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 (F_sulfide) 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 F_sulfide, 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 F _sulfide 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 F_sulfide.