Lucie estuary, we considered the north fork, the south fork, and the central estuary to be separate sites 

and we sampled three oyster reefs (= stations), or three potential oyster reef locations if no oysters were 

present, within each of those sites. Similarly, in the Loxahatchee River, we considered the south fork and 

the northwest fork to be separate sites and we sampled three oyster reefs within each of those sites. Three 

oyster reefs were also selected as study sites within Lake Worth Lagoon. This strategy resulted in a total of 

six separate study sites (St. Lucie-North, St. Lucie-Central, St. Lucie-South, Loxahatchee-North, 

Loxahatchee-South, and Lake Worth) each with three stations (i.e., oyster reefs) that were monitored. 

Station locations can be seen in Figure 1, and coordinates for each sampling reef are provided in Table 1. 

Table 1. Station locations for CERP oyster monitoring sites in southeast Florida. 

Site Station Latitude o

N Longitude oW 

St. Lucie North Fork 1 27 13.232 80 16.737 

St. Lucie North Fork 2 27 12.686 80 15.846 

St. Lucie North Fork 3 27 12.459 80 17.072 

St. Lucie Central Estuary 1 27 12.743 80 14.599 

St. Lucie Central Estuary 2 27 12.087 80 14.493 

St. Lucie Central Estuary 3 27 12.096 80 15.282 

St. Lucie South Fork 1 27 11.691 80 15.636 

St. Lucie South Fork 2 27 11.228 80 16.149 

St. Lucie South Fork 3 27 09.949 80 15.671 

Loxahatchee NW Fork 1 26 58.164 80 07.688 

Loxahatchee NW Fork 2 26 58.237 80 07.649 

Loxahatchee NW Fork 3 26 58.370 80 07.686 

Loxahatchee SW Fork 1 26 56.574 80 07.112 

Loxahatchee SW Fork 2 26 56.630 80 07.280 

Loxahatchee SW Fork 3 26 56.560 80 07.257 

Lake Worth Lagoon 1 26 40.181 80 02.618 

Lake Worth Lagoon 2 26 38.848 80 02.436 

Lake Worth Lagoon 3 26 35.851 80 02.417 

Adult Sampling 

Adult sampling was conducted in the late spring and fall of 2008. On each sampled reef, we 

haphazardly deployed fifteen replicate 1/4-m2

 quadrats and harvested all oysters within each quadrat for 
9

determination of the number of live and dead oysters with articulated shells. A maximum of 10 live oyster 

shell heights (SH = maximum linear distance from the umbo to the ventral shell margin) were also 

measured from each quadrat. 

Reproductive and Disease Monitoring 

 Oysters were collected for analysis of physiological condition, gonadal development state, and for 

the prevalence and intensity of the oyster diseases Perkinsus marinus (dermo) and Haplosporidium nelsoni 

(MSX) on a monthly basis. A sample of ten oysters from each of the three reefs within a study site (total N 

= ten oysters x three reefs x six sites = 180 per month if live oysters were available at all stations) were 

transported, live and chilled, to the FWRI laboratory for processing. Each individual oyster was measured 

(SH mm), shucked, and the tissues processed for reproductive stage, disease status and physiological 

condition according to the methods described below. 

For condition analysis, five of the oysters collected from each station within a site were processed 

by thoroughly cleaning each individual, measuring the shell height, weighing the whole animal, then 

shucking each oyster and obtaining the tissue wet weight. The shells and tissues were dried for 48 hours 

and then the shell and tissue dry weights were recorded. Oyster condition index was calculated as the ratio 

of tissue dry weight to shell dry weight. The remaining five animals collected from each station were 

processed for reproductive and disease analysis. 

 For Perkinsus marinus (dermo) disease analysis, prevalence and intensity was diagnosed with 

Ray’s fluid thioglycollate method (RFTM) as described by Bushek et al. (1994). Small (1 cm2

) pieces of 

gill and mantle tissue were incubated in RFTM media with antibiotics for seven days in the dark at 25o

C. 

Tissue pieces were then placed onto glass microscope slides, macerated with razor blades, stained with 

Lugol’s, and examined at 400x for the presence of hypnospores. Parasite density (infection intensity) was 

ranked using the Mackin scale, which ranges from 0 to 5 (Table 2). Average parasite densities were 

calculated for each sample. 

For reproductive and MSX disease analyses, the remaining oyster tissue was preserved in 

Dietrich’s fixative solution (Barber, 1996). Following 20 hours of fixation, the oyster tissues were 
10

Table 2. Mackin scale showing different stages of Perkinsus marinus (dermo) infection intensity. 

Stage Category Cell Number Notes 

0 Uninfected No cells detected 

0.5 Very light <10 cells in entire preparation 

1 Light 11-100 cells in entire preparation Cells scattered or in localized clusters of 

10-15 cells 

2 Light-moderate 

Cells distributed in local concentrations 

of 24-50 cells; or uniformly distributed 

so that 2-3 cells occur in each field at 

100X 

3 Moderate 3 cells in all fields at 100X Masses of 50 cells may occur 

4 Moderate heavy Cells present in high numbers in 

all tissues 

Less than half of tissue appears blueblack macroscopically 

5 Heavy Cells in enormous numbers Most tissue appears blue-black 

macroscopically 

thoroughly rinsed in tap water and preserved in 70% ethanol for subsequent histological preparation. 

Histological preparation consisted of dehydrating each oyster in 95% ethanol for a minimum of three hours, 

then embedding the tissue in paraffin. Depending upon the size of the individual oyster, a minimum of one 

to a maximum of six 3.5 micrometer sections were cut from each embedded sample using a microtome 

mounted with a glass knife, maintaining a minimum separation of 60 micrometers (the approximate 

maximum diameter of an oocyte) between sections. The sections were stained with hematoxylin and eosin, 

and then mounted onto pre-labeled glass slides for analysis. Resultant slides were examined at 200-400x 

on a compound microscope and each sample was assigned to a reproductive stage following the 

classification scheme (Table 3) modified from the work of Fisher et al. (1996). Finished slides were also 

examined at 100x for the presence of MSX parasites. 

Spat Recruitment 

 Juvenile oyster recruitment was monitored at each of the reefs within each study site. Three 

replicate spat monitoring arrays were deployed and retrieved at each station on a monthly schedule. Each 

array consisted of 12 axenic adult oyster shells (5 -10 cm SH) strung onto two separate lengths of 

galvanized wire. The shells were oriented with their inner surface facing downward when suspended off 

the bottom, and spat recruitment was estimated by counting the number of settled spat on the underside of 

the strung shells. 
11

Table 3. Qualitative reproductive staging criteria for oysters collected from Florida waters. 

Value Observations__________________________________________________________ 

0 Neuter or resting stage with no visible signs of gametes 

1 Gametogenesis has begun with no mature gametes 

2 First appearance of mature gametes to approximately one-third mature gametes in 

follicles 

3 Follicles have approximately equal proportions of mature and developing gametes 

4 Gametogenesis progressing, but follicles dominated by mature gametes 

5 Follicles distended and filled with ripe gametes, limited gametogenesis, ova 

compacted into polygonal configurations, and sperm have visible tails 

6 Active emission (spawning) occurring; general reduction in sperm density or 

morphological rounding of ova 

7 Follicles one-half depleted of mature gametes 

8 Gonadal area is reduced, follicles two-thirds depleted of mature gametes 

9 Only residual gametes remain, some cytolysis evident 

10 Gonads completely devoid of gametes, and cytolysis is ongoing 

______________________________________________________________________________________ 

Juvenile Growth Monitoring 

 Juvenile growth monitoring was initiated at each study site in March 2008. Growth arrays were 

constructed of 12.7-mm-mesh plastic-coated wire mesh with dimensions of approximately 0.6-m L x 0.6-m 

W. On each of these growth arrays, 25 adult, axenic oyster shells were attached by fishing line to serve as 

substrate for wild juvenile oysters. Cage sampling involved measuring the shell heights of 30 haphazardly 

selected live oysters (or all live oysters if <30). This monitoring was conducted in concurrence with the 

monthly recruitment sampling at all sites. Monitoring of juvenile oyster growth continued until February 

2009 at all sites. 

