ABSTRACT Charophytes are green macroalgae that lay the foundation of important habitats in shallow, low‐salinity, and sheltered areas of the Baltic Sea. Charophyte habitats provide food, shelter, and reproduction sites for diverse benthic communities, fish, and waterbirds. Due to human pressures, charophytes have declined in the Baltic Sea. They are sensitive to water turbidity and eutrophication. To protect and restore charophytes and their habitats, knowledge of the species' environmental requirements is fundamental. We examined differences in sediment and porewater properties in areas with and without the large charophyte species Chara tomentosa in five shallow sheltered bays in the northern Baltic Sea. Sediment samples were analysed for total contents of phosphorus (P), nitrogen (N) and carbon (C), grain size, water content, loss‐on‐ignition, and a set of easily soluble elements (Al, B, Ba, Ca, Cd Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Sr., and Zn) from deionised water extraction. From the porewater, dissolved inorganic P (DIP) and N in nitrate and nitrite (NO 2,3 ‐N) were analysed. The results showed significantly lower porewater DIP concentrations and finer grain sizes in the sediments from areas with C. tomentosa compared to those without it. The variability of the organic content and the total contents of P, N, and C was lower in the sediments from the seafloor areas occupied by C. tomentosa than in those without it. The contents of easily soluble elements were similar in all sediments. We conclude that the sediment grain size and organic content should be considered in charophyte restoration. The results show that C. tomentosa habitats influence phosphorus cycling in the sediment–water interface. How microbial communities associated with charophytes influence phosphorus cycling needs further attention to estimate the link between charophyte habitats and eutrophication.
Puttonen et al. (Thu,) studied this question.