THE DIATOM NITZSCHIA-SICULA (CASTR) HUST AND NAUPLIAR FECAL MINIPELLETS IN THE ADRIATIC SEA

Nitzschia sicula (CASTR.) HusT., a planktonic pennate diatom, dominate d the microphytoplankton (10(4)-5 x 10(4) cells . l-1) in the central, oligotrophic Southern Adriatic Pit waters (20-50 m layer) in May 1990 . Nauplii dominated the microzooplankton (3 indiv. . l-1 in the 0-50 m layer, 130 mum mean body length) and produced a considerable amount o f faecal (mini)pellets (1900 . l-1 at 50 m, mean diameter 47 mum), whi ch were compactly filled with N. sicula frustules. The subsurface accu mulation of diatoms and nauplii was probably the result of both produc tive activity under specific hydrographic conditions and the convergen ce in a relatively strong southern Adriatic cyclonic gyre; the advecti ve transport of populations along the shear zone, at the boundary betw een water masses of different thermo-haline characteristics may also h ave played a role. The amount of minipellets was high if compared with the data available from other seas. This was probably due to high pro duction of minipellets, their slow sinking rate, and/or an absence of manipulation of the faecal material by zooplankton.