Water Quality Monitoring 

 Monthly water quality sampling was conducted in conjunction with field sampling at all stations 

within each study site. Recorded parameters included water depth, temperature, salinity, turbidity, pH, 

dissolved oxygen concentration and chlorophyll a concentration. Water depth was determined with a 

sounding line and turbidity was obtained using a standard Secchi disk. All other parameters were measured 

with a YSI 6820 multi-parameter data logger. 
12

Results 

St. Lucie Estuary

Adult Sampling

 During the spring oyster survey in May 2008, mean live oyster densities exceeded 100 m-2 at all 

sampled stations in the north fork, the central estuary and at one station in the south fork (Figure 2). These 

live densities were among the highest recorded in the St. Lucie estuary since the start of our oyster 

monitoring program in early 2005. Mean shell heights of these oysters ranged from approximately 50 to 80 

mm (Figure 3) suggesting that oysters were surviving many months and thus achieving larger sizes. 

Unfortunately, storm activity coupled with water releases into the estuary caused salinities to fall below 5 

ppt at all stations in September and October. As a result, during the fall survey in October 2008, we found 

that oyster populations were completely decimated in the north and south forks and reduced by 

approximately 99% in the central estuary. Numbers of dead oysters greatly increased and almost mirrored 

the live densities that were present during the previous spring survey (Figure 2). Because only dead oysters 

with articulated shells are counted during our surveys, some recently dead oysters may not have been 

accounted for in the north fork. Oysters in this study site are present in clumps rather than in a more 

traditional benthic reef structure. When oysters in these areas die-off, the shells become disarticulated 

much more rapidly than would typically occur at a reef site. As previously mentioned, not all oysters were 

lost in the central study site. However, mean shell heights at these three stations only ranged from 

approximately 5 to 14 mm (Figure 3). The small size of these oysters suggests that they were actually new 

recruits that were able to settle onto the reefs before the end of the spawning season. 

Disease Monitoring 

When present, live adult oysters were collected monthly from each station in the St. Lucie estuary 

for determination of the prevalence and intensity of the oyster diseases Perkinsus marinus (dermo) and 

Haplosporidium nelsoni (MSX). However, at Station 3 in the St. Lucie-South study site, oysters were not 

collected during the months of January through March 2008. This was due to an oyster die-off that 

occurred at that station in November 2007. This die-off can be attributed to decreased salinities resulting 

from increased freshwater flow into the area from the nearby canal. Oysters were also not collected for 
13

analyses from all stations in all three study sites during the months of September through December 2008. 

This is the die-off mentioned in the previous section that resulted from a combination of local storm events 

and freshwater releases. Those few live oysters that survived and were present in the central estuary during 

the fall were too young to exhibit any degree of disease infection or prevalence and as such were not 

collected for these analyses. 

Dermo infection was almost absent from oysters collected from the north and south fork study 

sites but was present in low to moderate levels in oysters from the St. Lucie-Central study site (Figures 4 

and 5). Even these infection levels were relatively low with mean intensities remaining below 1, indicating 

that sampled oysters were only lightly infected with the parasite. The prevalence of dermo reached 

moderate levels in oysters collected from the central estuary, especially at Station 1 where 60% of the 

oysters collected during the months of March through July were infected with dermo. Infection levels 

began declining in August which suggests that the lower salinities associated with the wet season were 

inhibiting the parasite. At this time we have found no evidence of Haplosporidium nelsoni (MSX) 

infection in any of the oysters collected from the St. Lucie estuary. 

Physiological Condition and Reproductive Development 

 When present, live adult oysters were collected monthly from each site in the St. Lucie estuary for 

determination of physiological condition and reproductive development. As previously mentioned, oyster 

collections did not occur at Station 3 in St. Lucie-South from January through March and at all stations in 

all study sites from September through December. Reproductive tissues were sexed (neuter, male or 

female) and then assigned a reproductive development stage (Table 3). Analysis of gonadal tissues 

indicated that most oysters are in various stages of reproductive development including gametogenesis, 

active spawning and gonadal recycling throughout most of the year (Figures 6, 7, and 8). However, most 

oysters collected in January and February were found in a resting (i.e., neuter) stage with gonads 

completely devoid of gametes. In previous years’ studies, we found that oysters are in this resting state 

throughout the winter which would also include the month of December. Changes in physiological 

condition generally reflected these reproductive patterns in that condition index values were typically 
14

higher in the spring at the beginning of the spawning season and slowly declined throughout the summer as 

fall approached (Figure 9). 

 

Spat Recruitment

 Juvenile oyster recruitment was monitored at each station in the estuary on a monthly basis. Three 

replicate spat monitoring arrays were deployed at each station for a period of approximately 28 days and 

then collected and returned to the lab for processing. Larval recruitment is reported as the mean number of 

spat counted per shell per month. Peak larval oyster recruitment rates occurred in July followed by a 

second peak in November at most stations (Figure 10). In St. Lucie-Central larval recruitment occurred 

almost continuously from April through November. The spat recorded at Station 3 in January are likely 

recruits from the 2007 spawning season. Recruitment occurred over a slightly shorter time frame in both 

the north and south forks ranging from May or June through November. Overall these recruitment rates are 

fairly low, rarely exceeding 1 spat/shell/month. This reflects a decrease in mean recruits at most stations 

when compared with peak recruitment rates in 2006 and 2007. 

Juvenile Growth Monitoring 

 The juvenile growth monitoring component of our project was initiated in March 2008. At each 

station within a site, one growth array containing 25 adult, axenic oyster shells attached by fishing line was 

deployed directly on the reef or monitoring location. Each shell was checked monthly for the presence of 

wild oyster spat and up to 30 shell height measurements were recorded at each station. Juvenile oysters 

first appeared in St. Lucie-Central in May, in the south fork in June and in the north fork in July (Figure 

11). This closely corresponds to the spat recruitment results in that juvenile oysters were found in each 

study site exactly one month following the first recruits recorded in each study site. However, survival of 

these juveniles was short-lived as the wet season progressed and by September juveniles at most stations 

were dead. Fortunately, salinities recovered quickly and recruits from the last pulse of the spawning season 

settled onto the growth arrays in each study site. These juveniles exhibited a rapid growth rate adding an 

average of over 5 mm shell height per month. When the study was completed in February 2009, the 

juvenile oysters had reached a mean size of approximately 30 mm shell height. 
15

Water Quality Monitoring

 Several water quality parameters were recorded in conjunction with monthly field sampling at 

each station. These parameters included water depth, temperature, salinity, turbidity, pH, dissolved oxygen 

concentration and chlorophyll a concentration. Salinity was highly variable in the estuary ranging from a 

low of nearly 0 ppt to almost 25 ppt throughout the year. Salinities were near the optimal range (15-20 ppt) 

for oyster survival and recruitment success during the first six months of the year, but decreased rapidly in 

July leading to the subsequent oyster die-off at all sites in the fall (Figure 12). In November, the rapid 

increase in salinities coincided with the last recruitment peak of the year. As a result, it’s likely that those 

spat will survive to grow and spawn in the 2009 and allow for some recovery of oyster populations in the 

estuary. Temperatures recorded at each station in the estuary exhibited a typical seasonal pattern with 

values ranging from approximately 16o

C to 32oC over the year (Figure 13). Dissolved oxygen 

concentrations and pH measurements in the estuary were as expected ranging from approximately 4 mg/L 

to 11 mg/L and 6.3 to 8.2 respectively (Figures 14 and 15). Chlorophyll a concentrations ranged from a 

minimum of 6 ug/L to a maximum of 28 ug/L (Figure 16). Turbidity in the water column is presented as a 

Secchi penetration value which is calculated as the percentage of the water column through which the 

Secchi disk could be seen. Turbidity was low at most stations from January through August but began 

increasing in the fall when freshwater inputs into the estuary increased (Figure 17). 
16

Mean Live

Oysters per m2

0

200

400

600

800

1000

St. Lucie-Central

Spring 2008 Fall 2008

Mean Dead 

Oysters per m2

0

200

400

600

800

1000

Mean Live

Oysters per m2

0

50

100

150

200

250

Station 1

Station 2

Station 3

St. Lucie-North

Spring 2008 Fall 2008

Mean Dead 

Oysters per m2

0

50

100

150

200

250

Mean Live

Oysters per m2

0

50

100

150

200

250

Spring 2008 Fall 2008

Mean Dead 

Oysters per m2

0

50

100

150

200

250

St. Lucie-South

Figure 2. Mean number (+ S.D.) of live and dead oysters present at stations in the St. Lucie-North (top), 

St. Lucie-Central (middle) and St. Lucie-South (bottom) study sites during the spring and fall 2008 surveys. 

