LOSS-CONTROLLED PHYTOPLANKTON PRODUCTION IN NUTRIENT-POOR LITTORAL WATERS OF THE NW MEDITERRANEAN - IN-SITU EXPERIMENTAL-EVIDENCE

The role of losses in controlling phytoplankton biomass and production off Blanes Bay (NW Mediterranean) were examined, using in situ dialys is cultures, during a period of very low nutrient concentration. The a mbient community experienced a negative net growth rate (-0.05 +/- 0.0 3 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 con trolled by consumers, even at a time of very low nutrient concentratio ns. 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 sl ightly. 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 g roups may achieve. These results clearly provide evidence that the bio mass and production of larger (>5 mu m) phytoplankton was controlled t op-down by metazoan consumers, which were excluded from the dialysis b ags, while picophytoplankton biomass was likely to be controlled by pr otists. This supports the notion that whether autotrophic production i s channelled through metazoan herbivores or through the 'microbial loo p' is closely dependent on the size (greater and smaller than about 5 mu m, respectively) of the autotrophs, and that top-down control of ph ytoplankton production may occur even when nutrients are very scarce.