Bottom-up controls on bacterial production in tropical lowland rivers

The importance of organic carbon and inorganic nutrients in controlling bacterial production was investigated in two tropical lowland rivers draining undisturbed forested catchments. Glucose (C), ammonium (N), phosphate (P), leaf leachate, and algal leachate were added alone or combined to water collected from one clear-water (Cataniapo) and one black-water (Autana) river of the Middle Orinoco basin, and bacterial production (BP)was measured at 0, 8, 24, and 36 h of incubation. The rivers have low pH (3.8-5.9) and conductivity (6.3-9.1 muS cm(-1)) and abundant nitrogen (total N, 273-314 mug L-1) compared to phosphorus (total P, 3.6-5.5 mug L-1). BP was significantly stimulated by additions of P, CP NP, and CNP in both river waters. N or C alone or combined did not stimulate BP, suggesting that P rather than carbon is the primary limiting nutrient in these rivers.. Higher responses to CNP and CP amendments (3-7 times over controls), compared to P and NP (1.3-4 times over controls), indicate that carbon is an important secondary constraint to bacterial production. Responses to NP and P were more frequent in the Autana than in the Cataniapo, suggesting that bacteria in the clear-water river were less resource limited than those in the black-water river. Responses to added leaf leachate, which had high concentrations of P and dissolved organic carbon (DOC), were generally greater than responses to added algal leachate. Seasonal patterns in response to nutrient addition suggest that the size of the labile fraction of DOC may increase during periods of low and rising water.