Filled bars represent the number of live oysters and the hollow bars represent the number of dead oysters 

with articulated shells. Please note the differences in the y-axis range among the different study sites. 
17

St. Lucie-South

Spring 2008 Fall 2008

Mean Shell Height (mm)

0

20

40

60

80

100

120

Spring 2008 Fall 2008

Mean Shell Height (mm)

0

20

40

60

80

100

120

Spring 2008 Fall 2008

Mean Shell Height (mm)

0

20

40

60

80

100

120

Station 1

Station 2

Station 3

St. Lucie-North

St. Lucie-Central

* * *

* * *

Figure 3. Mean shell height (+ S.D.) of live oysters present at stations in the St. Lucie-North (top), St. 

Lucie-Central (middle) and St. Lucie-South (bottom) study sites during the spring and fall 2008 surveys. 

Asterisks denote stations where no live oysters were present for measurement. 
18

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Intensity

0

1

2

3

4

5

Station 1

Station 2

Station 3

*** *** *** ***

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Intensity

0

1

2

3

4

5

*** *** *** ***

St. Lucie-South

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Intensity

0

1

2

3

4

5

*** *** ***

St. Lucie-North

St. Lucie-Central

* * * ***

Figure 4. Monthly mean infection intensity (+ S.D.) of oysters infected with Perkinsus marinus at stations 

in the St. Lucie-North (top), St. Lucie-Central (middle) and St. Lucie-South (bottom) study sites. Asterisks 

denote stations where no live oysters were present for collection and disease analyses.
19

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Prevalence

0

20

40

60

80

100

Station 1

Station 2

Station 3

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Prevalence

0

20

40

60

80

100

St. Lucie-South

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Prevalence

0

20

40

60

80

100

*** *** *** ***

*** *** ***

*** *** *** ***

***

St. Lucie-North

St. Lucie-Central

* * *

Figure 5. Monthly mean prevalence (%) of oysters infected with Perkinsus marinus at stations in the St. 

Lucie-North (top), St. Lucie-Central (middle) and St. Lucie-South (bottom) study sites. Asterisks denote 

stations where no live oysters were present for collection and disease analyses. 
20

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

% Frequency 0

20

40

60

80

100

Female

Male

Neuter

0

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40

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100

0

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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0

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Stage 8 - Follicles 2/3 depleted of mature gametes

Stage 9 - Only residual gametes remain

Stage 10 - Gonads completely devoid of gametes

Stage 6 - Active spawning occuring

Stage 7 - Follicles 1/2 depleted of mature gametes

Stage 5 - Follicles distended and filled with ripe gametes

Stage 4 - Follicles dominated by mature gametes

Stage 2 - Up to 1/3 mature gametes

Stage 1 - Early gametogenesis

Stage 3 - Equal numbers of mature and immature gametes

Figure 6. Reproductive development of oysters collected from the St. Lucie-North study site in 2008. The 

gray shaded areas represent the time period when live oysters were not available for collection from any of 

the three stations. 
21

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

% Frequency

0

20

40

60

80

100

Female

Male

Neuter

0

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100

0

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0

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0

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

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0

20

40

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0

20

40

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100

Stage 8 - Follicles 2/3 depleted of mature gametes

Stage 9 - Only residual gametes remain

Stage 10 - Gonads completely devoid of gametes

Stage 6 - Active spawning occuring

Stage 7 - Follicles 1/2 depleted of mature gametes

Stage 5 - Follicles distended and filled with ripe gametes

Stage 4 - Follicles dominated by mature gametes

Stage 2 - Up to 1/3 mature gametes

Stage 1 - Early gametogenesis

Stage 3 - Equal numbers of mature and immature gametes

Figure 7. Reproductive development of oysters collected from the St. Lucie-Central study site in 2008. 

The gray shaded areas represent the time period when live oysters were not available for collection from 

any of the three stations. 
22

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

% Frequency 0

20

40

60

80

100

Female

Male

Neuter

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Stage 8 - Follicles 2/3 depleted of mature gametes

Stage 9 - Only residual gametes remain

Stage 10 - Gonads completely devoid of gametes

Stage 6 - Active spawning occuring

Stage 4 - Follicles dominated by mature gametes Stage 7 - Follicles 1/2 depleted of mature gametes

Stage 2 - Up to 1/3 mature gametes

Stage 1 - Early gametogenesis

Stage 3 - Equal numbers of mature and immature gametes

Stage 5 - Follicles distended and filled with ripe gametes

Figure 8. Reproductive development of oysters collected from the St. Lucie-South study site in 2008. The 

gray shaded areas represent the time period when live oysters were not available for collection from any of 

the three stations. 
23

St. Lucie-South

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Condition Index

0

2

4

6

8

*** *** *** ***

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Condition Index

0

2

4

6

8

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Condition Index

0

2

4

6

8

Station 1

Station 2

Station 3

*** *** ***

*** *** *** ***

* * *

***

St. Lucie-North

St. Lucie-Central

Figure 9. Monthly mean condition index (+ S.D.) of oysters collected from stations in the St. Lucie-North 

(top), St. Lucie-Central (middle) and St. Lucie-South (bottom) study sites. Asterisks denote stations where 

no live oysters were present for collection and physiological condition analysis. 
24

St. Lucie-South

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Spat/Shell/Month

0

1

2

3

4

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Spat/Shell/Month

0

1

2

3

4

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Spat/Shell/Month

0

1

2

3

4

Station 1

Station 2

Station 3

St. Lucie-North

St. Lucie-Central

Figure 10. Mean number (+ S.D.) of oyster recruits collected per shell each month from stations in the St. 

Lucie-North (top), St. Lucie-Central (middle) and St. Lucie-South (bottom) study sites. 
25

Month

Mar 2008 

Apr 2008 

May 2008 

Jun 2008 

Jul 2008 

Aug 2008 

Sep 2008 

Oct 2008 

Nov 2008 

Dec 2008 

Jan 2009 

Feb 2009 

Mar 2009 

Mean Shell Height (mm)

0

20

40

60

80

Mean Shell Height (mm)

0

20

40

60

80

Mean Shell Height (mm)

0

20

40

60

80

Station 1

Station 2

Station 3

St. Lucie-North

St. Lucie-South

St. Lucie-Central

Figure 11. Monthly mean shell height (+ S.D.) of wild juvenile oysters settled onto growth arrays at 

stations in the St. Lucie-North (top), St. Lucie-Central (middle) and St. Lucie-South (bottom) study sites. 

