Stephen W. Golladay

Response of freshwater mussel assemblages (Bivalvia:Unionidae) to a record drought in the Gulf Coastal Plain of southwestern Georgia

Abstract Freshwater mussel assemblages in the Flint River Basin (FRB) of southwestern Georgia are among the most diverse in the southeastern Coastal Plain of North America. Historically, 29 species, including 7 endemics, occurred in the FRB. A drought during the summer of 2000 caused record low flows and many perennial streams dried or became intermittent. Predrought surveys conducted in 1999 allowed an assessment of the impact of the drought on mussel assemblages. During 2001, 21 stream reaches that had abundant or diverse mussel assemblages in 1999 were resurveyed. Study sites were classified as flowing or non-flowing during the drought based on data from stream gauging stations or visual observation of study reaches. Mussels were classified by conservation status, either stable, special concern, or federally endangered. Greater than 90% of the mussels observed in the lower FRB were species with stable conservation status. Special-concern species represented 5 to 6% and endangered species represented 1% of mussel abundance. Sites that ceased flowing during the drought had significant declines in the abundance of stable species and in taxa richness. Endangered species also showed evidence of a decline in non-flowing sites. Sites that maintained flow had increases in stable species and no change in special concern, endangered species, or species richness through the drought. Sites that showed declines in mussel abundance had a significantly lower frequency of wood debris than other sites. Field observations suggested that shallow depressions beneath wood debris may act as refuges for freshwater mussels during stream drying. Greatest declines in mussel abundance usually occurred in the mid-reaches of the major tributaries of the lower Flint River. These reaches depend on the Upper Floridan aquifer, which is heavily used for irrigation, to maintain base flows. Declines in mussel populations appear to be associated with unusual climatic conditions and increasing demand on the area streams and the regional aquifer system for irrigation water supply.

Hydroperiod Influence on Breakdown of Leaf Litter in Cypress-gum Wetlands

Abstract Many cypress-gum wetlands in the southeast United States are isolated from rivers and streams and are seasonally inundated by rainfall. Organic matter processing in these wetlands is caused primarily by biotic components (i.e., microbes and invertebrates), which are influenced by timing and duration of seasonal inundation, and low dissolved oxygen levels. Using litter bags, we examined breakdown of cypress (Taxodium spp.) and gum (Nyssa sylvatica var. biflora) leaves in three wetlands with different hydroperiods: (1) flooded exposed (FE; 5 mo flooded/6 mo litter exposed), (2) multiple flooded exposed (MFE; 6 mo flooded/exposed/flooded/exposed) and (3) permanently flooded (PF; 11 mo flooded). Breakdown was fastest in the MFE wetland suggesting cycles of wetting and drying accelerated decomposition by promoting microbial activity through aeration. Even though ergosterol content, an indicator of fungal biomass on the litter, was similar among wetlands, we hypothesized that within the MFE wetland microbial activity was promoted by exposed conditions, but during subsequent flooding microbial biomass was kept at a low level by invertebrate consumers. Macroinvertebrate density and biomass were comparable between litter types, but were highest in the PF wetland, followed by MFE, then FE wetlands. Chironomids, oligochaetes, Caecidotea and Crangonyx were the dominant taxa indicating litter inputs are vital in maintaining the aquatic foodweb in this system. Cypress litter (k = −1.61 y−1) had faster breakdown rates than gum litter (k = −1.02 y−1), most likely because of plant morphology and greater surface area available to microbial decomposers. Ergosterol (mg g−1 AFDM leaf material) levels were higher on cypress (34.5) than gum (22.5) litter. In both litter types initial C:N and N:P ratios were >20, and C:P ratios were >500, indicating a possible P or N/P co-limitation in cypress-gum wetlands. Elemental gains or losses in litter were influenced predominantly by litter type and to a lesser extent by hydrologic regime. Gum leaves accumulated P, N, Ca and K and lost Mg, whereas cypress leaves had initial declines of these elements, followed by some accumulations in P and K. Temporal patterns of P showed that the drier sites (FE and MFE) immobilized more P than the wetter site (PF), suggesting that exposed conditions promoted microbial activity. In addition, N and P accumulations on gum leaves were highest in the summer at the time when wetlands would normally dry, indicating a seasonal period when moisture and temperature conditions are optimal for microbial growth. Net flux rates to the 1+ y-old component of litter indicated that the FE wetland is accumulating more organic matter (172 g m−2 y−1) than the other wetlands (65 and 72 g m−2 y−1), which we attributed to higher cypress litter production. We concluded that hydrologic regime influences breakdown rates and element accumulations, but that net productivity is more important in determining litter accumulation rates.

