Freshwater inflow effects on larval fish and crab settlement onto oyster reefs.
KEY WORDS: freshwater inflow, oyster reef settlement, larval distribution, decapods, fishes
Access to oyster reef (Crassostrea virginica) habitat by larvae of resident organisms is largely a function of any transport/retention mechanisms at work. Because net estuarine flow is seaward, estuarine species with meroplanktonic larvae have developed behavioral mechanisms that enhance retention and/or immigration and subsequent settlement to adult habitats (Olmi 1995). Such mechanisms exploit the 2-layer estuarine circulation that results from the interaction of freshwater inflow and tides. Brachyuran crabs inhabiting oyster reefs are either retained within estuaries, relying on vertical migration to limit dispersal downstream (Latz & Forward 1977, Cronin 1982, Lambert & Epifanio 1982, Sulkin et al. 1983, Morgan 1987), or, if exported to coastal waters, immigrate up estuary to appropriate settlement habitats using vertical migration and floodtide transport (Tankersley et al. 1995, Christy & Morgan 1998, Welch & Forward 2001, Tankersley et al. 2002). Larvae of some oyster reef-resident fishes may also rely on retention strategies, seeking out low-flow conditions to maintain position near reefs (Breitburg et al. 1995).
Less well studied is the potential influence of extreme freshwater inflow (i.e., freshets, large freshwater releases resulting from water management practices) on larval distribution and subsequent settlement to benthic habitats, including oyster reefs. Southwest Florida estuaries are subjected to seasonal extremes in precipitation, resulting in highly variable freshwater inflow. Such extremes of inflow may not only limit the distribution and abundance of organisms inhabiting oyster reefs through osmoregulatory challenges (Wells 1961), but also may impact the settlement of larvae onto reefs in the first place. Elevated freshwater inflows can advect larvae downstream and away from oyster reefs, thus creating a spatial gap between larval distribution and settlement habitats.
Because export of larvae from an estuary can potentially decouple local larval production from successful recruitment to nearby reefs (see Caley et al. 1996), larval dispersion (i.e., increased connectivity among oyster reef habitats) must be balanced by larval import or retention to maintain populations locally (Boylan & Wenner 1993). Decapods inhabiting oyster reefs exhibit complex life histories, with different developmental stages responding differently to environmental cues (Forward 1990, Forward & Tankersley 2001, Lopez-Duarte & Tankersley 2009). For example, zoeae of both the blue crab Callinectes sapidus and fiddler crabs Uca spp. develop in the lower estuary or on the continental shelf; yet, megalopae of these species use selective tidal stream transport to migrate up estuary to settlement habitats (Tankersley et al. 1995, Hasek & Rabalais 2001). It is unclear to what degree the larvae of decapods inhabiting oyster reefs are exported out of the estuary and onto the continental shelf, and subsequent up-estuary movement by postlarvae may not be present in all species (Christy & Morgan 1998).
This study investigates the influence of freshwater inflow on the oyster reef commensal community (i.e., resident decapods and fishes) and examines settlement within the context of variable freshwater inflow. Specifically, this study (1) examines relationships between freshwater inflow and the abundance of specific life stages (planktonic larvae, juveniles, and adults) of organisms residing on oyster reefs, (2) identifies seasonality in both the spawning of oyster reef organisms and subsequent recruitment to reefs, (3) investigates whether living oyster density influences the abundance and diversity of reef-resident organisms and whether these measures are linked to freshwater inflow, and (4) evaluates the potential for settlement on oyster reefs resulting from variation in proximity of planktonic larvae to settlement habitats.
MATERIALS AND METHODS
Estero Bay receives freshwater inflow from a number of small estuarine tributaries: Hendry and Mullock creeks to the north, the Estero River and Spring Creek to the east, and the Imperial River to the south (Fig. 1). Of these, the Imperial River, Mullock Creek, and the Estero River represent the largest watersheds, with drainage basins of 218 [km.sup.2], 206 [km.sup.2], and 182 [km.sup.2] (Byrne & Gabaldon 2008), respectively. The remaining tributaries have drainage basins of less than 50 [km.sup.2] each.
The bay experiences extreme seasonal variation in freshwater inflow. Seasonal rains coupled with tropical weather systems reduce salinities in the bay June through September, and hypersaline conditions can occur late in the dry season (April and May). Estero Bay is shallow (mean depth, 0.9 m (Byrne & Gabaldon 2008)) and microtidal, and the oyster reefs it contains are distributed intertidally, exhibiting limited vertical relief (<1 m). Effective tidal and wind mixing limits density stratification to deeper channels. Oyster reefs here are concentrated in the northern part of the bay, especially downstream of the confluence of Hendry and Mullock creeks (Fig. 1).
Collection of Larval Oyster Reef-Resident Fishes and Decapods
Sampling of larval oyster reef commensal fishes and decapods was conducted at 16 fixed locations distributed across the survey area: the open waters of Estero Bay, inside the passes connecting the bay to the Gulf of Mexico, and near the mouths of estuarine tributaries, including Hendry and Mullock creeks and the Estero and Imperial rivers (Fig. 1, Table 1). Stations were sampled monthly using plankton nets for 24 mo beginning January 2005. Each month's sampling was conducted over a 2-day period under daytime flood-tide conditions. Flood-tide sampling favored the collection of larvae invading the survey area using flood-tide transport or attempting to maintain position using other retention strategies. A total of 384 plankton samples (24 mo x 16 stations) were collected during the survey.
Zooplankton were sampled using a 500-[micro]m Nitex mesh, 0.5-m mouth diameter, conical plankton net (see Peebles et al. 2007) equipped with a mechanical flowmeter. Net deployment consisted of a 3-step oblique tow that divided the net's fishing time equally among bottom, mid-depth, and surface waters. Tow duration and speed were 5 min and 1.0-1.5 m/sec. Organism abundance was normalized to volume of water filtered as calculated from flowmeter readings. Ichthyoplankton and invertebrate zooplankton were fixed in the cod-end jar by adding 50 mL formalin (37% formaldehyde) to 500-700 mL ambient water. Samples were transported to the laboratory, rinsed, and transferred to 50% isopropanol for preservation.
