Benthic–pelagic coupling in the oligotrophic Cretan Sea

Abstract

During the EU-MAST2 project CINCS (pelagic-benthic Coupling IN the oligotrophic Cretan Sea), sediment community oxygen consumption (SCOC) was measured during the winter and summer of 1995. Satellite CZCS images showed a different phytoplankton biomass in the surface water during these two periods. SCOC was measured in-situ with a benthic lander at depths ranging from 40 to about 1600 m. In conjunction to the SCOC measurements, microbial biomass and chlorophyll content of the surface layer of the sediment were also determined. SCOC, microbial biomass and the chlorophyll concentration displayed similar spatial and temporal trends, i.e. a steady decrease with increasing water depth in both seasons, with lower values occuring below 40 m depth during the summer. In winter SCOC ranged from 438 μmol m −2 h −1 at 40 m depth to 37 μmol m −2 h −1 at 1570 m which is equivalent to carbon mineralisation varying from 107 to 9 mg C m −2 d −1, respectively. SCOC values in the summer were about half the winter values except at the 40 m station where the opposite was found. Short-term deployments of a sediment trap 4 m above the sea floor showed diminished vertical fluxes of phytodetritus were lower in summer at all stations including 40 m depth. It is argued, partially on the basis of oxygen microprofiles, that the enhanced sedimentary concentration and decreased vertical flux of chlorophyll in summer at 40 m are primarily caused by benthic primary production. The results from the other stations show that in an highly oligotrophic sea such as the Cretan Sea, pelagic-benthic coupling exists although the amplitude of the seasonal signal is small. Moreover, the bathymetric trends in benthic microbial biomass and SCOC indicate that there is no substantial horizontal transport of labile organic material down the slope.