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

Abstract

To analyse and quantify the seasonal particulate vertical flux of faecal pellets and microplankton in the oligotrophic Cretan Sea, sediment traps were deployed at 200 m and 1500 m depth on the shelf north of Crete. Microscopical analysis revealed numerous zooplankton faecal pellets and intact cells of foraminifera and phaeodarian radiolarians, but few phytoplankton cells among the sedimented material. The greatest share among the identified particles was comprised by small (30–65 μm) and medium (80–200 μm) sized elliptical faecal pellets. A maximum flux of large (250–400 μm) and medium sized elliptical pellets was recorded during March to May at 200 m, but in general faecal pellet flux was rather uniform and increased at depth. On a seasonal basis the estimated carbon flux from identifiable particles was calculated to 0.175 and 0.303 mg C m −2 d −1 at 200 m and 1500 m depth, respectively. This was only about 6% of the vertical POC flux. The relative contribution of faecal pellets decreased from 93% to 84% of the carbon content of the identified sedimenting particles at 200 m and 1500 m depth, respectively. The increased faecal pellet flux recorded at 200 m depth during spring may probably reflect changes in the balance between the ‘classical’ grazing and microbial food web in the euphotic zone, followed by significant retention during periods when the microbial food web dominates. However, the vertical flux at depth, receives only a small contribution directly from the surface layer production at the investigation site. Vertical flux at depth reflects advective transport of biogenic matter across the shelf and slope, as well as resuspension and faecal pellets from a deep-dwelling zooplankton community. Signals of direct and rapid deposition of surface water appear buffered and greatly modified by mid- and deep-water zooplankton communities on the Cretan Sea shelf.