Natural plankton communities from Masnou, a locality 20 km north of Barcelona (NW Mediterranean coast), were enclosed in 30 l microcosms to test the effect of different availability of nitrogen (N) and phosphorus (P) on the biomass of the main microplankton groups, and the biochemical composition (DNA, protein and chlorophyll concentration) of microbial communities. Immediately after enclosure in microcosms, three different nutrient enrichments were performed: N-deficient, P-deficient and nutrient-balanced. N and P deficiencies affected the structure and the biochemical composition of the microbial communities. Phytoplankton assemblages showed similar temporal patterns under the three nutrient treatments, although the relative contribution of the different groups was notably affected. The lowest DNA concentration was measured in the P-deficient treatment, suggesting that P availability imposes the limits on the DNA levels in the ecosystem. The availability of N in the P-deficient microcosms allowed relatively high synthesis of chlorophyll and protein until the end of the experiment. Significantly high chlorophyll: DNA and protein: DNA ratios characterized the P-deficient treatment (where N was available) compared to the N-deficient microcosms. From the results obtained, we suggest that the protein: DNA ratios may constitute a biochemical indicator of the P versus N availability in natural ecosystems L.A. was supported by the ICI (Instituto de Cooperacidn Iberoamericana) fellowship. This work was supported by EU grants EV5V-CT92-0215 and MAS2-CT93-0063