Nutrient reduction scenarios cannot offset climate-driven habitat deterioration of Posidonia oceanica

Seagrasses are marine plants and key indicators of water quality. Changes in environmental conditions may lead to seagrass deterioration with potential loss of associated ecosystem services (e.g., sediment accretion and stabilisation, carbon sequestration, habitat and food provisioning for important commercial and economic species of fish and invertebrates). In this study, we set up a habitat suitability model (MaxEnt) for Posidonia oceanica in the Mediterranean Sea to explore the interplay between shifts in environmental drivers, namely nutrients, and seagrass habitat suitability. We run the model for present and a future climate projection (RCP 4.5 scenario up to 2050). For future projections, we run three different nutrient input scenarios (business as usual – BAU – and two scenarios with reductions in riverine nitrogen and phosphorus loads). Results suggest that assuming full protection of P. oceanica suitable habitat, i.e., no anthropogenic pressures, it is expected a decrease of about 11% (due to climate change only) of suitable habitat area in relation to the estimated present suitable habitat area. However, by implementing upstream policy measures that reduce the nutrient input in the basin (a 6% reduction in Nitrogen load and a 3% reduction in Phosphorous load), this decrease is reduced to about 6% of suitable habitat area in comparison to the estimated area for present conditions. Under the ambitious nutrient reduction (less 21% of Nitrogen load and less 44% of Phosphorous load), projected losses in suitable habitat area were similar to those under the BAU scenario, with decreases of about 11% of suitable habitat area by 2050 in relation to the present estimations. These results suggest: i) a non-linear response of seagrass habitat to nutrient load reductions with benefits limited up to a certain (currently unquantified) threshold and ii) nutrient reduction policies targeting coastal eutrophication in the Mediterranean Sea, can only marginally offset climate-driven decline in P. oceanica habitat suitability. As such, additional conservation and climate-adaptation measures will likely be required to address projected climate-driven habitat suitability declines. Our results reinforce the vulnerability of P. oceanica meadows to a global changing environment and the need of complex and holistic transboundary management actions towards its recovery.