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Influence of salinity on the habitat use of oyster reefs in three Southwest Florida estuaries.

ABSTRACT To gauge the influence of salinity on the habitat value of oyster reefs, spatial and seasonal patterns of the presence of reef-resident fishes and decapods were assessed in the Caloosahatchee, Estero, and Faka-Union estuaries of Southwest Florida. Lift nets (1 [m.sup.2]) containing 5 L of oyster clusters were deployed on intertidal reefs at three sites along the salinity gradient of each estuary. Nets were deployed during three seasonally dry and three seasonally wet months for a period of 30 d. Oyster densities were estimated at each site and a number of community metrics were calculated as a measure of habitat use (e.g., organism density, biomass, diversity, dominance, richness). Several metrics increased downstream in one or more systems (e.g., organism density, biomass, diversity) and in general appeared to be more related to salinity than to the density of living oysters present. Although organism density was higher during the wet season for all three systems, biomass was higher during the dry season in the Caloosahatchee. In the Caloosahatchee and the Estero, measures of biodiversity tended to be higher during the dry season. These results suggest that the salinity requirements of the organisms that inhabit oyster reefs should be considered in the planning of oyster-reef restoration or enhancement projects or in the management or alteration of freshwater inflow into estuaries.

KEY WORDS: oyster reefs, habitat use, decapods, fishes, salinity

INTRODUCTION

The Eastern oyster Crassostrea virginica is highly valued as food, yet its ecological significance remains under-appreciated (Coen et al. 1999). Individual oysters filter 5 L of water [h.sup.-1] [g.sup.-1] dry mass (Newell 1988) removing phytoplankton, particulate organic carbon, sediments, pollutants, and microorganisms from the water column. This process results not only in greater light penetration downstream but also in the mineralization of nutrients (Dame et al. 1985), thus promoting the growth of submerged aquatic vegetation (see also Peterson & Heck 2001). Oysters assimilate the bulk of the organic matter that they filter; the remainder is deposited on the bottom where it provides food for benthic organisms. Oysters and the complex, three-dimensional, reef structure they form, attract numerous species of invertebrates and fishes.

These "ecosystem engineers" (sensu Jones et al. 1994) attract predators such as mud crab (Panopeus herbstii) (Meyer 1994), black drum (Pogonias cromis) (Ingle & Smith 1956), and crown conch (Melongena corona) (Woodburn 1965), which feed on the living oysters themselves. Oyster shell serves as a site for egg laying and nesting in the crown conch (M. corona) (personal observation, Tolley) and Florida blenny (Chasmodes saburrae) (Peters 1981), respectively. Intertidal oyster reefs provide refuge from predation (McDonald 1982) and desiccation (Grant & McDonald 1979) for mud crabs (Xanthidae), and subtidal reefs may offer a safe haven from hypoxia (Lenihan et al. 2001).

To date, over 300 species have been identified as depending, either directly or indirectly, on intertidal oyster reefs (Wells 1961; see also Crabtree & Dean 1982, Wenner et al. 1996, Coen et al. 1999). Based on the relative degree of dependence, oyster-reef fauna can be classified as reef residents, facultative residents, and transients (Breitburg 1999). Many of these organisms in turn serve as forage for important fisheries species (e.g., spotted seatrout, Cynoscion nebulosus, Tabb & Manning 1961, McMichael & Peters 1989, red drum, Sciaenops ocellatus, Peters & McMichael 1987, bluefish, Pomatomus saltatrix, Harding & Mann 2001), and birds (e.g., yellow-crowned night-heron, Nycticorax violaceus, Watts 1988). Not surprisingly, oyster reefs have been identified as essential fish habitat (Coen et al. 1999) as defined by the Magnuson-Stevens Fishery Conservation and Management Act, where fish are defined as fin-fish, mollusks, and crustaceans (USDOC 1997).

Although Wells (1961) suggested a role for salinity in determining the composition of oyster-reef communities, limited empirical work has been undertaken to quantify this effect. Only Gorzelany (1986) has since addressed the issue to any real extent by sampling the communities of oyster-reef associated organisms at several points along the salinity gradients of temperate estuaries in the Big Bend region of northwest Florida.

In South Florida, because no commercial harvesting of oysters exists, it is the ecologic function of oysters and oyster reefs, the filtration of the water column, the coupling of benthic and pelagic environments through the transfer of organic matter, and the provision of secondary habitat, that is of primary interest. Furthermore, the influence of freshwater inflow on oysters and oyster reef-habitat is of special interest in the region where the Comprehensive Everglades Restoration Plan, one of the largest ecosystem restoration projects in the world is currently underway. This massive project includes the rerouting and reallocation of freshwater from and into many estuaries, including those in Southwest Florida.

