Response of Different Zooplankton Taxa to Instantaneous Salinity Changes.
Supervisor: Lodewijk van Walraven (Royal Netherlands Institute for Sea Research (NIOZ) and Utrecht University).
Salinity variations are key factors influencing the distribution of estuarine biota and constitute a considerable stress on living organisms. In the ocean, zooplankton can experience rapid salinity changes due to heavy precipitation, ice and snow melting, storms or their own vertical migration in stratified waters. However, this rapid natural salinity change is smooth, rather than instantaneous changes in salinity of an anthropogenic nature, such as periodic discharges of water from dams separating seas and freshwater lakes. A novel cause of anthropogenic salinity variation is Blue Energy production.

In Blue Energy production, energy is generated from the mixing of waters with different salinities in a process called Reverse Electrodialysis (RED). The world’s first Blue Energy power plant is currently being tested in the Netherlands. Organisms passing through a RED plant are subjected to multiple stressors, i.e. salinity shock that zooplankton experience when sea water and freshwater mix. Rapid salinity reduction or increases may have negative consequences for planktonic organisms: mortality, decrease in growth rates and impaired feeding ability, but knowledge on this is currently limited.

We tested potentially negative impacts of Blue Energy generation on zooplankton by setting experiments and reviewing available literature regarding the impact of abrupt salinity changes on zooplankton. In a series of controlled long-term experiments using natural zooplankton assemblages collected in the western Wadden Sea we studied the impact of instantaneous salinity changes on survival ability at different temperature regimes. To test the assumption that freshwater organisms do not withstand instantaneous increases in salinity, complementary experiments on the response of freshwater zooplankton were also performed.

We found that western Wadden Sea marine zooplankton species are able to tolerate abrupt salinity reductions. However, it appears that at higher temperature regimes the mortality rates of organisms increase. In this regard, the influence of the temperature and related abiotic factors is considered. On the basis of long-term observations, it can be concluded that freshwater zooplankton do not survive instantaneous increases in salinity. For future studies we recommend to focus on temperature-related factors such as dissolved oxygen control, as well as on the range of instantaneous salinity change.

Key words: zooplankton, mortality, salinity shock, salinity change, Blue Energy, reverse electrodialysis, Wadden Sea