Summer reproduction of the planktonic copepod Calanus sinicus in the Yellow Sea: influences of high surface temperature and cold bottom water

We have observed that Calanus sinicus retreated from neriticareas in the Yellow Sea and concentrated in the Yellow Sea ColdBottom Water (YSCBW) area in summer. To investigate the summerreproductive strategy of C. sinicus in this situation, effectsof high temperature on reproduction and hatching, as well asgeographical variation of in situ egg production rate, werestudied by onboard incubation in August 2001. Diel verticalmigration (DVM) of females was investigated within and outsidethe YSCBW, respectively. Onboard incubation at 27°C (i.e.surface temperature) resulted in lower fecundities than thatat 9.8 and 12°C (i.e. bottom temperature inside and outsidethe YSCBW) together with decreased hatching rates and increasednaupliar malformation. Egg production was more active at stationsoutside the YSCBW than inside, where chlorophyll-a concentrationwas also relatively low. Females inside the YSCBW underwentDVM although they rarely entered the surface layer, but DVMwas not observed outside the YSCBW. We conclude that surfacetemperature in summer has deleterious effects on C. sinicusegg production and hatching, and that it cannot reproduce successfullyover the whole area. Inside the YSCBW, egg production is depressedby low food availability, while females outside suffer fromhigh temperatures because of strong vertical mixing.