2013 SOUTHEASTERN NATURALIST 12(1):171–196 Seasonality and Spatial Variation in Nekton Assemblages of the Lower Apalachicola River Robert Gorecki1,* and Matthew B. Davis1 Abstract - The results of statistical analyses conducted on monthly trawl and seine data collected from a multiyear fisheries-independent monitoring program, between July 2006 and March 2009 on the lower Apalachicola River, FL were used to determine which regions of the river, and what time of year should be further studied to determine if and how freshwater flow alterations affect the nekton. Differences in nekton community structure were clear between each of the 3 predefined habitat regions for shoreline seine catches. Deepwater trawled habitats showed a distinct difference between the upper river channel and both distributary regions, and less pronounced differences between the two distributary regions. The strongest pattern in seasonality of nekton communities coincides with seasonal fluctuations in recruitment of juveniles into the estuary and the periods of greatest salinity differences in the marshgrass-dominated lower distributary region for shoreline and deepwater habitats. Seasonal variations in community structure were evident and mostly likely dominated by recruitment, whereas the response of organisms to fluctuations in salinity may be dictated by their relative position within the lower reaches of the Apalachicola River system. Our results suggest that future studies of the effects of changes in flow on nekton assemblages in the lower Apalachicola River would best be performed during the dry season in the upper and lower portions of the distributary region. Introduction The Apalachicola River and Apalachicola Bay system in the Florida Panhandle is one of the most biologically productive and relatively undeveloped river-dominated estuarine systems in the southeastern United States. The lower Apalachicola River encompasses varying habitats from freshwater hardwood swamps to saltwater marshes (Livingston 1983). The area supports a thriving sport and commercial fishery that lands 90% of the state’s oysters and has the third-highest shrimp catch in Florida (Chanton and Lewis 1999). The Apalachicola River basin drains an area of roughly 30,900 km² and includes tributaries from Florida, Georgia, and Alabama. It has the greatest river flow rate in Florida and is the only river in Florida that originates in the Piedmont of the Appalachian Mountains. The alluvial freshwater flow of the Apalachicola River is the most important factor controlling seasonal nutrient and salinity levels within the Apalachicola Bay system (Livingston 1983). As the global population increases human needs and demands for freshwater continue to grow, the ecological consequences can be substantial if the needs of freshwater ecosystems and species are neglected in water-management programs ¹Fish and Wildlife Research Institute, Florida Fish and Wildlife Conservation Commission, 350 Carroll Street, Eastpoint, FL 32328. *Corresponding author- Bob.Gorecki@ myfwc.com. 172 Southeastern Naturalist Vol. 12, No. 1 (Richter et al. 2003). On the Apalachicola River and throughout the world, reduced periods of rainfall and increased water withdrawal in the upper portions of watersheds have raised concerns about the effects of reduced freshwater flow on biodiversity and freshwater ecosystems (Chanton and Lewis 1999, Livingston 1997, Livingston et al. 1997, Richter et al. 2003). Flow alterations due to anthropogenic actions and changing climate may reduce biological production and further harm already depleted populations in the Apalachicola River and Bay system (Livingston et al. 1997). The broad range of variability in physical and biological parameters of estuarine systems necessitates the use of long-term data sets in order to detect meaningful patterns and trends in each system (Tsou and Matheson 2002). Little is known about the diversity of the fish and mobile macroinvertebrates (nekton) in the tidal reaches of the Apalachicola River (FWC-FMRI 2000), despite previous nekton research conducted in Apalachicola Bay (Chanton and Lewis 1999; FDEP-FMRI 1998; Livingston 1997; Livingston et al. 1977, 1997; Subrahmanyam and Coultas 1980). We investigated seasonal and spatial variations in the nekton community of the lower Apalachicola River basin to assess what time of year and which regions of the river should be studied further to determine how and whether alterations in freshwater flow affect the nekton of the Apalachicola River. Study Area The sampling area is located in the lower Apalachicola River and its distributaries and was stratified into three regions based on differences in habitat and salinity (Fig. 1). The marshgrass-dominated lower distributary region (MLD), which begins at river mile 0 and extends to the tree-line on the river located near river mile 5, typically has higher salinities (0.2–11.4 ppt) than the other regions. The shoreline vegetation in this region consists of varying compositions of marsh grasses: Spartina spp. (cordgrass), Scirpus spp. (bulrush), Phragmites australis Cavanilles, Trinius, and Steudel (Common Reed), Typha spp. (cattails), and Cladium spp. (sawgrass). The bottom substrate consists of sandy mud with patchy areas of bottom vegetation including Chara spp. (muskgrass), Vallisneria spp. (tapegrass), and Ruppia spp. (widgeon grass). The forested upper distributary region (FUD), which begins at the tree-line (river mile 5) and extends to the point of division of the main river channel into the distributary portion of this river system (river mile 11), is characterized by mostly mixed hardwood, as well as cypress and tupelo swamp shorelines, with oligohaline conditions (0.1–4.7 ppt). Bottom substrate and vegetation are similar to those of the MLD region, except that Ruppia spp. was not detected. The upper river channel region (URC; river miles 11–22) is characterized