This study investigates the influence of salinity on spatial and seasonal patterns of oyster-reef habitat use by decapod crustaceans and fishes in three Southwest Florida estuaries, all of which will be impacted by Greater Everglades restoration. Special attention is given to examining the potential influence of salinity on the abundance and composition of these organisms. Understanding the relationships between salinity (a proxy for freshwater inflow) and metrics associated with oyster-reef habitat use will provide a valuable tool that can be used to improve coastal resource management and restoration efforts. Resource managers can use this approach to regulate salinity zones within an estuary to optimize habitat utilization of oyster reefs by associated organisms. Further, such relationships can be used prior to the commencement of oyster-reef restoration or enhancement efforts to identify those areas within an estuary that are likely to provide an appropriate salinity regime for the development of a robust assemblage of associated organisms.

METHODS

Study Location

Oyster reefs were selected in 3 estuarine systems within Southwest Florida for comparison: the Caloosahatchee and Estero rivers and the Faka-Union Canal (Fig. 1). These study areas exhibit tidal amplitudes of [less than or equal to] 2 m and are therefore considered microtidal. Further, the living oyster reefs examined are intertidal and are therefore of limited vertical relief. The Caloosahatchee watershed (3,700 [km.sup.2]; Science Subgroup 1996) is highly altered and highly managed: it was augmented during the late 19th century via the creation of a permanent connection with Lake Okeechobee and the Kissimmee River (Antonini et al. 2002). As part of the Okeechobee Waterway that traverses the state, the upper Caloosahatchee has been converted from a meandering river into a canal over much of its length, and water in the river is impounded behind a series of control structures. Significant freshwater releases into the River from Lake Okeechobee occur as a means of flood prevention during the rainy season and as a result of periodic drawdowns in the Lake to manage aquatic vegetation.

[FIGURE 1 OMITTED]

Lying south of the Caloosahatchee estuary is Estero Bay, the State's first aquatic preserve. Even though much of the lower estuary is protected, its tributaries, including the Estero River (170 [km.sup.2]; Janicki 1999), are increasingly subjected to development in the upper portions of their watersheds. Farther southeast is the Faka-Union Canal that empties into the Ten Thousand Islands adjacent to the western boundary of the Everglades National Park. The Faka-Union (518 [km.sup.2]; Governor's Commission for the Everglades 1999) is the least developed of the systems considered but has a highly augmented watershed--the result of a failed real estate development in the 1960s (i.e., Southern Golden Gate Estates) that created 86 km of canals to drain freshwater wetlands.

Seasons in Southwest Florida are determined as much by rainfall as by temperature. Seasonal rains are prevalent from mid-June through mid-October and coupled with occasional tropical weather systems reduce salinities in local estuaries. In contrast, hypersaline conditions may occur in the downstream portion of these estuaries during the remainder of the year.

Field Sampling and Analysis

To examine the influence of salinity (freshwater inflow) on the habitat use of oyster reefs, a spatiotemporal comparison of reef-resident fishes and decapod crustaceans was conducted during three seasonally dry (mid-March through mid-June 2002) and three seasonally wet (mid-July through mid-October 2002) months in the three estuaries.

Three stations were selected for sampling along the salinity gradient of each estuary. Stations were selected at sites that were morphologically homologous among the systems: an upper station located within the tidal river itself, a middle station located near the mouth of the river, and a lower station located well below the mouth of the river. This approach is supported by the work of Gorzelany (1986) who found greater similarity among oyster-reef associates found at comparable sites (inshore, middle, offshore) in different tidal rivers than among different sites within the same tidal river.

For each sampling effort (6 sampling periods x 3 systems x 3 stations) salinity and water temperature were recorded, and triplicate lift nets (Crabtree & Dean 1982) were deployed intertidally, just above mean low water, on living oyster reefs for a duration of approximately 30 d. (Previous collections using Hester-Dendy samplers suggested that this period was sufficient for macroinvertebrate recruitment). Lift nets (1 [m.sup.2]) were constructed using 3.2-cm PVC frames and 6.4-mm delta-weave netting dipped in vinyl to minimize wear and tear resulting from constant contact with oyster shell. The bag on each net measured 0.5 m in height, and the bottom was made using 1.6-mm netting to prevent the escape of small organisms. Upon deployment, a 1-[m.sup.2] area of the substrate was cleared of oyster shell. The net walls were then collapsed as each lift net was pinned to the substrate using 45-cm lengths of PVC attached to PVC T-fittings. Approximately 5 L (volume displacement) of live oyster clusters were collected from adjacent portions of the reef and were then placed in each net. Because each net would be deployed for a period of 30 d, no effort was made to remove existing fauna from these oyster clusters. Upon retrieval of the nets oyster clusters were removed and any associated decapods and fishes were extricated using forceps. Any remaining decapods and fishes were then either removed from the net by hand or by using dip nets to sweep the interior of the lift net. These organisms were then transported on ice back to the laboratory for identification. Specimens were stored in 70% isopropanol for archiving and further analysis.

