Enviro-Days 1997 Relative Importance of Bacteria and Phytoplankton to Higher Trophic Levels

Cannon, T. A., Wu, K. W., Blum, L. K.

The relative importance of phytoplankton and bacteria to consumers was compared in tidal creeks of a net autotrophic and a net heterotrophic estuary. Grazing of bacterial cells was compared to grazing of phytoplankton cells using a dilution technique. Standing stock and production of both bacteria and phytoplankton were also measured. Bacterial abundance, biovolume, and productivity were all significantly greater in autotrophic water, but bacterial standing stock turnover times were slower than in heterotrophic water. Grazing constants for consumption of bacteria were consistently higher in the heterotrophic (mean=0.034/h) than in autotrophic (mean=0.014/h) creeks. The phytoplankton standing stocks were consistently greater in the autotrophic system than in the heterotrophic estuary (25.6 +/- 1.6 and 12.0 +/- 0.3 um chl-a/L, respectively for June 1995). Phytoplankton production was at least twice as high for the autotrophic water. In contrast to measures of grazing on bacteria, consumption of phytoplankton appears to balance production in the autotrophic estuary, while no measurable grazing of phytoplankton was observed in the heterotrophic estuary. These results suggest that bacteria in autotrophic waters function primarily in nutrient cycling, but that when phytoplankton standing stock and production are low, bacteria may function as important secondary producers if allochthonous sources of carbon are available.

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