The presence of submerged macrophytes is a desired environmental target for coastal freshwater ecosystems. Maintaining a rich community of these species can be challenging as salinization by sea-level rise poses an increasing threat to ecosystem integrity. We tested the effect of salinization on the growth and germination of freshwater macrophytes experimentally using field sediment. In a 56-day experiment, a macrophyte community was exposed to salinity treatments representing seasonal water management scenarios (a decreasing salinity from 1500 to 300 mg L-1 NaCl, a stable salinity of 300 mg L-1 NaCl, an increasing salinity from 300 to 1500 mg L-1 NaCl and a stable salinity of 1500 mg L-1 NaCl), crossed with treatments simulating periodic turbidity pulses. All species except Elodea nuttallii grew poorly on the saline and eutrophic sediment, reflecting the challenges of growth in eutrophic coastal systems. Surprisingly, the highest community biomass was achieved in the salinity scenario of 1500 mg L-1 NaCl. In a second experiment, field-collected sediments were incubated at 300 and 1500 mg L-1 NaCl salinity (representing summer and winter scenarios), and the germination capacity of the existing seedbank was quantified. Most germinated seedlings did not reach maturity irrespective of salinity treatment. This indicated that sediment salinity, rather than water column salinity, determined seedling establishment success. Interestingly, the established species were characteristic of freshwater habitats, thus indicating maladaptation of the seedbank. Our results show that a mismatch between the high salinity level of eutrophic sediment and the overlaying freshwater may hamper macrophyte growth. Furthermore, target species in coastal eutrophic freshwaters should be evaluated carefully. Elodea nuttalli, which has a wide tolerance range for nutrients and salinity, outperformed other macrophyte species in our study. Thus, species with similar traits may be most successful in establishing macrophyte stands in coastal eutropic wetlands.

This depositited dataset contains the data and scripts underlying these results.