Response of copepod grazing and reproduction to different taxa of spring bloom phytoplankton in the Southern Yellow Sea

The responses of copepod grazing and reproduction to the spring phytoplankton bloom were studied in the temperate shelf water of the Southern Yellow Sea in March–April, 2009. Two different algal blooms were found during the cruises. A diatom-dominated bloom at Station Z11, and a dinoflagellate-dominated bloom at Station Z4. The gut pigment contents indicated that different sized copepods exhibited different responses to different-species phytoplankton blooms. Large copepods (LC: body size larger than 1000 μm) and medium copepods (MC: body size ranging from 500 to 1000 μm), grazed actively on diatom blooms, but inactively on dinoflagellate blooms, although the chlorophyll-a concentrations of dinoflagellate blooms were twice as high as than those of the diatom blooms. For small copepods (SC: body size smaller than 500 μm), however, there was no significant difference in gut pigment contents between the two different algal blooms. Among the three size groups, LCs were the major grazers on the diatom bloom, while SCs were major grazers on the dinoflagellate bloom. Grazing impacts of copepod assemblages on phytoplankton blooms were low, only being equivalent to 1% day−1, or less, of the chlorophyll-a standing stock. The egg production rates of a large copepod, Calanus sinicus, were on average, 11.3 egg ind.−1 day−1, which was among the higher levels recorded in the study area, especially at the two stations where phytoplankton was blooming (21.8 and 14.9 egg ind.−1 day−1 at Stations Z11 and Z4, respectively). However, C. sinicus could only obtain sufficient food to support this high reproduction from the diatom bloom, but could not if relying only on the apparently unpalatable dinoflagellate bloom. Our analysis of copepod grazing and reproduction suggests that, although the spring blooms do enhance the reproduction of copepods, the taxa changed during spring blooms from large diatoms to small dinoflagellates would change the pathway of primary production. This would restructure secondary-producers (e.g. copepods) community structure, and have important ramifications through various marine trophic levels in the Southern Yellow Sea.