Use of RNA:DNA ratios to evaluate the condition and growth of the copepod Calanus sinicus in the Southern Yellow Sea

Calanus sinicus, a dominant calanoid copepod in the Yellow Sea, is an important link in the food web between phytoplankton and higher trophic levels. Its populations typically start to develop in later winter with a maximum of individuals in early summer. To study the correlation between changes in the abundance of this species and changes in food resources and the physical environment, RNA and DNA concentrations and egg production rates (EPR) were measured, and RNA:DNA ratios were calculated as indices of growth and nutritional conditions of copepods collected in the Yellow Sea from February to July. We observed pronounced seasonal and spatial variations of RNA concentrations and resulting RNA:DNA ratios. There was a positive correlation between the EPR and RNA:DNA ratios. The copepods collected in March and April, when phytoplankton were more abundant, had high RNA:DNA ratios, and contained more RNA than copepods collected during the other months. There was no significant correlation between the growth indices (RNA:DNA ratios and EPR) and chlorophyll-a concentrations (Chl a) or temperature at large temporal and spatial scales. We tracked the development of two phytoplankton blooms in April, which were dominated in turn by diatoms and dinoflagellates. We observed high concentrations of RNA and a high RNA:DNA ratio at both bloom sites during the respective blooms. During the diatom bloom, the RNA:DNA ratios in copepods increased at the onset of the bloom and decreased thereafter. In addition, we observed a positive correlation (P<0.001) between RNA-based indices and Chl a. Our results suggest that food availability plays a more important role than temperature in controlling the growth of C. sinicus in the field. Thus, the spring phytoplankton blooms in the Yellow Sea are important regulators of copepod abundance.