Salinity
Marine holoplanktonic invertebrates are cold-blooded and have body fluids isotonic with the surrounding water. For this reason they are limited to the narrow salinity ranges of oceanic water and have a limited tolerance to changes in salinity. Most species cannot survive in waters such as Black Sea, deltaic or estuarian regions where salinities are much lower than those of open ocean waters. Whereas, a few species including epiplanktonic Creseis spp. and Limacina trochiformis (d’Orbigny) have a better adaption to salinity variation and lower salinity. In Daya bay, there were heavy rains from May 18 to June 9 2020 (http://www.tianqihoubao.com/weather/top/shenzhen.html), which induced a quick drop of surface water salinity. Based on buoy measurements, the surface water salinity decreased approximately 3 within 3 days and maintained at a low level for about 2 weeks until June 24 (Fig. 6). As mentioned above, compared with other pteropods,Creseis species are known to be able to tolerate violent fluctuations of water salinity as well as lower salinity, therefore, it is possible that C. acicula survived in this rainy season, while its competitor or predator did not. Therefore, this could be another possible trigger for the C. acicula bloom.