Effects of point source of untreated sewage waters on seagrass (Zostera marina and Z. noltii) beds in the South-Western Black Sea

• First study of biogeochemical conditions in Black Sea seagrass meadows.
• Seagrass dynamics and sediment response to untreated sewage.
• Signs of eutrophication observed as nutrient increase in seagrass leaves.
• Zostera noltii was absent at polluted site.

Tourist development is rapid on the South-West coast of the Black Sea, increasing the pressure on the coastal ecosystem. Zostera marina and Zostera noltii are abundant seagrasses in the Black Sea, but little is known about their sensitivity towards coastal developments. In this study plant nutrient content (N and P), sulphide intrusion, sediment organic matter, pore water pools of nutrients and sulphides, sulphate reduction rates as well as water column nutrients and chlorophyll-a concentrations were studied in a mixed seagrass meadow (Z. marina L. and Z. noltii Hornem.) located in an area receiving untreated sewage waters from a point source in Sozopol Bay, South-Western Black Sea. In addition, studies included population level metrics like density, biomass and shoot morphology. The point source impacted water quality in the area leading to local nutrient enrichment, development of filamentous macroalgae and reduced water transparency. Although sulphide intrusion was detected in Z. marina and thus indicating existence of sulphide pressure on the plants, there were no significant effects on the seagrass distribution except for Z. noltii being absent from the meadows close to the point source. The nutrient levels were low in the plants and in the sediments compared to global Z. marina and Z. noltii meadows, but the observed blooms of filamentous macroalgae, increase in sulphide intrusion and higher N contents of the plants during the peak tourist season are signs of eutrophication in Sozopol Bay most likely as a result of increased recreational activity on the coast.