Benthic response to particulate fluxes in different trophic environments: a comparison between the Gulf of Lions-Catalan Sea (western-Mediterranean) and the Cretan Sea (eastern-Mediterranean)

Quantitative information on particle fluxes, sedimentary OM composition, mi crobial and meiofaunal parameters is summarised from various stations in th e Gulf of Lions and Catalan Sea (North-Western Mediterranean) and in the Cr etan Sea (Eastern Mediterranean), investigated between 1993 and 1996. Benth ic responses in relation to the different trophic conditions in the two are as were compared in terms of: (1) temporal and spatial variability of the m ass fluxes; (2) pelagic-benthic coupling in organic matter composition (suc h as proteins, lipids, soluble carbohydrates and CPE); (3) microbial respon se to changes in organic matter composition and POC fluxes; (4) meiofaunal response to changes in organic matter composition and particle fluxes, (5) relative significance of bacteria and meiofauna, The two areas were also co mpared to identify ratios between productivity, vertical fluxes and benthic standing stocks. Mass fluxes at equal depths were up to two orders of magn itude higher in the Western than in the Eastern Mediterranean. Clear season al changes were reported in both areas, although mass fluxes and variabilit y were consistently higher in the North-Western Mediterranean, From primary production estimates in the Western and Eastern Mediterranean (140-160 vs. approximate to 19-60 mgC m(-2) d(-1), respectively) it has been calculated that a carbon export from the euphotic layer to 1000 m depth is equivalent to about 10% in the Gulf of Lions and 2-3% in the Cretan Sea. Chlorophyll- a concentrations, at similar depths, were 2-3 times higher in the Western b asin. Carbohydrates were the dominant biochemical component in the Cretan S ea sediments while total amino-acids represented an important fraction of t he biodegradable material in the Gulf of Lions-Catalan Sea. In the Western Mediterranean, bacterial densities (annual mean 7.9x10(8) cells g(-1)) were about 4 times higher than in the Cretan Sea (annual mean 2.1x10(8) cells g (-1)), indicating that, in deep-sea sediments, different trophic conditions are influencing bacterial densities. Meiofaunal abundance and biomass were similar on the continental shelves of both areas (940-2558 ind./10 cm(2)) but, at bathyal depths, densities in the Cretan Sea (range: 60-120 ind./10 cm(2)) were 4-25 times lower than those in the NW-Meditenanean (500-1500 in d./10 cm(2)). In contrast to what was observed in the North-Western Mediter ranean, deep-sea meiofaunal assemblages of the Cretan Sea did not react (in terms of density or biomass) to the seasonal variations in food inputs. He re, the bacterial to meiofaunal biomass ratio displayed much higher values (up to >20), possibly causing competition for food sources with small metaz oans. The efficiency with which the POC fluxes were being exploited was est imated. In the NW-Mediterranean about 0.14 mgC/d are available to each meio faunal individual compared to the 0.07 mgC ind.(-1) of the Eastern Mediterr anean. Similarly, 3.3 mgC were provided daily per 1 mgC of bacterial biomas s in the Western Mediterranean, compared to 0.07 in the Eastern Mediterrane an. These data indicate that the benthic components in the Cretan Sea, are subject to more limiting trophic conditions, and so might have a higher eff iciency in exploiting the particulate organic fluxes. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.