Growth arrays were planted in March 2008. Months with no data indicate months when no juvenile 

oysters were present on the growth arrays. 
26

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Salinity

0

5

10

15

20

25

30

35

40

Station 1

Station 2

Station 3

St. Lucie-North

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Salinity

0

5

10

15

20

25

30

35

40

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Salinity

0

5

10

15

20

25

30

35

40

St. Lucie-South

St. Lucie-Central

Figure 12. Monthly salinity recorded at stations in the St. Lucie-North (top), St. Lucie-Central (middle) 

and St. Lucie-South (bottom) study sites. 
27

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature (oC)

0

5

10

15

20

25

30

35

40

Station 1

Station 2

Station 3

St. Lucie-North

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature (oC)

0

5

10

15

20

25

30

35

40

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature (oC)

0

5

10

15

20

25

30

35

40

St. Lucie-South

St. Lucie-Central

Figure 13. Monthly temperature recorded at stations in the St. Lucie-North (top), St. Lucie-Central 

(middle) and St. Lucie-South (bottom) study sites. 
28

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

DO (mg/L)

0

5

10

15

20

Station 1

Station 2

Station 3

St. Lucie-North

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

DO (mg/L)

0

5

10

15

20

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

DO (mg/L)

0

5

10

15

20

St. Lucie-South

St. Lucie-Central

Figure 14. Monthly dissolved oxygen concentration recorded at stations in the St. Lucie-North (top), St. 

Lucie-Central (middle) and St. Lucie-South (bottom) study sites. Missing data is due to our malfunctioning 

YSI unit. 
29

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

pH

5

6

7

8

9

10

Station 1

Station 2

Station 3

St. Lucie-North

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

pH

5

6

7

8

9

10

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

pH

5

6

7

8

9

10

St. Lucie-South

St. Lucie-Central

Figure 15. Monthly pH recorded at stations in the St. Lucie-North (top), St. Lucie-Central (middle) and St. 

Lucie-South (bottom) study sites. Missing data is due to our malfunctioning YSI unit. 
30

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Chl 

a (ug/L)

0

10

20

30

40

50

Station 1

Station 2

Station 3

St. Lucie-North

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Chl 

a (ug/L)

0

10

20

30

40

50

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Chl 

a (ug/L)

0

10

20

30

40

50

St. Lucie-South

St. Lucie-Central

Figure 16. Monthly chlorophyll a concentration recorded at stations in the St. Lucie-North (top), St. 

Lucie-Central (middle) and St. Lucie-South (bottom) study sites. Missing data is due to our malfunctioning 

YSI unit. 
31

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

% Secchi Penetration

0

20

40

60

80

100

Station 1

Station 2

Station 3

St. Lucie-North

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

% Secchi Penetration

0

20

40

60

80

100

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

% Secchi Penetration

0

20

40

60

80

100

St. Lucie-South

St. Lucie-Central

Figure 17. Monthly percent Secchi penetration recorded at stations in the St. Lucie-North (top), St. LucieCentral (middle) and St. Lucie-South (bottom) study sites. 
32

Loxahatchee River

Adult Sampling 

 Live oyster densities in the Loxahatchee River reached a mean of nearly 300 m-2 at all stations in 

the northwest fork and over 200 m-2 at two of the three stations in the south fork during the spring oyster 

survey in May 2008 (Figure 18). In the northwest study site, these numbers are comparable to densities 

seen in previous years but in the south fork these values were slightly higher with approximately 80 to 100 

more live oysters m-2 present. Oyster shell heights in the Loxahatchee were similar to those from previous 

years with a mean size of approximately 40 mm in both study sites (Figure 19). During the fall survey in 

October 2008, live oyster densities more than doubled at all stations in the Loxahatchee-North study site 

yielding a mean of over 700 live oysters m-2 (Figure 18). Live densities also increased at two of the three 

stations in the south fork with an average of more than 300 m-2 at a Stations 1 and 3. Mean density at 

Station 2 did not decrease but was identical to the mean from the spring survey. In the Loxahatchee, fall 

mean shell heights are typically lower than those from the spring surveys due the presence of recently 

settled larval oysters. Fall 2008 was no exception with mean shell heights in the northwest fork ranging 

from 33 mm to 38 mm at each of the stations and in the south fork from 34 mm to 41 mm (Figure 19). 

Disease Monitoring 

Live adult oysters were collected monthly from each station in the Loxahatchee River for 

determination of the prevalence and intensity of the oyster diseases Perkinsus marinus (dermo) and 

Haplosporidium nelsoni (MSX). Dermo infection was consistently present in oysters collected from both 

the northwest and south fork study sites. Prevalence of the disease in was moderate to high with anywhere 

from 7% to 67% of oysters from the northwest fork and 20% to 92% from the south fork infected each 

month (Figure 21). However, infection intensity was low with levels rarely exceeding a score of 1 (light 

infection) on the Mackin scale (Figure 20). At this time we have found no evidence of Haplosporidium 

nelsoni (MSX) infection in any of the oysters collected from the Loxahatchee River.
33

Physiological Condition and Reproductive Development 

 Live adult oysters were collected monthly from each site in Loxahatchee River for determination 

of physiological condition and reproductive development. Reproductive tissues were sexed (neuter, male 

or female) and then assigned a reproductive development stage (Table 3). Analysis of gonadal tissues 

indicated that most oysters are in various stages of reproductive development including gametogenesis, 

active spawning and gonadal recycling throughout most of the year (Figures 22 and 23). However, the 

majority of oysters collected in January, February, November and December were found in a resting (i.e., 

neuter) stage with gonads completely devoid of gametes. Changes in physiological condition generally 

reflected these reproductive patterns in that condition index values were slightly higher during the winter 

months when oysters are able to devote energy to somatic growth (Figure 24). 

 

Spat Recruitment

 Juvenile oyster recruitment was monitored at all six stations in the Loxahatchee River on a 

monthly basis. Three replicate spat monitoring arrays were deployed at each station for a period of 

approximately 28 days and then collected and returned to the lab for processing. Larval recruitment is 

reported as the mean number of spat counted per shell per month. Recruitment rates peaked twice in both 

study sites the Loxahatchee River. In the northwest fork, there was a recruitment peak in June and 

September while in the south fork the peaks occurred in April and September (Figure 25). In both study 

sites, recruitment occurred continuously from April through December. The spat recorded at 5 of the 6 

stations in January are likely recruits from the 2007 spawning season. Overall, the peaks and duration of 

the recruitment season are comparable to rates seen in previous years at the Loxahatchee River study sites. 

Juvenile Growth Monitoring 

 The juvenile growth monitoring component of our project was initiated in March 2008. At each 

station within a site, one growth array containing 25 adult, axenic oyster shells attached by fishing line was 

deployed directly on the reef or monitoring location. Each shell was checked monthly for the presence of 

wild oyster spat and up to 30 shell height measurements were recorded at each station. Juvenile oysters 

first appeared in Loxahatchee-South in April and in Loxahatchee-North in May (Figure 26). This 
34

corresponds to the spat recruitment results in that the first recruits recorded in each study site were found in 

April. In the northwest fork, juvenile oysters at stations 1 and 2 exhibited a moderate growth rate adding an 

average of 3.5 mm shell height per month while juveniles at station 3 achieved a growth rate of only 1.5 

mm per month. In the south fork, growth rates were similar to the stations 1 and 2 in the northwest site 

with rates ranging from 3.4 mm to 4.1 mm per month. When the study was completed in February 2009, 

the juvenile oysters had reached a mean size of approximately 30 mm shell height in the northwest fork and 

42 mm shell height in the south fork. 

