SEAGRASS SEDIMENT SULFUR DYNAMICS IN RESPONSE TO GRAZING BY GREEN TURTLE HERBIVORES

Disturbances, both natural and anthropogenic, are important drivers of ecosystem functions and services. Sediment sulfur (S) biogeochemistry plays an important role in coastal seagrass habitats, due to the prevalence of sulfate reduction and its byproduct, dissolved hydrogen sulfide (DS). Sulfide is a phytotoxin, and DS accumulation could therefore influence the health of primary producer communities and resultant ecosystem services. As marine herbivores that consume seagrasses, green turtles may influence these dynamics. Green turtle grazing decreases seagrass metabolic rates and oxygen production, which could lead to increased sediment DS. To investigate effects of grazing on sediment S dynamics, we measured porewater DS and inorganic S pools in seagrass meadows with and without active green turtle grazing along Florida’s west coast. Neither porewater DS (range 1-502 uM) nor sediment inorganic S pools (range 1-37 umol S cm^-3) differed between grazed and ungrazed seagrass areas, though both varied across sites and with sediment depth. Inorganic S pool results suggest that sedimentary characteristics may drive among-site differences in DS. Additionally, porewater DS was significantly higher in disturbed, unvegetated sediments (mean 849 uM) than in seagrass meadows (p<0.01), suggesting that grazed seagrass may still produce enough oxygen to suppress sediment DS accumulation. Understanding these types of biogeochemical responses is critical for predicting how seagrass ecosystem services will be affected by future changes in herbivory and anthropogenic disturbance.

Presentation Preference: Oral

Primary Presenter: Robert Johnson, University of Wisconsin-Madison (robert.a.johnson@wisc.edu)

Authors:

Robert Johnson, University of Wisconsin-Madison  (robert.a.johnson@wisc.edu)

Peter Conowall, University of Wisconsin-Madison  (conowall@wisc.edu)

Eric Roden, University of Wisconsin-Madison  (eroden@geology.wisc.edu)