by tupelo, cypress, and hardwood swamp, steeper banks, freshwater salinities (0.1 ppt), and an array of small narrow tributaries. Bottom substrates are similar to those of the other regions, while bottom vegetation is less widespread and consists mainly of Chara spp. and Limnophila 2013 R. Gorecki and M.B. Davis 173 sessiloflora Vahl and Blume (Asian Marshweed). While the URC is longer than the other two regions, this is largely due to there being a lack of visible differences in habitat and no measured differences in salinity throughout this region. The downstream extent of the URC is at the division of the main river channel into the distributary portion of this river system where the FUD begins, and the upstream extent of the URC at mile 22 coincides with the upstream extent of the tidal signal in this river system. Methods Sampling Nekton was sampled monthly from July 2006 through March 2009 with a 21.3-m center-bag seine (3.2-mm stretched mesh) and a 6.1-m otter trawl (38-mm stretched mesh and a 3-mm mesh liner) that targeted smaller fishes Figure 1. Map of the lower Apalachicola River s h o w i n g t h e three sampling r e g i o n s . T h e sampled waters are highlighted, and the black lines delineate borders between the 3 sampling regions. 174 Southeastern Naturalist Vol. 12, No. 1 (generally of 15–100 mm standard length). Seines were deployed in shoreline habitats with water depths ≤1.8 m; while some of the Apalachicola river system’s shoreline has steeper banks with water depths >1.8 m, these areas were not sampled with the seine due to the limitations of the gear. Trawl collections were deployed in deepwater habitats at depths of 1.8–7.6 m. Seine and trawl samples were collected monthly at sites selected according to a stratified random sampling design. Each of the three regions was divided into grid cells of 0.1′ lat × 0.1′ long, and 32 of these cells were randomly selected for sampling each month in the MLD (11 sites per month, 4 trawl tows, 7 seine hauls), FUD (9 sites per month, 3 trawl tows, 6 seine hauls), and URC (12 sites per month, 6 trawl tows, 6 seine hauls) regions. To sample the steep banks of the Apalachicola River and its distributaries, seines were deployed by boat in a semi-ellipse along the shoreline, and then hand-pulled onshore (approximate sampling area = 68 m²). The otter trawl was deployed in the river channel from a slowly moving vessel (0.5–1.0 m/s). Trawl tows were made against current flow, which varied with tide and wind conditions. Boat speed was adjusted in an attempt to cover 185 m during a 5-min tow, but actual tow distance was calculated from GPS coordinates. Catch per unit effort (CPUE) for both types of sampling gear are reported as number of animals per 100 m². All fish and selected invertebrates (recreationally or commercially important species, e.g., Litopenaeus setiferus L. [White Shrimp] and Callinectes sapidus Rathbun [Blue Crab]) were identified in the field to the lowest possible taxonomic level (usually species). Juveniles of several species (Farfantepenaeus spp. <15 mm postorbital head length, Lepomis spp. <20 mm standard length, Eucinostomus spp. <40 mm standard length, and Gobiosoma spp. <20 mm standard length) were identified only to genus because of difficulty in accurately identifying them to species in the field (e.g., Matheson and McEachran 1984). Additionally, all Menidia spp. and Brevoortia spp. were identified only to genus due to frequent hybridization and the impracticality of diagnostic characters (Duggins et al. 1986, Echelle and Echelle 1997, Greenwood et al. 2007). For each species collected on a given sampling trip, representative samples were retained to verify field identifications. Any specimens that could not be positively identified in the field were taken back to the laboratory for identification. Water chemistry data (temperature, salinity, dissolved oxygen, and pH) were recorded starting at a depth of 0.2 m and subsequently at every 1.0-m interval to just above the bottom. Mean values of measured water-chemistry parameters were used in analyses. Data analyses We conducted multivariate analyses to investigate the temporal and spatial patterns of nekton assemblages in the Apalachicola River (PRIMER v6, PRIMER-E Ltd., UK; Clarke and Gorley 2006). Data from trawl tows and seine hauls were analyzed separately utilizing square-root-transformed monthly mean densities (fish per 100 m²) to improve normality and reduce the 2013 R. Gorecki and M.B. Davis 175 influence of patchy but abundant species. Bray-Curtis similarities (Bray and Curtis 1957) were calculated for each pair of samples, and the relationships between all samples were displayed graphically using non-metric multidimensional scaling (MDS). MDS was also used to determine seasonal patterns in nekton assemblages in conjunction with CLUSTER analysis as shown in Clarke (1993). Two-way and one-way analysis of similarities (ANOSIM; explained in Clarke 1993) were used to examine temporal (i.e., seasonal) and spatial (i.e., river region) differences in nekton community structure. The sampling data were stratified into three regions based on the differences in habitat and salinity discussed earlier. Based on MDS plots and CLUSTER dendrograms (Figs. 2–5), sampling data were divided into seasons. For most habitats and river regions there were two seasons: the wet season (January– April) and the dry season (May–December). The cutoff for selecting clusters was set at 40% since this level seemed to show a temporal trend in both the CLUSTER dendrograms and the CLUSTER overlays on the MDS plots and showed three seasons: the wet season (January–April), a warm dry season (May–September), and a cool dry season (October–December). Shoreline habitats in the URC had three seasonal groupings with a cluster selection cutoff of 60% showing the best temporal trend. Thus, the monthly catch data dictated the division between