Flow Disturbance of Macroinvertebrates Inhabiting Sediments and Woody Debris in a Prairie Stream

Abstract We studied the effect of an engineered flow disturbance on macroinvertebrates in an intermittent N Texas stream. To augment a drinking water supply, water is being diverted through the natural stream channel. Quantitative sampling of sediments and woody debris was conducted immediately before water diversion, immediately after and periodically during recovery, for three diversion events over a 2-yr period. Reduction in total density of macroinvertebrates was dependent on type of substrate, averaging 76% in sediments as compared with 66% on woody debris. Recovery was rapid for most taxa, which often reached predisturbance density in less than 1 mo. For the entire macroinvertebrate assemblage in 1992 and chironomids only in 1991, resilience was greater on woody debris than on sediments. Compared with sand, the common inorganic substrate in this stream, woody debris was more stable, due in part to its retention by debris dams. It appears that woody debris is an important refuge and source of recolonizers to this sandy prairie stream after flow disturbances.

Stable isotopic signatures, tissue stoichiometry, and nutrient cycling (C and N) of native and invasive freshwater bivalves

Abstract Filter-feeding mussels historically comprised most of benthic biomass in many streams. They contribute to stream ecosystem functioning by linking the water column and benthic habitats. Both native and nonnative species coexist in many streams, but their ecological roles are not well quantified. The invasive bivalve, Corbicula fluminea, has the potential to alter profoundly organic matter dynamics and nutrient cycling in streams. We compared stable isotope ratios and tissue and biodeposit stoichiometry of the native freshwater mussel, Elliptio crassidens, and C. fluminea in a Coastal Plain stream (Ichawaynochaway Creek, a tributary to the lower Flint River, Georgia, USA) to assess their trophic niche space and potential effects on nutrient cycling. We hypothesized that C. fluminea would assimilate a larger range of materials than E. crassidens. To determine dietary overlap of C. fluminea and E. crassidens, we measured the elemental and stable isotopic compositions (δ13C and δ15N) of their tissue. Corbicula fluminea showed lower trophic fidelity than E. crassidens and was able to acquire and assimilate a wide range of resources, as illustrated by their wide range of δ13C values. Corbicula fluminea also might alter nutrient cycling in the benthic environment of streams because they retain less N than E. crassidens, as reflected by their higher tissue C∶N. In the laboratory, we measured C and N in biodeposits (feces and pseudofeces) from the 2 species. Corbicula fluminea released more N through their biodeposits relative to E. crassidens by mass, a result implying that C. fluminea might modify nutrient cycling in streams. Our results show important differences in the food resources assimilated and the nutrients deposited as feces and pseudofeces by these 2 bivalves. Furthermore, our results demonstrate how invasive species, such as C. fluminea, can alter aquatic environments through differences in species traits within a functional group.

Drought Responses of Freshwater Mussels (Unionidae) in Coastal Plain Tributaries of the Flint River Basin, Georgia

ABSTRACT During extreme drought conditions, mussel survival and habitat conditions were monitored weekly at nine locations representing a gradient in stream size in the lower Flint River basin, Georgia, USA. Cumulative unionid mortality ranged from 13 to 93% among sites, and was associated with low flow velocity (below 0.01 m/s) and dissolved oxygen concentrations below 5 mg/L. Species assemblages demonstrated differential mortality under declining dissolved oxygen conditions. Riffle and medium-large stream mussel assemblages had greater mortality than generalist assemblages under reduced dissoloved oxygen (DO < 5 mg/L). Mussel community composition at medium-sized sites shifted toward greater dominance of generalist species and lower proportions of riffle and medium-large stream species. At other sites, community structure changed little, likely due to the dominance of drought-resilient species in small streams and less detrimental changes in stream habitat conditions in large streams. Low flow conditions and severe drought adversely affected mussel distributions and assemblages, particularly in high diversity, medium-sized streams.

Prescribed Fire's Impact on Water Quality of Depressional Wetlands in Southwestern Georgia

Abstract Depressional wetlands are a natural feature of the longleaf pine-wiregrass ecosystem on the southeastern Coastal Plain. Fire is an essential part of the longleaf pine forest with prescribed burns occurring at 1–3 y intervals. In 2000 and 2001 we sampled wetlands whose surrounding uplands had been burned and reference wetlands (i.e., no fire) to determine the short-term changes (<1 mo) in surface water quality. In 2000 pH, alkalinity and dissolved inorganic carbon (DIC) were higher in burned wetlands than reference sites based on ranked ANOVA. In 2001 dissolved organic carbon (DOC) and NH4-N were higher in burned wetlands than reference ones. Differences between years suggest that field conditions are very important in determining fires affect on water quality. To clarify our findings we conducted a laboratory experiment where we looked at changes in water quality when exposed to material (wiregrass, dead pine needles and soil) that had undergone simulated fire (muffle furnace at 340 C for 1 h). Results indicated that water exposed to burned soil had elevated pH, alkalinity, DOC, NH4-N and soluble reactive phosphorus (SRP) compared to unburned soil. Burned wiregrass and pine needles had lower DOC and DIC levels compared to unburned material, but burned wiregrass had higher NH4-N and SRP concentrations than the unburned treatment. Overall our results suggest that the linkage of fire and water quality of wetlands is through fires effect on soils rather than vegetation.