Organisms were identified to the lowest taxon practical, which included developmental stage. Most taxa were enumerated by examining successive aliquots under a stereomicroscope. For taxa too abundant to enumerate directly, abundances were estimated from a ~30-50-mL subsample decanted immediately after multiple inversions of the entire sample. Inversions were made in a 1,000-mL graduated cylinder, allowing the calculation of a multiplier value for converting counts from subsamples to counts estimated for the entire sample.
Collection of Juvenile and Adult Oyster Reef-Resident Fishes and Decapods
Oyster reef-resident fishes and decapods were sampled from reefs located near the mouths of 5 of the bay's estuarine tributaries (Estero and Imperial rivers, and Hendry, Mullock, and Spring creeks; Table 1), each experiencing a different amount of freshwater inflow. Compared with oyster reef sampling conducted along estuarine salinity gradients (Tolley et al. 2005, Tolley et al. 2006), the current approach was more compatible with zooplankton sampling, as the locations of reefs sampled exhibited some spatial overlap with the geographical coverage of zooplankton sampling. Reefs were sampled monthly using lift nets for a period of 24 mo beginning March 2005 (2 mo after the start of plankton-net surveys), producing a total of 360 samples (24 mo x 5 sites x 3 replicates). Lift nets (0.5 [m.sup.2] with 6.4-mm side mesh and 1.6-mm bottom mesh) were selected to target small reef and facultative residents (Breitburg 1999). For each sampling effort (station and month), lift nets were deployed in triplicate at mean low water on living oyster reefs (Tolley et al. 2005, Tolley et al. 2006). After an area of bottom approximating 0.5 [m.sup.2] was cleared of all oyster shell, the lift net was deployed and filled with approximately 3-L volume displacement of oyster clusters taken from adjacent areas of the reef. On net retrieval after ~30 days, oyster clusters were removed, and associated fishes and decapods were extricated. Target organisms remaining in the net were also removed. Organisms were transported on ice to the laboratory, where all fishes and decapods were identified to the lowest taxon practical, measured to the nearest 0.1 mm (fishes, standard length; crabs, carapace width; shrimp, carapace length), and weighed to the nearest 0.01 g wet mass.
Water-quality profiles were taken after each plankton-net tow using a calibrated multiparameter sonde. Temperature, salinity, and dissolved oxygen (DO) were measured at the surface (0.2 m), bottom, and 0.5-m intervals. Salinity, temperature, and DO measurements were also recorded (0.2 m depth only) on both deployment and retrieval of lift nets.
Inflow data for 4 of the 5 estuarine tributaries sampled--Estero and Imperial rivers, Spring Creek, and Ten Mile Canal, a major contributor to Mullock Creek were collected by the U.S. Geological Survey and provided by the South Florida Water Management District. Daily stream flows were averaged to estimate mean daily inflow (measured in cubic meters per second). Daily inflow data from all tributaries were summed and then averaged to serve as a proxy for total inflow entering the bay. This proxy underestimated actual inflow because of the existence of an ungauged watershed area and the unavailability of stream flow data for Hendry and Mullock creeks during a significant portion (17 mo) of the survey period.
Ambient oyster density (i.e., outside lift nets) was estimated by enumerating the number of living oysters contained in a 0.25-m quadrat. Each site was sampled once at the end of the survey period by enumerating oysters bounded by 4 randomly placed, replicate quadrats. Measured density was normalized to 1 [m.sup.2] of bottom. In addition, 50 oysters were selected randomly from each quadrat and measured to the nearest millimeter (longest hinge-to-lip distance).
Water-quality measurements associated with plankton-net tows were averaged vertically, whereas measurements taken at the beginning and end of replicate lift-net deployments were averaged temporally. Community metrics from lift-net data (abundance, biomass, species richness, diversity, and dominance) were examined using ANOVA. Homogeneity of variance was tested using the Levene statistic, with significant differences (P [less than or equal to] 0.05) resolved using multiple comparison tests (Day & Quinn 1989): Fisher's least significant difference in cases of equal sample size and equal variance, Hochberg's GT2 method in cases of unequal sample size but equal variance, and the Games-Howell test in cases of unequal variance. Unless otherwise noted, data are presented as mean [+ or -] SD.
Distribution and abundance of oyster reef commensal larvae were visualized via kriging using Surfer 8 (Golden Software, Inc.). Mean densities of oyster reef commensals (lift-net data) and their larvae (plankton-net data) were compared with freshwater inflow (measured in cubic meters per second) using Pearson's correlation; tributary-specific inflow was used with juvenile and adult densities on reefs and mean daily total estuarine inflow was used with larval densities. Flows were lagged at periods of 0-4 mo with l-mo time steps: a lag of 0 mo corresponded to inflow measured the day of sampling, a lag of 1 mo corresponded to inflow averaged over the 30 days prior to (or during, in the case of lift nets) sampling, a lag of 2 mo corresponded to freshwater inflow averaged over 31-60 days prior to sampling, and so on. Lags longer than 4 mo were not considered because they represented time periods older than the developmental stages of the majority of organisms examined and therefore were not likely to be meaningful biologically. Lagged flows were averaged over 30-day periods to represent mean flow conditions coinciding with monthly sampling frequency.
Freshwater Inflow and Water Quality
Inflow generally tracked the annual oscillation between dry and wet seasons typical of Southwest Florida; however, the wet season began approximately 4-6 wk earlier in 2005 than in 2006, as reflected in both daily and cumulative inflow (Fig. 2). Total cumulative inflow was 42% higher in 2005 compared with 2006 as a result, in part, of the passage of Hurricane Wilma on October 24, 2005. Inflow increased substantially at this point and continued above 28 [m.sup.3]/sec through early November.
Inflow data from August 2004 through September 2005, which included data from all tributaries, indicated that Mullock Creek and its tributary Ten Mile Canal contributed the majority of the bay's inflow (52%), followed by the Imperial River (35%). In contrast, the Estero River and Spring and Hendry creeks contributed 10% or less to the total. As a result of the confluence of the high-flow Mullock and low-flow Hendry creeks, lower Hendry Creek (including the lift-net site) was subjected to reduced salinities during wet months despite low levels of inflow delivered by Hendry Creek itself.
Mean salinity was lower and SD higher at plankton stations associated with estuarine tributaries (Table 1), and salinity was typically lower in the northern bay than the southern bay as expected as a result of higher inflow delivered by Mullock Creek and Ten Mile Canal. Salinity measured at lift-net stations was highest at Spring Creek, followed by the Estero River, and was lowest at Hendry and Mullock creeks (Table 1); salinity was greatly reduced during the rainy season, especially at the Hendry, Mullock, and Imperial sites.