Community metrics of the decapods and fishes recruited into the oyster clusters were examined: density, biomass, diversity (Shannon-Wiener Index, H'), dominance (% occurrence of the most abundant species), and species richness. The ratio of the porcelain crab Petrolisthes armatus to the mud crab Eurypanopeus depressus (STENO:EURY) has also been suggested as a useful metric for assessing the effects of freshwater inflow on oyster-reef communities (Shirley et al. 2004) and was therefore used in this study. P. armatus is a stenohaline organism that is less tolerant of reduced salinities (Shumway 1983) compared with the more euryhaline E. depressus. In the laboratory, organisms were identified, measured to the nearest 0.1 mm (shrimp: carapace length, crabs: carapace width, fishes: standard length) and were weighed to the nearest 0.01 g wet mass (WM). Length-weight regressions were calculated for each species and were subsequently used to estimate biomass based upon the mean size of each species collected in each sample. Oyster densities were estimated by enumerating the number of living oysters contained in a 0.25 [m.sup.2] quadrat. Each site was sampled once during the study with four replicates being made. Measured density was normalized to 1-[m.sup.2] of bottom.

Response variables (e.g., density, biomass, diversity, etc.) were examined using 1-way analysis of variance with season (wet vs. dry), estuary, and station as factors. Homogeneity of variance was tested using the Levene statistic; when variances were deemed unequal, the Welch ANOVA was used. Significant differences detected by ANOVA (P [less than or equal to] 0.05) were resolved using multiple comparison tests (Day & Quinn 1989): Fisher's Least Significant Difference in cases of equal sample size and equal variance; the GT2 method in cases of unequal sample size but equal variance; and the GH test in cases of unequal variance (regardless of sample size). Unless otherwise specified, data are presented as mean [+ or -] standard deviation.

RESULTS

Density and Biomass

Decapod crustaceans dominated the samples both numerically and in terms of biomass. Nine species of decapods were collected, with the porcelain crab Petrolisthes armatus and the mud crabs Eurypanopeus depressus and Panopeus sp. being the most abundant for all three systems (Table 1). Fishes were more diverse with 16 species (Table 1), but were typically 1-2 orders of magnitude less abundant than decapods. Based on their relative percent occurrence in the samples, the Florida blenny (Chasmodes saburrae) and gulf toadfish (Opsanus beta) were considered common on Caloosahatchee reefs; the skilletfish (Gobiesox strumosus), crested goby (Lophogobius cyprinoides), and gulf toadfish were common on Estero reefs; and the code goby (Gobiosoma robustum) was the only fish to occur commonly on Faka-Union oyster reefs.

Significant spatial and temporal variation in both organism density and biomass were detected for all three estuaries. During the dry season, organism density tended to be higher downstream (Fig. 2): densities in Estero Bay increased significantly downstream; densities in Faka-Union Bay were higher at the middle and lower stations compared with the upper station; and densities in the Caloosahatchee were significantly higher at the middle station compared with the upper station. Wet-season densities exhibited even greater differences between stations and also increased downstream (Fig. 2.): Caloosahatchee densities overlapped but still increased significantly downstream; Estero densities were significantly higher at the middle and lower stations; and Faka-Union densities increased significantly downstream.

[FIGURE 2 OMITTED]

Patterns in biomass were similar to those exhibited by organism density (Fig. 3). During the dry season, biomass of decapods and fishes on Caloosahatchee reefs increased significantly downstream, and biomass on Estero reefs was significantly higher at the downstream station. No among-station differences in biomass were detected in the Faka-Union. Wet-season biomass was significantly higher at the middle and lower stations compared with the upper station for all three estuaries. Although biomass did not vary significantly among systems, organism density was significantly lower in the Caloosahatchee (109.04 [+ or -] 55.47 [m.sup.-2]) compared with the Faka-Union (161.04 [+ or -] 149.04 [m.sup.-2]) (Welch ANOVA: F = 3.503, P = 0.034, df = 153).

[FIGURE 3 OMITTED]

Comparing seasons, density was significantly lower during dry months for each estuary examined (Table 2). Although biomass was found to be significantly higher during the dry season, no between-season differences were detected for the other two estuaries.

