Satilla River Research Articles

Assessing Blue Catfish population dynamics across varying introduction timelines in Georgia

AbstractObjectiveUnderstanding the dynamic rate functions of introduced fish populations is essential in guiding management strategies. Blue Catfish Ictalurus furcatus population dynamics have been well studied; however, there is a gap in knowledge from populations that have been introduced into novel systems.MethodsBlue Catfish were collected from the Altamaha, Ocmulgee, Oconee, and Satilla rivers in Georgia. Age estimates were obtained through otolith analysis. Multimodel growth analysis was used to describe growth patterns, and early life growth was modeled with linear regression. Instantaneous mortality of each population was estimated and compared by using a generalized linear model approach.ResultThe four introduced Blue Catfish populations exhibited rapid growth and low mortality rates during establishment; however, we found that populations with earlier introduction histories generally had slower growth rates and lower mortality. The Satilla River population is comparatively the most recently introduced population (first discovered in 2011) that exhibited the fastest growth and highest mortality.ConclusionOur results suggest that Blue Catfish in our study systems are in an intermediate stage of invasion and have not yet reached an equilibrium state. As density continues to increase in these river systems, we expect a decline in growth rates, delayed sexual maturation, and a greater observed maximum age. However, additional monitoring is needed to identify how population dynamics and reproductive characteristics vary over time.

Two Distinct Life History Strategies of Atlantic Sturgeon in the Ogeechee River, Georgia

Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) is an anadromous, widely distributed, highly migratory sturgeon subspecies that occurs in rivers and marine waters along the North American Atlantic Coast. This fish has shown widespread declines and has been afforded conservation protections, including some based in the U.S. on the delineation of Distinct Population Segments (DPS) under the Endangered Species Act. The management of Atlantic sturgeon will benefit from the knowledge of its population structure as well as the ability to uniquely identify individuals (in relation to population origin) that are vulnerable to anthropogenic stressors at sites outside of their natal estuaries. We used microsatellite analysis to estimate the genetic population structure of Atlantic sturgeon from 13 spawning rivers ranging from the St. Lawrence River, Quebec to the Satilla River, Georgia. Individual Based Assignment (IBA) testing was used to estimate the contribution of each population to mixed-stock aggregations in coastal waters and in a non-natal estuary. An unexpected finding was the discovery of two distinct genetic clusters of juvenile Atlantic sturgeon in the Ogeechee River, Georgia, with specimens in the two clusters differing significantly in terms of mean total length. Additionally, three distinct genetic clusters were detected within the Satilla River juvenile collection, along with two clusters within the Edisto River sample. In F1ST and FST analyses, the extent of the pairwise genetic differentiation between the two genetic clusters in the Ogeechee River and the three in the Satilla River was greater than that between all other pairwise comparisons among rivers in the South Atlantic DPS. In contrast, we found no evidence of the genetic partitioning of juvenile sturgeon within the neighboring Savannah or Altamaha river populations. Using IBA, we found that the overall Ogeechee River population made a moderate contribution (8.3%) to the overall mixed-stock collections (n = 1512) from coastal North Carolina to the Bay of Fundy. Surprisingly, all of the Ogeechee River-assigned specimens (n = 125) in these mixed-stock aggregations were representatives of only one of the two Ogeechee River genetic clusters. These results suggest that the two Ogeechee River genetic clusters exhibit significantly different life history strategies, with one being resident and the second being highly migratory.

The Detection and Prevalence of Microsporidia in Shrimp from the Satilla River Estuary

The Detection and Prevalence of Microsporidia in Shrimp from the Satilla River Estuary

Status and Genetic Character of Atlantic Sturgeon in the Satilla River, Georgia

AbstractThe Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus is an important component of biodiversity along the Atlantic coast of North America, but most populations have been decimated by habitat degradation and chronic overfishing. Historically, spawning populations existed in all major Atlantic coast rivers from the St. Lawrence River, Quebec, to the St. Johns River, Florida, but fisheries surveys conducted in the past two decades suggest that several populations at the southern extent of this range are now extirpated or have declined to remnant status. Our objective was to assess the abundance and genetic character of Atlantic Sturgeon in the Satilla River, Georgia. Using entanglement gears, we expended over 2,800 h of sampling effort and captured a total of 193 Atlantic Sturgeon in tidally influenced reaches of the river during 2008–2010. Of the 157 fish that were collected in 2010, 72 were identified as river‐resident juveniles (ages 0–1). Genetic analyses of a subset (n = 61) of these juveniles revealed (1) depauperate levels of mitochondrial DNA (mtDNA) haplotype diversity and (2) the presence of large family units based on microsatellite DNA multilocus genotypes, collectively suggesting that very few parents produced the 2008 year‐class. The mtDNA and microsatellite analyses both indicated that juveniles in the Satilla River population were genetically distinct from other populations in the South Atlantic Distinct Population Segment. Atlantic Sturgeon life history characteristics and the present results suggest that sampled juveniles from the 2008 year‐class were the offspring of a small remnant pool of Satilla River adults; however, a full description of the population's genetic character and origin will require additional juvenile samples from future year‐classes.

