Organic matter composition of the continental shelf and bathyal sediments of the Cretan Sea (NE Mediterranean)

The seasonal, spatial and bathymetric changes in the distribution of chloro plastic pigments (Chl a, phaeopigments and CPE), TOC, TON, ATP, bottom wate r nutrient content and the main biochemical classes of organic compounds (l ipids, proteins and carbohydrates) were recorded from May 1994 to September 1995 over the continental margin of northern Crete. The concentration of c hloroplastic pigment equivalents (CPE) was always low, dropping dramaticall y along the shelf-slope gradient. Microbial activity (ATP) also dropped sha rply beyond the continental shelf following a distribution pattern similar to TOC and TON. Lipid, protein and carbohydrate concentrations, as well as biopolymeric carbon were comparable to those reported for other more produc tive areas, however, the quality of the organic matter itself was rather po or. Thus, carbohydrates, the dominant biochemical class, were characterised by being highly (80-99%) refractory, as soluble carbohydrates represented (on annual average) only 6% of the total carbohydrate pool. Protein and lip id concentrations strongly decreased with depth, indicating depletion of tr ophic resources in the bathyal zone. Proteins appeared to be the more degra dable compounds and indeed the protein to carbohydrate ratios were found to decrease strongly in the deeper stations. Organic matter content and quali ty decreased both with increasing distance from the coast and within the se diment. All sedimentary organic compounds were found to vary between sampli ng periods, with the changes being more pronounced over the continental she lf. The different temporal patterns of the various components suggest a dif ferent composition and/or origin of the OM inputs during the different samp ling periods. The amount of material reaching the sediments below 540 m is extremely low, suggesting that most of the organic material is decomposed a nd/or utilised before reaching the sea floor. In conclusion, the continenta l shelf and bathyal sediments of the Cretan Sea can be considered, from a t rophic point of view, as two different subsystems. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.