Water Quality Monitoring

 Several water quality parameters were recorded in conjunction with monthly field sampling at 

each station. These parameters included water depth, temperature, salinity, turbidity, pH, dissolved oxygen 

concentration and chlorophyll a concentration. Salinity was highly variable in both Loxahatchee study 

sites ranging from a low of 2 ppt to over 30 ppt throughout the year. Salinities were near the optimal range 

(15-20 ppt) for oyster survival and recruitment success in early months of the year, but throughout the rest 

of the year salinities were erratic and often either too high (increased predation and disease) or too low 

(Figure 27). Temperatures recorded at each station in the estuary exhibited a typical seasonal pattern with 

values ranging from approximately 17o

C to 33oC over the year (Figure 28). Dissolved oxygen 

concentrations and pH measurements in the estuary were as expected ranging from approximately 4 mg/L 

to 8 mg/L and 7.3 to 8.3 respectively (Figures 29 and 30). Chlorophyll a concentrations ranged from a 

minimum of approximately 3 ug/L to a maximum of 30 ug/L (Figure 31). Turbidity in the water column is 

presented as a Secchi penetration value which is calculated as the percentage of the water column through 

which the Secchi disk could be seen. Secchi penetration was 100% at all six stations in the Loxahatchee 

River for almost the entire year (Figure 32). The only exception was in Loxahatchee-South during the 

months of October and November. Even so, Secchi penetration was still greater than 75% of the water 

column in both months.
35

Mean Live

Oysters per m2

0

200

400

600

800

1000

Loxahatchee-South

Spring 2008 Fall 2008

Mean Dead 

Oysters per m2

0

200

400

600

800

1000

Mean Live

Oysters per m2

0

500

1000

1500

2000

Station 1

Station 2

Station 3

Loxahatchee-North

Spring 2008 Fall 2008

Mean Dead 

Oysters per m2

0

500

1000

1500

2000

Figure 18. Mean number (+ S.D.) of live and dead oysters present at stations in the Loxahatchee-North 

(top) and Loxahatchee-South (bottom) study sites during the spring and fall 2008 surveys. Filled bars 

represent the number of live oysters and the hollow bars represent the number of dead oysters with 

articulated shells. Please note the differences in the y-axis range between the two study sites. 
36

Spring 2008 Fall 2008

Mean Shell Height (mm)

0

20

40

60

80

100

120

Spring 2008 Fall 2008

Mean Shell Height (mm)

0

20

40

60

80

100

120

Station 1

Station 2

Station 3

Loxahatchee-North

Loxahatchee-South

Figure 19. Mean shell height (+ S.D.) of live oysters present at stations in the Loxahatchee-North (top) 

and Loxahatchee-South (bottom) study sites during the spring and fall 2008 surveys. 
37

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Intensity

0

1

2

3

4

5

Station 1

Station 2

Station 3

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Intensity

0

1

2

3

4

5

Loxahatchee-North

Loxahatchee-South

Figure 20. Monthly mean infection intensity (+ S.D.) of oysters infected with Perkinsus marinus at 

stations in the Loxahatchee-North (top) and Loxahatchee-South (bottom) study sites. 
38

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Prevalence

0

20

40

60

80

100 Station 1

Station 2

Station 3

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Prevalence

0

20

40

60

80

100

Loxahatchee-North

Loxahatchee-South

Figure 21. Monthly mean prevalence (%) of oysters infected with Perkinsus marinus at stations in the 

Loxahatchee-North (top) and Loxahatchee-South (bottom) study sites. 
39

% Frequency 0

20

40

60

80

100

Female

Male

Neuter

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Stage 8 - Follicles 2/3 depleted of mature gametes

Stage 9 - Only residual gametes remain

Stage 10 - Gonads completely devoid of gametes

Stage 6 - Active spawning occuring

Stage 7 - Follicles 1/2 depleted of mature gametes

Stage 5 - Follicles distended and filled with ripe gametes

Stage 4 - Follicles dominated by mature gametes

Stage 3 - Equal numbers of mature and immature gametes

Stage 2 - Up to 1/3 mature gametes

Stage 1 - Early gametogenesis

Figure 22. Reproductive development of oysters collected from the Loxahatchee-North study site in 2008. 
40

% Frequency 0

20

40

60

80

100

Female

Male

Neuter

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Stage 8 - Follicles 2/3 depleted of mature gametes

Stage 9 - Only residual gametes remain

Stage 10 - Gonads completely devoid of gametes

Stage 6 - Active spawning occuring

Stage 7 - Follicles 1/2 depleted of mature gametes

Stage 5 - Follicles distended and filled with ripe gametes

Stage 4 - Follicles dominated by mature gametes

Stage 3 - Equal numbers of mature and immature gametes

Stage 2 - Up to 1/3 mature gametes

Stage 1 - Early gametogenesis

Figure 23. Reproductive development of oysters collected from the Loxahatchee-South study site in 2008. 
41

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Condition Index

0

2

4

6

8

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Condition Index

0

2

4

6

8

Station 1

Station 2

Station 3

Loxahatchee-North

Loxahatchee-South

Figure 24. Monthly mean condition index (+ S.D.) of oysters collected from stations in the LoxahatcheeNorth (top) and Loxahatchee-South (bottom) study sites. 
42

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Spat/Shell/Month

0

5

10

15

20

25

Station 1

Station 2

Station 3

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Spat/Shell/Month

0

5

10

15

20

25

Station 1

Station 2

Station 3

Loxahatchee-North

Loxahatchee-South

Figure 25. Mean number (+ S.D.) of oyster recruits collected per shell each month from stations in the 

Loxahatchee-North (top) and Loxahatchee-South (bottom) study sites. 
43

Month

Mar 2008 

Apr 2008 

May 2008 

Jun 2008

Jul 2008 

Aug 2008 

Sep 2008 

Oct 2008 

Nov 2008 

Dec 2008 

Jan 2009 

Feb 2009 

Mar 2009 

Mean Shell Height (mm)

0

20

40

60

80

Mean Shell Height (mm)

0

20

40

60

80

Station 1

Station 2

Station 3

Loxahatchee-North

Loxahatchee-South

Figure 26. Monthly mean shell height (+ S.D.) of wild juvenile oysters settled onto growth arrays at 

stations in the Loxahatchee-North (top) and Loxahatchee-South (bottom) study sites. Growth arrays were 

planted in March 2008. Months with no data indicate months when no juvenile oysters were present on 

the growth arrays. 
44

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Salinity

0

5

10

15

20

25

30

35

40

Station 1

Station 2

Station 3

Loxahatchee-North

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Salinity

0

5

10

15

20

25

30

35

40

Loxahatchee-South

Figure 27. Monthly salinity recorded at stations in the Loxahatchee-North (top) and Loxahatchee-South 

(bottom) study sites. 
45

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature (oC)

0

5

10

15

20

25

30

35

40

Station 1

Station 2

Station 3

Loxahatchee-North

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature (oC)

0

5

10

15

20

25

30

35

40

Loxahatchee-South

Figure 28. Monthly temperature recorded at stations in the Loxahatchee-North (top) and LoxahatcheeSouth (bottom) study sites. 
46

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

DO (mg/L)

0

5

10

15

20

Station 1

Station 2

Station 3

Loxahatchee-North

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

DO (mg/L)

0

5

10

15

20

Loxahatchee-South

Figure 29. Monthly dissolved oxygen concentration recorded at stations in the Loxahatchee-North (top) 

and Loxahatchee-South (bottom) study sites. Missing data is due to our malfunctioning YSI unit.
47

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

pH

5

6

7

8

9

10

Station 1

Station 2

Station 3

Loxahatchee-North

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

pH

5

6

7

8

9

10

Loxahatchee-South

Figure 30. Monthly pH recorded at stations in the Loxahatchee-North (top) and Loxahatchee-South 

(bottom) study sites. Missing data is due to our malfunctioning YSI unit.
48

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Chl a (ug/L)

0

10

20

30

40

50

Station 1

Station 2

Station 3

Loxahatchee-North

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Chl a (ug/L)

0

10

20

30

40

50

Loxahatchee-South

Figure 31. Monthly chlorophyll a concentration recorded at stations in Loxahatchee-North (top) and 

Loxahatchee-South (bottom) study sites. Missing data is due to our malfunctioning YSI unit.
49

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

% Secchi Penetration

0

20

40

60

80

100

Station 1

Station 2

Station 3

Loxahatchee-North

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

% Secchi Penetration

0

20

40

60

80

100

Loxahatchee-South

Figure 32. Monthly percent Secchi penetration recorded at stations in the Loxahatchee-North (top) and 

Loxahatchee-South (bottom) study sites. 
50

Lake Worth Lagoon

Adult Sampling 

In Lake Worth Lagoon, mean live oyster densities during the spring survey were approximately 

250 m-2 at the large reef found at Station 2, 50 m-2 at Station 1 and 20 m-2 at Station 3 (Figure 33). Spring 

oyster densities have been steadily decreasing in Lake Worth Lagoon over past few years with densities at 

Station 2 almost 50% reduced since spring 2006. In stark contrast to the spring survey, fall densities at 

Station 2 increased by over 4-fold with a mean density of over 1100 m-2, while densities at the other two 

stations were both greater than 300 m-2 (Figure 33). This substantial increase in live oysters can likely be 

attributed to the large recruitment event that occurred from the months of April through October. The 

influence of new recruits can be further demonstrated by comparing the mean shell heights of live oysters 

from both surveys. Mean shell heights of oysters at all stations in the spring were approximately 34 mm 

which is comparable to sizes seen in previous years (Figure 34). However, in fall 2008 the mean shell 

height of oysters ranged from approximately 13 mm to 25 mm at the three stations in Lake Worth Lagoon 

(Figure 34). 

