LIMITING NUTRIENTS AND BIOCHEMICAL COMPOSITION OF NATURAL MICROBIAL COMMUNITIES IN THE NW MEDITERRANEAN  <B>LIMITING NUTRIENTS AND BIOCHEMICAL COMPOSITION OF NATURAL MICROBIAL COMMUNITIES IN THE NW MEDITERRANEAN </B> <HR><P> Berdalet, E., Arin, L., Fara, A., Marrasé, C., Cruzado*, A. & Estrada, M.<BR> Institut de Ciències del Mar (CSIC), P. Joan de Borbó s/n, 08039 Barcelona (Spain)<BR> * Centre d'Estudis Avançats de Blanes, Cami de Santa Bàrbara, Blanes (Spain)</B><P><P> The role of N and P as limiting factors of the primary production and biomass in the Mediterranean Sea is still controversial. Laboratory experiments performed with natural planktonic communities from the NW Mediterranean coast suggest that some biochemical indicators can be used to identify N versus P deficiency. In particular, high N:P ratios were associated to high protein:DNA and chlorophyll:DNA ratios; the opposite trends were observed under low N:P ratios. In addition, low chlorophyll:protein ratios characterized N limited communities.<BR> <BR> Based on these results, we investigated the spatial and temporal changes of chlorophyll, protein and DNA concentrations (and their ratios) in open sea waters of the NW Mediterranean Sea during the stratification season. Several stations located along a transect between Barcelona and the Balearic Islands and with different hydrographic characteristics were repeatedly visited in June 1993 and June 1995.<P><P> In all stations, N and P were almost depleted in the euphotic zone, chlorophyll concentration presented deep maxima (DCM, between 40 and 60 m), DNA was high above or at the vicinity of the DCM and, protein concentration was high between surface and the DCM. Such high protein and DNA concentration at surface, where chlorophyll concentration was low, may be related to the high abundance of heterotrophic organisms (bacteria and flagellates) estimated in this part of the water column.<P><P> The high chlorophyll:protein ratios measured at the DCM could be related to a) a dominance of autotrophic organisms, b) the physiological conditions (low light levels and probably higher nutrient availability) that favor chlorophyll synthesis or, c) the specific composition of the phytoplankton (with a high diatom dominance at the DCM).<P><P> High protein:DNA ratios were found at all depths and stations. The comparison of the data with the experiments performed with natural communities suggest that both bacteria and phytoplankton are P limited in the open sea waters of the NW Mediterranean.<P><P>