seasons for further analyses, which correlated to apparent historic seasonal changes in water flow and rainfall according to the data in Light et al. (2006), while the URC seemed to also be influenced by other factors during the dry season. We used similarity percentage (SIMPER) analysis to determine characteristic groups of nekton, based on definitions provided by Deegan et al. (2000), for each region. Correlations between nekton abundance and environmental variables (measured during biological sampling) were investigated by comparing Bray-Curtis similarities with normalized salinity, dissolved oxygen, and temperature measurements using the BIOENV routine (explained by Clarke 1993) in PRIMER. Finally, a distance matrix was created for monthly samples following the methods of Tsou and Matheson (2002), wherein the distance between November and December and the distance between November and October was one, the distances between November and January and between November and September was two, etc. This matrix and the respective Bray-Curtis similarity matrix for each habitat (collection method) were then applied to the RELATE routine in Clarke and Warwick (2001) to investigate correlations between seasonal patterns and community structure, similar to the process used in Tsou and Matheson (2002) and Idelberger and Greenwood (2005). Utilizing this procedure, the greater and closer to 1 that ρ is indicates a stronger monthly relationship to changes in nekton abundance. Likewise, for smaller ρ-values temporal cycles of change in nekton abundance are much weaker. Comparing temporal relationships in this manner was done to obtain greater confidence in ANOSIM tests for seasonality due to the high stress levels on MDS plots. 176 Southeastern Naturalist Vol. 12, No. 1 Figure 2. Shoreline data [seined habitats] MDS plots displaying the relationship of nekton species composition by month for each region: a) marshgrassdominated lower distributary, b) forested upper distributary, and c) upper river channel, for seine catches in the lower Apalachicola River, J u l y 2 0 0 6 – March 2009 (2D stress = 0.18–0.21). The numbers represent months (1 = January, 2 = February, etc.). The circles overlaid on these plots represent g r o u p s f r o m cluster analyses that are ≥40% similar. 2013 R. Gorecki and M.B. Davis 177 Figure 3. Shoreline data [seined habitats] CLUSTER dendrograms displaying the relationship of nekton species composition by month for each region: a) marshgrassdominated lower distributary, b) forested upper distributary, and c) upper river channel, for seine catches in the lower Apalachicola River, July 2006–March 2009. The numbers along the x-axis represent months (1 = January, 2 = February, etc.) and symbols denote seasons. 178 Southeastern Naturalist Vol. 12, No. 1 Figure 4. Deepwater data MDS plots displaying the relationship of nekton species composition by month for each region: a) marshgrassd o m i n a t e d lower distributary b) forested upper distributary, and c) upper river channel, for trawl catches in the lower Apalachicola River, J u l y 2 0 0 6 – March 2009 (2D stress = 0.18–0.21). The numbers represent months (1 = January, 2 = February, etc.). The circles overlaid on these plots represent groups f rom clus ter analyses that are ≥40% similar. 2013 R. Gorecki and M.B. Davis 179 Figure 5. Trawl data, deepwater habitats, CLUSTER dendrograms displaying the relationship of nekton species composition by month for each region: a) marshgrass-dominated lower distributary, b) forested upper distributary, and c) upper river channel, for seine catches in the lower Apalachicola River, July 2006–March 2009. The numbers along the x-axis represent months (1 = January, 2 = February, etc.) and symbols denote seasons. 180 Southeastern Naturalist Vol. 12, No. 1 Results Water temperatures at sampling sites in the Apalachicola River ranged from 10.2 to 31.1 °C and were similar in all three sampling regions each season (Fig. 6a). The MLD region was the only region to show a seasonal salinity trend, typically low in winter (0.2–0.4 ppt) and higher throughout the rest of the year (1.2–11.4 ppt) (Fig. 6b). In the FUD region, salinity ranged from 0.1 to 4.7 ppt with a mean of 1.3 ppt, and salinity in the URC was consistent throughout the study at 0.1 ppt (Fig. 6b). Dissolved oxygen was similar in all regions, ranging from 4.4 to 10.9 mg/l (Fig. 6c). Dissolved oxygen minima coincided with periods of high temperature and elevated salinity. Figure 6. Variability in mean a) water temperature, b) salinity, and c) dissolved oxygen concentration recorded at the sample sites, July 2006–March 2009. Error bars indicate standard error. Data are divided by region: marshgrass-dominated lower distributary (MLD), forested upper distributary (FUD), and upper river chann el (URC). 2013 R. Gorecki and M.B. Davis 181 A total of 314,042 animals was collected from 626 seine hauls and 424 trawl tows (Table 1). A total of 122 taxa, including 114 species of fish and 8 selected species of macroinvertebrates, was represented. Species richness was generally greatest from August through November (77–89 taxa collected) and least from January through April (57–64 taxa collected) (Appendix 1). A total of 107,374 animals, representing 113 species, was collected in seine hauls, accounting for a mean density estimate of 252 animals/100 m2 (Table 1). The 10 most abundant taxa (n = 84,139) represented 78.4% of the total seine catch. Anchoa mitchilli (Bay Anchovy) (n = 19,949) and Trinectes maculatus (Hogchoker) (n = 19,416) were the most abundant taxa collected, accounting for 36.7% of the total catch; T. maculatus was the most frequent taxon caught (65% occurrence) (Table 1). A total of 206,668 animals, represented by 94 species, were collected in trawl tows, accounting for a mean density estimate of 65.9 animals/100 m2 (Table 1). A. mitchilli (n = 141,420) was the most abundant species