Bacteria-protist interactions and organic matter degradation under P-limited conditions: Analysis of an enclosure experiment using a simple model

An enclosure experiment was performed in the brackish layer of a Norwegian fjord, a system where both phytoplankton and bacterial growth have previous ly been hypothesized to be phosphorus (p) limited. All enclosures had a suc cession pattern characterized by an initial autotrophic phase with positive net community production, increasing phytoplankton primary production, and chlorophyll, followed by a heterotrophic phase with decline in primary pro duction, negative net community production, increased community respiration , bacterial production. and biomass. Daily additions of phosphate increased the amplitudes of both the autotrophic and the heterotrophic phases of thi s succession. Because no stimulating effect of glycine addition on bacteria l production or on community respiration was observed, the delay in bacteri al response could not easily be explained as a delay in production or avail ability of carbon substrates for the bacteria. An alternative hypothesis is that bacterial production is regulated by a combination of P-limited bacte rial growth rate and predatory control of bacterial biomass. A simple numer ical model based on this hypothesis was able to reproduce the main features of the observed pattern of succession.