Increased seawater viscosity, Phaeocystis globosa spring bloom and Temore longicornis feeding and swimming behaviours

The suggested influence of increased seawater viscosity on the feeding and swimming behaviours of adult females of the calanoid copepod Temora longicornis was investigated during a Phaeocystis globosa spring bloom in the coastal waters of the eastern English Channel. Adult female gut contents did not exhibit any significant correlation with chlorophyll concentration or seawater excess viscosity over the course of the bloom. Instead, the highest gut contents were observed when the seawater viscosity was maximum (up to 4.6 centipoise [cP]), after a 5-fold decrease in chlorophyll concentration related to the formation of foam. This demonstrates that even high viscosity did not mechanically hamper zooplankton grazing. Gut contents were controlled by the taxonomic availability rather than the quantitative availability of phytoplankton-based food. This is consistent with the observed sustained egg production rates despite drastic changes in the composition of protist resource over the course of the bloom. Before and after the bloom (in the absence of P. globosa), T. longicornis exhibited similar swimming paths characterized by their large spatial extent and low curviness. In contrast, during the bloom their movements were spatially more localised, significantly slower and more convoluted. This behaviour is suggested as an adaptive strategy to optimise foraging activity during P. globosa blooms, which have been recently shown to generate high level of phytoplankton patchiness.