Dynamics of meiofaunal assemblages on the continental shelf and deep-sea sediments of the Cretan Sea (NE Mediterranean): relationships with seasonal changes in food supply

Quantitative information on metazoan meiofaunal abundance and biomass was o btained from three continental shelf (at 40, 100 and 200 m depth) and four deep-sea stations (at 540, 700, 940 and 1540 m depth) in the Cretan Sea (So uth Aegean Sea, NE Mediterranean). Samples were collected on a seasonal bas is (from August 1994 to September 1995) with the use of a multiple corer. M eiofaunal abundance and biomass on the continental shelf of the Cretan Sea were high, in contrast to the extremely low values reported for the bathyal sediments that showed values comparable to those reported for abyssal and hadal environments. In order to explain the spatial and seasonal changes in metazoan meiofauna these data were compared with: (1) the concentrations o f 'food indicators' (such as proteins, lipids, soluble carbohydrates and CP E) (2) the bacterial biomass (3) the flux of labile organic compounds to th e sea floor at a fixed station (D7, 1540 m depth). Highly significant relat ionships between meiofaunal parameters and CPE, protein and lipid concentra tions and bacterial biomass were found. Most of the indicators of food qual ity and quantity (such as CPE, proteins and carbohydrates) showed a clear s easonality with highest values in February and lowest in September. Such ch anges were more evident on the continental shelf rather than at deeper dept hs. On the continental shelf, significant seasonal changes in meiofaunal de nsity were related to changes in the input of labile organic carbon whereas meiofaunal assemblages on the deep-sea stations showed time-lagged changes in response to the food input recorded in February 95. At all deep-sea sta tions meiofaunal density increased with a time lag of 2 months. Indications for a time-lagged meiofaunal response to the food inputs were also provide d by the increase in nauplii densities during May 95 and the increase in in dividual biomass of nematodes, copepods and polychaetes between February an d May 1995. The lack of strong seasonal changes in deep sea meiofaunal dens ity suggests that the supply of organic matter below 500 m is not strong en ough to support a significant meiofaunal development. Below 700 m depth >92 % of the total biomass in the sediment was represented by bacteria. The rat io of bacterial to meiofaunal biomass increased with increasing water depth indicating that bacteria are probably more effective than meiofauna in exp loiting refractory organic compounds. These data lead us to hypothesise tha t the deep-sea sediments of the Cretan Sea are largely dependent upon a ben thic microbial loop. (C) 2000 Elsevier Science Ltd. All rights reserved.