The origin, composition, and reactivity of dissolved iron(III) complexes in coastal organic- and iron-rich sediments

The redox chemistry and speciation of Fe in both solid and dissolved phases were characterized in the organic- and Fe-rich sediments of the Satilla River estuary in South-East Georgia (USA) on a series of four cruises between July 2007 and January 2008. Results indicate that dissolved Fe is present in relatively high concentration in the overlying waters at the freshwater end of the estuary and flocculates along the river as the salinity increases downstream. Soluble organic–Fe(III) complexes comprise the majority of dissolved Fe (<0.2μm) in the suboxic pore waters of the upriver stations that are characterized by high concentrations of poorly crystalline Fe(III) (oxy)hydroxides. In contrast, SO42−-reducing conditions downstream prevent the accumulation of organic–Fe(III) in the pore waters by titrating Fe from the sediment. Separation of dissolved Fe by size exclusion chromatography revealed that Fe(II) is complexed by organic ligands in the pore waters while the organic–Fe(III) complexes are either small or highly reactive with the column matrix. Finally, dissimilatory Fe(III) reduction, stimulated by inoculating anaerobic sediments with a Fe(III)-reducing bacterium (FeRB), Shewanella putrefaciens strain 200, increased production of soluble organic–Fe(III) complexes, and addition of reactive Fe(III) hydroxides accelerated the non-reductive dissolution of Fe(III) (oxy)hydroxides irrespective of the presence of exogenous FeRB. These findings suggest soluble organic–Fe(III) complexes in suboxic pore waters may be produced both as intermediates during the dissimilatory reduction of Fe(III) (oxy)hydroxides by Fe(III)-reducing microorganisms and during the oxidation of organic–Fe(II) complexes by Fe(III) (oxy)hydroxides. These soluble organic–Fe(III) complexes are stable in pore waters and may flux from the sediments to the continental shelf.

Landscape and seasonal factors influence Salmonella and Campylobacter prevalence in a rural mixed use watershed

Salmonella and Campylobacter prevalence in stream networks of the Satilla River Basin (SRB) were monitored monthly from August 2007 to August 2009 to study relationships between these pathogens and land use, presence of poultry houses and wastewater treatment plant (WWTP) discharge. Salmonella and Campylobacter were detected at all 10 stream sites and the three sites at the sole wastewater treatment plant (WWTP) in the study area. In all, 43% (129/299) and 62% (96/156) of samples were positive for Salmonella and Campylobacter, respectively, with detection frequency increasing in downstream sites with more poultry production and influence of WWTP discharge. Both Salmonella and Campylobacter detection frequencies were positively associated with the number of poultry houses in the subwatersheds, but agricultural land use as a proportion of the watershed was not a significant predictor of either pathogen. Fecal indicator bacterial levels were assessed and evaluated for their ability to predict the presence of pathogens. Of those examined, enterococci was most predictive; of the 129 samples positive for Salmonella, 88% (113/129) were detected when enterococci were above EPA single sample threshold (61 CFU 100 ml-1); and of the 96 samples positive for Campylobacter, 90% (86/96) were detected when enterococci levels exceeded this level. Comparatively, Escherichia coli concentrations were above EPA single sample thresholds in 38% (49/129) of the positive Salmonella samples. Detection of the pathogens throughout the watershed indicated that there was potential for waterborne transmission especially in downstream areas that were more likely to have recreational users.