Water temperature varied seasonally but differed little among stations (both plankton- and lift-net surveys; Table 1). DO concentrations less than 4 mg/L were recorded at 5 of the plankton stations, including 3 associated with estuarine tributaries: Hendry Creek, Imperial River, and Fish Trap Bay, a small, restricted bay located just downstream of the Imperial River (Table 1). In general, low DO was recorded from July to October; however, hypoxia (<2 mg/L) was not encountered during the plankton survey. No significant differences in DO concentrations were detected among lift-net stations (Table 1). Although DO less than 4 mg/L was recorded 1 or more times at each station (10.8%), hypoxic levels were recorded only once, at the Spring Creek site.
Ambient density of living oysters varied significantly among tributary-specific oyster reefs (F[4,14] = 6.32, P = 0.0034) and was higher at the Estero River (908 [+ or -] 167.4/[m.sup.2]) and Spring Creek (781 [+ or -] 254.8/[m.sup.2]) sites compared with the other sites (Hendry, 437 [+ or -] 66.2/[m.sup.2]; Imperial, 289 [+ or -] 78.2/[m.sup.2]; Mullock, 19 [+ or -] 13.9/[m.sup.2]). The number of living oysters per clump was also greater for the Estero River (6.2 [+ or -] 1.1) and Hendry (6.3 [+ or -] 1.0) and Spring Creek (6.8 [+ or -] 0.5) sites compared with the Mullock Creek (2.6 [+ or -] 0.8) and Imperial River (2.3 [+ or -] 0.1) sites (F[4,14] = 7.71, P = 0.0017). Oysters ranged in size from 2.7-93.5 mm, but mean (and median) size did not vary by site. Although higher oyster densities were associated with higher mean salinities, the relationship was not significant; however, oyster density was related negatively to the coefficient of variation of salinity (r = -0.89, P = 0.04, n = 5).
Larval Composition (Plankton-Net Data)
Fishes were represented by 134 taxa and decapods by 105 taxa in the zooplankton samples. Fish and decapod data were subsequently filtered to exclude taxa that were not collected as juveniles or adults in the lift-net samples, taxa that were rarely encountered, and family-level taxonomic categories that would not sufficiently exclude nonoyster reef species (e.g., Alpheidae, Gobiidae). Among the remaining fishes (Table 2), Gobiesox strumosus larvae were the most abundant potential oyster reef recruits and, together with Gobiosoma spp. and Chasmodes saburrae larvae, represented 89% of the total oyster reef commensal fishes; however, these 3 taxa represented only 5% of the total number of larval fishes collected. Among the decapods (Table 2), Rhithropanopeus harrisii, Eurypanopeus depressus, Petrolisthes armatus, and Panopeus spp., in order of abundance, comprised 62% of the total collected and 98% of the decapods identified as oyster reef commensals. Florida stone crab Menippe mercenaria was also relatively abundant in the zooplankton catch, and juveniles are commonly found on Southwest Florida oyster reefs.
Larval Distribution and Seasonality (Plankton-Net Data)
Preflexion larval skilletfish G. strumosus were found primarily near or within estuarine tributaries (Fig. 3A), whereas the gobies Bathygobius soporator and Gobiosoma spp. and the Florida blenny Chasmodes saburrae were collected in the open bay. Zoeae of the Harris mud crab R. harrisii were also found primarily in association with estuarine tributaries (Fig. 3B) along with zoeae of E. depressus (Fig. 3C), but the latter were also abundant in open bay waters. Zoeae of the mud crabs Panopeus spp. were predominantly collected from open bay waters (Fig. 3D), and P. armatus zoeae occurred both in the open bay and near passes (Fig. 3E). Florida stone crab M. mercenaria zoeae were found primarily near passes (Fig. 3F).
The majority of species examined exhibited protracted spawning, with early developmental stages being present throughout the year or nearly so. For example, preflexion skilletfish were present in every month except September, and zoeae of flatback mud crabs were present in all months. Nonetheless, apparent spawning activity was not evenly distributed throughout the year. The decapods E. depressus and P. armatus exhibited peak larval densities in late spring (end of the dry season) and again in late summer/early fall (wet season; Fig. 4A, B). In contrast, R. harrisii larval density peaked in the month following the onset of the wet season before decreasing at the end of the wet season (Fig. 4C). Seasonal use of the estuary by larvae of the Florida stone crab was highly variable and inconsistent between years (Fig. 4D). Larvae of different oyster reef fishes were abundant at different times of the year (Fig. 5): The fish G. strumosus was more abundant in winter and spring; C. saburrae, in late spring and mid to late summer; Gobiosoma spp., during summer (wet) months; and B. soporator, toward the end of the wet season. This pattern was evident in 2005 data only, as peak larval densities in 2006 overlapped considerably.
Juvenile and Adult Composition on Reefs (Lift-Net Data)
Of the 11 species of decapods collected from Estero Bay oyster reefs, the xanthids E. depressus and R. harrisii and the porcellanid P. armatus were most abundant (Table 3), together representing 94% of the total decapod catch. Relatively abundant were the bigclaw snapping shrimp Alpheus heterochaelis and mud crabs of the genus Panopeus. Fishes were more diverse, with 15 species represented (Table 3). The crested goby Lophogobius cyprinoides was most abundant, followed by G. strumosus, gobies of the genus Gobiosoma, and the gulf toadfish Opsanus beta. Together these comprised 91% of the total fish catch. In general, decapods were at least an order of magnitude more abundant than resident fishes on the reefs, with 20,902 individuals collected compared with 1,423 fishes.
Variation in Juveniles and Adults Among Reefs (Lift-Net Data)
Three patterns of spatial distribution for oyster reef commensals were apparent (Table 3). First, E. depressus and the goby L. cyprinoides occurred commonly at all sites. Second, organisms found predominantly on reefs associated with reduced freshwater inflow and higher mean salinities (i.e., Estero River and Spring Creek) included P. armatus, Panopeus spp., A. heterochaelis, and Gobiosoma robustum. Juvenile M. mercenaria, although much less abundant, also exhibited this pattern and, similarly, O. beta was most abundant at the Estero River site. Third, in contrast, R. harrisii, G. strumosus, and Gobiosoma bosc were found primarily at the Hendry Creek, Mullock Creek, and Imperial River sites, sites associated with lower mean salinities.
