Copepod gut contents, ingestion rates and grazing impact on phytoplankton in relation to size structure of zooplankton and phytoplankton during a spring bloom

Abstract

Copepod gut contents, ingestion rates a s well as the grazing impact on phytoplankton were estimated in coastal waters off Plymouth (SW England) before and during the spring phytoplankton bloom in 1989. A size-fractionation approach was applied, estimating phytoplankton biomass as chlorophyll a concentration in 3 size fractions [pico(0.2 to 2 pm), nano(2 to 10 pm) and microplankton (10 to 100 pm)] and copepod abundances, gut pigment contents and gut evacuation rates in 3 size categories (small, 200 to 350 pm; medium, 350 to 710 pm; and large. > 710 pm). Results showed a direct relation between copepod feeding and the size structure of zooplankton and phytoplankton. Gut pigment contents were higher for the larger copepods and lower for the smaller ones, and increased in parallel with the increase of chlorophyll a concentration and dominance of large cells (> 10 pm) during the spring bloom. Estimations of the grazing impact of the copepod community showed that 5 to 8 % of the total phytoplankton standing stock, and up to between 30 and 40 % of the fraction > 10 Ltm, was grazed during the exponential growth phase of phytoplankton prior to the spring bloom, in contrast to < 2 % and < 5 %, respectively, during other periods before and after the bloom. The medium-sized copepods were responsible for most of this grazing impact, a s a result of their numerical dominance in these coastal waters. Different aspects affecting these estimations of copepod community grazing impact are discussed.