Biodiversity and Composition

Measures of biodiversity exhibited clear trends in the Caloosahatchee estuary but not in the other two systems. Diversity increased significantly downstream during the dry season and was greater at the middle and lower stations compared with the upper station during the wet season (Fig. 4). Richness was significantly higher during the dry season at the lower station in the Caloosahatchee (Fig. 5) and significantly higher at the middle station compared with the upper station during the wet season. Mean richness at the lower station was also higher than at the upper station but the difference was not significant. Dominance, which is inversely related to diversity, was greatest at the upper and middle stations in the Caloosahatchee during the dry season (Fig. 6) and decreased downstream during the wet season. In Estero Bay, diversity was actually higher at the upper station compared with the middle station (Fig. 4). No other significant differences were detected for any measures of biodiversity in either Estero or Faka-Union Bays.

[FIGURES 4-6 OMITTED]

Although no differences in diversity were detected among systems, richness of associated organisms was significantly greater on Caloosahatchee oyster reefs (6.7 [+ or -] 2.7 species) than on reefs in the other estuaries (Estero: 4.4 [+ or -] 1.3 species; Faka-Union: 4.6 [+ or -] 2.0 species) (Welch ANOVA: F = 14.424, P = 0.000, df = 153). In addition, species dominance was significantly lower on Estero Bay oyster reefs (60.60% [+ or -] 8.99) than on either Caloosahatchee (68.88% [+ or -] 20.53) or Faka-Union reefs (67.86% [+ or -] 13.48) (Welch ANOVA: F = 7.056, P = 0.001, df = 153).

Comparing seasons, diversity was significantly greater during the dry season for both the Caloosahatchee and the Estero (Table 2); however, no such difference was detected for the Faka-Union (Table 2). Although data pooled for all three systems suggested that species richness was also greater during dry months (5.58 + 2.63 species) compared with wet months (4.84 [+ or -] 1.92 species) (Welch ANOVA: F = 3.997, P = 0.47, df = 153), no seasonal differences were detected within individual estuaries (Table 2). In Estero Bay, dominance exhibited an inverse pattern to diversity, with higher values occurring during the wet season (Table 2), but Caloosahatchee and Faka-Union values did not vary seasonally (Table 2).

In general, the ratio of the porcelain crab Petrolisthes armatus to the mud crab Eurypanopeus depressus increased downstream for all three systems (Fig. 7). In fact, no individuals of P. armatus were found at the upper station in the Caloosahatchee, and in the Faka-Union P. armatus was only collected during the dry season at the upper station (Fig. 7). The ratio of P. armatus to E. depressus varied significantly among estuaries with lower values calculated for Caloosahatchee reefs (0.51 [+ or -] 0.67) than for either Estero (1.00 [+ or -] 0.93) or Faka-Union reefs (1.51 [+ or -] 1.705) (Welch ANOVA: F = 10.280, P = 0.000, df = 153). No significant seasonal variation in this ratio was detected for any of the systems examined (Table 2).

[FIGURE 7 OMITTED]

Environmental Factors

Oyster density was greatest at the middle station in the Caloosahatchee (Fig. 8) and was higher downstream in the Faka-Union (Fig. 8). Oyster density did not vary spatially in the Estero; however, oyster density was significantly higher in the Estero (1474 [+ or -] 624 [m.sup.-2]) compared with either the Caloosahatchee (858 [+ or -] 482 [m.sup.-2]) or Faka-Union (842 [+ or -] 549 [m.sup.-2]) (ANOVA: F = 5.066, P = 0.012, df = 35).

[FIGURE 8 OMITTED]

Salinity increased significantly downstream in both the Caloosahatchee and Faka-Union during the wet season; however, no among-station differences were detected either for the Estero during the wet season or for any of the systems during the dry season (Fig. 9). Although mean salinity was higher in the Estero (25.29 [+ or -] 10.10 psu) compared with the Caloosahatchee (15.90 [+ or -] 9.18 psu) or Faka-Union (9.69 [+ or -] 7.86 psu) during the wet season (ANOVA: F = 8.947, P = 0.001, df = 35), no significant differences among estuaries were detected during the dry season. All three estuaries exhibited significant seasonal variation in salinity with higher values occurring during the dry season (Table 2).

[FIGURE 9 OMITTED]

Temperature did not vary significantly among stations for any of the estuaries examined, nor among systems during either wet or dry seasons. Mean temperature was significantly greater during the wet season compared with the dry in both the Estero and Faka-Union (Table 2); no seasonal differences were identified for the Caloosahatchee.

