Aquatic Macroinvertebrate Community Responses to Drought Conditions in a Coastal Plains Floodplain

Coastal floodplains are productive ecosystems that depend on a regular hydrologic regime that includes a predictable flooding event. Since 2011, ~60-76% of Georgia has experienced abnormally dry to extreme drought conditions. To investigate how an extended drought affects a floodplain ecosystem, we characterized benthic macroinvertebrate communities and organic matter standing stocks on a floodplain of the Altamaha River at the Moody Forest Natural Area. Macroinvertebrate communities were sampled monthly from December 2011-April 2012 and characterized by functional feeding group (FFG) during pre(December-January); peak(February-March); and post(April) flood conditions. There were observed differences in FFG composition over the course of the study. Collector-gatherers were the dominant FFG during pre-flood comprising 85-87% of the community. During peak-flood, filtering macroinvertebrates became more abundant (2547% of total community). We observed monthly differences in dissolved oxygen (DO) with a trend of increasing DO with increasing flood stage and consequently decreasing as the waters receded. Changes in abiotic factors such as DO are predicted to contribute to changes in consumer communities. While there was a noticeable hydrological event at the area, gauge height readings indicated that flood stage (approx. 4m) was not reached and discharge remained under the 42 year average during the entire sampling period. Ongoing studies in this area will allow for a comprehensive assessment of the effects of extreme events such as drought on floodplain communities. Furthermore, this study will provide insight into the response and potential resiliency of one of Georgia’s unique ecosystems to a commonly occurring disturbance in the region. Introduction Floodplains are characterized by a predictable flooding event called the flood pulse. In the Southeastern Coastal Plain, this flood pulse typically occurs during the winter months (December-February) and is coupled with decreased evapotranspiration from riparian vegetation (Smock, 1999). During the flood pulse there is increased habitat and food resource availability which renders the floodplain an area of high biological productivity (Benke, 2001). Aquatic macroinvertebrates serve as a critical link between primary production and higher trophic levels (Wissinger, 1999). Macroinvertebrate production is high in floodplains due to the large surface area, habitat availability, and rapid turnover of generations (Benke et al., 1984; Benke, 2001). High levels of secondary (i.e., animal) productivity provides energy as prey to larger aquatic consumers such as predatory macroinvertebrates, fish, and birds. In addition, some aquatic macroinvertebrates that have an emergent aerial phase heavily subsidize the diets of many terrestrial consumers (Baxter et al., 2005). Their productivity is linked directly to the occurrence of regular, predictable flooding events. The Altamaha River is a high order river that runs through the Coastal Plains region of southeastern Georgia. The Altamaha contains no impoundments on the main stem of the river, which makes it a valuable study system (Benke and Cushing, 2010). Without artificially altered hydrology, the Altamaha provides a unique opportunity to study the effects of natural hydrological variations on aquatic ecosystems. The Moody Forest is a protected area of 1,794 hectares bordered by approximately 3 kilometers of the Altamaha and adjoining floodplain. The habitat contains one of Georgia’s few protected bottomland hardwood forests and cypress-tupelo sloughs. Since 2011, 60-76% of the state of Georgia has experienced severe to extreme drought conditions (U.S. Drought Monitor). Natural disturbances, such as drought, have negative impacts on biological communities. These disturbances can be defined as an event that varies in frequency, intensity, and duration from expected values (Resh et al., 1988; Lake, 2000). Drought in aquatic ecosystems can result in loss of habitat and food resource availability, which consequently leads to lowered biodiversity or productivity (Boulton, 2003). Due to the unpredictable nature of drought disturbances, there is a lack of baseline data about ecological communities before and during a disturbance (Lake, 2003;2008). The following study characterized the aquatic macroinvertebrate community in a floodplain area along the Altamaha during the 2011-2012 flood season. It is hypothesized that the aquatic macroinvertebrate community will change in concert with the hydrology of the area. This study is part of an ongoing effort to assess the ecosystem responses to supraseasonal disturbances, such as drought, which are expected to increase in frequency and intensity with predicted changes in climate. Methods Site Selection The study site consisted of a backwater area connected to the main channel of the Altamaha River. Sites were chosen from floodplain areas that contained water prior to the start of the 2011 flood pulse and from areas known to regularly inundate during peak flood stages. The main sampling area in 2011 was located at 17N 0506772 354434 UTM. Physicochemical Parameters Temperature (°C), dissolved oxygen (mg/L), conductivity (mS/cm), and pH were recorded monthly using a YSI Professional Plus Multi-parameter Water Quality Meter (YSI, Inc). Hydrologic data (discharge, gauge height) were obtained from USGS gauge 02225000 near Baxley, GA. Aquatic Macroinvertebrates Invertebrates were collected at shallow depths (< 1m) using a stovepipe benthic corer with a sampling area of 0.032 m and a maximum sampling depth of 0.61 m. Three core samples were collected monthly by inserting the core into the substrate and removing all the materials enclosed in the sampler to a depth of ~10cm into a 5 gallon bucket.