DISTRIBUTIONS AND FLUXES OF PHOSPHORUS, NITROGEN, AND ORGANIC CARBON IN A SOUTHEASTERN COASTAL PLAIN RIVER: IMPLICATIONS OF RIVER FLOW AND FLOODPLAIN INTERACTIONS (NUTRIENTS)

Much of our present understanding of nutrient dynamics in stream ecosystems is based on information from low-order, upland streams. However, the complexity of nutrient cycling becomes more enigmatic in downstream segments. In high-order, coastal plain rivers, patterns of nutrient distribution and transport are functions of in-stream cycling, benthic interactions, tributary inputs and exchanges with broad vegetated floodplains during periods of inundation. Spatial and seasonal trends in nutrient distributions in these systems remain poorly understood.; I examined how hydrologic regime, distributions of forested floodplains and sub-basin drainage tributary interact to control spatial distributions and fluxes of nutrients.; Major fractions of C, N, and P were studied for 18 months (March 1983 - August 1984) in the Lower Santee coastal-plain River which represents the last 140 km segment of a 41,000 km('2) watershed. Twenty-seven percent of its 1,942 km('2) sub-basin drainage area constitutes forested floodplain. I have introduced the concept of Wetland Factor to describe the distribution of floodplains along each segment.; Annual mean concentration of Total Phosphorus was 4.17 (+OR-) 0.14 ug at/L and was dominated by particulate fractions (PP = 57%). Dissolved Organic Phosphorus (DOP) and orthophosphate (O-PO4) constituted 24 and 19 percent of the annual mean. O-PO4 showed well defined temporal variability. Inputs of O-PO4 during initial floods was significant (P < 0.04). DOP significantly decreased downstream (P < 0.008) during summer flow while DOP imports from floodplain sources correlated with positive net fluxes. The floodplain was a sink for O-PO4 and PP.; Annual mean concentration of Total Nitrogen was 13.55 (+OR-) 0.5 ug at/L. Nitrate-Nitrite represented 46% while Particulate-Nitrogen (PN), Dissolved-Organic-Nitrogen (DON) and Ammonia-Nitrogen represented 31, 12 and 11 percent respectively. NO(,3)-NO(,2) correlated well with flow and growing season while the inorganic sub-species significantly decreased (P < 0.006; P < 0.012) downstream.; Total Organic Carbon annual mean concentration was 8.48 (+OR-) 0.24 mg/L with Dissolved Organic Carbon (DOC) representing 77% and 23% represented by Particulate Organic Carbon (POC). Significant DOC increase downstream (P < 0.02) correlated well with floodplain and tributary sources of this subspecies. The floodplain was a sink for POC.; The mechanisms which modulated the distributions and fluxes of the nutrients included the variable inputs from the source water and drainage tributaries, seasonal exchanges between the river and floodplain and variations in metabolism.