DISCUSSION

The assemblage of fishes and decapod crustaceans collected in association with oyster clusters during this study is similar to those reported previously from more temperate waters. For example, the skilletfish (Gobiesox strumosus), feather blenny (Hypsoblennius hentz), and snapping shrimp (Alpheus heterochaelis) have all been previously collected from oyster reefs ranging from the Carolinas to Virginia and Maryland (Breitburg 1999, Coen et al. 1999, Posey et al. 1999) and were represented on reefs in Southwest Florida. Likewise, the mud crab Eurypanopeus depressus occurs commonly on North Carolina reefs (Meyer 1994) and was the dominant decapod collected on oyster reefs within the St. Martins Aquatic Preserve on the central Gulf coast of Florida (Glancy et al. 2003). This species was the second most abundant organism found in the current study. In other cases, more temperate species occurring on oyster reefs from the Carolinas to New York are replaced in Southwest Florida by more tropical to subtropical congeners: the striped blenny Chasmodes bosquianus (Breitburg 1999, Coen et al. 1999) is replaced by the Florida Blenny Chasmodes saburrae, and the oyster toadfish Opsanus tau (Breitburg 1999, Coen et al. 1999) is replaced by the Gulf toadfish Opsanus beta. Although the green porcelain crab Petrolisthes armatus, considered an invasive exotic along much of the South Atlantic Bight, has been previously reported throughout the Gulf of Mexico (Knott et al. 1999, Glancy et al. 2003), its range prior to 1994 was apparently limited on the US Atlantic coast to south of Cape Canaveral (Knott et al. 1999). In this study, Petrolisthes was the numerically dominant organism collected, and its abundance increased from north to south among the estuaries sampled.

Nekton (transients) were not specifically targeted for sampling in this study; however, juveniles of a number of commercially and recreationally important species of nekton were sampled using lift nets. Young pinfish Lagodon rhomboides were collected on Caloosahatchee reefs, and sheepshead Archosargus probatocephalus were found on oyster reefs in all three estuaries. Large juveniles and adult sheepshead are known to feed on oysters in the northern Gulf of Mexico (Benson 1982). Juvenile gray snapper Lutjanus griseus and lane snapper Lutjanus synagris were found in samples from Estero and Faka-Union bays. Winstead et al. (2004) inferred the presence of nektonic species based on the parasite and symbiont fauna found in oysters collected from the Caloosahatchee estuary. Infection of oysters by the digenetic trematode Bucephalus suggested the presence of either the definitive host Lepisosteus (gars of the family Lepisosteidae) or the second intermediate host Mugil cephalus, the striped mullet. The latter species was seen schooling adjacent to Caloosahatchee oyster reefs in the present study. Furthermore, the spotted eagle ray Aetobatis nari-nari and the cownose ray Myliobatis bonasus--known molluscan predators--are definitive hosts for some species of the cestode Tylocephalum. This parasite was also identified from Caloosahatchee oysters and both of these rays were seen in the vicinity of one or more of the Caloosahatchee reefs during the current study.

Some of the species collected in this study were found at all stations sampled and occurred in a variety of salinities (e.g., Alpheus heterochaelis, Eurypanopeus depressus, Panopeus sp.); however, the crested goby Lophogobius cyprinoides was found only at the uppermost station in the Estero and Faka-Union, and other species appeared unsuccessful in colonizing oyster clusters at the uppermost station in one or more systems (e.g., Menippe mercenaria, Petrolisthes armatus). Wells (1961) suggested that a majority of species inhabiting oyster reefs are limited in their upstream distribution by the reduced salinities occurring there.

The distribution of oyster reefs is determined by a number of factors: predation and substrate type limit settling success of recruiting spat (MacKenzie 1970), and food availability and salinity influence the health and fitness of individual oysters (Mackin 1959, Wilbur 1992). What is less clear is how oyster-reef structure and location--including the local salinity regime--shape the assemblages of organisms found there. Oysters grow and reproduce optimally at intermediate salinities: prolonged exposure to freshwater inhibits oyster growth (Shumway 1996, White & Wilson 1996), and higher salinity waters not only harbor a suite of marine predators (Dame et al. 1984) but are also correlated with an increased susceptibility of oysters to the potentially lethal parasite Perkinsus marinus (Chu & Volety 1997, La Peyre et al. 2003). Sprinkel (1986) found mean oyster densities ranging from 116.2-659.2 [m.sup.-2] on nearshore reefs, from 49.2-740.8 [m.sup.-2] on mid reefs, and from 4.8457.2 [m.sup-2] on offshore reefs in several estuaries along the central Gulf coast of Florida. In his study (Sprinkel 1986), sampling was conducted beginning from the mouth of each tidal river and extending offshore. Based on oyster density and other metrics, Sprinkel (1986) also reported a decreasing trend of "successful oyster-producing reefs with greater distance offshore."

