Evaluation of the influence of submarine outflows of Waste Water Treatment Plants on Posidonia oceanica meadows in the Balearic Island.
Supervisor: Iris E. Hendricks IMEDEA
Posidonia oceanica meadows are ecosystems of vital importance in Mediterranean Sea. However, many stressors of anthropogenic origin endanger the functionality of the meadows, between the most relevant here eutrophication. For instance the inputs of wastewater rich in nutrients and organic matter through sewage could affect Posidonia meadows negatively. Here, we evaluate the ecological status of Posidonia oceanica meadows near sewage outfalls in the Balearic Islands using guidelines set by the WFD to monitor the water quality with P. oceanica as a biological indicator. To evaluate the ecological status reference values from Pergent et. al., (2005) were used to calculate the ecological quality ratio (EQR) of meadows monitored by the Agència Balear de l’Aigua i de la Qualitat Ambiental (ABAQUA) between 2018 and 2021. To identify drivers behind meadow status, information on possible drivers in outflow of treatment plants was obtained from 12 wastewater treatment plants with submarine outlets close to Posidonia oceanica meadows in Mallorca, Ibiza, Formentera, and Menorca. We evaluated nutrients in outflow water (total nitrogen and total phosphorous), suspended solids, biochemical oxygen demand, and volume of water entering the marine environment as possible drivers, and also took the distance between the meadow and the and the wastewater outflow into account. We used a multivariable analysis to establish the relationship between anthropogenic drivers related to the sewage outflow and P. oceanica meadow status. Furthermore, we assess the direct influence of sewage on Posidonia oceanica ecosystem function in two sites of the Balearic Islands, Son Veri (outflow of the Llucmajor WWTP in Mallorca) and La Savina in Formentera. We contrasted ecosystem functioning in an area with sewage presence with that at a similar and nearby control site in an area without anthropogenic influence (For Son Veri: Cabrera Archipelago Maritime-Terrestrial National Park; For la Savina: Punta Rasa, Formentera). In this part we evaluate the effect of measured sedimentation rates (total and organic matter), sulfide concentrations in the sediment, meadow parameters (density) and productivity of the meadow as metabolic rates. Finally, the outcome of this study can be used to establish new management guidelines to improve waste water treatment plans, reducing the most harmful agents for Posidonia oceanica meadows.

Keywords: Ecological indicators, Posidonia oceanica, eutrophication, sedimentation, organic matter, waste water treatment plants, water quality, water framework directive