collected, accounting Table 1. Catch statistics for 10 dominant taxa collected in six hundred twenty-six 21.3-m centerbag seine samples and four hundred twenty-four 6.1-m otter trawl samples during Apalachicola River stratified-random sampling, July 2006–March 2009. Percent (%) is the percent of the total catch represented by that taxon; percentage occurrence (% occur) is the percentage of samples in which that taxon was collected. Fish caught Species n % % occur 21.3-m center-bag seine Anchoa mitchilli 19,949 19.0 26.0 Trinectes maculatus 19,416 18.0 66.5 Brevoortia spp. 10,753 10.0 9.4 Menidia spp. 9713 9.0 29.4 Cyprinella venusta 7502 7.0 34.0 Lucania parva 4159 3.9 22.4 Eucinostomus spp. 3289 3.1 31.3 Notropis petersoni 3170 3.0 29.2 Mugil cephalus 3131 2.9 7.0 Notropis texanus 3057 2.8 22.5 Subtotal 84,139 78.0 Totals 107,374 100.0 6.1-m otter trawl Anchoa mitchilli 141,420 68.0 41.5 Litopenaeus setiferus 14,865 7.2 18.4 Micropogonias undulatus 14,682 7.1 26.9 Trinectes maculatus 9174 4.4 51.4 Cynoscion arenarius 7813 3.8 21.5 Eucinostomus spp. 3445 1.7 26.2 Ictalurus punctatus 2989 1.4 34.9 Leiostomus xanthurus 3073 1.5 22.2 Callinectes sapidus 1529 0.7 43.6 Notropis texanus 1400 0.7 23.3 Subtotal 200,390 97.0 Totals 206,668 100.0 182 Southeastern Naturalist Vol. 12, No. 1 for 68.3% of the trawl catch (Table 1). The taxa most frequently collected in trawl tows were T. maculatus (51.4% occurrence) and C. sapidus (43.6% occurrence). Community structure in shoreline habitats The spatial variation in nekton communities associated with shoreline habitats (sampled by seining) of the lower Apalachicola River was significantly different between the three river-study regions as shown by the two-way ANOSIM (season and region) results (ANOSIM R = 0.670, P = 0.01). Overall, pairwise comparisons revealed that the difference in community structure between adjacent regions increased with distance upstream. There were major differences in community structure, with some species overlap, between the MLD and FUD regions (ANOSIM R = 0.468, P = 0.01) and somewhat more pronounced differences between the FUD and URC regions (ANOSIM R = 0.569, P = 0.01). Significant and pronounced differences were evident between community structures in the MLD and URC regions (ANOSIM R = 0.993, P = 0.01). The ANOSIM results and MDS plot suggest a relationship between abundance and river region that is related to changes in salinity (Fig. 7). The two-dimensional stress for the MDS plot of seine catches for all regions was moderate (0.16), indicating that two dimensions do not fully represent the similarities/dissimilarities of the regions. Results from SIMPER analysis defined the shift from dominance by marine transients to freshwater species in shoreline habitats with increasing distance upstream. In the MLD region, the community was characterized by marine Figure 7. Shoreline data MDS plots displaying the relationships between nekton species composition and salinity by river region (M = marshgrass-dominated lower distributary, F = forested upper distributary, U =upper river channel) for seine catches in the lower Apalachicola River, July 2006–March 2009 (2D stress = 0.16). The size of the bubble surrounding each data point corresponds to the average monthly salinity for that region of the river; larger bubbles indicate relatively greater salinity. 2013 R. Gorecki and M.B. Davis 183 transients (Menidia spp., C. sapidus, Eucinostomus spp., A. mitchilli, Ctenogobius boleosoma [Darter Goby], and Lagodon rhomboides [Pinfish]). In the FUD region, the community comprised a mixture of marine transients (T. maculatus, C. sapidus, Eucinostomus spp., Menidia spp.) and freshwater species (Lepomis macrochirus [Bluegill], Micropterus salmoides [Largemouth Bass], Notropis petersoni [Coastal Shiner], and Lepomis microlophus [Redear Sunfish]). In the URC region, the community was dominated by freshwater species (Cyprinella venusta [Blacktail Shiner], Notropis texanus [Weed Shiner], L. macrochirus, N. petersoni, and Labidesthes sicculus [Brook Silverside]) and two marine transients (T. maculatus and Callinectes sapidus). Because of the differences in nekton assemblages between the three regions, each region was examined for seasonal variations separately by both one-way ANOSIM and comparisons of Bray-Curtis similarity matrices for the monthly distance matrix and nekton community matrices using the RELATE procedure. The farthest downstream MLD region showed the strongest trend in seasonal variation (ANOSIM R = 0.600, P = 0.01; RELATE ρ = 0.535, P = 0.001), with January–April (the rainy season) displaying a different nekton assemblage from that seen throughout the rest of the year (Figs. 2a, 3a). Community differences along seasonal scales were also seen in the FUD region (ANOSIM R = 0.331, P = 0.01; RELATE ρ = 0.395, P = 0.001), although not as pronounced (Figs. 2b, 3b). The URC region showed small seasonal differences (ANOSIM R = 0.243, P = 0.016) reinforced by the evidence of a small difference between months based on the RELATE procedure (ρ = 0.311, P = 0.001) (Figs. 2c, 3c). Pairwise comparisons between the 3 seasons suggested by cluster analysis indicates no difference between the warm dry and cool dry seasons (ANOSIM R = 0.110, P = 0.065), slight differences between the wet and warm dry season (ANOSIM R = 0.217, P = 0.002), and major differences in community structure between wet and cool dry seasons (ANOSIM R = 0.480, P = 0.001). SIMPER analysis indicates that the differences in the community structure between the wet and dry seasons in the MLD and FUD regions are a result of lower abundances of Menidia spp., A. mitchilli, Eucinostomus spp., Lucania parva (Rainwater Killifish), N. petersoni, C. sapidus, and C. boleosoma and greater abundances of Brevoortia spp., T. maculatus, Gambusia holbrooki (Eastern Mosquitofish), L. rhomboides, and Mugil cephalus (Striped Mullet) during the winter. SIMPER analyses indicated that differences in community structure between the wet, warm dry, and cool dry months in the URC region were due to increased abundance of T. maculatus and M. cephalus and decreased abundance of most other taxa during the winter wet season. The results also indicated that increased abundance of mostly freshwater taxa (C. venusta, N. texanus, L. macrochirus, N. petersoni, L. sicculus, and Opsopoeodus emilia [Pugnose Minnow]), as well as a few marine transients (Eucinostomus spp., C. sapidus, and A. mitchilli), began in the warm dry season and continued through the cool dry season, and that the abundance of these taxa were greatly reduced in the wet season. 