Mesozooplankton community prey selectivity and grazing impact in the Celtic Sea

Mesozooplankton play a pivotal role in transferring carbon and energy from microplankton to higher trophic levels. An understanding of the ecological and biogeochemical impacts of mesozooplankton communities can be obtained by upscaling data from single species experiments conducted at isolated points in space and time.However, this approach is best suited to systems that are dominated by one, or only a few, species. Quantifying the roles of diverse mesozooplankton communities, such as those found in shelf sea ecosystems, requires an alternative approach. Our study investigated prey preferences and grazing impacts of small (200-500 μm) and large (>500 μm) sized mesozooplankton communities at two stations (Central Site and Shelf Edge) in the Celtic Sea during April and July2015. Particle-removal experiments were conducted using size-fractionated aliquots of Wild-caught zooplankton assemblages. Other protist plankton’ were typically Ingested proportionally to their ambient concentrations although modest preference for diatoms was apparent in July at the Central Site.  Both mesozooplankton size fractions ingested ≤3 % of their body carbon daily, suggesting that poor feeding conditions were encountered throughout the study period. Total mesozooplankton community ingestion rates were higher in April (<0.1–2.9 μg C [μg body C] day-1) than July (<0.1-0.2 μgC [μg body C]day-1) at the Central Site, whereas the opposite occurred at the Shelf Edge(0.2-0.5 and 2.0-2.7 μg C [μg body C]day-Y1 inApril and July respectively). Despite this spatio-temporal variability in ingestion rates, the mesozooplankton community grazing impact was negligible at all stations, accounting for ≤3 % of 14C-derived estimates of primary production. Our data indicate that mesozooplankton grazing in the Celtic Sea is not the main sink for primary production. This suggests that other loss processes, potentially including microzooplankton grazing and advection, may be important for the ecological and biogeochemical functioning of this region.