Red tides in Masan Bay, Korea, in 2004–2005: III. Daily variations in the abundance of mesozooplankton and their grazing impacts on red-tide organisms

• We investigated the role of mesozooplankton in the dynamics of red tides in Masan Bay, Korea.
• We measured the abundance of mesozooplankton in daily samples collected from June 2004 to May 2005.
• Mesozooplankton were abundant in the winter, but rare in the summer, and had a range of abundance of 3–52,843 ind. m−3.
• The biomass of copepods was significantly positively correlated with that of DO and some phototrophic and heterotrophic dinoflagellates.
• Acartia can have a considerable grazing impact on the populations of Pfiesteria-like dinoflagellates and Prorocentrum minimum.

To investigate the role of mesozooplankton in the dynamics of red tides in Masan Bay, Korea, we measured the abundance of mesozooplankton in daily samples collected from June 1, 2004 to May 31, 2005. Mesozooplankton were abundant in the winter, but rare in the summer, and had a range of abundance of 3–52,843 ind. m−3. Similarly, both copepods and cladocerans were abundant in the winter, but rare in the summer, and had ranges of abundance of 0–48,817 ind. m−3 and 0–10,951 ind. m−3, respectively. Invertebrate larvae were abundant in the fall but not in other seasons. The biomass of copepods was significantly positively correlated with salinity, dissolved oxygen, the biomass of the phototrophic dinoflagellates Heterocapsa triquetra and Prorocentrum minimum, and the biomass of the heterotrophic dinoflagellate Gyrodinium glaucum, but negatively correlated with water temperature and the biomass of heterotrophic bacteria and small algae. In addition, the biomass of cladocerans was significantly positively correlated with salinity and the biomass of euglenophytes and G. glaucum, but negatively correlated with water temperature. The biomass of invertebrate larvae was significantly positively correlated with water temperature, but negatively correlated with dissolved oxygen. These observations suggest that copepods and cladocerans may increase their populations by feeding on large phytoplankton in cold water, whereas invertebrate larvae may prefer warm water. The grazing coefficients for the copepods Acartia spp. on co-occurring Pfiesteria-like dinoflagellates (PLDs), P. minimum, Skeletonema costatum, H. triquetra, Heterosigma akashiwo, and Scrippsiella trochoidea were 0.104, 0.083, 0.042, 0.034, 0.033, and 0.030 d−1, respectively. These results suggest that grazing by Acartia populations in Masan Bay can have a considerable impact on the populations of PLDs and P. minimum, but only a moderate impact on S. costatum, H. triquetra, S. trochoidea, and H. akashiwo.