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Aquatic ecosystems provide vital services, and macrophytes play a critical role in their functioning. Conceptual models indicate that in shallow lakes, plants with different growth strategies are expected to inhabit contrasting habitats. For shallow peat lakes, characterized by incohesive sediments, roles of growth forms, life-history strategies and environmental factors in determining the occurrence of aquatic vegetation remain unknown. In a field survey, we sampled 64 points in a peat lake complex and related macrophyte occurrence to growth forms (floating-leaved rooted and submerged), life-history strategies for overwintering (turions, seeds, rhizomes) and environmental factors (water depth, fetch, and porewater nutrients). Our survey showed that macrophyte occurrence relates to water depth, wind-fetch, and nutrients, and depends on growth form and life-history strategies. Specifically, rooted floating-leaved macrophytes occur at lower wind-fetch/shallower waters. Submerged macrophytes occur from low to greater wind-fetch/water depth, depending on life-history strategies; macrophytes with rhizomes occur at greater wind-fetch/depth relative to species that overwinter with seeds or turions. We conclude that growth form and life-history strategies for overwintering predict macrophytes occurrence regarding environmental factors in peat lakes. Therefore, we propose an adapted model for macrophyte occurrence for such lakes. Altogether, these results may aid in species-selection to revegetate peat lakes depending on its environment.

Seagrass meadows are threatened biodiversity hot spots that provide essential ecosystem services. Green sea turtles may overgraze meadows, further enhancing seagrass decline. However, we observed an unexpected, remarkable recovery of seagrasses in a previously overgrazed meadow with abundant unattached branched coralline algae, suggesting that turtle grazing had ceased. We hypothesize that this recovery is due to an effective grazing-protection mutualism, in which the spiny coralline algae structures protect the seagrass meadows from overgrazing, while the seagrasses protect the algae from removal by currents and waves. Removing coralline algae from recovered seagrass plots allowed the turtles to resume grazing, while addition of coralline algae to grazed plots caused cessation of grazing. Coralline algae that were placed on bare sand were quickly displaced by wave action, whereas those placed in grazed or ungrazed seagrass remained. Our experiments demonstrate a grazing-protection mutualism, which likely explains the witnessed recovery of an overgrazed seagrass meadow. To our knowledge, this is the first account of a plant–plant grazing-protection mutualism in an aquatic environment. Our findings show that grazing-protection mutualisms can be vital for the maintenance and recovery of ecosystems shaped by habitat-structuring foundation species, and highlight the importance of mutualisms in coastal ecosystems. As seagrasses, sea turtles and coralline algae share habitats along tropical shores worldwide, the mutualism may be a global phenomenon. Overgrazing is expected to increase, and this mutualism adds a new perspective to the conservation and restoration of these valuable ecosystems.

A plethora of oomycetes was isolated mostly from Zostera marina but also from other halophilic plants originating from several locations including the Wadden Sea, Limfjord (Denmark), the Dutch Delta area (the Netherlands), Thau lagoon (France), Lindholmen (Sweden) and Chesapeake Bay (Virginia, U.S.). Based on ITS sequences, seven different groups could be distinguished. The largest group was assigned to Phytophthora gemini (Germany, Sweden, the Netherlands, U.S.). The CoxI sequences of all P. gemini strains were identical indicating that P. gemini is probably an invasive species in the Wadden Sea. A second group was identified as P. inundata (the Netherlands, Denmark), that was also isolated from the halophilic plants Aster tripolium and Salicornia europaea. Four strains, originating from Chesapeake Bay clustered in a monophyletic clade with high bootstrap support at the ITS as well as the CoxI loci. They are phylogenetically closely related to P. gemini and are considered to represent a new species described here as Phytophthora chesapeakensis sp. nov. In addition, Salisapilea sapeloensis was isolated from Zostera noltii. Eleven other strains belonging to three unidentified taxa, originating from the Wadden Sea, the Dutch Delta area and Thau lagoon, clustered each in a monophyletic clade with high bootstrap support at the ITS locus, including Halophytophthora vesicula, the type species of the genus Halophytophthora. Hence, these strains were considered to belong to the Halophytophthora sensu stricto group and probably represent three new Halophytophthora species, informally designated here as Halophytophthora sp-1, Halophytophthora sp-3 and Halophytophthora sp-4 sensu Nigrelli and Thines. Halophytophthora sp-2 was not detected in this study. In addition, P. gemini and Halophytophthora sp-3 were obtained by baiting from locations in the Wadden Sea and Halophytophthora sp-1 was obtained by baiting from the Delta area.