Zooplankton eat what they need: Copepod selective feeding and potential consequences for marine systems

Herbivores are generally faced with a plethora of resources which differ in quality. Therefore, they should be able to select foods which most closely match their metabolic needs. Here, we tested the hypothesis that copepods of the species Acartia tonsa select prey cells based on quality differences within prey species. We assessed age-specific variation in feeding behaviour and evaluated the potential consequences of such variation for nutrient cycles. Nauplii (young) stages characterized by a low nitrogen to phosphorus (N:P) ratio in their body tissue selected for phosphorus-rich food, while older copepodite stages with higher body N:P selected for nitrogen-rich food. Further, the analysis of a 35-year data set in the southern North Sea revealed a positive correlation between the abundance of nauplii and the ratio of dissolved inorganic N:P, thus suggesting that P-availability for primary producers declines with the population densities of nauplii. Our findings demonstrate that a combination of stage-specific selective feeding and body stoichiometry has the potential to affect cycling of limiting nutrients when consumer populations change in composition.