Seasonal variation of particulate organic carbon, dissolved organic carbonand the contribution of microbial communities to the live particulate organic carbon in a shallow near-bottom ecosystem at the Northwestern Mediterranean Sea

Microbial planktonic communities (i.e. bacteria and protozoa), phytoplankto n, dissolved organic carbon (DOC) and particulate organic carbon (POC) were seasonally examined at Medes Islands (Northwestem Mediterranean) to assess their variation in abundance and composition throughout the year in a near -bottom littoral ecosystem. From October 1995 to November 1996, samples wer e collected between two and six times per month at 0.5 m above the bottom. Mean DOC and POC values throughout the year were 2560 +/- 180 (SE) and 387 +/- 35 mu g C l(-1), respectively. All year, detrital organic carbon (detri tal = total POC - live carbon) represented the main POC fraction, and mean live carbon was 24 +/- 9 mu g C l(-1). Winter and spring had maximum values of POC, and spring and summer had maximum values of DOC. Heterotrophic bac teria, with a mean abundance of 5.16 +/- 0.08 x 10(5) cells ml(-1), were th e main contributor to live carbon (26 +/- 7%). During winter, heterotrophic bacterial biomass decreased 40% due to a decrease in mean biovolume per ce ll. Synechococcus sp. and Prochlorococcus sp. abundance were 2.24 rt 0.09 x 10(4) and 1.05 +/- 0.07 x 10(4) cells ml(-1), respec tively. However, whil e Synechococcus sp. were present all year, Prochlorococcus sp. were not obs erved from April to July. Mean phytoplankton (i.e. diatoms and dinoflagella tes) abundance was 2.06 +/- 0.40 x 10(4) cells l(-1) with biomass at a maxi mum during the winter months, the period with the lowest temperature and th e highest nutrient concentration. The size composition of live carbon showe d two clearly distinct periods: from December to March, live carbon was dom inated in biomass by microplankton, while from April to November, pico- and nanoplankton cells were dominant. Overall, the dynamics of the near-bottom planktonic communities was characterized by a low biomass of heterotrophic and autotrophic bacteria, phytoplankton and ciliates in contrast to previo us water column studies. This pattern and the high temporal heterogeneity o f the different planktonic communities are discussed in relation to the phy sical and chemical characteristics of the environment, as well as to the po tential role that benthic communities may be exerting in the control of the near-bottom planktonic communities.