Disease Monitoring 

Live adult oysters were collected monthly from each station in Lake Worth Lagoon for 

determination of the prevalence and intensity of the oyster diseases Perkinsus marinus (dermo) and 

Haplosporidium nelsoni (MSX). Dermo infection was present almost every month in oysters collected 

from all three stations in Lake Worth with prevalence ranging from 7% to 80% (Figure 36). Despite the 

persistent presence of dermo, infection intensity levels were low with scores rarely exceeding a rank of 

light infection on the Mackin scale (Figure 35). At this time we have found no evidence of 

Haplosporidium nelsoni (MSX) infection in any of the oysters collected from the Lake Worth Lagoon. 

Physiological Condition and Reproductive Development 

Live adult oysters were collected monthly from Lake Worth Lagoon for determination of 

physiological condition and reproductive development. Reproductive tissues were sexed (neuter, male or 

female) and then assigned a reproductive development stage (Table 3). Analysis of gonadal tissues 
51

indicated that most oysters are in various stages of reproductive development including gametogenesis, 

active spawning and gonadal recycling throughout most of the year (Figure 37). However, the majority of 

oysters collected in January, February, March, November and December were found in a resting (i.e., 

neuter) stage with gonads completely devoid of gametes. When compared to oysters from St. Lucie 

estuary and Loxahatchee River, this appears to be a somewhat protracted period of time for oysters to 

remain in the resting state. In the other estuaries, condition index values are typically higher in the winter 

months when oysters are devoting energy to somatic growth. In Lake Worth Lagoon oysters, condition 

index values exhibited the opposite pattern with peak values occurring in the summer and fall months 

(Figure 38). The impacts of this gonadal development pattern will be demonstrated in the following 

section. 

 

Spat Recruitment

Juvenile oyster recruitment was monitored at all three stations in Lake Worth Lagoon on a 

monthly basis. Three replicate spat monitoring arrays were deployed at each station for a period of 

approximately 28 days and then collected and returned to the lab for processing. Larval recruitment is 

reported as the mean number of spat counted per shell per month. At all three stations in Lake Worth, 

recruitment occurred from the months of April through November with peak recruitment rates occurring in 

August (Figure 39). This is in stark contrast to recruitment patterns observed in the other estuaries in that 

they typically exhibit a bimodal pattern with peaks occurring in the both spring and fall. In previous years, 

recruitment patterns in Lake Worth also appeared to be bimodal but this pattern was much less pronounced 

than those seen in the Loxahatchee River. The reduced period of reproductive development described in 

the previous section is likely responsible for the recruitment pattern observed in Lake Worth in 2008. As 

far as the recruitment season duration and peak recruitment rates, the values recorded in 2008 are 

comparable to previous years in the Lake Worth Lagoon study site. 

Juvenile Growth Monitoring 

The juvenile growth monitoring component of our project was initiated in March 2008. At each 

station within a site, one growth array containing 25 adult, axenic oyster shells attached by fishing line was 
52

deployed directly on the reef or monitoring location. Each shell was checked monthly for the presence of 

wild oyster spat and up to 30 shell height measurements were recorded at each station. Juvenile oysters 

first appeared at all stations in Lake Worth in May (Figure 40). Initial growth estimates appeared high with 

an average of over 10 mm of shell height added in the first month. However, after June mean sizes of 

juvenile oysters began decreasing. Data is missing from station 1 for the months of September and October 

because the growth array became untethered and drifted behind the nearby mangrove. The growth array 

was relocated in November during a low tide sampling trip. When the study was completed in February 

2009, the juvenile oysters had only reached a mean size of approximately 26 mm shell height in the Lake 

Worth study site. 

Water Quality Monitoring 

Several water quality parameters were recorded in conjunction with monthly field sampling at 

each station. These parameters included water depth, temperature, salinity, turbidity, pH, dissolved oxygen 

concentration and chlorophyll a concentration. Salinity was highly variable, but generally higher than 

salinities recorded in other estuaries, in the Lake Worth study site ranging from a low of 12 ppt to 35 ppt 

throughout the year (Figure 41). Salinities were typically higher than the optimal range (15-20 ppt) which 

likely impacted Lake Worth oysters by increasing the incidence of predation and disease infection (dermo). 

Temperatures recorded at each station in the estuary exhibited a typical seasonal pattern with values 

ranging from approximately 17o

C to 31oC over the year (Figure 42). Dissolved oxygen concentrations and 

pH measurements in the estuary were as expected ranging from approximately 4 mg/L to 8 mg/L and 7.7 to 

8.1 respectively (Figures 43 and 44). Chlorophyll a concentrations ranged from a minimum of 

approximately 3 ug/L to a maximum of 29 ug/L (Figure 45). Turbidity in the water column is presented as 

a Secchi penetration value which is calculated as the percentage of the water column through which the 

Secchi disk could be seen. Secchi penetration was 100% at all three stations in Lake Worth for almost the 

entire year (Figure 46). The only exception was during the months of April, September, October and 

November.
53

Mean Live

Oysters per m2

0

500

1000

1500

2000

Spring 2008 Fall 2008

Mean Dead 

Oysters per m2

0

500

1000

1500

2000

Lake Worth

Figure 33. Mean number (+ S.D.) of live and dead oysters present at stations in the Lake Worth study site 

during the spring and fall 2008 surveys. Filled bars represent the number of live oysters and the hollow 

bars represent the number of dead oysters with articulated shells. 

Lake Worth

Spring 2008 Fall 2008

Mean Shell Height (mm)

0

10

20

30

40

50

Station 1

Station 2

Station 3

Figure 34. Mean shell height (+ S.D.) of live oysters present at stations in the Lake Worth study site 

during the spring and fall 2008 surveys. 
54

Lake Worth

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Intensity

0

1

2

3

4

5

Station 1

Station 2

Station 3

Figure 35. Monthly mean infection intensity (+ S.D.) of oysters infected with Perkinsus marinus at 

stations in the Lake Worth study site. 

Lake Worth

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Dermo Infection Prevalence

0

20

40

60

80

100 Station 1

Station 2

Station 3

Figure 36. Monthly mean prevalence (%) of oysters infected with Perkinsus marinus at stations in the 

Lake Worth study site. 
55

% Frequency 0

20

40

60

80

100

Female

Male

Neuter

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Stage 8 - Follicles 2/3 depleted of mature gametes

Stage 9 - Only residual gametes remain

Stage 10 - Gonads completely devoid of gametes

Stage 6 - Active spawning occuring

Stage 7 - Follicles 1/2 depleted of mature gametes

Stage 5 - Follicles distended and filled with ripe gametes

Stage 4 - Follicles dominated by mature gametes

Stage 3 - Equal numbers of mature and immature gametes

Stage 2 - Up to 1/3 mature gametes

Stage 1 - Early gametogenesis

Figure 37. Reproductive development of oysters collected from the Lake Worth study site in 2008. 

