Ecosystem Metabolism and Turnover of Organic Carbon along a Blackwater River Continuum

Variation in ecosystem metabolism and organic carbon turnover length was examined in the Ogeechee River (Georgia, USA) basin, a sixth—order blackwater river, to determine if the patterns observed in other rivers are also seen along a river continuum characterized by warm water temperatures, low stream gradient, extensive riparian swamps, and tea—colored water. The study was in two parts: an intensive study of seasonal variation in metabolism and organic carbon turnover length at a fourth—order site, which can be compared with an earlier study at a sixth—order site, and a study of the variation in system metabolism at five second— through sixth—order sites along the continuum. Metabolism varied seasonally at the fourth—order site that was studied most intensively; mean gross primary production (GPP, measured as O 2 ) was 0.8 g°m — 2 °d — 1 and mean community respiration (CR, also measured as O 2 ) was 4.1 g°m — 2 °d — 1 . The community production/respiration ratio (P/R) ranged from 0.02 to 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 organ carbon turnover length at the fourth—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 sixth—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 (DOC), which is less efficiently retained. Seasonal variation in turnover length reflected variation in transport. At five sites along the Ogeechee River continuum ranging from second to sixth order, GPP, CR, and P/R increased with stream order, as observed in other rivers; yet P/R was always <1, and net daily metabolism was always negative and became more negative with increasing stream order. In contrast to other river continua studied, 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. Although first—through third—order streams represent 32% of the channel area in the Sixth—order basin, only 4% of basin—wide GPP and 9% of basin—wide CR occurs in them. Higher order streams are less efficient in organic carbon processing (longer turnover lengths), but are nonetheless responsible for most of the metabolic activity in this river basin.