Estimation of Daily Streamflow of Southeastern Coastal Plain Watersheds by Combining Estimated Magnitude and Sequence

AbstractCommonly used methods to predict streamflow at ungauged watersheds implicitly predict streamflow magnitude and temporal sequence concurrently. An alternative approach that has not been fully explored is the conceptualization of streamflow as a composite of two separable components of magnitude and sequence, where each component is estimated separately and then combined. Magnitude is modeled using the flow duration curve (FDC), whereas sequence is modeled by transferring streamflow sequence of gauged watershed(s). This study tests the applicability of the approach on watersheds ranging in size from about 25‐7,226 km2 in Southeastern Coastal Plain (U.S.) with substantial surface storage of wetlands. A 19‐point regionalized FDC is developed to estimate streamflow magnitude using the three most selected variables (drainage area, hydrologic soil index, and maximum 24‐h precipitation with a recurrence interval of 100 years) by a greedy‐heuristic search process. The results of validation on four watersheds (Trent River, North Carolina: 02092500; Satilla River, Georgia: 02226500; Black River, South Carolina: 02136000; and Coosawhatchie River, South Carolina: 02176500) yielded Nash‐Sutcliffe efficiency values of 0.86‐0.98 for the predicted magnitude and 0.09‐0.84 for the predicted daily streamflow over a simulation period of 1960‐2010. The prediction accuracy of the method on two headwater watersheds at Santee Experimental Forest in coastal South Carolina was weak, but comparable to simulations by MIKE‐SHE.

Stock Origin of Migratory Atlantic Sturgeon in Minas Basin, Inner Bay of Fundy, Canada, Determined by Microsatellite and Mitochondrial DNA Analyses

AbstractFive distinct population segments of Atlantic sturgeon Acipenser oxyrinchus were recently listed (April 2012) as endangered or threatened under the U.S. Endangered Species Act. Atlantic sturgeon are anadromous, spawning occurs in rivers from the St. Lawrence River, Quebec, to the Satilla River, Georgia, and subadults and adults undertake extensive coastal migrations. Bycatch of Atlantic sturgeon in coastal fisheries may have resulted in the slowed or failed rebuilding of many populations despite the imposition of a U.S. federal moratorium on their harvest in 1998. Canada's Bay of Fundy hosts weir and trawl fisheries which bycatch Atlantic sturgeon of unknown origin. Additionally, tidal power development projects for the Bay of Fundy have been proposed which could detrimentally impact migratory sturgeon. We hypothesized that the Atlantic sturgeon that occur in Minas Basin in the Bay of Fundy are of local Saint John River, New Brunswick, origin with little or no U.S. contribution. We used microsatellite DNA (11 loci) and mitochondrial DNA control region sequence analysis along with previously determined characterizations of nine reference spawning populations to quantify their stock origin. We determined that the summer assemblage of Atlantic sturgeon collected within Minas Basin was of mixed origin, with a greater than 60% contribution from the nearby Saint John River but with a substantial (34–36%) contribution from the Kennebec River, Maine, and a smaller (1–2%) contribution from the Hudson River, New York. There was significant genetic heterogeneity between smaller (&lt;130 cm) and larger individuals (≥130 cm) in Minas Basin; however, the smaller specimens were not exclusively of proximal Saint John River origin. Our results indicate that Atlantic sturgeon of U.S. origin are vulnerable to anthropogenic impacts in the Bay of Fundy, particularly those of Kennebec River origin.

Non-Indigenous Range Expansion of the Blue Catfish (Ictalurus furcatus) in the Satilla River, Georgia

Abstract Here we present evidence of the first field observation of the nonnative Ictalurus furcatus (Blue Catfish) occurring on the Satilla River, GA, in May 2011, and additional collections since then. This is the second large, non-native riverine catfish to be found in the Satilla River basin. Pylodictis olivaris (Flathead catfish) was first collected from the Satilla River in May 1996. The ecological effects of Blue Catfish on native mussel and fish species in the Satilla River are currently unknown, but competition with native catfishes is likely.

Effects of Salinity on Denitrification and Greenhouse Gas Production from Laboratory-incubated Tidal Forest Soils

We measured ambient and potential denitrification in tidal freshwater floodplain forest (tidal forests) soils along the Altamaha, Ogeechee, and Satilla Rivers in southeast Georgia to characterize nitrogen removal from these understudied systems. Further, we measured the response of denitrification, greenhouse gas production (CO2, CH4, N2O), and the formation of reduced inorganic sulfur species following laboratory incubations simulating saltwater intrusion with salinity levels of 2 and 5. Ambient denitrification ranged from 0.03 μg N2O/g/hr in the Satilla River to 0.40 μg N2O/g/hr in the Altamaha River. Potential denitrification was two to eight times greater than ambient denitrification. Denitrification from Satilla River soils increased with salinity, while Altamaha and Ogeechee River soils were unaffected. Methane production, however, was inhibited by salinity, decreasing by 77% in the 2 treatment and 89% in the 5 treatment whereas CO2 generally increased with salinity, though exhibited a variable response between the three rivers. Formation of reduced sulfur species followed a similar trend to CO2 production. Our findings suggest that tidal forests are effective sinks for N, capable of producing anywhere from 0.032 to 1.9 μg N2O/g soil/hr. Overall, salinity increased the amount of C mineralized suggesting that low levels of salinity can alter the short-term C dynamics of tidal freshwater forests.