Densities of juvenile and adult commensals varied considerably during the survey period, but some trends were apparent. Peak densities of both E. depressus and P. armatus generally occurred in late summer and late fall. Densities of R. harrisii also generally peaked in the summer and fall. Densities of fishes inhabiting oyster reefs did not exhibit any clear seasonal patterns with the exception of L. cyprinoides, which exhibited bimodal peaks during both years at the Mullock and Spring sites.
Community-Level Variation in Juveniles and Adults Among Reefs (Lift-Net Data)
Several community metrics varied among reefs (Table 4). Both mean abundance (density) and biomass were related positively to mean oyster density at each site (Fig. 6), with oyster density being a better predictor of commensal biomass ([R.sup.2] = 96%) than abundance ([R.sup.2] = 84%). Abundance, biomass, and richness were all greater at the Estero River and Spring Creek sites compared with the Hendry Creek, Mullock Creek, and Imperial River sites. Richness was also related positively to mean oyster density ([R.sup.2] = 84%, P = 0.0291, y = 3.2695 + 0.0032x). The Shannon index (H') was highest for samples collected from Spring Creek, followed by Estero River, Mullock Creek, Imperial River, and Hendry Creek. Last, dominance, the ratio of the abundance of the most abundant species present to total organism abundance (expressed as a percentage), was higher at the Hendry Creek and Imperial River sites and lowest at Spring Creek.
Inflow Relationships (Both Gear Types)
Comparing contour maps of salinity and larval density revealed differences in distribution that likely represented responses to freshwater inflow. For several species (E. depressus, Panopeus spp., and P. armatus), the center of organism distribution moved away from the influence of the high-flow northern and southern estuarine tributaries during the wet season and returned to the mouths of these tributaries during dry months. In contrast, larval M. mereenaria were concentrated primarily in moderate to high salinities, largely away from estuarine tributaries, and larval R. harrisii were found primarily in association with estuarine tributaries, even during wet months.
Although larval densities of E. depressus, Panopeus spp., and P. armatus were related negatively to inflow, time lags involved (3-4 mo) were outside the duration of larval development. In contrast, P. armatus larval density was related positively to inflow at a lag of 1 mo (r = 0.44, P = 0.0304). Density of R. harrisii larvae also exhibited a positive relationship with inflow, with best fits (based on correlation coefficients) present for lags of 1-2 mo (r = 0.71, P = 0.0001). The only fish to exhibit a relationship with freshwater inflow was G. strumosus, with density of preflexion larvae related negatively to inflow at lags of 0-1 mo (r = 0.49, P = 0.0157).
Juveniles and adults of several species also exhibited significant relationships with tributary-specific inflow (Table 5). Mean density of E. depressus recruits ([less than or equal to] 5 mm CW) at the Estero site was positively related to Estero River inflow at lags of 2-4 mo (Fig. 7A); recruits from the Imperial site were positively related to Imperial River inflow at lags of 2-4 mo (Fig. 7B); and recruits from the Spring site were positively related to Spring Creek inflow at a lag of 2 mo (Fig. 7C). Density of P. armatus, found primarily at the Estero and Spring sites, was also related to inflow, with recruits at the Spring site being positively related to Spring Creek inflow at lags of 0-3 mo (Fig. 8). In general, inflow relationships for newly settled recruits (carapace width, [less than or equal to] 5 mm) were stronger than those for individuals of all sizes (i.e., total density). The density of G. strumosus (all sizes) was related negatively to freshwater inflow at lags of 3-4 mo, but only for Mullock Creek samples.
Our central focus was to investigate the effect of freshwater inflow on the settlement of resident fishes and decapods to oyster reef habitat. Juveniles and adults of many of these species were found either in association with reefs near the mouths of low-flow tributaries (Estero River, Spring Creek) or high-flow tributaries (Hendry Creek, Mullock Creek, Imperial River), but not both (Hendry Creek is grouped with high-flow tributaries here because of the influence of Mullock Creek). Using multivariate analysis, Tolley et al. (2006) found that community structure on reefs exposed to reduced freshwater inflow and higher salinities was distinct from that on reefs experiencing greater inflow and lower mean salinities. Although salinity and its covariates have been identified as being important factors in shaping oyster reef communities (Wells 1961, Tolley et al. 2005, Tolley et al. 2006), the impact of variable freshwater inflow on larval transport and settlement on oyster reef habitat is less well understood. The results of this work suggest that the interaction of variable freshwater inflow and larval transport mechanisms also has the ability to shape community structure on oyster reefs by influencing settlement locally (i.e., supply-side ecology; see Lewin (1986)).
Densities of larval E. depressus and P. armatus peaked at the end of the dry season, fell precipitously with the onset of the wet season (which differed in timing between years), and then increased at seaward locations throughout the remainder of the wet season. In contrast, R. harrisii larval density peaked immediately after the onset of the wet season. The decapod R. harrisii often inhabits the extreme upper end of estuaries (Wurtz & Roback 1955, Ryan 1956), and it is likely that the center of abundance for the spawning population was located upstream of the survey area, as indicated by juvenile and adult distributions. Larvae spawned upstream would be advected downstream into the survey area by high wet-season inflows, or the spawning population may have expanded downstream during the wet season. Either process would result in increased larval densities near reefs.
Larval densities of R. harrisii and G. strumosus were related positively to inflow at lags of 0-2 mo, which brackets the duration of larval development. The upper range of these lags may reflect a recruitment response to inflow resulting from variable survival rates related to bottom-up processes (i.e., food availability). However, because peak densities of these 2 species occurred within or near the mouths of tidal tributaries, shorter lags may simply reflect increased catchability as larvae were advected downstream and into the study area (for discussions of inflow mechanisms see Drinkwater and Frank (1994), Peebles (2005), and Robins et al. (2005)).
Densities of juvenile and adult P. armatus and E. depressus occurring on oyster reefs were also related positively to freshwater inflow typically at lags of 2-4, mo, which approximates the age of juveniles. As planktonic larvae, commensals are trophically dependent on plankton production, which can be enhanced by freshwater inflow through nutrient loading. As a juvenile and adult, E. depressus is an omnivore (McDonald 1982) known to feed on small oysters and other bivalves (McDermott 1960, Milke & Kennedy 2001) as well as on algae and detritus (Gibbons & Blogoslawski 1989). Although P. armatus is primarily a filter feeder, it can feed on detrital food sources through deposit feeding (Caine 1975, Kropp 1981). For these species, bottom-up processes resulting from increased inflow would have increased food availability, thus enhancing survival or growth of recruits on the reefs.
