Floodplains are low gradient areas adjacent to the main channel of a river that are characterized by a predictable hydrological event in which lateral overbank flooding occurs. In the southeastern Coastal Plain of the US seasonal flooding typically begins in the winter months (December-March) and is coupled with decreased evapotranspiration and water usage from riparian vegetation (Smock 1999). During inundation, floodplains experience a large increase in surface area which renders them areas of high biological productivity and a major energetic contributor to riverine ecosystems (Benke 2001). High availability of snag habitat (i.e., deposited woody debris), which is characteristic of river floodplains in the southeastern US, facilitates macroinvertebrate production that can at times exceed that of the main channel (Benke et al. 1984; Benke 2001). Benthic macroinvertebrate communities in southeastern floodplains are typically dominated in abundance and biomass by Oligochaeta, Isopoda, Chironomidae larvae, mollusks and crustaceans (Batzer and Wissinger 1996; Smock 1999; Benke 2001). This macroinvertebrate biomass can be an important energetic linkage to higher trophic levels such as fish (Ross and Baker 1983; Batzer and Wissinger 1996). Floodplains provide other important ecosystem services such as habitat availability, food resources, organic matter processing and energy export and subsidies (Junk et al. 1989; Benke 2001, Bush et al. 2017). Ecosystem functions provided by floodplains are valued at approximately $25,682 ha− 1 yr− 1, which is second only to ecosystem services provided by estuaries (de Groot et al. 2012; Costanza et al. 2014).
The valuable economic and ecological functions of floodplain ecosystems are maintained by periodic seasonal flooding. Flood pulse events, as described by Junk et al. (1989), provide increased habitat and nutrient availability as well as organic matter transport to the main channel (Cuffney 1988; Jones and Smock 1991; Benke 2001). Tockner et al. (1999) identified three distinct phases of the flood pulse: biotic interaction, primary productivity, and transport. The biotic interaction phase occurs prior to flooding and are characterized by stagnation, nutrient limitation and competition by phytoplankton and microcrustacean. The primary productivity phase occurs during the inundation, which creates an open system with high surface area and high levels of algal biomass and primary production. Transport phase occurs when there is high water level in the main stem of the river and consequent overflow into the floodplain. The ability of a flood pulse to succeed in reaching the primary production and transport phase is dependent on several characteristics of river flow, e.g., timing, magnitude, and frequency. Components of river flow are often considered the key variables in riverine ecosystems due to their importance in maintaining ecological integrity and biological diversity (Resh et al. 1998; Poff and Ward 1989, Poff et al. 1997). For floodplains, where lateral connectivity is integral to ecosystem function, disruption in overbank flooding can lead to a loss of diversity and thus negatively affect the ecosystem function and services they provide (Ward and Stanford 1995; Ward et al. 1999). Consequences of alteration to the flow regime can be observed across taxonomic groups and in certain species/habitat interactions (Bunn and Arthington 2002).
Large river systems without major impoundments are increasingly rare as there are an estimated 2.8 million dams worldwide and 91,000 dams in the continental US (Lehner et al. 2011; US Army Corps of Engineers 2019). Dams frequently homogenize flow regimes, disrupting the historical patterns of flow regimes, and disconnect main stem rivers from floodplains (Poff et al. 2007; Bolland et al. 2012). Unlike in other major river basins in the southeastern Coastal Plain (e.g., Savannah River) and the rest of the continental US, there are no major impoundments on the main stem of the Altamaha River (Reese and Batzer 2007). Despite not having major impoundments, floodplain systems like the Altamaha, are still susceptible to the effects of climate change, such as prolonged drought. Supra-seasonal droughts are becoming more prevalent with human modified flow regimes, low precipitation years, and climate change (Lake 2003; Lake 2008; Bloschl et al. 2017). From 2009–2013, Georgia experienced a multi-year, supra-seasonal drought disturbance (US Drought Monitor 2013). During this time, discharge on the main stem of the Altamaha River near Baxley, Georgia, was well below the 42-year average (USGS 2013). Extended periods of drought can have negative consequences on structure and function of aquatic communities (Parasiewicz et al. 2019). Investigating aquatic communities in the transition period between prolonged drought and flooding can provide insight to the potential of the community to recover.
To assess patterns of aquatic macroinvertebrate community structure in a floodplain following a drought, we compared macroinvertebrate samples collected during a flood pulse event from a floodplain of the Altamaha River during years in which the river experienced drastically different flows. The first year of sampling (December 2011-April 2012), hereafter referred to as the “drought year,” was characterized by and preceded by two years of severe to extreme drought. The second year of sampling (December 2012-April 2013), hereafter referred to as the “flood year,” was characterized by higher than average discharge at the main stem. In order to determine how invertebrate biomass and community composition differed in a forested wetland area during drought and flooding conditions, we quantified hydrology and water quality; macroinvertebrate richness, abundance, biomass, community composition, and functional feeding group (FFG) and benthic organic matter standing stocks. We hypothesized that aquatic macroinvertebrate community composition would differ based on overall habitat availability (presence/absence of flooded area) and availability of organic matter as a source of food. We predicted that there would be taxonomically distinct invertebrate communities in each sampling year and higher productivity (biomass) in the flood year. Taxonomic richness is predicted to increase over the course of the sampling period during both years.