Ecosystem metabolism and turnover of organic carbon along a blackwater river continuum

Abstract

Variation in ecosystem metabolism and organic carbon turnover length was examined in the Ogeechee River, Georgia, basin, a 6th order blackwater river. Metabolism varied seasonally at the 4th order site studied most intensively; mean gross primary production (GPP, measured as O2) was 0.8 g.m-2.d-1 and mean community respiration (CR, also measured as O2) was 4.1 g.m-2.d-1. The community production/respiration rate (P/R) ranged from 0.02-0.4 with an annual mean of 0.2. This is clearly a heterotrophic stream reach, where high rates of community respiration are supported by inputs of organic matter from extensive riparian swamps. Mean annual organic carbon turnover length at the 4th order site was 570 km, although on most dates the length was much less, with a median of 114 km. Turnover lengths are significantly less than those at the 6th order site, but much greater than observed in other rivers of similar size, probably because most of the organic carbon export in this river is dissolved organic carbon, which is less efficiently retained. At 5 sites along the Ogeechee River continuum ranging from 2nd-6th order, GPP, CR and P/R increased with stream order, yet P/R was always <1, and net daily metabolism was always negative and became more negative with increasing stream order. Metabolism in this low-gradient blackwater river becomes more dependent on allochthonous organic carbon with increasing stream size despite downstream increases in GPP; floodplains and riparian swamps appear to be the source of allochthonous carbon. Riparian influences increase rather than decrease along the continuum in this river. -from Authors