Egg-to-egg generation times for some xanthid and porcellanid crabs allow for the production of multiple generations per year (McDonald 1982, Hollebone & Hay 2007), and females have the ability to carry more than 1 brood per year (Hines 1982, McDonald 1982, Lardies et al. 2004). Likewise, fishes distributed at low latitudes often exhibit protracted spawning, with multiple batches produced annually (Houde 1989). What can result is a nearly continuous supply of larvae in the water column available for settlement on oyster reefs (as evidenced herein). Successful recruitment to reefs is, therefore, not only dependent on spawning stock reproductive capacity (including stock size), spawner condition, and food availability (Peebles et al. 1996), but also on the presence of conditions favorable for settlement, as revealed by relationships between abundance and inflow.
Comparisons of larval distributions between dry and wet seasons indicate that larvae of the dominant oyster reef crabs E. depressus, Panopeus spp., and P. armatus moved toward the center of the bay and seaward during the wet season, being positioned away from oyster reefs associated with the mouths of high-flow northern and southern tributaries. This movement also placed these larvae closer to the mouth of low-flow Spring Creek and may have enhanced settlement on reefs there. The recruit--inflow relationship was more irregular and asymptotic for the Estero River located farther north, except during low flows when larvae would have been more proximal to the Estero River oyster habitat. The occurrence of high larval densities just inside the passes on an incoming tide suggests overall larval abundances were actually higher during the wet season, but this pattern may have been obscured as a result of the export of larvae from the study area and out into continental shelf waters. An alternate explanation is that reduced reproductive output upstream, resulting from salinity stress in adults, may have resulted in decreased larval densities during periods of high inflow.
Larval dispersal is an important mechanism in maintaining connectivity among oyster reef habitats, and seaward dispersal of larvae in estuaries has been identified as a potential adaptation for reducing predation in crab larvae (Christy & Morgan 1998). Yet, Cowen et al. (2000) suggest that larval exchange rates in demographically open populations are often overestimated and that local larval retention may be of "great importance" in maintaining population structure. Our data suggest that seaward dispersal of larvae resulting from extreme freshwater inflow creates a spatial gap between larval distribution and oyster reef habitat, and has the potential to cause inconsistent settlement. During high inflow rates, exported larvae would be required to immigrate back to Estero Bay from the Gulf of Mexico (i.e., larval densities near inlets were high during flood tides), whereas larvae retained within the bay would simply face longer migrations. This effect would be most pronounced for larval settlement on oyster reefs exposed to greater freshwater inflow and could result in spatial differences in recruitment.
Sulkin et al. (1983) demonstrated that postlarvae of E. depressus, a dominant member of oyster reef communities in Southwest Florida, exhibited negative geotaxis and a high degree of barokinesis; these authors suggested that this combination of behavioral responses should allow for precise depth regulation and might facilitate tidal stream transport. However, Christy and Morgan (1998) found no evidence for progressive upstream transport in postlarval E. depressus, even though such evidence was present for Panopeus herbstii, a congener of species found on Southwest Florida reefs. Although Tilburg et al. (2010) found no evidence for selective tidal stream transport in zoeae of P. armatus, another dominant decapod on Southwest Florida reefs, larvae were more abundant near the bottom, suggesting that larval retention was enhanced by net upstream tidal flow. Information regarding transport in post-larvae is lacking. It is therefore unclear whether these decapods have the ability to immigrate back into estuaries through tidal stream transport or some other mechanism (e.g., wind forcing) once exported out onto the continental shelf. Last, the most abundant decapod larvae in the study area--those of R. harrisii--were retained within the tidal rivers and upper estuary. In contrast to E. depressus, Panopeus spp., and P. armatus, there was no evidence in the current study that larvae of this species were ever exported out of the estuary. This latter pattern was also evident for larvae of the skilletfish G. strumosus.
Depending on temperature and salinity, total time of larval development (hatching through megalopa) in the laboratory is 11-41 days (20-30[degrees]C) for R. harrisii (Costlow et al. 1966, Christiansen & Costlow 1975) and 18-52 days (20-30[degrees]C) for P. herbstii (Costlow et al. 1962). Larvae displaced seaward and away from oyster reefs would therefore have a reduced window of opportunity within which to find substrate suitable for settlement. In the absence of suitable substrate, some crab larvae delay metamorphosis from the megalopa stage, thereby extending the opportunity for successful settlement; however, such delays come at the expense of reduced postsettlement survival and growth (Gebauer et al. 1999, Pechenik 2006).
Marine and estuarine fisheries yields have been correlated with freshwater inflows throughout the world (Drinkwater 1986, Day et al. 1989, Baisre & Arboleya 2006). Nutrient loading associated with increased freshwater input increases primary productivity within estuaries until the point where dystrophy (hypereutrophication) is reached (Caddy 1993). Yet increased inflow may also exacerbate the spatial separation between larvae and their settlement habitats, allowing extrinsic population factors such as aberrant drift and increased predation losses to play a greater role in determining ultimate settlement success.
This investigation also produced 2 observations that can be viewed within a broader ecological context. First, we observed living oyster density to be related positively to total density, biomass, and species richness of reef-resident crabs and fishes. This suggests living oyster density is a reasonable ecological indicator of the health of the oyster reef community; however, positive associations such as these may be inconsistent among estuaries (May 1974, Tolley et al. 2005, Kimbro & Grosholz 2006, Bergquist et al. 2006) and should be viewed with caution. This first observation serves as a reminder that recruitment success is dependent on habitat quality as well as quantity.