Lake Worth

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Condition Index

0

2

4

6

8

Station 1

Station 2

Station 3

Figure 38. Monthly mean condition index (+ S.D.) of oysters collected from stations in the Lake Worth 

study site. 
56

Lake Worth

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Mean Spat/Shell/Month

0

10

20

30

40

50

Station 1

Station 2

Station 3

Figure 39. Mean number (+ S.D.) of oyster recruits collected per shell each month from stations in the 

Lake Worth study site. 

Month

Mar 2008 

Apr 2008 

May 2008 

Jun 2008 

Jul 2008 

Aug 2008 

Sep 2008 

Oct 2008 

Nov 2008 

Dec 2008 

Jan 2009 

Feb 2009 

Mar 2009 

Mean Shell Height (mm)

0

20

40

60

80

Station 1

Station 2

Station 3

Lake Worth

Figure 40. Monthly mean shell height (+ S.D.) of wild juvenile oysters settled onto growth arrays at 

stations in the Lake Worth study site. Growth arrays were planted in March 2008. Months with no data 

indicate months when no juvenile oysters were present on the growth arrays. 
57

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Salinity

0

5

10

15

20

25

30

35

40

Station 1

Station 2

Station 3

Lake Worth

Figure 41. Monthly salinity recorded at stations in the Lake Worth study site.

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Temperature (oC)

0

5

10

15

20

25

30

35

40

Station 1

Station 2

Station 3

Lake Worth

Figure 42. Monthly temperature recorded at stations in the Lake Worth study site.
58

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov

DO (mg/L)

0

5

10

15

20

Station 1

Station 2

Station 3

Lake Worth

Figure 43. Monthly dissolved oxygen concentration recorded at stations in the Lake Worth study site. 

Missing data is due to our malfunctioning YSI unit.

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

pH

5

6

7

8

9

10

Station 1

Station 2

Station 3

Lake Worth

Figure 44. Monthly pH recorded at stations in the Lake Worth study site. Missing data is due to our 

malfunctioning YSI unit.
59

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Chl 

a (ug/L)

0

10

20

30

40

50

Station 1

Station 2

Station 3

Lake Worth

Figure 45. Monthly chlorophyll a concentration recorded at stations in Lake Worth study site. Missing 

data is due to our malfunctioning YSI unit.

Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

% Secchi Penetration

0

20

40

60

80

100

Station 1

Station 2

Station 3

Lake Worth

Figure 46. Monthly percent Secchi penetration recorded at stations in the Lake Worth study site.
60

Discussion 

 This study continued a previous monitoring effort (Arnold, Parker, and Stephenson, 2008) in three 

estuaries: St. Lucie Estuary, Loxahatchee River, and Lake Worth Lagoon. Four estuaries were 

discontinued: Tampa Bay, Biscayne Bay, Sebastian River, and Mosquito lagoon. Monitoring oysters in 

the estuaries of southeast Florida was initiated in January 2008 and consisted of four major tasks: spatial 

and size distribution patterns of adult oysters, distribution and frequency patterns of the oyster diseases 

Perkinsus marinus (dermo) and Haplosporidium nelsoni (MSX), reproduction and recruitment, and 

juvenile oyster growth. 

Oysters in each estuary were subjected to large variations in salinity. Salinities in SLE were near 

the optimal range (15-20 ppt) for oyster survival and recruitment success during the first six months of the 

year, decreased rapidly in July leading to the subsequent oyster die-off at all sites in the fall and recovered 

quickly enough to allow successful settlement of the last recruits of the year. Salinities in LOX were near 

the optimal range (15-20 ppt) for oyster survival and recruitment success in the early months of the year, 

but throughout the rest of the year salinities were erratic and often either too high resulting in increased 

predation and disease, or too low for survival. Salinity was highly variable in the LWL, but was generally 

higher than salinities recorded in the other estuaries, typically higher than the optimal range (15-20 ppt) 

ranging from a low of 12 ppt to a high of 35 ppt throughout the year (Figure 41). At no point were 

salinities low enough that they should have negatively impacted growth or recruitment, but high salinities 

likely impacted Lake Worth oysters by increasing the incidence of predation and disease infection (dermo). 

The estuary most heavily impacted by an extreme salinity event was the St. Lucie Estuary. 

Although there was nearly a complete mortality of adult oysters, and failure of the recruit class during midsummer, the event was less severe than that observed during 2005. In 2005, the freshwater event 

associated with repeated hurricane events continued throughout the fall recruit season, such that most 

oyster reefs in SLE were devoid of oysters throughout the winter of 2006, and only showed minimal adult 

densities in the fall. The result was that an entire year of oyster growth was lost - potentially having serious 

implications for siltation and subsidence of the reef. After the die-off in fall 2008, the salinity increased 

sufficiently to allow some settlement and growth of oysters in the fall. We anticipate that there were 
61

oysters over-wintering on the reef, and that there will be a much more rapid recovery of the reefs in the 

estuary during 2009. 

The eastern oyster thrives in salinities from 10-30 ppt but does poorly when salinity falls below 5 

ppt for a period of from one to five months (Shumway, 1996). Our adult density measures for the SLE 

reflect these salinity tolerance limits, particularly during our spring and fall samplings in 2008. Oysters 

were present at all nine stations in numbers which generally exceeded any seen during the 2005-2007 

monitoring. By the fall survey, the numbers of dead oysters increased and almost mirrored the live 

densities that had been present in the spring, while only a scattering of live oysters were found on the 

central estuarine reefs, none in either fork. Both the LOX and LW sites also experienced salinity 

fluctuations, but none sufficient to cause large mortalities. As a result, both estuaries had increased density 

from spring to fall 2008 and oyster density greatly exceeded the average densities observed during the 

2005-2007 study. 

The mean shell heights observed during the spring 2008 survey were roughly comparable to those 

observed in the fall of 2007, with each site showing modest growth over the winter. In both Loxahatchee 

and Lake Worth, there were modest declines, reflective of the large recruit class over the summer entering 

the population. In St. Lucie, the mean shell height declined dramatically. No large oysters were detected, 

indicating that virtually all of the observed oysters in the fall survey were recent recruits to the reef. 

The patterns of disease intensity and prevalence are markedly different between the three estuaries 

studied. Both intensity and prevalence were much higher in LOX and LWL, where salinities were both 

higher and more stable, than in St. Lucie, where a protracted period of low salinity occurred. Those 

patterns match the 2005-2007 study. The most straightforward explanation is that the salinities experienced 

in much of the SLE are too low for completion of the life history of the Perkinsus parasite. Perkinsus

prevalence and intensity in LOX and LWL are comparable with those patterns observed in our 2005-2007 

study in Mosquito Lagoon and Tampa Bay, sites anticipated to be unaffected by Everglades restoration 

activities and large salinity fluctuations. The severity of dermo in southeast Florida estuaries remains 

considerably less than it is in most Gulf of Mexico populations (Craig et al., 1989). We have not detected 

MSX (Haplosporidium nelsoni) in any sample examined. 
62

 Reproductive development showed some similarities among our study sites, and some notable 

differences. In general, there were few oysters in what would be considered developmental or fully ripe 

stages (Stage 1-5) in any estuary. Most oysters collected in January, February, and December were 

classified as Stage 10, completely devoid of gametes, or resting. There were some oysters in the resting 

period as late as March, or as early as November. Each site also had some Stage 10 oysters present in midsummer. Those may represent young-of-the-year oysters which have not matured, or could represent early 

spawning oysters which were completely spent by the end of the spring 

There are also important differences between study sites, even within estuaries. The greatest 

variation observed was related to the nearly complete mortality of oysters throughout the St. Lucie Estuary, 

making collection of samples for assessment of reproductive development impossible from September 

through December. No such mortality was observed in either LOX or LWL. In the St. Lucie-Central and 

Loxahatchee-North sites, there is a gradual and regular progression of the oyster population having 

increasing frequencies of spawning activity (Stages 6-8) beginning in January, peaking in April and May, 

and declining through August. At the remaining sites, St. Lucie-North, St. Lucie-South, LoxahatcheeSouth, and Lake Worth, there appears to be a less regular pattern, with suggestions of bimodal peak in 

March/April and again in July/August. 