In comparison, oyster densities measured in Southwest Florida were generally greater than those reported by Sprinkel (1986): mean densities ranged from 251-1,387 [m.sup.-2] for reefs within tidal rivers, from 1,148-1,548 [m.sup.-2] for reefs at the mouths of tidal rivers, and from 467-1,487 [m.sup.-2] for reefs located downstream from the tidal river mouths. In the Estero, a much smaller watershed than the other two systems and with limited freshwater input (mean salinity 30.53 psu), oyster densities were generally high but did not vary significantly among stations. In the Caloosahatchee and the Faka-Union, both of which experience considerably higher freshwater input, oyster densities were low upstream. In the Caloosahatchee (mean salinity 25.43 psu), oyster abundance was highest at the intermediate site sampled along the salinity gradient and was reduced once again at the station farthest downstream. In the Faka-Union, which has a highly augmented watershed and which experiences the greatest degree of freshwater input of the estuaries examined (mean salinity 22.24 psu), oyster densities were significantly greater at the two downstream stations. May (1972) also reported that reefs in upper Mobile Bay, which is subjected to severe freshets, had reduced oyster densities compared with those downstream.

Although oyster density and the biomass of oyster-reef associates were significantly higher at the two downstream stations in the Faka-Union during the wet season, in general the density of living oysters present at each site failed to explain the observed patterns in community metrics. It should be noted that oyster size was not considered in this study. May (1974) also reported that there was little evidence relating mud crab (Xanthidae) abundance to oyster density on Alabama reefs.

In contrast, the spatial and seasonal patterns detected in oyster-reef community metrics suggest that salinity plays an important role in structuring the assemblage of decapods and fishes found on Southwest Florida reefs. A number of community metrics varied upstream to downstream in this study. These patterns were strongest in the Caloosahatchee where greater between-station distances resulted in a greater distinction among stations with respect to salinity. The Caloosahatchee, with a watershed area 22 times that of the Estero and 7 times that of the Faka-Union, also exhibited significantly greater species richness, possibly reflecting the much larger geographic area of this system. MacArthur and Wilson (1967) proposed a mechanism explaining the greater number of species present on larger islands compared with smaller ones, and more recently Koel (1997) found that species richness of stream fishes in the Red River basin was correlated with both stream length and watershed drainage area.

There are two sources of seasonal programming at work in Southwest Florida estuaries: seasonal variation in air and water temperature and seasonal variation in rainfall and water releases. When they interact, salinity and temperature can also confound the interpretation of results: seasonal rains typically occur from mid-June through mid-October in Southwest Florida resulting in reduced salinities at a time of elevated water temperatures. The density of reef-resident decapods and fishes was significantly higher in all three estuaries during the wet season. This increase in abundance observed during the wet season is likely a result of the substantial recruitment of young Petrolisthes armatus that occurs during this time of the year. In contrast, evidence from the Caloosahatchee and Estero indicate that biodiversity is greater during the dry season, prior to the onset of seasonal rains. Limitations to the upstream distribution of oyster-reef organisms due to reduced salinities (Wells 1961) would be expected to extend even farther downstream during the wet season when estuarine-wide salinities are reduced. In addition, juveniles of a number of fishes can be found on Southwest Florida oyster reefs during the spring (e.g., Lagodon rhomboides, Archosargus probatocephalus, Eucinostomus sp., Bairdiella chrysoura). Both of these influences would tend to increase diversity during the dry season.

Among the metrics calculated for oyster reefs in Faka-Union Bay, only organism density exhibited any significant seasonal variation, with higher densities occurring during the wet season. The difference in mean salinity between wet and dry seasons in this system was a striking 22.03 psu, suggesting that this system was more highly influenced by seasonal rains compared with either the Caloosahatchee or Estero.

Water-resource management increasingly involves the identification and conservation of important habitats. From a fisheries perspective, Stalnaker et al. (1995) suggest that for management to be effective, "fishery resource managers must become water and habitat managers." From an ecosystem perspective, Beck and Odaya (2001) proposed that the most effective way to conserve biodiversity in the Gulf of Mexico is to focus on key habitats--including oyster reefs--"and on the ecological processes that affect their variability." Oysters and the reefs they form are managed according to one or both of these perspectives in different parts of the United States. In Southwest Florida, where oysters are not harvested as a food resource, ecologic function is of greater interest.

