Effect of nutrient supply on the biomass structure of planktonic communities: an experimental test on a Mediterranean coastal community

The hypothesis that increasing nutrient supply increases the biomass of aut otrophs proportionately more than the biomass of heterotrophs was tested by increasing (0, 1-, 2-, 4-, 8-, and 16-fold over the background loading of 5 mmol N m(-2) d(-1), 1.6 mmol Si m(-2) d(-1), and 0.25 mmol P m(-2) d(-1)) the addition of nutrients to large (33 000 1) mesocosm units enclosing an oligotrophic coastal Mediterranean planktonic community. Autotrophic plankt on biomass increased 50-fold along the range of nutrient inputs, whereas he terotrophic biomass increased only 10-fold. Heterotrophic biomass increased as the 1/5 power of the increase in the biomass of autotrophs, implying th at the ratio of heterotroph to autotroph biomass (HB/AB ratio) declined rap idly as the biomass of autotrophs increases with increasing nutrient inputs . The biomass distribution within the community shifted from an 'inverted p yramid' distribution, involving greater biomass of heterotrophs than that o f autotrophs, at low nutrient inputs, to the conventional 'upward' pyramid pattern, where the biomass of autotrophs exceeds that of consumers, at the highest nutrient inputs. This shift stabilized after 4 d, and the pyramids remained quite constant for the rest of the experiment. The experimental te st presented supports the hypothesis that the relative biomass distribution between heterotrophs and autotrophs is regulated by nutrient supply.