A STUDY OF THE EFFECTS OF MIGRATORY ZOOPLANKTON ON MICROBIAL-POPULATIONS IN SURFACE WATERS OF THE NW MEDITERRANEAN

We examined the hypothesis that metazoan zooplankton may directly infl uence on microbial populations in an oligotrophic marine system. Diel variations of microbial populations, relative to changes in the biomas s and feeding activity of metazoan zooplankton, were studied at an off shore station in the North Western Mediterranean Sea. Two different pa rts of the water column, representing relatively low, 0.05 mu g l(-1), and high, 0.12 mu g l(-1), chlorophyll zones were repeatedly sampled. At 3 h intervals, samples were obtained from 5 m and 40 m for estimat es of microbial population abundances, size-fractions of chlorophyll a nd bacterial production; metazoan zooplankton were sampled concurrentl y with vertical net hauls between 0 and 50 m. Overall, there were no l arge day/night differences or clear diel rhythms in parameters of micr obial populations. Bacterial concentrations peaked in the early evenin g at 40 m and later at 03:00 h at 5 m depth. Cell specific thymidine i ncorporation rates showed a distinct peak in the early morning hours a t 5 m and were nearly constant at 40 m. Photosynthetic and heterotroph ic nanoflagellates showed dissimilar temporal trends both relative to one another and between the two sampling depths. Chlorophyll, in the < 10 mu m or < 3 mu m size-fractions, increased at both 5 m and 40 m ov er the study period. Chlorophyll > 10 mu m was a negligible part of th e chlorophyll crop at 5 m but represented about 50% of the chlorophyll at 40 m and showed considerable variability. In contrast to parameter s of microbial populations, we found clear and large day/night differe nces in both abundances and feeding activities of metazoan zooplankton . Abundances of total zooplankton were higher by about a factor of 4 d uring nightime hours. Metazoan zooplankton peak abundances occurred fr om midnight to 03:00 h corresponding with peak activities of the diges tive enzymes amylase and trypsin in total zooplankton as well as gut f luorescence in the most abundant copepod, Centropages typicus. There w ere few significant relationships detectable between parameters of mic robial populations and metazoan zooplankton. The Link between metazoan zooplankton and microbial populations, even in low-chlorophyll surfac e waters was apparently weak. (C) 1998 Elsevier Science B.V.