EFFECT OF CTENOPHORE PREDATION ON MESOZOOPLANKTON DURING A SPRING OUTBURST OF PLEUROBRACHIA-PILEUS

A spring outburst of the ctenophore Pleurobrachia pileus in Dutch coastal waters was studied by almost daily sampling. Till the end of May, small ctenophores (<5 mm), probably originating from the Wadden Sea, dominated the population. Then, recruitment of small ctenophores stopped, and an average diameter increase of ∼0.5 mm per day followed. Densities ranged from ∼1 to 20 individuals per m3, with peaks in May and early June. By the middle of June, Pleurobrachia had disappeared. Frequent analysis of pharynx-contents, and measurements of digestion rate, showed that large ctenophores ate 10 to 20 copepods per day; average population consumption was maximal by the end of May with 20 to 40 copepods eaten per m3 per day. Copepod and meroplankton densities were also measured frequently, all showing steady percentual increases over the whole period of ctenophore predation. In our area and year of observation, predatory impact of ctenophores on mesozooplankton abundance was obviously small. Differences from, and similarities to, other Pleurobrachia studies are discussed and the importance of the independent timing of predator waves and prey population developments as a possible cause of highly variable predatory impact is stressed. © 1990.