Grazing of adults of the copepods Acartia spp., Temora longicornis, Centropages hamatus and nauplii of Balanus spp. on natural particulate matter from the Oosterschelde was studied using a Coulter counter. Two types of particulate matter distributions were found to occur in the Oosterschelde: 1. distributions with distinct peaks in the >20 µm size range (A) and 2. more flattened distributions which were bell-shapened or contained multiple small peaks spread over the entire 4-100 µm size range (B). In A-type distributions, peak tracking was performed in all species studied, especially in Acartia spp. and T. longicornis. In B-type distributions, the animals tended to spread their grazing activity towards the smaller particles. Quantitative microscopial analysis of the phytoplankton distributions shows that A- and B-type distributions were caused by the presence (A) or absence (B) of blooms of phytoplankton species with an SED of >20 µm, reaching concentrations of >1,40·106 µm³·cm-3. The demonstrated clearance rate distributions are shown to result in a concentration of phytoplankton in the ingested material as compared to the medium for all copepod species studied. Balanus spp. nauplii did not demonstrate this systematic selection of phytoplankton. When B-distributions occurred, only Acartia spp. switched its feeding activity towards smaller particles to such a degree that a substantial contribution of the <20 µm size range to the ingested material resulted. Whether this behaviour represents a higher selection capacity for small phytoplankton species or a detritivorous behaviour remains an open question.
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