Effects of down- and up-shocks from rapid changes of salinity on survival and growth of estuarine phytoplankters

In the laboratory, we investigated the effects of rapid changes of salinity on the survival and growth of 13 species of phytoplankton inhabiting an estuary where there are wide fluctuations in salinity. These phytoplankton were exposed to rapid decreases in salinity (down-shocks) from 32 to 5, 10, 15, or 20 psu, acclimatized to these lower salinities, and thereafter exposed to rapid increases (up-shocks) from the lower salinities to 32 psu. The phytoplankters tested proliferated in decreases and increases of salinity between 20 and 32 psu in a manner similar to controls (i.e. from 32 to 32 psu), but showed diagnostic responses such as a long lag-time before growth or mortality following drastic decreases or increases between 5-15 and 32 psu. The responses differed among phytoplankton species inhabiting the same estuary. Some flagellates formed morphologically abnormal cells after some treatments involving decreased salinity. The species-specific responses to drastic changes of salinity may contribute to the short-term succession of phytoplankton in estuaries.