Zooplankton food limitation and grazing impact in a eutrophic brackish-water tropical pond (Côte d'Ivoire, West Africa)

Gut flurorescence, feeding and egg production rates of zooplankton assemblages were measured in a shallow, eutrophic brackish-water pond for 24 days. #Brachionus plicatilis$, #Hexarthra intermedia$ and #Apocyclops panamensis$ successively developed and exhibited differencs in food selectivity. Rotifers selected small particles but also had a preference for larger particles (15-21 micrometers, Equivalent Spherical Diameter, ESD). #B. plicatilis$ appeared less selective than #H. intermedia$, which fed mostly on particles less than 6 micrometers. #A. panamensis$ adults showed a selectivity for 6-21 micrometer ESD particles. Laboratory experiments suggested that #A. panamensis$ adults were able to shift from seston to carnivorous feeding, depending on the availability of these food resources. Measurements of gut fluorescence and grazing gave comparable ingestion rates. Rotifers displayed the highest ingestion rates (up to 486% body C/d). Despite high total phytoplankton and seston biomasses, the high ingestion rates and selective grazing of rotifers induced auto food-limitation phenomena and caused major changes in seston abundance and size structure. Grazing impact was less important when #A. panamensis$ dominated. (Résumé d'auteur)