Sodium chloride solutions not a good experimental substitute for sea water

Climate change is leading to rising sea levels and increased storm activity, which in turn lead to oceanic storm surges and the flooding of coastal ecosystems. In order to help effectively manage coastal areas, scientists must understand the effect of increased saltwater flooding on flood-prone plant communities. The unpredictable nature of storm surges, however, makes studying this phenomenon in situ nearly impossible. Instead, it is studied under laboratory conditions and plants are artificially flooded. Because seawater is naturally variable in its composition, substitutes are often used in the name of consistency. Sometimes, these are simple sodium chloride solutions, while in other experiments, marine salt (MS) solutions are used. Despite being commonly used, the validity of these substitutes in terms of their effect on plants has not been tested.

As part of an Annals of Botany special issue on coastal flooding and storm risks, Mick E. Hanley and colleagues set out to assess the physiological response of white clover to immersion in true sea water, NaCl solution, and MS solution. A second experiment examined the effects of seawater and MS solution on six perennial herb species from two habitats prone to flooding: Spanish sand dunes and British coastal grasslands.

The first experiment produced near-complete mortality in plants exposed to NaCl solution, regardless of whether the solution had been calibrated to the same salinity or the same ionic strength as typical seawater. Plant responses to MS solution versus true seawater, on the other hand, were very similar and had a much lower mortality rate. “Although the greatest impact of seawater flooding on plant performance may stem from the ionic and osmotic stress imposed by Na+ and Cl−,” the authors write, “our results suggest that other seawater constituents moderate these effects.” The second experiment showed consistent reactions within each genus to the treatments involved, but significant differences between genera. This suggests different impacts may be felt by plants within a coastal community and there may be weakening of community structure due to decline and loss of some species. Though all species reacted negatively to MS and seawater flooding, each showed a tolerance for up to four days of root immersion in seawater.

These results demonstrate the unsuitability of sodium chloride solutions for experiments involving simulated seawater flooding, and the much more representative results achieved with a marine salt solution. The authors note that this potentially calls into question the soundness of previous findings in which NaCl solutions were used to study plant responses to soil salinity. Increased risk of storm surges and the need to anticipate their ecological fallout makes this work timely. “The global importance of plant communities to coastal defence, at a time when they also face increased flood risk,” write the authors, “gives urgency to our need to better understand how acute seawater inundation affects component species and ecosystem processes.”