184 Southeastern Naturalist Vol. 12, No. 1 Community structure in deepwater habitats In the deepwater habitats sampled by trawling, spatial community structure was significantly different between river study regions as shown by the two-way ANOSIM (season and region) results (ANOSIM R = 0.517, P = 0.01). Overall, the greatest difference in community structure occurred between the MLD and URC regions (ANOSIM R = 0.903, P = 0.01), followed by differences between the URC and FUD regions (ANOSIM R = 0.368, P = 0.01), and the FUD and MLD regions (ANOSIM R = 0.238, P = 0.01). Similar to the pattern seen in shoreline habitats, the differences in community structure between regions increased linearly with distance upstream. The MDS plot and ANOSIM results suggest that differences in these distinct regionally based groupings are related to changes in salinity (Fig. 8). The two-dimensional stress for the trawl MDS plot was 0.18, again indicating that the two dimensions do not fully represent the similarities/dissimilarities of the different regions. SIMPER analyses defined a community shift from dominance by marine transients to dominance by freshwater species with increasing distance upstream. In the MLD region, community structure was characterized by the marine transients (A. mitchilli, Micropogonias undulatus [Atlantic Croaker], L. setiferus, Cynoscion arenarius [Sand Weakfish], and C. sapidus). In the FUD region, the community comprised mostly marine transients (T. maculatus, C. sapidus, Eucinostomus spp., A. mitchilli) and one freshwater species (Ictalurus punctatus [Channel Catfish]). Meanwhile, the URC region was Figure 8. Deepwater data MDS plots displaying the relationship of nekton species composition by region (M = marshgrass-dominated lower distributary, F = forested upper distributary, U = upper river channel) for trawl catches in the lower Apalachicola River, July 2006–March 2009 (2D stress = 0.14). The bubble surrounding each data point corresponds to the average monthly salinity for that region of the river; larger bubbles indicate relatively greater salinity. 2013 R. Gorecki and M.B. Davis 185 dominated by two freshwater species (I. punctatus and N. texanus) and a marine transient (T. maculatus). We also separately examined seasonal variations in community structure for each river region in deepwater habitats by one-way ANOSIM and the previously discussed use of the RELATE procedure and monthly distance matrix because of the differences in nekton assemblages between regions. The nekton communities in the MLD region (ANOSIM R = 0.545, P = 0.01; RELATE ρ = 0.409, P = 0.001; Figs. 4a, 5a) and the FUD region (ANOSIM R = 0.559, P = 0.01; RELATE ρ = 0.333, P = 0.001; Figs. 4b, 5b) showed the strongest difference in seasonal variation, with January–April (the rainy season) clearly displaying a different nekton assemblage from that seen throughout the rest of the year. The URC region showed a very small seasonal difference (ANOSIM R = 0.266, P = 0.01; RELATE ρ = 0.307, P = 0.001; Figs. 4c, 5c). SIMPER analysis suggests that temporal shifts in community structure are due to decreased abundance of most dominant taxa (i.e., N. texanus, T. maculatus, I. punctatus, A. mitchilli, Eucinostomus spp.) during the wet season in all three river regions, whereas abundance of M. undulatus and Leiostomus xanthurus (Spot) in the MLD region of the river increased during the rainy season. Correlations between fish abundance and environmental variables Each region was examined separately for seasonal variations, and all regions were analyzed collectively to evaluate correlations between spatial variation in nekton assemblages and physical factors. As expected, of all measured environmental factors, salinity was the most strongly correlated with spatial differences in nekton assemblages between the three river regions in both shoreline (BIOENV ρ = 0.459) and deepwater (BIOENV ρ = 0.309) habitats and supported the MDS plots (Figs. 7, 8). The inclusion of other measured physical factors in the analysis resulted in smaller BIOENV coefficients. In shoreline habitats of the URC region, nekton assemblages were most highly correlated with the combination of temperature and dissolved oxygen (BIOENV ρ = 0.273). The inclusion of salinity had no effect on the BIOENV coefficient, probably due to the lack of variation during the study. The pattern in nekton assemblages was most highly correlated with the combination of temperature and salinity (BIOENV ρ = 0.357) for shoreline habitats in the FUD region, whereas the pattern in the MLD region was most highly correlated with a combination of salinity, temperature, and dissolved oxygen (BIOENV ρ = 0.461). In deepwater habitats, the pattern in nekton assemblages was most highly correlated with temperature (BIOENV ρ = 0.314) in the URC region, with the combination of salinity and temperature (BIOENV ρ = 0.310) in the FUD region, and with a combination of salinity, temperature, and dissolved oxygen (BIOENV ρ = 0.544) in the MLD region. The RELATE procedure indicated that the influence of seasonal changes on nekton communities was greatest in the MLD region (ρ = 0.535) and decreased moving upstream through the FUD (ρ = 0.395) and URC (ρ = 