The trophic role of the marine cladoceran Penilia avirostris in the Gulf of Trieste : Cladocera: the biology of model organisms

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 June to December, but reached its greatest abundance in the summer months. A significant correlation between the gut pigment content and chlorophyll a concentration in the surface layer was established. Observations with an epifluorescence microscope revealed that the guts were filled with fluorescing nanoplankton and picoplankton (cyanobacteria). Quantitative estimates indicated that P. 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 during some periods in September. It can therefore be concluded that P. avirostris, together with planktonic protists, plays an important role within the microbial loop.