Seagrasses

This study investigated the influence of high concentrations of porewater H2S (~100 μM) on recruitment of the tropical dominant seagrass species, Thalassia testudinum, following mortality events or "die-offs" in Florida Bay. Major seagrass die-off events (>50 km2) are occurring globally in coastal regions with mortality frequently linked to hypoxia and sediment-derived hydrogen sulfide (H2S) exposure, a well-known phytotoxin. In tropical carbonate environments, such as Florida Bay, low iron in sediments promote H2S accumulation and subsequent intrusion into seagrass meristematic tissue through roots, and root-shoot junctions. While H2S intrusion into meristematic tissue is a leading hypothesis for large-scale seagrass mortality events, it is less clear if H2S contributes to a decline in seagrass recruitment following large-scale seagrass die-off events. Herein, I examined tissue stable sulfur isotope signatures (d34S), belowground tissue biomass partitioning, and internal O2/H2S dynamics of newly recovering shoots over seasons at a western Florida Bay site with recurrent die-off events. Tissue results showed less H2S accumulation in tissue samples of shoots recruiting into bare sediment patches compared to tissue samples from adjacent T. testudinum and H. wrightii seagrass meadows. Additionally, internal gas dynamics of recruits showed high pO2 during the day, and no detection of meristematic H2S intrusion, despite meristem hypoxia for several hours during the night. Recruiting shoots consistently have low root biomass, likely contributing to a lack of meristem H2S intrusion, as young, minimally developed, or lack of roots in recruiting shoots limit H2S intrusion. These results lead me to suggest that high H2S levels in porewater of western Florida Bay does not limit T. testudinum recruitment into open bare patches following major die-off events, supported by the recovery, albeit slow, of this species based long-term monitoring of seagrass in the Bay.

Recent evidence indicates P-limitation in tropical seagrass beds, but to date no data are available on P kinetics of the dominant tropical seagrass Thalassia testudinum. In this study, T. testudinum leaves and roots exhibited Michaelis-Menten saturation kinetics under high (0.5 to 25 muM SRP) and low (0.5 to 5 muM SRP) P concentrations. Leaf Vmax was similar under light and dark conditions. Root Vmax under the high range was slightly lower (30%) in the dark and 2 to 3-fold lower than leaves in the light and dark. Leaf and root P uptake rates were similar at low substrate concentrations. A two-phase kinetic system appears to be functioning with lower values of leaf Vmax and Km at low P levels, indicating a higher affinity for P by leaves at lower P concentrations. In conclusion, T. testudinum leaf and root tissues may contribute similarly to plant P uptake at low concentrations.