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.