Sulfide invasion in the seagrass Posidonia oceanica at Mediterranean fish farms: assessment using stable sulfur isotopes

The effect of organic enrichment of sediments on the composition of stable sulfur isotopes (δ34S), sulfide invasion (Fsulfide) and concentrations of total sulfur (TS) and elemental sulfur (S0) in the seagrass Posidonia oceanica was investigated along transects from 3 Mediterranean fish farms in Spain, Italy and Greece. The δ34S decreased and Fsulfide, TS and S0 decreased with distance from the fish farms indicating a higher invasion of sulfide in seagrasses close to the farms. Changes in plant sulfur parameters were linked to sedimentation of organic carbon, sediment organic matter pools and sediment sulfide production (sulfate reduction rates), but relationships were not statistically significant. The most significant changes in seagrass sulfur parameters took place in the roots and rhizomes, whereas leaves showed minor or no changes along the transects and among farms. Roots had the lowest δ34S, indicating that sulfide entered the plants here and moved to the other plant compartments. Significant correlations between S0 and Fsulfide suggested that sulfide derivatives were accumulating inside the plant and isotopic analysis confirmed that the δ34S signal of S0 extracted from the plants was similar to the δ34S of sediment sulfide. The mortality of P. oceanica was negatively correlated to δ34S in the plant, indicating higher plant mortality with increasing sulfide invasion. The usability of stable sulfur isotopes as indicators of seagrass sulfide exposure is good, except in situations with high variation in δ34S of the sulfur sources, as observed at the fish farm in Spain. This variation may be adjusted for by calculating Fsulfide.