The flux of soluble organic‐iron(III) complexes from sediments represents a source of stable iron(III) to estuarine waters and to the continental shelf

Iron speciation in the Satilla River Estuary was investigated using competitive ligand equilibration–adsorptive cathodic stripping voltammetry (CLE‐ACSV) with the ligand 1‐nitroso‐2‐naphthol (1N2N). The blackwater Satilla River contains high concentrations of dissolved organic matter and iron, suggesting that it could provide a source of dissolved iron to the continental shelf. Total dissolved iron in the water column decreases along the estuary, likely due to flocculation and precipitation processes associated with the increase in salinity. Simultaneously, the percentage of dissolved organic‐Fe(III) complexes measured by CLE‐ACSV in overlying waters of sediment cores increases with salinity. The speciation of dissolved iron in the pore waters indicates that these complexes originate in the underlying sediments. Diffusive fluxes calculated from depth profiles in the sediments indicate that only 8% of the sedimentary flux of Fe(III) is delivered to the continental shelf during low riverine discharge. During normal flow conditions, however, the sediment flux of Fe(III) represents 63% of the total riverine flux of Fe(III). These findings suggest that the flux of dissolved iron to the continental shelf is controlled by the sedimentation of iron in the estuary and the remobilization of soluble organic‐Fe(III) complexes produced either during iron reduction deep in the sediment or oxidation of Fe(II) close to the sediment–water interface. The Satilla River Estuary generates five to eight times higher concentrations of dissolved Fe(III) than the average major world rivers, suggesting that it is, along with other small blackwater rivers, currently underrepresented in world average river flux calculations.

The effect of riverine discharge on biogeochemical processes in estuarine sediments

A recently developed autonomous benthic lander equipped with an underwater potentiostat and a micromanipulator for in situ voltammetric depth profiling of main redox species in pore waters was deployed for 3 yr at multiple stations along the Satilla River estuary (Georgia, U.S.A.). These measurements revealed that biogeochemical processes in estuarine sediments vary seasonally and are influenced by riverine discharge. A prolonged drought decreased river discharge, altered the salinity gradient in the estuary, and profoundly affected anaerobic respiratory processes in the underlying sediments. Under normal hydrologic conditions, iron reduction was the dominant anaerobic respiratory process across the estuary, likely due to the significant supply of iron from the coastal plane and low salinity of the estuary. Under drought conditions, the salinity of the estuary increased to full seawater strength and carbon remineralization was enhanced significantly. Yet, sulfate reduction was only observed near the mouth of the estuary, whereas a substantial increase in iron reduction was distinctive upriver. Evidence indicates that the increase in iron‐reducing activity during the drought resulted mainly from the deposition of mineral particles in the upper estuary when the salinity gradient increased. Altogether, this study demonstrates that the biogeochemical response of estuarine sediments to natural perturbations is rapid and that respiration processes are controlled by a combination of temperature, supply of inorganic and organic substrates, and hydrological processes. These findings also suggest that an increase in the frequency of droughts as a result of climate change may enhance the resiliency of iron‐reducing bacteria in river‐fed estuarine sediments.

Observation of Atlantic Sturgeon Predation by a Flathead Catfish

AbstractSturgeons (family Acipenseridae) are a highly threatened taxon worldwide; populations have declined for several reasons, including overfishing and habitat modification. It has been suggested that the widely introduced, highly piscivorous, flathead catfish Pylodictis olivaris is responsible for the decline of certain sturgeon populations in North America. Here we present the first field observation of flathead catfish predation on a sturgeon of any species, an Atlantic sturgeon Acipenser oxyrinchus in the Satilla River, Georgia. Our finding demonstrates a potential impact of flathead catfish predation on sturgeon populations; however, more research is needed to describe the full magnitude of such effects on sturgeon populations.