A second observation of general interest is the suggestion of resource partitioning in reef-resident fishes, as evidenced by the peaks in larval densities of different fishes occurring at different times of the year. Deegan and Thompson (1985) noted that juveniles of different estuarine fish species occupied Louisiana tidal creeks at different times of the year, creating the appearance of seasonal resource partitioning. Oyster reef-resident fishes may live several years (e.g., Malca et al. 2009), and abiotic seasonal cues may be responsible for the onset of spawning behavior in individual fish. However, of the oyster reef fishes identified in the current study, G. strumosus, C. saburrae, B. soporator, and G. bosc use empty oyster shell as sites for egg laying and nesting, with males guarding the attached eggs in some cases (Breder 1943, Dahlberg & Conyers 1973, Peters 1981, Crabtree & Middaugh 1982, Peters 1983). If the supply of clean oyster boxes (empty, yet still articulated oyster shells) is limited, one species may preempt the use of oyster boxes by others during spawning, causing the larvae of different species to occur in the water column at different times of year. Such temporal resource partitioning may be further enhanced by size-specific selection of oyster shell in these fishes (Crabtree & Middaugh 1982). The pattern found in the current study, although evident only in the 2005 data, provides an example of interspecific influences on recruitment success.
The current study illustrates how interaction between stationary (oyster reef) and dynamic (inflow) habitat can be selectively beneficial or detrimental, depending on the autecologies of individual species. In effect, variation in dynamic habitat produces winners and losers. Winners in this case represent larvae that, despite high levels of freshwater inflow, are able to settle successfully on oyster reef habitat, and losers represent those larvae that, once exported out of the estuary and/or away from oyster reef habitat, may not have sufficient time (larval development) or behavioral adaptations (e.g., tidal stream transport) to immigrate up estuary and find suitable settlement habitat. This impact may be reduced substantially in species that are less dependent on stationary, structural habitats.
We thank P. Doering of the South Florida Water Management District (SFWMD), B. Chamberlain of the St. Johns River Water Management District, and B. Howard of the U.S. Environmental Protection Agency (USEPA) for their support, as well as the anonymous reviewer for valuable suggestions. Thanks also go to E. Patino, M. Byrne, and C. Price of the U.S. Geological Survey and to K. Haunert (SFWMD) for providing inflow data. Sampling of oyster reefs was conducted by A. Booth and E. Rasnake with the help of a number of undergraduate interns: W. Bluhm, J. Haner, A. Myers, A. Walthier, M. Westphal, and N. Wingers. We especially thank R. Miner for her help in processing oyster reef commensal samples and B. Denkert, M. Andresen, and J. Cook for their careful reviews of the draft manuscript. This work was funded by SFWMD grant RS040975 and by congressional grant X7-96403504-0 awarded through the USEPA.
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S. GREGORY TOLLEY, (1) * BETHANY M. BROSIOUS, (2) JAMES T. EVANS, III, (3) JENNIFER L. NELSON, (1) LESLI H. HAYNES, (1) LACEY K. SMITH, (2) SCOTT E. BURGHART (4) AND ERNST B. PEEBLES (4)
(1) Florida Gulf Coast University, Coastal Watershed Institute, 10501 FGCU Boulevard South, Fort Myers, FL, 33965; (2) Passarella and Assoctates, Inc., 13620 Metropolis A venue, Fort Myers, FL, 33912; (3) The City of Sanibel, 800 Dunlop Road, Sanibel, FL, 33957; (4) University of South Florida, College of Marine Science, 140 7th Avenue South, St. Petersburg, FL, 33701
* Corresponding author. E-mail: email@example.com
TABLE 1. Sampling locations and associated water quality data presented as mean with SD in parentheses. Latitude Station Description (N) Plankton sampling 1 Tributary (Hendry 26[degrees]27.847' Creek) 2 Tributary (Mullock 26[degrees]28.012' Creek) 3 Tributary (Hendry/ 26[degrees]27.114' Mullock) 4 Open bay 26[degrees]26.222' 5 Open bay 26[degrees]26.055' 6 Pass (Matanzas Pass) 26[degrees]26.039' 7 Open bay 26[degrees]25.206' 8 Tributary (Estero 26[degrees]25.784' River)/bay 9 Tributary (Imperial 26[degrees]20.199' River) 10 Tributary/Fish Trap 26[degrees]20.384' Bay 11 Open bay 26[degrees]21.434' 12 Open bay 26[degrees]22.487' 13 Pass (New Pass) 26[degrees]22.912' 14 Open bay 26[degrees]23.516' 15 Open bay 26[degrees]24.340' 16 Pass (Big Carlos Pass) 26[degrees]24.773' Lift-net sampling E Oyster reef (Estero 26[degrees]25.739' River) H Oyster reef (Hendry 26[degrees]28.000' Creek) I Oyster reef (Imperial 26[degrees]20.598' River) M Oyster reef (Mullock 26[degrees]27.916' Creek) S Oyster reef (Spring 26[degrees]22.808' Creek) Salinity Longitude ([per Station Description (W) thousand]) Plankton sampling 1 Tributary (Hendry 81[degrees]52.325' 15.75 (2.05) Creek) 2 Tributary (Mullock 81[degrees]51.653' 12.33 (2.28) Creek) 3 Tributary (Hendry/ 81[degrees]52.344' 22.89 (1.66) Mullock) 4 Open bay 81[degrees]52.760' 29.28 (0.84) 5 Open bay 81[degrees]53.615' 29.51 (0.73) 6 Pass (Matanzas Pass) 81[degrees]54.435' 27.76 (1.00) 7 Open bay 81[degrees]53.468' 32.49 (0.51) 8 Tributary (Estero 81[degrees]51.909' 28.96 (1.29) River)/bay 9 Tributary (Imperial 81[degrees]50.108' 13.24 (2.07) River) 10 Tributary/Fish Trap 81[degrees]50.544' 19.46 (2.19) Bay 11 Open bay 81[degrees]50.739' 27.71 (1.78) 12 Open bay 81[degrees]51.357' 33.36 (0.56) 13 Pass (New Pass) 81[degrees]51.357' 33.30 (0.68) 14 Open bay 81[degrees]50.969' 32.15 (0.72) 15 Open bay 81'51.338' 32.48 (0.58) 16 Pass (Big Carlos Pass) 81[degrees]52.640' 33.40 (0.48) Lift-net sampling E Oyster reef (Estero 81[degrees]52.002' 25.91 (8.36) River) H Oyster reef (Hendry 81[degrees]52.439' 15.73 (10.93) Creek) I Oyster reef (Imperial 81[degrees]50.570' 20.36 (11.47) River) M Oyster reef (Mullock 81[degrees]51.872' 15.26 (11.70) Creek) S Oyster reef (Spring 81[degrees]50.400' 29.28 (6.14) Creek) Temperature Dissolved Station Description ([degrees]C) Oxygen (mg/L) Plankton sampling 1 Tributary (Hendry 25.05 (0.86) 5.96 (0.32) Creek) 2 Tributary (Mullock 25.42 (0.82) 6.56 (0.30) Creek) 3 Tributary (Hendry/ 25.55 (0.93) 7.00 (0.36) Mullock) 4 Open bay 25.56 (0.96) 7.63 (0.39) 5 Open bay 25.65 (0.95) 7.92 (0.44) 6 Pass (Matanzas Pass) 25.88 (0.96) 7.32 (0.47) 7 Open bay 25.59 (0.95) 7.29 (0.38) 8 Tributary (Estero 26.32 (0.95) 7.94 (0.48) River)/bay 9 Tributary (Imperial 25.74 (0.81) 5.30 (0.23) River) 10 Tributary/Fish Trap 26.06 (0.87) 6.24 (0.33) Bay 11 Open bay 25.87 (0.99) 7.66 (0.29) 12 Open bay 25.63 (0.94) 7.72 (0.48) 13 Pass (New Pass) 25.83 (0.95) 8.06 (0.49) 14 Open bay 25.70 (0.98) 8.12 (0.45) 15 Open bay 25.80 (0.99) 7.81 (0.37) 16 Pass (Big Carlos Pass) 25.95 (0.95) 7.91 (0.45) Lift-net sampling E Oyster reef (Estero 26.09 (4.32) 6.07 (1.41) River) H Oyster reef (Hendry 26.41 (4.68) 6.19 (1.52) Creek) I Oyster reef (Imperial 26.60 (3.82) 6.07 (1.68) River) M Oyster reef (Mullock 26.22 (4.06) 6.42(l.45) Creek) S Oyster reef (Spring 26.96 (3.99) 6.67 (2.33) Creek) TABLE 2. Abundance of zooplankton taxa also collected as juveniles and/or adults on oyster reefs. Developmental No. Collection Taxon Stage Collected Frequency Decapods Palaemonetes spp. Mysis larva 5,044 321 Postlarva 2,266 153 Petrolisthes armatus Zoea 185,187 318 Juvenile 109 19 Adult 1 1 Callinectes sapidus Zoea 1 1 Megalopa 30 8 Juvenile 2 2 Adult 1 1 Eurypanopeus depressus Zoea 280,305 329 Panopeusspp. Zoea 100,291 251 Menippe mercenaria Zoea 8,831 102 Megalopa 2 1 Rhithropanopeus harrisii Zoea 298,376 188 Fishes Eucinostomus spp. Postflexion 112 31 Juvenile 4 2 Archosargus probatocephalus Postflexion 24 7 Juvenile 1 1 Lagodon rhomboides Postflexion 2 2 Juvenile 54 23 Chasmodes saburrae Flexion 258 60 Postflexion 116 37 Juvenile 1 1 Hypsoblennius spp. Flexion 3 1 Postflexion 3 1 Juvenile 1 1 Lupinoblennius nicholsi Flexion 17 9 Postflexion 15 9 Gobiesox strumosus Preflexion 1,347 151 Flexion 336 44 Postflexion 316 25 Juvenile 199 14 Bathygobius soporator Preflexion 129 42 Flexion 40 14 Postflexion 12 4 Gobiosoma spp. Postflexion 755 84 Gobiosoma robustum Flexion 4 2 Juvenile 1 1 Adult 1 1 Mean Maximum density Density (no. (no. Developmental [10.sup.3]/ [10.sup.3]/ Taxon Stage [m.sup.3]) [m.sup.3]) Decapods Palaemonetes spp. Mysis larva 200.0 7,763.4 Postlarva 83.0 6,352.7 Petrolisthes armatus Zoea 7,041.9 156,941.5 Juvenile 3.9 297.2 Adult <0.1 14.6 Callinectes sapidus Zoea <0.1 14.7 Megalopa 2.4 493.2 Juvenile <0.1 13.8 Adult <0.1 12.7 Eurypanopeus depressus Zoea 10,727.8 307,109.8 Panopeusspp. Zoea 3,739.5 87,932.8 Menippe mercenaria Zoea 337.1 16,426.8 Megalopa 0.1 28.5 Rhithropanopeus harrisii Zoea 10,066.4 463,708.1 Fishes Eucinostomus spp. Postflexion 4.0 226.8 Juvenile 0.2 33.9 Archosargus probatocephalus Postflexion 0.9 135.7 Juvenile 0.1 37.2 Lagodon rhomboides Postflexion 0.1 13.4 Juvenile 3.4 413.4 Chasmodes saburrae Flexion 10.2 537.6 Postflexion 4.3 303.8 Juvenile <0.1 12.0 Hypsoblennius spp. Flexion 0.1 51.3 Postflexion 0.1 43.4 Juvenile <0.1 11.7 Lupinoblennius nicholsi Flexion 0.6 38.1 Postflexion 0.5 65.7 Gobiesox strumosus Preflexion 49.8 2,551.0 Flexion 12.2 1,154.6 Postflexion 11.0 2,308.8 Juvenile 8.0 1,224.5 Bathygobius soporator Preflexion 4.6 142.4 Flexion 1.5 73.5 Postflexion 0.4 75.5 Gobiosoma spp. Postflexion 30.7 2,613.4 Gobiosoma robustum Flexion 0.1 35.8 Juvenile <0.1 11.8 Adult 0.2 82.7 TABLE 3. Abundance of juvenile and adult decapod crustaceans and fishes collected from Estero Bay oyster reefs using lift nets. No. Collected Species Common Name EST HEN Decapods Farfantepenaeus sp. Penaeid shrimp 63 0 Palaemonetes spp. Grass shrimp 41 0 