Seasonal variations in phytoplankton growth and microzooplankton grazing in a temperate coastal embayment, Korea

Microzooplankton grazing on coastal phytoplankton was determined by the dilution method from May 2002 to April 2003 at a fixed site located in the Jinhae Bay, Korea. During the dilution experiments, our study site exhibited a wide range of chlorophyll a concentrations (0.29–127.42 μg l−1), and the species composition of the phytoplankton community changed dramatically over a year, shifting from the predominance of chain-forming diatoms, particularly Chaetoceros spp., Leptocylindrus danicus, Pseudonitzschia pungens, and Skeletonema costatum, between May and September 2002, to a massive bloom of the dinoflagellates, Alexandrium spp. in October 2002, to a dominance of cryptophytes (Chroomonas sp.) between November 2002 and March 2003, and then again to a prevalence of diatoms toward the end of the experiment. Both nutrients enriched (μn) and in situ phytoplankton growth rates (μ0) showed pronounced seasonal variations, ranging from 0.11 to 2.87 d−1 and from −0.63 to 2.08 d−1, respectively. With regard to both variables, the lowest values were obtained during the fall and winter seasons. The average ratio of μ0/μn was 0.96 (SE = 0.08), thereby indicating that phytoplankton growth in the study site was not nutrient-limited. Microzooplankton grazing rates showed the large fluctuations (0–3.86 d−1) over an annual cycle, with non-significant and/or negative grazing frequently (62% of 29 measurements) detected. Relatively high grazing rates did occur frequently at the times during which a large phytoplankton biomass and/or large-sized phytoplankton dominance were observed. Our results contribute to the growing body of evidence suggesting that microzooplankton are important phytoplankton consumers in communities dominated by large phytoplankton, and also bolster the notion that size-based models of food web relationships may be of limited predictive value. The observed large fluctuations in grazing rates over a year, coupled with frequent non-significant and/or negative grazing, indicate that intensive and long-term measurements of microzooplankton grazing are required for a better understanding of the interaction between phytoplankton and microzooplankton in a highly dynamic coastal ecosystem.