Distribution and retention of Petrolisthes armatus in a coastal plain estuary: The role of vertical movement in larval transport

Since transport of planktonic larvae is essential to the maintenance and expansion of many marine species, we examined the spatial and temporal distribution of green porcelain crab Petrolisthes armatus (Gibbes, 1850) larvae and the possible underlying physical and behavioral mechanisms using a combination of field observations and numerical modeling. The field study consisted of observations of larval abundance and distribution as well as hydrographic surveys of the Satilla River estuary on the east coast of the USA in August 2006. Larvae were found throughout the water column within the tributaries but primarily at depth in the main river. A numerical model was used to examine the effect of flow and possible larval behavior responsible for the observed distribution and the consequences for larval retention in the estuary. Model results that included downward larval movement are consistent with the field observations, supporting the hypothesis that P. armatus larvae vertically migrate within the water column, which aids in their retention within the estuary.

Complexity of the flooding/drying process in an estuarine tidal‐creek salt‐marsh system: An application of FVCOM

The tidal flooding/drying process in the Satilla River Estuary was examined using an unstructured‐grid finite‐volume coastal ocean model (FVCOM). Driven by tidal forcing at the open boundary and river discharge at the upstream end, FVCOM produced realistic tidal flushing in this estuarine tidal‐creek intertidal salt‐marsh complex, amplitudes and phases of the tidal wave, and salinity observed at mooring sites and along hydrographic transects. The model‐predicted residual flow field is characterized by multiscale eddies in the main channel, which are verified by ship‐towed ADCP measurements. To examine the impact of complex coastal geometry on water exchange in an estuarine tidal‐creek salt‐marsh system, FVCOM was compared with our previous structured‐grid finite difference Satilla River Estuary model (ECOM‐si). The results suggest that by failing to resolve the complex coastal geometry of tidal creeks, barriers and islands, a model can generate unrealistic flow and water exchange and thus predict the wrong dynamics for this estuary. A mass‐conservative unstructured‐grid model is required to accurately and efficiently simulate tidal flow and flushing in a complex geometrically controlled estuarine dynamical system.

The effect of secondary circulation on the salt distribution in a sinuous coastal plain estuary: Satilla River, GA, USA

An analysis of observational data suggests salt exchange in a sinuous coastal plain estuary is significantly impacted by counter-rotating residual horizontal eddies formed by channel curvature in meandering channels. The parts of adjacent eddies that advect material downstream follow the deep part of the channel where the flow continually criss-crosses from one side of the channel to the other and follows a relatively unimpeded trajectory to the sea. On the other hand, the parts of adjacent eddies that advect material upstream cross the channel at a different location where it encounters a series of shoals. In this case, the resulting upstream transport of salt is relatively inefficient and retards the rate at which salt can disperse upstream into the estuary. The strength of these circulations is modulated by the spring/neap cycle, allowing for a stronger gravitational mode of exchange to develop near neap tides, but has minimal impact on the length of the salt intrusion. It is suggested that the impeded upstream salt transport accounts for the observation that an impulse of river discharge advects a given isohaline 10 km downstream in 20 days, but that after the impulse, 70 days are required to return the isohaline to a similar position, counter to the notion of a simple dependence of intrusion length on river discharge.

Comparison of Native and Introduced Flathead Catfish Populations in Alabama and Georgia: Growth, Mortality, and Management

AbstractWe compared growth of flathead catfish Pylodictis olivaris from two native populations in Alabama (Coosa and Tallapoosa rivers) and two introduced populations in Georgia (Ocmulgee and Satilla rivers). We also compared mortality rates and potential outcomes of various management regimes (minimum length limits [MLLs]) among the populations. Total length–log10(age) regression slopes for introduced fish were higher than those for native fish, and von Bertalanffy growth coefficients (K) were greater for introduced fish (Ocmulgee: 0.195; Satilla: 0.201) than for native individuals (Coosa: 0.057; Tallapoosa: 0.059). Therefore, introduced flathead catfish grew more rapidly than those in their native range. Mortality (instantaneous mortality rate, Z) was higher in the Satilla River population (Z = −0.602) than in the Ocmulgee River (Z = −0.227) and Coosa River (Z = −0.156) populations. However, fish in the Satilla River population had been introduced for only 10 years and presumably did not reach their theoretical maximum age, potentially biasing the mortality estimate for that population. Simulation of management regimes in Fishery Analyses and Simulation Tools software predicted that maximum biomass of flathead catfish in the Ocmulgee (1,668 kg) and Satilla (1,137 kg) rivers was substantially larger than that in the Coosa (873 kg) and Tallapoosa (768 kg) populations. However, increased exploitation rates in the Ocmulgee and Satilla River populations resulted in dramatic declines in overall biomass, especially at lower MLLs (254 and 356 mm, respectively). Therefore, in systems where introduced flathead catfish represent an important recreational fishery but have dramatically reduced the abundance of native fishes through predation, minimal protection is recommended. We contend that rapid growth of introduced flathead catfish has major implications for their management and the conservation of native fishes.

