Organic inputs and ecosystem efficiency in the deep Mediterranean sea

Abstract

Most studies on the benthic–pelagic coupling have focussed on how energy flow and fluxes of organic matter might influence the production and structure of the benthic communities. The classical scenario depicts the input of organic material from the photic zone fertilising the seabed and the consequent benthic nutrient regeneration supplying again the water column to sustain primary production processes. More recent studies highlighted the role of merobenthos as cysts of planktonic organisms that spend part of their life quiescent in the sediments. Such benthic propagules, when environmental conditions are not anymore adverse, can supply back plankton communities. Marine canyons might accumulate cysts derived from shelf export and reverse the direction of the coupling in a bottom up perspective. In the Mediterranean Sea, Eastern and Western basins display different productivity, flux rates and benthic standing stocks, with mass fluxes and organic carbon inputs up to 80 times higher in the Western basin. The low inputs reaching the deep-sea sediments are apparently responsible for the lack of coupling in the oligotrophic regions of the Eastern Mediterranean. The poverty of the organic input below 500 m, thought insufficient to fully support growth at the metazoan level, is partially balanced by a higher organic matter quality (i.e. bioavailability) in the more oligotrophic environments. In the Eastern Mediterranean, benthic organisms (e.g., bacteria and small metazoans) are 2–3 times more efficient than in the Western Mediterranean in exploiting the available food sources. These conditions are reflected also by bioenergetic strategies. In the more oligotrophic eastern basin, meiofauna body composition (as lipid, protein and carbohydrate content) displays an apparent biochemical coupling with the composition of the organic food sources and is apparently able to cope with longer periods of scarce and highly unpredictable food inputs. Finally also climate changes might influence the pelagic-benthic coupling modifying primary productivity and sedimentation rates. Recent changes in deep water mass characteristics of the south Aegean, have determined the formation, apparently related to climate anomalies, of new colder and higher salinity water masses. These waters replaced and lifted up the older nutrient-reach deep waters, thus enhancing primary production and phytodetritus input to the deep sea. Such phenomenon has determined a clear accumulation (up to 10-times) of labile organic carbon in the deep sea. There is increasing evidence that greenhouse effects and transient climate changes might have rapid and profound effects also on pelagic–benthic coupling in deep-sea ecosystems.