Assessing the impact of sewage on ecosystem services provided by Posidonia oceanica meadows in the Balearic Islands

Abstract

The Posidonia oceanica meadows in the Mediterranean Sea are ecosystems of vital importance for the health of its waters. Posidonia provides, among other numerous benefits, oxygenation of the water column, capture of carbon dioxide as well as particulate carbon, that can be buried underneath he meadow. Therefore it plays an important role in climate change mitigation. Nevertheless, many stressors of anthropogenic origin endanger the functionality and the ecosystem services provided by Posidonia meadows. Between the most relevant is eutrophication due to the inputs of wastewater rich in nutrients and organic matter through sewage. This can affect the plants negatively, i.e. through enhanced bacterial sulfide production and subsequent intrusion. Here we assess the health of Posidonia meadows in areas with sewage presence and identify drivers behind meadow degradation. We determine the influence of sewage inputs on oxygen production and metabolic rates, at two sites in the Balearic Islands, one station in an area with sewage presence (Son Veri, Mallorca) and a control site in an area without anthropogenic influence, considered pristine (Cabrera Archipelago Maritime- Terrestrial National Park) during two field campaigns, in May 2021, before the maximum growth stage of Posidonia, and in September 2021, at the end of the growth stage. To identify drivers behind meadow degradation we evaluate sedimentation of particulate matter and sulfide concentrations in the sediment. Additional information on meadow status and possible drivers was obtained from monitoring reports provided by the Age¿ncia Balear de l'Aigua i de la Qualitat Ambiental (ABAQUA). The information of 9 wastewater treatment plants with submarine outlets close to Posidonia oceanica meadows was compiled from 2018 to 2020 for Mallorca, Ibiza and Menorca. Through a multivariable analysis we establish the relationship between anthropogenic drivers and meadow state and trajectory. The ecological status of the meadows was evaluated taking the distance to the wastewater outlet into account and comparing all the sites with the control station. Finally, this study intends to raise the awareness of the consequences of human activity and choices for the environment, such as the positioning of sewage outlets and the possibility to improve waste water treatments reducing the most harmful agents for Posidonia oceanica meadows. This knowledge can be used to establish a solid basis for new management guidelines.