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Landscape-scale variation in a sulfur-based sediment stress indicator for the seagrass Thalassia testudinum in Florida Bay, USA

Intrusion of sediment-derived hydrogen sulfide into above-ground tissues of seagrasses is correlated with reduced growth and has been linked to large-scale die-offs of Thalassia testudinum in Florida Bay, USA. In May of 2019, leaves from T. testudinum short shoots at 350 sites within 13 basins across Florida Bay were collected to investigate bay-wide variation in a novel, stable sulfur isotope-based indicator of hydrogen sulfide intrusion [(δ34Sleaf + 30)/total sulfur content]. The components of this sediment stress indicator (SSI), total sulfur content (% dry weight) and δ34S, were negatively correlated (R2 = -0.24), indicating greater sediment sulfide exposure in plants with higher sulfur content. Generalized additive model selection revealed that SSI was best predicted by a model which included T. testudinum cover, sediment depth, and Halodule wrightii cover (R2 = 0.24, weight = 0.48). Macrophyte communities dominated by dense T. testudinum climax communities and with deep sediments, which are characteristics associated with die-off, had the lowest SSI values, indicating greatest sulfide intrusion. Sites within the area of a recent (2015) seagrass die-off had significantly higher SSI values than nearby, non-die-off-affected sites (mean ± SE: 44.1 ± 1.5 vs. 40.2 ± 1.6, Wilcoxon p < 0.05). The recent die-off sites also had lower T. testudinum cover and higher H. wrightii cover than the non-die-off sites, indicating they were in the midst of recovery and at an early successional stage. Our findings indicate that SSI may provide a minimally destructive indicator of chronic sulfide intrusion in T. testudinum in Florida Bay.