Survival strategies of Calanus sinicus during harmful algal blooms in spring

Harmful algal blooms (HABs) have been a serious threat to estuarine and coastal ecosystems in recent decades. The adverse effects of HABs on zooplankton have been widely reported, but zooplankton’s strategies to cope with HABs remain unclear. Calanus sinicus, the dominant zooplankton species in the North Pacific Ocean during spring and early summer, frequently encounters blooms of Skeletonema costatum and Prorocentrum donghaiense. In this study, we conducted feeding behavioral experiments on C. sinicus to investigate its behavioral responses and survival strategies in a harmful algal bloom environment through feeding rate, fatty acid, and feeding behavioral parameters. The results showed low carbon ingestion rates of C. sinicus on harmful P. donghaiense and S. costatum 12.5 and 3.1 µg/(cop·d), respectively corresponding to 77.2% and 19.1% of the ingestion rate on the control alga Platymonas helgolandica 16.2 µg/(cop·d). After 5 days of mono-algae feeding, all C. sinicus failed to obtain sufficient polyunsaturated fatty acids from either HAB algae species. Behavioral observations showed that C. sinicus exhibited lower locomotor activity in a single S. costatum diet, with a higher proportion of passive sinking time. In contrast, it showed obvious escape behavior in a single P. donghaiense diet, characterized by higher jumping frequency and velocity. When other potential food sources were present around the harmful algae, C. sinicus was more active in foraging. We assumed that C. sinicus could adjust its behavior in blooms of S. costatum and P. donghaiense to conserve energy and seek additional energy from alternative food sources. The specific survival strategy of C. sinicus during HABs suggests that zooplankton possess important mechanisms for regulation and recovery in complex marine ecosystems, and can be used to predict the future development of zooplankton populations.