THE TROPHIC ROLE OF THE MARINE CLADOCERAN PENILIA-AVIROSTRIS IN THE GULF-OF-TRIESTE

The herbivory of the marine cladoceran Penilia avirostris was studied in the Gulf of Trieste (Northern Adriatic) from June 1993 to December 1994 using the gut fluorescence method. P. avirostris occurred from Ju ne to December, but reached its greatest abundance in the summer month s. A significant correlation between the gut pigment content and chlor ophyll a concentration in the surface layer was established. Observati ons with an epifluorescence microscope revealed that the guts were fil led with fluorescing nanoplankton and picoplankton (cyanobacteria). Qu antitative estimates indicated that 19 avirostris grazed less than 5% of the available chlorophyll a in more than half of all measurements, but removed most of the available chlorophyll a in the surface layer d uring some periods in September. It can therefore be concluded that P. avirostris, together with planktonic protists, plays an important rol e within the microbial loop.