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 influence on microbial populations in an oligotrophic marine system. Diel variations of microbial populations, relative to changes in the biomass and feeding activity of metazoan zooplankton, were studied at an offshore station in the North Western Mediterranean Sea. Two different parts 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 estimates of microbial population abundances, size-fractions of chlorophyll and bacterial production; metazoan zooplankton were sampled concurrently with vertical net hauls between 0 and 50 m. Overall, there were no large day/night differences or clear diel rhythms in parameters of microbial populations. Bacterial concentrations peaked in the early evening at 40 m and later at 03:00 h at 5 m depth. Cell specific thymidine incorporation rates showed a distinct peak in the early morning hours at 5 m and were nearly constant at 40 m. Photosynthetic and heterotrophic 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 over the study period. Chlorophyll > 10 mu m was a negligible part of the chlorophyll crop at 5 m but represented about 50% of the chlorophyll at 40 m and showed considerable variability. In contrast to parameters of microbial populations, we found clear and large day/night differences in both abundances and feeding activities of metazoan zooplankton. Abundances of total zooplankton were higher by about a factor of 4 during nightime hours. Metazoan zooplankton peak abundances occurred from midnight to 03:00 h corresponding with peak activities of the digestive enzymes amylase and trypsin in total zooplankton as well as gut fluorescence in the most abundant copepod, Centropages typicus. There were few significant relationships detectable between parameters of microbial populations and metazoan zooplankton. The Link between metazoan zooplankton and microbial populations, even in low-chlorophyll surface waters was apparently weak. (C) 1998 Elsevier Science B.V.