Hydrologic and geomorphic controls on suspended particulate organic matterconcentration and transport in Ichawaynochaway Creek, Georgia, USA

In low-gradient geologically unconstrained streams, organic particles deriv ed from floodplain soils are an important food source for aquatic life. Sin ce 1993, we have been measuring particulate organic matter (POM) concentrat ion in Ichawaynochaway Creek, a 5th order blackwater tributary of the lower Flint River, Georgia, USA. Monthly samples have been collected during stab le now periods at 7 stations ranging from near the headwaters downstream to the confluence with the Flint River. Physical measures of channel and floo dplain width at each station were used to calculate a floodplain index (flo odplain/channel width ratio). POM concentration ranged from 0.46 to 5.65 mg /L and was strongly correlated with floodplain index, being consistently hi gher in reaches with greater floodplain/channel width ratios. Using streamf low records at one of the sample sites, we found that POM concentration was significantly correlated with daily discharge. This relationship was used to develop a POM rating curve and estimate annual POM transport. During mos t years, a majority (55-85%) of annual transport occurred from January thro ugh June corresponding with seasonal flood cycles. We used our POM rating c urve and the long flow record (continuous since 1943) to investigate severa l hydrologic scenarios. A hydrologic analysis indicated consistently lower than average spring (March-June) and summer (July-September) daily discharg es since the early 1970s. Reduced daily discharges were associated with inc reasing agricultural water use and a drying trend in regional climate. Our rating curve predicted that POM transport would be substantially reduced in association with declining spring and summer discharges. Reductions in POM availability may cause long-term declines in secondary production, particu larly in trophic pathways involving amorphous detritus, non-filtering colle ctors, and their predators. We also examined how tropical storm frequency m ight affect POM transport. Years with 2 or more tropical storms had signifi cantly greater summer POM transport than years with 1 or fewer storms. Freq uent tropical storms may partially offset regional water use and drying, al though it is not clear how climate change will affect storm frequency in th e southeast. This study suggests a hierarchy of controls on POM concentrati on. At the regional scale, hydrology (i.e. magnitude of runoff) is the domi nant influence while, at the stream reach scale floodplain geomorphology co ntrols patterns of POM availability. Areas with well-developed floodplains appear to be source areas for organic particles. Management actions or wate r allocation formulae that systematically reduce the annual period of flood plain inundation may reduce organic particle transport from floodplains. Re ductions in organic concentration would mean lower food availability to sup port aquatic life.