Seasonal changes and biochemical composition of the labile organic matter flux in the Cretan Sea

Downward fluxes of labile organic matter (lipids, proteins and carbohydrate s) at 200 (trap A) and 1515 m depth (trap B), measured during a 12 months s ediment trap experiment, are presented, together with estimates of the bact erial and cyanobacterial biomasses associated to the particles. The biochem ical composition of the settling particles was determined in order to provi de qualitative and quantitative information on the flux of readily availabl e organic carbon supplying the deep-sea benthic communities of the Cretan S ea. Total mass flux and labile carbon fluxes were characterised by a clear seasonality. Higher labile organic fluxes were reported in trap B, indicati ng the presence of resuspended particles coming from lateral inputs. Partic ulate carbohydrates were the major component of the flux of labile compound s (on annual average about 66% of the total labile organic flux) followed b y lipids (20%) and proteins (13%). The biopolymeric carbon Aux was very low (on annual average 0.9 and 1.2 gC m(-2) y(-1), at trap A and B). Labile ca rbon accounted for most of the OC flux (on annual average 84% and 74% in tr ap A and B respectively). In trap A, highest carbohydrate and protein fluxe s in April and September, corresponded to high faecal pellet fluxes. The qu alitative composition of the organic fluxes indicated a strong protein depl etion in trap B and a decrease of the bioavailability of the settling parti cles as a result of a higher degree of dilution with inorganic material. Qu antity and quality of the food supply to the benthos displayed different te mporal patterns. Bacterial biomass in the sediment traps (on average 122 an d 229 mu gC m(-2) d(-1) in trap A and B, respectively) was significantly co rrelated to the flux of labile organic carbon, and particularly to the prot ein and carbohydrate fluxes. Cyanobacterial flux (on average, 1.1 and 0.4 m u gC m(-2) d(-1), in trap A and B, respectively) was significantly correlat ed with total mass and protein fluxes only in trap A. Bacterial carbon flux , equivalent to 84.2 and 156 mgC m(-2) y(-1), accounted for 5-6.5% of the l abile carbon flux (in trap A and B respectively) and for 22-41% protein poo l of the settling particles. These results suggest that in the Cretan Sea, bacteria attached to the settling particles represent a potential food sour ce of primary importance for deep-sea benthic communities. (C) 2000 Elsevie r Science Ltd. All rights reserved.