In terms of managing freshwater flow into estuarine systems, Mattson (2002) reviewed the use of community metrics as a measure of habitat quality that can be compared with such factors as salinity to understand the functional relationships involved. Salinity is itself a characteristic of estuarine habitat and is invaluable in the management of freshwater inflows "because it is well-defined and measurable, has ecological significance, integrates a number of important estuarine processes and properties, and is meaningful to a large number of constituencies" (Jassby et al. 1995).

The regulation of freshwater flow into Southwest Florida estuaries can be a useful tool for providing suitable habitat for oysters. For example, La Peyre et al. (2003) proposed the management of freshwater inflow into the Caloosahatchee estuary to reduce infection intensities of the oyster parasite Perkinsus marinus. However, salinity fields appropriate for the maintenance of healthy oysters, which tend to favor intermediate salinities, may not necessarily be coincident with salinity fields that maximize density, biomass, or biodiversity of oyster-reef organisms. It is therefore paramount to consider the responses of oysters and of reef-resident organisms to salinity not only when managing or altering freshwater inflow in estuaries but also when selecting suitable locations for oyster-reef restoration and enhancement.
TABLE 1. Decapod crustaceans and fishes collected on Southwest
Florida oyster reefs.

 Species Common Name

Decapods
 Alpheus heterochaelis Bigclaw snapping shrimp
 Eurypanopeus depressus Flatback mud crab
 Libinia dubia Longnose spider crab
 Menippe mercenaria Florida stone crab
 Palaemonetes sp. Grass shrimp
 Panopeus sp. Mud crab
 Penaeidae Commercial shrimp
 Petrolisthes armatus Green porcelain crab
 Portunas gibbesii Iridescent swimming crab
Fishes
 Archosargus probatocephalus Sheepshead
 Bairdiella chrysoura Silver perch
 Bathygobius soporator Frillfin goby
 Chasmodes saburrae Florida blenny
 Cyprinodon variegatus Sheepshead minnow
 Eucinostomus sp. Mojarra
 Gobiesox strumosus Skilletfish
 Gobiosoma bosc Naked goby
 Gobiosoma robustum Code goby
 Hypsoblenius hentz Feather blenny
 Lagodon rhomboides Pinfish
 Lophogobius cyprinoides Crested goby
 Lupinoblennius nicholsi Highfin blenny
 Lutjanus griseus Gray snapper
 Lutjanus synagris Lane snapper
 Opsanus beta Gulf toadfish

 Number Relative
 Collected Occurrence

Species CAL EST FUC CAL EST FUC
 Decapods
 Alpheus heterochaelis 107 49 41 F C C
 Eurypanopeus depressus 3442 2525 3040 F F F
 Libinia dubia 3 0 7 U U
 Menippe mercenaria 19 11 0 U U
 Palaemonetes sp. 43 6 0 C R
 Panopeus sp. 165 272 284 F F F
 Penaeidae 24 1 5 C R U
 Petrolisthes armatus 1343 3595 5136 F F F
 Portunas gibbesii 1 0 1 R R
Fishes
 Archosargus probatocephalus 1 4 4 R R U
 Bairdiella chrysoura 13 0 0 R
 Bathygobius soporator 3 0 3 U U
 Chasmodes saburrae 62 2 9 C R U
 Cyprinodon variegatus 0 4 0 R
 Eucinostomus sp. 16 22 32 U U U
 Gobiesox strumosus 59 14 42 F C C
 Gobiosoma bosc 2 0 0 R
 Gobiosoma robustum 177 7 56 F U C
 Hypsoblenius hentz 6 0 0 U
 Lagodon rhomboides 5 9 4 U U U
 Lophogobius cyprinoides 0 112 11 C U
 Lupinoblennius nicholsi 2 1 1 R R R
 Lutjanus griseus 0 5 4 U U
 Lutjanus synagris 0 3 0 R
 Opsanus beta 46 27 14 C C U

(CAL = Caloosahatchee; EST = Estero; FUC = Faka-Union Canal). Relative
occurrence in each estuary was scored as follows: frequent F = present
in >50% of samples; common C = present in 20% to 50% of samples;
uncommon U = present in 5% to 20% of samples; rare R = present in <5%
of samples.

TABLE 2. Between-season comparisons of metrics related to oyster-reef
associated decapods and fishes as well as environmental factors
considered during the study. Data for each season are presented as
mean values for all stations with standard deviation in parentheses.