0.311) regions. This 186 Southeastern Naturalist Vol. 12, No. 1 pattern of a greater correlation between variation in nekton community and seasonal changes was also evident in deepwater habitats from the URC region (ρ = 0.307) through the FUD region (ρ = 0.333) and into the MLD region (ρ = 0.409). This agrees with the pattern observed in the CLUSTER dendrograms, MDS plots, and ANOSIM results for both trawl and seine catches (Figs. 2–5). Discussion Multivariate statistical analyses of our data revealed patterns in the spatial distribution of species within both shoreline and deepwater habitats throughout the lower Apalachicola River basin. Species assemblages in the URC region were characterized by freshwater species and were significantly different from those of the brackish distributaries. Marine transient species dominated nekton assemblages in the mesohaline MLD region, whereas the oligohaline FUD region represented a transitional area of suitable conditions for both marine transient and freshwater species. These results are similar to findings by Gelwick et al. (2001) in Matagorda Bay estuary on the Texas Coast, where the system was separated into regions based on salinity differences ranging from fresh (salinity < 5) to estuarine water (salinity > 15). Between regions, significant differences in nekton assemblages were attributed to the dif ferences in salinity. Our results are largely based on two distinct seasons that corresponded to one wet and one dry season. This division is supported by the historical flow levels reported by Light et al. (2006); they reported that distinct peaks in flow occurred in January through April and that flow levels were typically lower throughout the rest of the year. Greenwood et al. (2007) found little evidence of nekton seasonality in the upper reaches of the Alafia River estuary, which they attributed to both reduced penetration of marine and estuarine transient species and to domination by low-salinity and freshwater assemblages. Similarly, the URC and the FUD regions of the Apalachicola River were dominated by freshwater taxa and marine transient species tolerant of low salinities. The strongest patterns in seasonality of nekton communities corresponded with the periods of greatest salinity fluctuations within the MLD region and are supported by high correlations for salinity in BIOENV results. A decreasing correlation between salinity and nekton assemblages as you go upstream is evident in BIOENV results, with salinity correlating or explaining some of the relationship between nekton assemblages and environmental factors in the FUD region, and correlating very little with nekton assemblages in the URC region. This finding underscores the need for further study of the effects of river flow in MLD and FUD portions of the system, since nekton assemblages were hardly influenced by salinity changes in the URC region throughout this study. Considering that salinity was the only environmental factor that varied strongly between regions, it is not surprising that it explained the majority of the differences in species abundances. Livingston (1997) suggested that changes in the nekton assemblages of East Bay, 2013 R. Gorecki and M.B. Davis 187 an estuarine part of the Apalachicola Bay system, may be more directly linked to biological interactions such as competition and predator–prey relationships. Our study did not investigate the effects of biological interactions on assemblages, but did find a strong correlation between environmental factors and nekton assemblages in both shoreline and deepwater habitats of the MLD region (BIOENV ρ = 0.461–0.544), and we feel that there is a direct link between salinity and changes in nekton assemblages within the MLD region. Biological interactions probably do not influence nekton assemblages in the MLD region of the river as strongly as Livingston (1997) suggests in adjacent East Bay. Results of SIMPER analyses for both shoreline and deepwater habitats indicate that the seasonal differences in nekton assemblages were primarily due to decreased abundance of most taxa and greater abundance of Brevoortia spp., T. maculatus, G. holbrooki, L.rhomboides, M. cephalus, M. undulatus, and L. xanthurus during the wet season. These annual fluctuations in abundance coincide with the period of juvenile recruitment for Brevoortia spp., M. cephalus, M. undulatus, L. rhomboides, and L. xanthurus. The recruitment of many other species of nekton increased overall recruitment during the river’s dry season, including C. arenarius, L. setiferus, A. mitchilli, Farfantepenaeus spp., Eucinostomus spp., and many centrarchid fishes (FWC-FMRI, St. Petersburg, FL, unpubl. data; Idelberger and Greenwood 2005; Kupschus and Tremain 2001). Tremain and Adams (1995) attributed changes in the fish assemblages of the Indian River Lagoon to variations in water temperature and estuarine fish recruitment. Idelberger and Greenwood (2005) also stated that recruitment of a majority of estuarine-dependent fish species occurred during the period of May/June through September/October, and that another group (e.g., L. rhomboides, L. xanthurus, and M. cephalus) showed a pattern of fall spawning and winter recruitment into the Myaka and Peace rivers. Juvenile recruitment seems to coincide with changes in salinity in the Apalachicola River system but is more likely a function of a more complex interaction of biotic (e.g., life histories and behavior) and abiotic (i.e., photoperiod and temperature changes) factors (Kupschus and Tremain 2001). Seine and trawl catches both displayed similar seasonal and spatial patterns in nekton assemblages. However, a complete survey of the area using both gear types would present a more thorough picture of effects on all species since these two gear types sample different habitats and do not collect comparable numbers of some species (FDEP-FMRI 1998; Appendix 1). Seasonal trends in nekton assemblages coincide with seasonal fluctuations in salinity in the MLD and FUD regions of the river, but there was no apparent trend or variation in salinity in the URC region. Salinity also appeared to be relatively low throughout all three river regions throughout the wet season. This finding indicates that further sampling and analyses during the dry season on the MLD and FUD regions of the river would be best for detecting whether and when changes in flow affect the river’s nekton assemblages. 