Rapid detection of human fecal contamination in estuarine environments by PCR targeting of Bifidobacterium adolescentis

Detection of Bifidobacterium adolescentis was used as an effective genetic marker of human fecal contamination in Georgia estuaries. Enterococci enumerations on mEI media indicated that a tributary to the Little Satilla River with 516 CFU/100 ml was the most polluted of all the rivers tested. Extracted DNA from eight river water samples was subjected to a two-step nested PCR protocol using genus and species specific primers for Bifidobacterium spp. and B. adolescentis. B. adolescentis was detected from extracted DNA in Dunbar River, Black Bank Creek, and in a Little Satilla River tributary which demonstrates the presence of human fecal contamination in these three rivers. In the five other estuaries tested including West Point–Federica River and the Altamaha River, which both had less than 16 CFU/100 ml of enterococci, B. adolescentis was not detected.

Variations in the spectral properties of freshwater and estuarine CDOM caused by partitioning onto river and estuarine sediments

The optical properties and geochemical cycling of chromophoric dissolved organic matter (CDOM) are altered by its sorption to freshwater and estuarine sediments. Measured partition coefficients ( K p) of Satilla River (Georgia) and Cape Fear River estuary (North Carolina) CDOM ranged from 19 to 233 L kg −1 when model freshwater and organic-rich estuarine sediments were added to solution (concentrations of 0.1, 1, or 10 g L −1), with the largest K p values measured in solutions with the lowest sediment concentrations. Sorption of Satilla River CDOM was augmented upon raising the ionic strength of solution by mixing with natural seawater, likely due to the ‘salting out effect’ of Ca +2 and Mg +2 ions. For turbid estuarine systems with particle loads of ∼100 mg L −1, we estimate that ∼1–2% of the CDOM pool sorbs to settling particles, facilitating the transfer to a potentially large colored particulate organic matter (CPOM) reservoir within sediments. Our results also indicate that >30% of the colored organic matter pool within sediment pore waters (sediment concentration > 10 g L −1) may exist as CPOM. Spectral slope coefficients (300–700 nm) of initial CDOM samples increased as much as 20% after mixing with 10 g L −1 sediment and 5% after mixing with 1 g L −1 sediment indicating that sorption to particles has the potential to significantly alter the optical properties of CDOM in the water column of turbid shallow environments or in areas of high benthic exchange.

Microsite Influences on Productivity and Nutrient Circulation Within Two Southeastern Floodplain Forests

At broad spatial scales, floodplain forests of the southeastern USA are often perceived as either nutrient poor or nutrient rich. Perceived differences in nutrient availabilities between oligotrophic blackwater and eutrophic redwater floodplain forests suggest rates of aboveground net primary production (ANPP) are lower on the former. However, for these floodplain types, microsite variation may influence relationships between nutrient circulation and ANPP to a greater extent than landscape position and geomorphology of their associated watershed. Therefore, our objectives were to compare ANPP, nutrient circulation through litterfall and decomposition, and the microbial biomass among microsites that differed in terms of soils and hydrology within two riverine floodplain forests. The two floodplains used in this study were located along the Satilla River, a Coastal Plain blackwater system, and the Altamaha River, a Piedmont redwater system, both of which are located in southeastern Georgia. Microsite influences on ANPP were not significantly different during 1999 and 2000 for the Satilla (SAT) and Altamaha (ALT) floodplains. Litterfall production and patterns of P and Ca circulation on the SAT floodplain were significantly lower for the driest microsite type during both years. Microbial biomass N and P were also lowest for the driest microsite, especially in 2000. Overall, microsites on the SAT displayed greater variation in P and Ca cycling than did those of the ALT. Litterfall production, nutrient circulation in litterfall and decomposition, and the microbial biomass were similar among microsites of the ALT floodplain during both years. These findings suggest that spatial variability in nutrient cycling may be greater for oligotrophic floodplain forests.