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.