Impacts of thermal effluent on Posidonia oceanica and associated macrofauna

Abstract

Posidonia oceanica is a dominant marine macrophyte in shallow coastal waters of the Mediterranean and arguably the most important habitat-forming species. While the effects of elevated seawater temperature on P. oceanica have been studied in laboratory experiments and observed in the field following marine heatwaves, it appears that no field surveys to assess long-term influence are available. Making use of the exponential temperature gradient (k = 0.05 m−1) created by thermal effluent from a fossil fuel power station that has been operating for 3 decades, we aimed to fill this knowledge gap by assessing the in situ effects of elevated temperature (ΔT = 0.39°C) on P. oceanica and associated motile macrofauna. Seagrass leaf area and biomass along- side epiphyte biomass rapidly decreased on moving towards the thermal outflow, but no such change was evident for leaf number and shoot density. The observed differences in seagrass shoot attributes may result from differential adaptation of various parts of the plant to thermal stress. Macrofaunal species richness increased on moving towards the effluent source but evenness decreased. These results predict a near-future scenario where even small rises in seawater temperature in the coming decades will impact some attributes of P. oceanica and its epiphytes and cause a shift in the community composition of associated macrofauna towards fewer dominant species.