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, microbial and meiofaunal parameters is summarised from various stations in the Gulf of Lions and Catalan Sea (North-Western Mediterranean) and in the Cretan Sea (Eastern Mediterranean), investigated between 1993 and 1996. Benthic responses in relation to the different trophic conditions in the two areas were compared in terms of: (1) temporal and spatial variability of the mass fluxes; (2) pelagic-benthic coupling in organic matter composition (such as proteins, lipids, soluble carbohydrates and CPE); (3) microbial response 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 compared to identify ratios between productivity, vertical fluxes and benthic standing stocks. Mass fluxes at equal depths were up to two orders of magnitude higher in the Western than in the Eastern Mediterranean. Clear seasonal changes were reported in both areas, although mass fluxes and variability 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 basin. Carbohydrates were the dominant biochemical component in the Cretan Sea sediments while total amino-acids represented an important fraction of the 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 ind./10 cm(2)). In contrast to what was observed in the North-Western Mediterranean, deep-sea meiofaunal assemblages of the Cretan Sea did not react (in terms of density or biomass) to the seasonal variations in food inputs. Here, the bacterial to meiofaunal biomass ratio displayed much higher values (up to >20), possibly causing competition for food sources with small metazoans. The efficiency with which the POC fluxes were being exploited was estimated. In the NW-Mediterranean about 0.14 mgC/d are available to each meiofaunal individual compared to the 0.07 mgC ind.(-1) of the Eastern Mediterranean. Similarly, 3.3 mgC were provided daily per 1 mgC of bacterial biomass in the Western Mediterranean, compared to 0.07 in the Eastern Mediterranean. These data indicate that the benthic components in the Cretan Sea, are subject to more limiting trophic conditions, and so might have a higher efficiency in exploiting the particulate organic fluxes. (C) 1999 Elsevier Science Ltd. All rights reserved.