Alpheus heterochaelis Bigclaw snapping shrimp 276 4 Petrolisthes armatus Green porcelain crab 2,767 3 Callinectes sapidus Blue crab 0 5 Eurypanopeas depressus Flatback mud crab 5,281 2,423 Menippe mercenaria Florida stone crab 27 0 Panopeus spp. Mud crabs 36 12 Panopeus lacustris Knotfinger mud crab 4 4 Panopeus obesus Saltmarsh mud crab 33 3 Panopeus simpsoni Oystershell mud crab 50 22 Rhithropanopeus harrish Harris mud crab 10 106 Fishes Opsanus beta Gulf toadfish 43 6 Gobiesox strumosus Skilletfish 3 29 Epinephelus itgiara Goliath grouper 0 1 Latianus griseus Gray snapper 2 1 Eucinostomus sp. Mojarra 2 2 Archosargus probatocephalus Sheepshead 1 4 Lagodon rhomboides Pinfish 0 3 Chasmodes saburrae Florida blenny 7 1 Hypsoblennius hentz Feather blenny 10 0 Lupinoblennius nicholsi Highfin blenny 2 1 Bath ygobius soporator Frillfin goby 4 8 Gobiosoma bose Naked goby 5 72 Gobiosoma robustum Code goby 27 2 Lophogobius cyprinoides Crested goby 33 83 Achirus lineatus Lined sole 1 0 No. Collected Species IMP MUL SPR Size (mm) Decapods Farfantepenaeus sp. 2 0 6 14.7 (7.9) Palaemonetes spp. 1 2 31 19.8 (9.9) Alpheus heterochaelis 7 6 191 23.7 (8.3) Petrolisthes armatus 24 3 1,574 5.2 (1.8) Callinectes sapidus 2 7 0 46.5 (22.3) Eurypanopeas depressus 2,618 1,464 2,866 8.5 (3.8) Menippe mercenaria 0 0 19 16.0 (13.0) Panopeus spp. 9 5 69 11.0 (8.3) Panopeus lacustris 2 0 8 15.0 (4.5) Panopeus obesus 3 2 66 21.5 (9.5) Panopeus simpsoni 43 1 112 20.3 (7.6) Rhithropanopeus harrish 97 488 4 5.3 (1.9) Fishes Opsanus beta 7 4 4 52.1 (25.5) Gobiesox strumosus 222 27 10 26.9 (5.0) Epinephelus itgiara 0 0 0 47.0 Latianus griseus 0 1 2 67.6 (20.9) Eucinostomus sp. 1 0 6 31.6 (13.7) Archosargus probatocephalus 0 7 10 46.7 (12.6) Lagodon rhomboides 2 0 1 34.5 (5.4) Chasmodes saburrae 3 1 8 27.2 (10.1) Hypsoblennius hentz 3 0 2 25.3 (8.4) Lupinoblennius nicholsi 1 0 0 40.7 (1.4) Bath ygobius soporator 21 4 6 48.0 (10.4) Gobiosoma bose 72 36 10 25.2 (5.0) Gobiosoma robustum 3 3 63 24.3 (6.0) Lophogobius cyprinoides 38 302 191 33.0 (8.7) Achirus lineatus 0 0 0 28.9 Size is presented as mean with SD in parentheses. EST, Estero River; HEN, Hendry Creek; IMP, Imperial River; MUL, Mullock Creek; SPR, Spring Creek (SPR). TABLE 4. Results of ANOVA presenting significant station effects for various metrics describing the assemblage of oyster reef commensals fishes and decapods. Station Estero Hendry Imperial Metric (n = 70) (n = 72) (n = 71) Density (no. 0.5 [m.sup.2]) * 125 (a) 39 (c,d) 45 (c) Biomass(g WM) 35.04 (a) 19.79 (b) 18.54 (b) Species richness * 6 (a) 4 (b,c) 4 (b) Diversity (H') * 0.878 (b) 0.472 (d) 0.658 (c) Dominance (%) * 64.1 (c) 87.0 (a) 80.8 (b) Station Mullock Spring Metric (n = 71) (n = 72) Density (no. 0.5 [m.sup.2]) * 34 (d) 73 (b) Biomass(g WM) 16.20 (b) 31.56 (a) Species richness * 4 (c) 6 (a) Diversity (H') * 0.810 (b) 1.114 (a) Dominance (%) * 66.3 (c) 56.1 (d) Metric F df P Density (no. 0.5 [m.sup.2]) * 222.24 4,296 <0.001 Biomass(g WM) 23.98 4,340 <0.001 Species richness * 58.49 4,296 <0.001 Diversity (H') * 64.53 4,296 <0.001 Dominance (%) * 77.82 4,295 <0.001 * Significant interaction present. Values represent means and superscripts identify significant differences among sites. WM, wet mass. TABLE 5. Significant relationships between density (no. 0.5 [m.sup.2]) of organisms collected on oyster reefs and lagged freshwater inflow. Lag Species Tributary (mo) r P Recruits ([less than or equal to] 5 mm) Petrolisthes armatus Spring 0 0.72 0.0001 1 0.77 <0.0001 2 0.85 <0.0001 3 0.57 0.0035 * Eurypanopeus depressus Estero 2 0.75 <0.0001 3 0.73 <0.0001 4 0.63 0.0010 * Imperial 2 0.51 0.0117 3 0.65 0.0006 * 4 0.47 0.0191 Spring 2 0.55 0.0052 * All size classes Petrolisthes armatus Spring 1 0.42 0.0427 2 0.57 0.0037 Eurypanopeus depressus Estero 2 0.56 0.0041 3 0.67 0.0004 4 0.57 0.0039 * Imperial 3 0.54 0.0065 * 4 0.54 0.0066 Mullock 3 0.45 0.0281 4 0.45 0.0274 * Gobiesox strumosus Mullock 3 -0.59 0.0022 * 4 -0.46 0.0236 * Lag Intercept Species Tributary (mo) a Slope b Recruits ([less than or equal to] 5 mm) Petrolisthes armatus Spring 0 1.12 0.431 1 0.85 0.525 2 0.66 0.609 3 4.11 0.364 Eurypanopeus depressus Estero 2 2.32 0.258 3 2.37 0.245 4 15.10 0.217 Imperial 2 -0.37 0.391 3 2.77 0.018 4 -0.27 0.367 Spring 2 3.33 0.432 All size classes Petrolisthes armatus Spring 1 2.87 0.103 2 2.78 0.147 Eurypanopeus depressus Estero 2 4.05 0.099 3 4.02 0.114 4 63.71 0.363 Imperial 3 29.62 0.047 4 2.80 0.172 Mullock 3 2.53 0.114 4 18.01 0.025 Gobiesox strumosus Mullock 3 0.60 -0.002 4 0.55 -0.002 * y = a + bx. Unless otherwise noted, relationships are of the type y = [ax.sup.b].
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|Author:||Tolley, S. Gregory; Brosious, Bethany M.; Evans, James T., III; Nelson, Jennifer L.; Haynes, Lesli H|
|Publication:||Journal of Shellfish Research|
|Date:||Aug 1, 2012|
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