 Season
 Response
 Variable System Dry Wet

Biomass (g WM) Caloosahatchee 48.87 (27.07) 35.02 (14.77)
 Estero 55.20 (33.30) 40.22 (22.38)
 Faka-Union 48.48 (25.50) 40.15 (27.76)
Density Caloosahatchee 78.63 (25.01) 143.25 (60.62)
 ([m.sup.-2]) Estero 95.96 (62.52) 182.65 (145.35)
 Faka-Union 121.00 (95.04) 201.07 (181.43)
Diversity (H') Caloosahatchee 1.106 (0.553) 0.737 (0.427)
 Estero 0.994 (0.219) 0.830 (0.143)
 Faka-Union 0.855 (0.373) 0.797 (0.273)
Dominance (%) Caloosahatchee 64.33 (21.95) 74.00 (17.88)
 Estero 58.05 (8.84) 63.49 (8.43)
 Faka-Union 67.84 (14.61) 67.89 (12.52)
Richness Caloosahatchee 7.3 (3.0) 6.0 (2.2)
 (no. species) Estero 4.5 (1.2) 4.2 (1.3)
 Faka-Union 4.8 (2.4) 4.4 (1.6)
STENO:EURY Caloosahatchee 0.46 (0.65) 0.56 (0.71)
 Estero 0.86 (0.84) 1.14 (1.02)
 Faka-Union 1.66 (1.79) 1.35 (1.64)
Salinity Caloosahatchee 32.65 (5.13) 15.90 (9.18)
 Estero 36.51 (3.20) 25.29 (10.10)
 Faka-Union 31.72 (9.87) 9.69 (7.86)
Temperature Caloosahatchee 26.40 (5.60) 30.45 (1.16)
 Estero 26.83 (2.01) 29.08 (1.79)
 Faka-Union 28.75 (0.81) 29.82 (0.83)

 Response Degrees
 Variable System F-statistic P-value Freedom

Biomass (g WM) Caloosahatchee 4.962 0.031 50
 Estero 3.324 NS 48
 Faka-Union 1.318 NS 53
Density Caloosahatchee 23.687 0.000 50
 ([m.sup.-2]) Estero 7.031 0.013 48
 Faka-Union 4.127 0.049 53
Diversity (H') Caloosahatchee 6.981 0.011 50
 Estero 9.341 0.004 48
 Faka-Union 0.423 NS 53
Dominance (%) Caloosahatchee 2.931 NS 50
 Estero 4.813 0.033 48
 Faka-Union 0.000 NS 53
Richness Caloosahatchee 3.143 NS 50
 (no. species) Estero 0.774 NS 48
 Faka-Union 0.754 NS 53
STENO:EURY Caloosahatchee 0.271 NS 50
 Estero 1.100 NS 48
 Faka-Union 0.443 NS 53
Salinity Caloosahatchee 28.183 0.000 20
 Estero 13.479 0.003 23
 Faka-Union 32.485 0.000 20
Temperature Caloosahatchee 4.047 NS 19
 Estero 7.692 0.012 21
 Faka-Union 6.725 0.020 17


ACKNOWLEDGMENTS

The authors thank Peter Doering of the Southwest Florida Water Management District (SFWMD) for his advice; Ananta Nath and Tomma Barnes of the District for their input; Sharon Thurston and Erin Rasnake for coordinating sampling and analysis; and a small army of undergraduate students and interns--Ben Andrews, Sherith Bankston, Mike Chichester, Julie Farineau, Rashel Grindberg, Matt Hooper, Ben Jacobs, Cecile Jauzein, Emily Lindland, Christy Linardich, Cedric Loret, Angelina Ruttan, Lacey Smith, and Jay Standiford--who contributed significantly to the field and laboratory work. Many of these students were supported by Congressional Grant P1 16Z010066 awarded through the US Department of Education. In addition, preliminary collections using Hester-Dendy samplers were performed by Arielle Poulos and Lesli Haynes as part of their senior research projects. The authors also thank Rebecca Totaro for her careful review of the manuscript; and Loren Coen of the South Carolina Department of Natural Resources, Mark Luckenbach of the Virginia Institute of Marine Science, Martin Posey of the University of North Carolina, Wilmington, and Roy Crabtree of the National Marine Fisheries Service, St. Petersburg, Florida, for their valuable suggestions and comments regarding the sampling of oyster-reef organisms. This work was supported by SFWMD grants C-12412-A1 and C-13252 and by an internal grant to the first author from Florida Gulf Coast University.

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S. GREGORY TOLLEY, * ASWANI K. VOLETY AND MICHAEL SAVARESE

Florida Gulf Coast University, College of Arts and Sciences, 10501 FGCU Blvd South, Fort Myers, Florida 33965

* Corresponding author. E-mail: gtolley@fgcu.edu
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