Copepod herbivory rate in a large arctic polynya and its relationship to seasonal and spatial variation in copepod and phytoplankton biomass

Large copepod species (Calanus spp.) overwinter in the North Water Polynya (NOW; ~75 to 79° N, ~68 to 78° W) and, via upward migration, can potentially exert a rapid and important grazing impact on the spring phytoplankton bloom. This study investigated the pattern and factors controlling copepod herbivory in the NOW from April through July 1998. Typically, there was a chlorophyll maximum between 50 m and the surface. We used incubation experiments to measure weight-specific herbivory rates (0 to 0.24 μg C μg C–1 d–1) representing the average for surface-layer copepod assemblages at each station, and we quantified 0 to 50 m in situ copepod biomass (20 to 3200 mg C m–2). Weight-specific herbivory rate was positively related to initial chlorophyll a concentration in experiments (r2 = 0.54). Maximum in situ copepod herbivory rate and in situ copepod biomass were larger and peaked earlier at stations dominated by Baffin Bay water in the southern and eastern NOW versus stations dominated by silica-rich arctic water in the northern and western NOW. We used a standard scaling model (I = aWb), where I = maximum daily ingestion rate and W = individual weight, to test the potential effect of size bias on our estimates of total in situ copepod herbivory, because the size structures of experimental and in situ copepod assemblages were often statistically different. Although these calculations found up to ±40% difference in our estimate of total in situ copepod herbivory, this had very little effect on the estimates of copepod impact on daily primary production (±1% PP), because copepod biomass was usually low relative to NOW phytoplankton biomass and productivity. During pre-bloom and spring-bloom conditions, total in situ copepod herbivory was low (<10% of PP). After the spring diatom bloom, we measured higher removal rates (~15 and 55% of PP) at 2 southern stations, where copepod biomass was high and PP was relatively low.