Vertical flux of faecal pellets and microplankton on the shelf of the oligotrophic Cretan Sea (NE Mediterranean Sea)

To analyse and quantify the seasonal particulate vertical flux of faecal pe llets and microplankton in the oligotrophic Cretan Sea, sediment traps were deployed at 200 m and 1500 m depth on the shelf north of Crete. Microscopi cal analysis revealed numerous zooplankton faecal pellets and intact cells of foraminifera and phaeodarian radiolarians, but few phytoplankton cells a mong the sedimented material. The greatest share among the identified parti cles was comprised by small (30-65 mu m) and medium (80-200 mu m) sized ell iptical faecal pellets. A maximum flux of large (250-400 mu m) and medium s ized elliptical pellets was recorded during March to May at 200 m, but in g eneral faecal pellet flux was rather uniform and increased at depth. On a s easonal basis the estimated carbon flux from identifiable particles was cal culated to 0.175 and 0.303 mg C m(-2) d(-1) at 200 m and 1500 m depth, resp ectively. This was only about 6% of the vertical POC flux. The relative con tribution of faecal pellets decreased from 93% to 84% of the carbon content of the identified sedimenting particles at 200 m and 1500 m depth, respect ively. The increased faecal pellet flux recorded at 200 m depth during spri ng may probably reflect changes in the balance between the 'classical' graz ing and microbial food web in the euphotic zone, followed by significant re tention during periods when the microbial food web dominates. However, the vertical flux at depth, receives only a small contribution directly from th e surface layer production at the investigation site. Vertical flux at dept h reflects advective transport of biogenic matter across the shelf and slop e, as well as resuspension and faecal pellets from a deep-dwelling zooplank ton community. Signals of direct and rapid deposition of surface water appe ar buffered and greatly modified by mid- and deep-water zooplankton communi ties on the Cretan Sea shelf. (C) 2000 Elsevier Science Ltd. All rights res erved.