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

Abstract

In low-gradient geologically unconstrained streams, organic particles derived from floodplain soils are an important food source for aquatic life. Since 1993, we have been measuring particulate organic matter (POM) concentration in Ichawaynochaway Creek, a 5 th order blackwater tributary of the lower Flint River, Georgia, USA. Monthly samples have been collected during stable flow periods at 7 stations ranging from near the headwaters downstream to the confluence with the Flint River. Physical measures of channel and floodplain width at each station were used to calculate a floodplain index (floodplain/channel width ratio). POM concentration ranged from 0.46 to 5.65 mg/L and was strongly correlated with floodplain index, being consistently higher in reaches with greater floodplain/channel width ratios. Using streamflow 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 most years, a majority (55-85 %) of annual transport occurred from January through June corresponding with seasonal flood cycles. We used our POM rating curve and the long flow record (continuous since 1943) to investigate several hydrologic scenarios. A hydrologic analysis indicated consistently lower than average spring (March-June) and summer (July-September) daily discharges since the early 1970s. Reduced daily discharges were associated with increasing 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, particularly in trophic pathways involving amorphous detritus, non-filtering collectors, and their predators. We also examined how tropical storm frequency might affect POM transport. Years with 2 or more tropical storms had significantly greater summer POM transport than years with 1 or fewer storms. Frequent tropical storms may partially offset regional water use and drying, although it is not clear how climate change will affect storm frequency in the southeast. This study suggests a hierarchy of controls on POM concentration. At the regional scale, hydrology (i.e. magnitude of runoff) is the dominant influence while, at the stream reach scale floodplain geomorphology controls patterns of POM availability. Areas with well-developed floodplains appear to be source areas for organic particles. Management actions or water allocation formulae that systematically reduce the annual period of floodplain inundation may reduce organic particle transport from floodplains. Reductions in organic concentration would mean lower food availability to support aquatic life.