Climate change and sea level rise can alter vegetation composition in aquatic ecosystems. Using actual seawater to study salinity effects in laboratory and greenhouse experiments is not always feasible, hence commercially available salts might be viable substitutes to mimic natural saltwater-freshwater systems. In this study, we evaluated the growth of vallisneria (Vallisneria americana) and hydrilla (Hydrilla verticillata) under four salinity levels (0.5, 1.0, 2.5 and 5.0 ppt) induced by four salt sources (seawater, Instant Ocean Aquarium Mix, laboratory-grade NaCl and Morton Sea Salt). Plants were grown in separate fertilized pots filled with one of two substrates (builders’ sand or field soil) and were submersed in 60-L mesocosms filled with pond water. Salinity levels were increased gradually and water level, salinity and pH were monitored every week. After 13 weeks exposure to target salinity levels, plants were visually evaluated on a 0 to 10 scale and harvested to record wet and dry weights. Elemental composition analysis was done on different saline solutions to compare different macro and micronutrients.
Results/Conclusions
Substrate had no effect on plant biomass or visual quality. Hydrilla cultured with Morton or NaCl had significant damage and reductions in biomass at 2.5 ppt and was eradicated after exposure to 5.0 ppt salinity. In contrast, hydrilla was less damaged when salinities of 2.5 and 5.0 ppt were induced using Instant Ocean or seawater and eradication did not occur. Vallisneria was not affected by different salt sources or salinity levels. Elemental composition analysis of different salt sources revealed that the main element responsible for cell osmotic disruption, Na, is around 16% higher in solutions produced using NaCl or Morton vs. Instant Ocean or seawater. This increase in Na could be above the threshold tolerated by hydrilla; which would explain the death of plants grown at 5.0 ppt with NaCl or Morton as a salt source and could be responsible for the reduction of vallisneria biomass under the same conditions. These results suggest that Instant Ocean Aquarium Mix is an appropriate proxy for inducing salinity in mesocosm experiments as its effects on plant biomass were similar to those in seawater.