188 Southeastern Naturalist Vol. 12, No. 1 The lower Apalachicola River represents an ecologically diverse transition zone between the estuarine Apalachicola Bay and the freshwater Apalachicola River. The transition between estuarine and freshwater systems is most evident in the salinity gradient and corresponding differences in nekton assemblages. Seasonal fluctuations in juvenile recruitment into the estuary seem to dominate seasonal shifts in community structure, whereas fluctuations in salinity may dictate whether and when some species enter the lower portion of the river. With increased anthropogenic demands for freshwater in the northern part of the watershed, insights from our study may influence impending changes in water management practices on the Apalachicola River. The results presented here suggest future studies on the effects of changes in freshwater flow on nekton assemblages in the lower Apalachicola River could be performed using both trawl and seine sampling data and concentrating sampling effort to the dry season in the FUD and MLD regions of the river would be a more efficient use of time and funding. Acknowledgments We thank fisheries-independent monitoring personnel from the FWRI Apalachicola Field Laboratory for collecting data and assisting with the preparation of this paper. We especially thank P.W. Stevens, D. Tremain, D.A. Blewett, K. Flaherty, B. McMichael, and C. Guenther for their insightful help in editing and discussing this document. This project was made possible thanks to the generous funding support provided by the Florida Fish and Wildlife Conservation Commission’s Florida Wildlife Legacy Initiative and the US Fish and Wildlife Service’s State Wildlife Grant program (T-9 Grant Number SWG05- 017). Additional funding support for this project was provided by the Department of the Interior, US Fish and Wildlife Service, Federal Aid for Sportfish Restoration Project Number F-43, and Florida saltwater fishing license monies. Literature Cited Bray, J.R., and J.T. Curtis. 1957. An ordination of upland forest communities of southern Wisconsin. 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Listing of taxa collected in the lower Apalachicola River, Florida in July 2006 – March 2009 by region (MLD = Marshgrass-dominated Lower Distributary, FUD = Forested Upper Distributary, URC = Upper River Channel, X = presence) with catch totals by season (dry season = May–December, wet season = January–April) and gear. Region Dry season Wet season Taxon Common name MLD FUD URC Seine Trawl Seine Trawl Total Stomolophidae Stomolophus meleagris Agassiz Cannonball Jelly X 0 1 0 0 1 Palaemonidae Macrobrachium ohione Smith Ohio Shrimp X X X 0 1 0 7 8 Penaeidae Farfantepenaeus aztecus Ives Brown Shrimp X X 2 33 0 0 35 Farfantepenaeus duararum Burkenroad Pink Shrimp X 0 7 0 0 7 Farfantepenaeus spp. Penaeid Shrimp X X 531 1373 0 14 1918 Litopenaeus setiferus L. White Shrimp X X 904 14,863 0 2 15,769 Rimapenaeus constrictus Stimpson Roughneck Shrimp X X 1 250 0 0 251 Xiphopenaeus kroyeri Heller Atlantic Seabob X 0 5 0 0 5 Portunidae Callinectes sapidus Rathbun Blue Crab X X X 1965 1303 420 226 3914 Callinectes similis Williams Lesser Blue Crab X 0 2 0 0 2 Achiridae Trinectes maculatus Bloch & Schneider Hogchoker X X X 11,519 8708 7897 466 28,590 Acipenseridae Acipenser oxyrinchus desotoi Mitchill Gulf of Mexico Sturgeon X X 0 2 0 0 2 Amiidae Amia calva L. Bowfin X X 6 1 3 1 11 Aphredoderidae Aphredoderus sayanus Gilliams Pirate Perch X X 7 0 14 0 21 Ariidae Ariopsis felis L. Hardhead Sea Catfish X X 1 143 0 4 148 Bagre marinus Mitchill Gafftopsail Sea Catfish X 0 9 0 0 9 Atherinidae Menidia spp. Silverside X X X 8958 4 755 10 9727 192 Southeastern Naturalist Vol. 12, No. 1 Region Dry season Wet season Taxon Common name MLD FUD URC Seine Trawl Seine Trawl Total Atherinopsidae Labidesthes sicculus Cope Brook Silverside X X X 878 0 119 0 997 Membras martinica Valenciennes Rough Silverside X X 4 0 2 0 6 Batrachoididae Porichthys plectrodon Jordan & Gilbert Atlantic Midshipman X 0 27 0 0 27 Belonidae Strongylura marina Walbaum Atlantic Needlefish X X 3 0 0 0 3 Strongylura spp. Needlefish X X X 16 0 0 0 16 Carangidae Caranx hippos L. Crevalle Jack X X 0 5 0 0 5 Caranx latus Agassiz Horse-eye Jack X 1 0 0 0 1 Chloroscombrus chrysurus L. Atlantic Bumper X 0 4 0 0 4 Oligoplites saurus Bloch & Schneider Leatherjacket X X X 69 0 0 0 69 Selene vomer L. Lookdown X 0 4 0 0 4 Catostomidae Carpiodes cyprinus Lesueur Quillback X X 0 0 2 6 8 Erimyzon sucetta Lacépède Lake Chubsucker X X 3 0 1 0 4 Minytrema melanops Rafinesque Spotted Sucker X X X 46 0 4 0 50 Moxostoma spp. Redhorse X X 20 1 1 0 22 Centrarchidae Enneacanthus gloriosus Holbrook Bluespotted Sunfish X X 43 0 4 0 47 Lepomis auritus L. Redbreast Sunfish X X 322 0 100 1 423 Lepomis gulosus Cuvier Warmouth X X X 8 0 2 0 10 Lepomis macrochirus Rafinesque Bluegill X X X 1553 16 655 13 2237 Lepomis microlophus Günther Redear Sunfish X X X 438 27 164 36 665 Lepomis punctatus Valenciennes Spotted Sunfish X X X 163 0 46 1 210 Lepomis spp. Sunfish X X X 229 0 28 0 257 Micropterus salmoides Lacépède Largemouth Bass X X X 1014 19 437 5 1475 Pomoxis nigromaculatus Lesueur Black Crappie X X 2 0 4 0 6 Clupeidae Alosa alabamae Jordan & Evermann Alabama Shad X 1 2 0 0 3 Alosa chrysochloris Rafinesque Skipjack Shad X 1 0 0 0 1 2013 R. Gorecki and M.B. Davis 193 Region Dry season Wet season