Bacterial response to seasonal changes in labile organic matter composition on the continental shelf and bathyal sediments of the Cretan Sea

Bacterial abundance, biomass and cell size were studied in the oligotrophic sediments of the Cretan Sea (Eastern Mediterranean), in order to investiga te their response to the seasonal varying organic matter (OM) inputs. Sedim ent samples were collected on a seasonal basis along a transect of seven st ations (ranging from 40 to 1570 m depth) using a multiple-corer. Bacterial parameters were related to changes in chloroplastic pigment equivalents (CP E), the biochemical composition (proteins, lipids, carbohydrates) of the se dimentary organic matter and the OM flux measured at a fixed station over t he deep basin (1570 m depth). The sediments of the Cretan Sea represent a n utrient depleted ecosystem characterised by a poor quality organic matter. All sedimentary organic compounds were found to vary seasonally, and change s were more evident on the continental shelf than in deeper sediments. Bact erial abundance and biomass in the sediments of the Cretan Sea (ranging fro m 1.02 to 4.59 x 10(8) cells g(-1) equivalent to 8.7 and 38.7 mu gC g(-1)) were quite high and their distribution appeared to be closely related to th e input of fresh organic material. Bacterial abundance and biomass were sen sitive to changes in nutrient availability, which also controls the average cell size and the frequency of dividing cells. Bacterial abundance increas ed up to 3-fold between August '94 and February '95 in response to the incr eased amount of sedimentary proteins and CPE, indicating that benthic bacte ria were constrained more by changes in quality rather than the quantity of the sedimentary organic material. Bacterial responses to the food inputs w ere clearly detectable down to 10 cm depth. The distribution of labile orga nic compounds in the sediments appeared to influence the vertical patterns of bacterial abundance and biomass. Cell size decreased significantly with water depth. Bacterial abundance and biomass were characterised by clear se asonal changes in response to seasonal OM pulses. The strong coupling betwe en protein flux and bacterial biomass together with the strong bacterial do minance over the total biomass suggest that the major part of the carbon fl ow was channelled through the bacteria and the benthic microbial loop. (C) 2000 Elsevier Science Ltd. All rights reserved.