 Recruitment patterns did not necessarily match the predicted spawning season based on histology. 

At each St. Lucie site, there were protracted periods with minimal but non-zero recruitment. There was 

also a large distinct peak of recruitment in July, suggesting that the large population of oysters in the central 

estuary supplied larvae throughout the system. In the Loxahatchee system, there were also protracted 

periods with minimal but non-zero recruitment. However, each fork had two strong peaks in recruitment. 

Both forks had a strong fall peak (September), but the spring peak occurred much earlier in the south fork 

(April) than in the north fork (June). In Lake Worth Lagoon maturation occurred latest accompanied by the 

latest peak in recruitment. Though less regular than other estuaries, the protracted reproductive period in 

LWL resulted in a large and protracted recruitment event - lasting from June through October. 

The source of recruits that reestablished the SLE oyster population remains a mystery - similar to 

the event that occurred in 2005-2006. Although we could find almost no live oysters in SLE during the fall 

survey, it is probable that a small relict population remained in the estuary. This population may have 
63

contributed the recruits that were detected in both November 2005 and November 2008 in the central 

portion of SLE and at all sites during mid-2006. It is also possible that those recruits were contributed by 

oysters living outside of the SLE, but there is no direct evidence to support either alternative. The source of 

these recruits is of more than academic interest. Whether the larval source was autochthonous or 

allochthonous, it is possible that continued coastal development will degrade the oyster population of 

southeast Florida to the point that recruits are no longer available at some sites. This condition does not 

have to remain for long, as evidence from consecutive oyster reef mapping efforts in the SLE suggest that 

at least the smaller reefs can subside in only a few years. This subsidence will eliminate the essential larval 

settlement substrate provided by an exposed reef, and once removed from the system it may require 

decades to rebuild (Mann and Powell, 2007) without intervention. 

In both the St. Lucie estuary sites and the Loxahatchee sites physiological condition began the 

year high, and gradually declined, representative of the transfer of energetic reserves into gamete 

production. In the Lake Worth Lagoon sites, physiological condition actually climbed gradually 

throughout most of the spawning season and only began to decline as summer turned to fall. This pattern is 

suggestive of one where metabolic demands are low enough and food supply high enough that acquired 

food is sufficient for both growth and reproduction. This result corresponds well with the observation that 

adult density increased tremendously in 2008 between spring and fall. In 2005-2007, most sites, when 

oysters were present, exhibited the pattern of high condition early in the year, with a gradual rate of decline 

as the reproductive season progressed. In some years the condition would begin to climb again in 

November and December, after reproduction had fallen off for the year. At one CERP impacted site in one 

of the three years, LWL in 2006, oysters exhibited the pattern of highest condition in mid-summer, at the 

peak of spawning season. This pattern was also observed in some years in our outlier populations, 

Tampa Bay and Mosquito Lagoon, suggesting it may be a characteristic of populations in higher salinity 

habitats. 

In our 2005-2007 study, the oysters, when present, exhibited a more gradual increase in 

reproductive development, rather than the rapid progression observed in 2008. Gradual development 

should lead to a prolonged recruit period whereas rapid development may lead to a shorter recruit period. 

In general, the population with a broader recruit period would be more resilient to modest departures from 
64

ideal water quality because the settling spat would have multiple chances to opportunistically settle 

throughout the estuary. The rates of development and patterns in recruitment may correlate with certain 

physical parameters, may have threshold values below or above which no development and recruitment 

occurs, or may have complex relationships with multiple physical parameters. Interpretations of the 

variations in observed development and recruitment patterns of oysters in these estuaries can only be 

formulated after multiple years of data have been compiled. 

 Juvenile growth rate was high in both the St. Lucie estuary and the Loxahatchee River. Observed 

juvenile growth rate was low in LWL. This may be explained by the higher levels of spat recruitment that 

began in June. If new recruits were settling to the growth array in June, July and August, their smaller sizes 

would decrease the mean shell height recorded during those months. This is one drawback to setting up a 

growth study in this manner in that there is no efficient way to mark and follow the same cohort of oysters 

throughout the experiment. The steady growth rates in the SLE and LOX sites are reflective of our ability 

to track the growth of a single recruit class of oysters which settled in relatively discrete peaks. Following 

recruitment, the juvenile oysters grew very rapidly and at most sites will exceed 40 mm SH by the 

following spring. These growth rates are typical of oysters in the southeastern United States and 

particularly in the Gulf of Mexico, but are more rapid than those typically reported for more northern 

populations (Shumway, 1996). Consistent with other bivalves, the more rapid growth of oysters in 

southern latitudes may be attributed to the longer growing season rather than to an inherently more rapid 

growth rate (Jones et al., 1990). This distinction is important, because factors other than temperature also 

influence growth rate. In particular, oysters do not grow well at salinities less than 10 ppt (Loosanoff, 

1953). Ramifications of growth variation can be significant, as faster growing oysters will more quickly 

escape the ravages of size-limited predators and also would be expected to reallocate energy from growth 

to reproduction at an earlier age. 

Water quality parameters generally fell within expected ranges with the exception of the low 

salinity event we have noted above. Temperatures rose and fell almost as expected, with the exception of a 

slight cooling in April. The water column at each study site remained well oxygenated. pH was usually 

near or slightly below 8, even during low salinity events. Chlorophyll a concentration was perhaps most 

variable. At the St. Lucie and Lake Worth Lagoon sites, chlorophyll a generally remained at or below 10 
65

ug/L. The exceptions to these relatively low and stable concentrations occurred at one station in St. LucieSouth and at both Loxahatchee sites. At LOX, all six stations began the year with relatively high chl a

concentrations suggesting that a source of nutrients had initiated a plankton bloom, which appeared to have 

been grazed down by May. Unfortunately, all chl a data was lost in June-August when the chlorophyll 

probe of our YSI meter was inoperable. Water clarity was generally good, with Secchi depth penetrating to 

the bottom at most sites. The notable exceptions were at the St. Lucie sites impacted by low salinity. 

Interestingly, the low visibility seems to have persisted longer than the lowest salinity readings, but did not 

initiate a large phytoplankton bloom during the fall despite the probable large input of nutrients to the 

system and reduced filtering capacity of the benthic community. 

Comparing parameter values recorded within our study estuaries to a range of values typical of 

Florida estuaries (not affected by CERP activities) provides a pragmatic and real-time view of the 

consequences of water management practices that affect the study estuaries. Our results from the 2008 

monitoring effort and the 2005-2007 study indicate that the CERP impacted estuaries may undergo more 

frequent low-salinity periods than outlier populations. Outlier populations, although not perfect because of 

physical, geological, and landscape differences, nonetheless provide some context within which events in 

the study estuaries can be evaluated. This is in agreement with accepted statistical practices regarding the 

analysis and interpretation of event-driven research and monitoring programs (Underwood and Chapman, 

2003). 

This report summarizes oyster population monitoring at six sites within three estuaries in southeast 

Florida within the South Florida Water Management District’s domain. Results indicate that the patterns of 

oyster abundance, health, and population ecology within two (LOX and LWL) of the study estuaries 

generally fell within the bounds expected for central and south Florida oyster populations. The impact of 

anthropogenic events, particularly introductions of freshwater into the estuaries to manage inland water 

levels, forced salinity values outside of expected and suitable ranges in the St. Lucie Estuary. Detailed 

examination of those events and their consequences continues as we correlate actual control structure flow 

rates with the biological and ecological responses of the oysters. In the meantime, it is apparent that such 

freshwater releases must be carefully considered with respect to their impact on a host of downstream 

organisms but particularly oysters. Because the oyster repertoire includes habitat engineering, impacts to 
66

oyster populations will have uniquely broad-ranging consequences for a host of ancillary organisms and for 

the oysters themselves, whose recovery from such events may be serially degraded to the point of no return. 
67

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