Factors controlling the summer development of copepod populations in the southern bight of the North Sea

In two consecutive years an intensive sampling programmed was implemented at a fixed station in Dutch coastal waters to obtain a detailed record of the summer development of copepod populations in relation to phytoplankton and macroplankton abundance. The central question was whether densities of copepods are controlled by predation, in particular by invertebrate pelagic carnivores, or by food limitation. Methods applied to estimate daily predation by observed stocks of carnivores included analysis of gut contents and digestion rate, extrapolation of experimental feeding rates and of literature data on daily rations and maintenance needs. Chlorophyll-a and cell concentrations served as a rough measure for algal food supply.Since a decline in copepod densities manifested itself most clearly in decreasing naupliar numbers in both years, populations were assumed to be regulated mainly by recruitment or survival of these early life stages. Naupliar declines coincided with maximum densities of the hydromedusa Phialidium hemisphaericum, which dominated the macroplankton both in abundance and in biomass and reached a maximum density of 467 specimens·m⁻³ or 7 mg C·m⁻³. Copepod eggs appeared by far the most frequent prey item in their guts. However, these eggs are digested very slowly, if at all, and may often be ejected without any visible damage. The effect of egg predation on naupliar recruitment seems therefore relatively unimportant. Predation on swimming copepod stages was generally low. There was no evidence of selective feeding on nauplii. The maximum values of calculated predation pressure exerted by Phialidium populations matched daily copepod production only by way of exception. Impact of other invertebrate carnivores was negligible. As predation did not play a significant role, food availability seems the key factor underlying copepod population dynamics. The consequences of food limitation (reduced egg production, production of diapause eggs and enhanced cannibalism) are discussed. The observed coincidence of maximum predator abundance and minimum chlorophyll-a and diatom concentrations does not support the hypothesis that carnivores are able to indirectly benefit phytoplankton growth by reducing grazing pressure of herbivores.