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 previously been hypothesized to be phosphorus (p) limited. All enclosures had a succession 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 production, 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 this succession. Because no stimulating effect of glycine addition on bacterial production or on community respiration was observed, the delay in bacterial response could not easily be explained as a delay in production or availability of carbon substrates for the bacteria. An alternative hypothesis is that bacterial production is regulated by a combination of P‐limited bacterial growth rate and predatory control of bacterial biomass. A simple numerical model based on this hypothesis was able to reproduce the main features of the observed pattern of succession.