Rver Continuum

Variation in ecosystem metabolims and organic carbon turnover length was examie in the Ogeeechee river ( Geórgia, usa) basin a sixth order blackwater river, to determine IF the patterns observed in the 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 seasional variation in metabolism and organic carbon turnover length at a fourth-order site, which can be compared with an carlier study at a sixth-order site, and a study of the variation in system metabolism at five second-through sixt-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) was 0.8g and mean community respiration (cr, also mensured as o2) was 4.1g . the community production/ respiration ratio (p/r) ranged from 0.02 t 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 ate the fourth-order site was 570km although on most dates the the length was much less, with a median of 114km. turnover lengths are significantly less than those at the sixth-order site, but much greater than observed in the other rivers 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 vaiation 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 size despite downstream increases in GPP floodplains and riparian swamps appear to be he source of allochthonous carbon. Riparia 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 GPPand 9% of basin-wide CR occurs in them. Higher order streams are less efficien in organic carbon processing (longer turnover lengths), but are nonetheless responsible for most of the metabolic activity in this river basin .

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