HOW HYDROLOGY, HABITAT TYPE, AND LITTER QUALITY AFFECT LEAF BREAKDOWN IN WETLANDS ON THE GULF COASTAL PLAIN OF GEORGIA

Since the 1970s, southwestern Georgia has seen significant decreases in annual stream flows and changes in seasonal rainfall patterns (i.e., slightly wetter winters and drier springs) that have implications for leaf breakdown processes in wetlands. In four separate study years (1999–2002), we examined leaf breakdown processes in two types of forested wetlands, floodplain and depressional. Using a reciprocal transfer design, litter bags containing black gum (Nyssa syhatica biflora) and water tupelo (Nyssa aquatica) were placed in three wetlands of each type. We predicted open wetlands (floodplain) would have faster breakdown rates than isolated wetlands (depressional), and water tupelo leaves acquired from high-nutrient wetlands (floodplain) would be less refractory and decompose faster than black gum leaves obtained from low-nutrient wetlands (depressional). There was no consistent pattern of one wetland type having faster breakdown rates than the other. Breakdown of water tupelo (k = −0.57 to −1.77 yr−1) was faster than black gum (k = −0.53 to −1.31 yr−1) in three out of eight instances (i.e., same wetland type and year); otherwise, no difference was detected between species. Results from the drier years (1999, 2000, and 2001) suggest that litter species was more important than wetland type in determining P levels in litter and in some instances N concentrations, but in the wetter year (2002) N and P patterns in litter differed by wetland type. This multiyear study allowed us to conclude that in drought years, moisture levels primarily influenced breakdown of organic matter, but in years with prolonged flooding, faster breakdown rates occurred in the wetland type and litter species with the higher nutrient levels. A 1.4 to 3-fold difference in breakdown rates among study years (for the same wetland type and litter) suggest that caution should be used in making conclusions based on a single decomposition study.

Amphibian Distributions within Three Types of Isolated Wetlands in Southwest Georgia

ABSTRACT Conservation of isolated wetlands is critical for maintaining regional biodiversity within the southeastern U.S. However, relatively little is known about the ecological communities of these wetland systems, particularly within the karst wetlands of the southeastern Coastal Plain. In southwestern Georgia seasonal isolated wetlands include marshes, cypress savannas and cypress/gum swamps, which have fundamental differences in vegetation and soils, hydrology, water chemistry and invertebrate abundance and diversity. We examined the relationship between wetland type and the distribution of amphibians in 28 relatively undisturbed, seasonally flooded isolated wetlands in southwestern Georgia, USA. We sampled wetlands for amphibians in the winter, spring and summer using aquatic traps, dipnetting, PVC pipe refugia and automated frog call recording devices (frogloggers). Mean amphibian species richness among study wetlands was 12.7 ± 0.5 species (range 7–18). Both species richness and composition varied among wetland types, with different wetland types supporting different amphibian assemblages. Our results highlight the importance of wetland diversity in promoting regional amphibian diversity.

Weather and Landscape Factors Associated with Adult Mosquito Abundance in Southwestern Georgia, U.S.A.

ABSTRACT: Mosquito community composition and population dynamics were compared to weather variables and land use/cover data during 2008 to determine which variables affected population dynamics at the J. W. Jones Ecological Research Center in southwestern Georgia. Models relating adult mosquito distributions to weather variables and time of year were compared using Akaikes Information Criterion (AIC) model selection. Precipitation, temperature, humidity, and Keetch-Byram Drought Index were important factors correlated with mosquito abundance or presence/absence for the species considered. A cluster analysis, which grouped eight sites based on the percentages of land use/cover and hydric soils located in a 1-km radius surrounding collection sites, and an indicator species analysis were used to investigate the associations among 11 mosquito species and sites with similar land use/cover. Aedes albopictus (Skuse), Culex coronator Dyar & Knab, Culex quinquefasciatus Say, and Culex salinarius Coquillett were associated with sites that had the most anthropogenic influence, while Coquillettidia perturbans (Walker) and Psorophora ferox (von Humboldt) were associated with natural land cover such as wetlands and forested land. This study demonstrates that regional climate and land use/cover data can be predictive of the population dynamics of certain mosquito populations and is the first to examine how the distribution of Cx. coronator adults relate to land use/cover in the southeastern United States.

AQUATIC INVERTEBRATES IN HARDWOOD DEPRESSIONS OF SOUTHWEST GEORGIA

Abstract Hardwood depressions in the southeastern United States have been extensively altered due to agriculture and other land management practices. They are small isolated wetlands dominated by oaks that typically become flooded every couple years for a few weeks to several months. We sampled the aquatic invertebrate assemblages of six depressions in 1998 and five depressions in 2001 and found they were composed primarily of clam shrimp, cladocerans, calanoid copepods, and chironomids. The primary functional feeding group was collector-filterers, which comprised >60% of the total numbers. Eubranchiopoda were well represented by two species of clam shrimp (Lynceus gracilicornis and Limnadia lenticularis) and one species of fairy shrimp (Streptocephalus seali). L. lenticularis is the first record of this species in Georgia. Consideration should be made for the conservation of hardwood depressions because of the rare invertebrates they accommodate.