Abstract
The role of losses in controlling phytoplankton biomass and production off Blanes Bay (NW Mediterranean) were examined, using in situ dialysis cultures, during a period of very low nutrient concentration. The ambient community experienced a negative net growth rate (-0.05 ± 0.03 divisions d−1), compared to relatively high community growth rates (0.78 ± 0.07 div. d−1) when metazoan grazers and physical losses, which appeared to be negligible, were excluded, indicating that loss rates (0.83 div. d−1) exceeded growth rates. This provided evidence that the biomass and production of the phytoplankton community was controlled by consumers, even at a time of very low nutrient concentrations. The community structure changed in response to the suppression of losses, leading to an enhanced growth of nano- and microphytoplankton, while the abundance of picoplanktonic autotrophs tended to decline slightly. Yet, the biomass attained by nano- and microphytoplankton was similar despite large differences in their initial density, indicating the existence of a resource limitation of the biomass the different groups may achieve. These results clearly provide evidence that the biomass and production of larger (>5 μm) phytoplankton was controlled top-down by metazoan consumers, which were excluded from the dialysis bags, while picophytoplankton biomass was likely to be controlled by protists. This supports the notion that whether autotrophic production is channelled through metazoan herbivores or through the 'microbial loop' is closely dependent on the size (greater and smaller than about 5 μm, respectively) of the autotrophs, and that top-down control of phytoplankton production may occur even when nutrients are very scarce.