Taxon Common name MLD FUD URC Seine Trawl Seine Trawl Total Brevoortia spp. Menhaden X X X 766 24 9987 121 10,898 Dorosoma cepedianum Lesusur American Gizzard Shad X X X 10 3 1 0 14 Dorosoma petenense Günther Threadfin Shad X X X 483 147 0 5 635 Harengula jaguana Poey Scaled Herring X 110 0 0 0 110 Cynoglossidae Symphurus plagiusa L. Blackcheek Tonguefish X X 113 710 0 0 823 Cyprinidae Cyprinella venusta Girard Blacktail Shiner X X X 5486 29 2016 0 7531 Cyprinus carpio L. Common Carp X X X 2 4 2 2 10 Notemigonus crysoleucas Mitchill Golden Shiner X X X 681 0 16 0 697 Notropis longirostris Hay Longnose Shiner X 1 0 30 0 31 Notropis maculatus Hay Taillight Shiner X X 85 0 30 0 115 Notropis petersoni Fowler Coastal Shiner X X X 2793 27 377 1 3198 Notropis texanus Girard Weed Shiner X X X 2887 1359 170 41 4457 Opsopoeodus emiliae Hay Pugnose Minnow X X 118 2 34 0 154 Cyprinodontidae Cyprinodon variegatus Lacépède Sheepshead Minnow X 1 0 0 0 1 Dasyatidae Dasyatis sabina Lesueur Atlantic Stingray X X X 3 27 0 0 30 Elassomatidae Elassoma okefenokee Bohlke Okefenokee Pygmy Sunfish X 5 0 1 0 6 Elassoma zonatum Jordan Banded Pygmy Sunfish X X 35 0 2 0 37 Eleotridae Dormitator maculatus Bloch Fat Sleeper X 2 0 0 0 2 Elopidae Elops saurus L. Ladyfish X 3 0 0 1 4 Engraulidae Anchoa hepsetus L. Broad-striped Anchovy X X 407 4 0 0 411 Anchoa lyolepis Evermann & Marsh Shortfinger Anchovy X 1 0 0 0 1 Anchoa mitchilli Valenciennes Bay Anchovy X X X 19,533 137,067 416 4353 161,369 Ephippidae Chaetodipterus faber Broussonet Atlantic Spadefish X 0 5 0 0 5 194 Southeastern Naturalist Vol. 12, No. 1 Region Dry season Wet season Taxon Common name MLD FUD URC Seine Trawl Seine Trawl Total Esocidae Esox niger Lesueur Chain Pickerel X X X 13 0 27 0 40 Fundulidae Adinia xenica Jordan & Gilbert Diamond Killifish X 67 0 5 0 72 Fundulus chrysotus Günther Golden Topminnow X X 18 0 21 0 39 Fundulus confluentus Goode & Bean Marsh Killifish X X 26 0 19 0 45 Fundulus grandis Baird & Girard Gulf Killifish X 57 0 10 0 67 Lucania goodei Jordan Bluefin Killifish X X X 928 0 1273 0 2201 Lucania parva Baird and Girard Rainwater Killifish X X X 3331 9 828 129 4297 Gerreidae Eucinostomus gula Quoy & Gaimard Jenny Mojarra X 19 7 0 0 26 Eucinostomus harengulus Goode & Bean Tidewater Mojarra X X X 478 320 0 9 807 Eucinostomus spp. Mojarra X X X 3283 3413 6 32 6734 Gobiidae Bathygobius soporator Valenciennes Frillfin Goby X 1 0 0 0 1 Tenogobius boleosoma Jordan & Gilbert Darter Goby X X X 1271 192 236 60 1759 Gobionellus oceanicus Pallas Highfin Goby X X 47 16 0 3 66 Gobiosoma bosc Lacépède Naked Goby X X 517 68 127 24 736 Gobiosoma spp. Goby X X 289 45 33 5 372 Microgobius gulosus Girard Clown Goby X X 210 24 11 1 246 Microgobius thalassinus Jordan & Gilbert Green Goby X X 33 905 1 6 945 Haemulidae Orthopristis chrysoptera L. Pigfish X 22 18 0 0 40 Ictaluridae Ameiurus catus L. White Catfish X X X 6 10 0 2 18 Ictalurus furcatus Valenciennes Blue Catfish X X X 3 90 2 24 119 Ictalurus punctatus Rafinesque Channel Catfish X X X 0 2787 0 202 2989 Noturus gyrinus Mitchilli Tadpole Madtom X 2 0 0 0 2 Noturus leptacanthus Jordan Speckled Madtom X 0 0 1 0 1 Lepisosteidae Lepisosteus oculatus Winchell Spotted Gar X X X 34 1 11 0 46 Lepisosteus osseus L. Longnose Gar X X X 10 9 2 5 26 2013 R. Gorecki and M.B. Davis 195 Region Dry season Wet season Taxon Common name MLD FUD URC Seine Trawl Seine Trawl Total Lutjanidae Lutjanus griseus L. Grey Snapper X X X 59 40 0 0 99 Lutjanus synagris L. Lane Snapper X 0 3 0 0 3 Moronidae Morone saxatilis Walbaum Striped Bass X X 0 0 0 4 4 Mugilidae Mugil cephalus L. Striped Mullet X X X 1203 0 1928 4 3135 Mugil curema Valenciennes White Mullet X 4 0 0 0 4 Ophichthidae Myrophis punctatus Lutken Speckled Worm-eel X X 3 1 0 0 4 Paralichthyidae Citharichthys macrops Dresel Spotted Whiff X 0 1 0 0 1 Citharichthys spilopterus Günther Bay Whiff X X 4 28 0 9 41 Etropus crossotus Jordan & Gilbert Fringed Flounder X 0 17 0 0 17 Paralichthys lethostigma Jordan & Gilbert Southern Flounder X X X 12 28 2 15 57 Percidae Ammocrypta bifascia Williams Florida Sand Darter X 0 0 0 0 0 Etheostoma edwini Hubbs & Cannon Brown Darter X X 87 0 17 0 104 Etheostoma fusiforme Girard Swamp Darter X X 50 0 2 0 52 Etheostoma swaini Jordan Gulf Darter X 5 1 2 0 8 Percina nigrofasciata Agassiz Blackbanded Darter X X 19 15 2 5 41 Poeciliidae Gambusia holbrooki Girard Eastern Mosquitofish X X X 677 0 662 1 1340 Heterandria formosa Girard Least Killifish X X 72 0 137 0 209 Poecilia latipinna Lesueur Sailfin Molly X X 83 0 41 0 124 Pomatomidae Pomatomus saltatrix L. Bluefish X 1 0 0 0 1 Sciaenidae Bairdiella chrysoura Lacépède Silver Perch X X 584 21 0 8 613 Cynoscion arenarius Ginsburg Sand Weakfish X X X 64 7679 0 134 7877 Cynoscion nebulosus Cuvier Spotted Seatrout X X 76 181 0 4 261 Leiostomus xanthurus Lacépède Spot X X 125 854 253 2219 3451 196 Southeastern Naturalist Vol. 12, No. 1 Region Dry season Wet season Taxon Common name MLD FUD URC Seine Trawl Seine Trawl Total Menticirrhus americanus L. Southern Kingfish X X 0 175 0 0 175 Menticirrhus saxatilis Bloch & Schneider Northern Kingfish X 0 1 0 0 1 Micropogonias undulatus L. Atlantic Croaker X X 81 7793 41 6889 14804 Pogonias cromis L. Black Drum X 0 7 0 2 9 Sciaenops ocellatus L. Red Drum X X 11 6 0 10 27 Scombridae Scomberomorus maculatus Mitchill Spanish Mackerel X 1 16 0 0 17 Sparidae Archosargus probatocephalus Walbaum Sheepshead X X X 27 83 3 67 180 Lagodon rhomboides L. Pinfish X X 565 164 1069 24 1822 Syngnathidae Hippocampus erectus Perry Lined Seahorse X 1 1 0 0 2 Syngnathus louisianae Günther Chain Pipefish X X 3 16 0 0 19 Syngnathus scovelli Evermann & Kendall Gulf Pipefish X X 111 16 9 8 144 Synodontidae Synodus foetens L. Inshore Lizardfish X X 17 67 0 0 84 Tetraodontidae Sphoeroides nephelus Goode & Bean Southern Puffer X 0 1 0 0 1 Triglidae Prionotus tribulus Cuvier Bighead Searobin X X 10 100 0 0 110 TOTAL 314,042