Biochemical composition of sedimentary organic matter and bacterial distribution in the Aegean Sea: trophic state and pelagic-benthic coupling

Biochemical composition of sedimentary organic matter (OM), vertical fluxes and bacterial distribution were studied at 15 stations (95-2270 m depth) i n the Aegean Sea during spring and summer. Downward fluxes of labile OM wer e significantly higher in the northern than in the southern part and were h igher in summer than in spring. Primary inputs of OM were not related to se dimentary OM concentrations, which had highest values in summer. Sedimentar y chlorophyll-a concentrations were similar in the northern and southern pa rts. Carbohydrates, the main component of sedimentary OR;I, were about 1.2 times higher in the southern part than in the northern; without significant temporal changes. Total proteins were higher in summer and about double in the northern part. Sedimentary proteins appeared more dependent upon the d ownward flux of phytopigment than of proteins. Sedimentary OM was character ised by a relatively large fraction of soluble compounds and showed better quality in the northern part. The lack of a depth-related pattern in sedime ntary OM and the similar concentrations in the two areas suggest that diffe rences in sedimentary OM quality in the Aegean basin are dependent on syste m productivity; the bulk of sedimentary OM is largely conservative. Sedimen tary bacterial density was about double in the northern part and higher in spring than in summer, but bacterial size was about three times higher in s ummer, resulting in a larger bacterial biomass in summer. Bacterial density was coupled with total and protein fluxes, indicating a rapid bacterial re sponse to pelagic production. Bacterial biomass was significantly correlate d with sedimentary protein and phytopigment concentrations, indicating a cl ear response to accumulation of labile OM in the sediments. In all cases ba cteria accounted for <5% of the organic C and N pools. The efficiency of be nthic bacteria in exploiting protein pools, estimated as amounts of protein available per unit bacterial biomass, indicates a constant ratio of about 70 mu g proteins/mu g C. This suggests a similar bacterial efficiency all o ver the area studied, unaffected by different trophic conditions. (C) 1999 Elsevier Science B.V. All rights reserved.