PHYTOTOXIC HYDROGEN SULFIDE DOES NOT EXPLAIN VARIATION IN RESPONSES OF SEAGRASS AND MACROALGAL COMMUNITY COMPOSITION TO GREEN TURTLE GRAZING

Green turtles (Chelonia mydas) are marine herbivores that consume seagrass for their diet, and whose grazing strategy can alter floral community composition in seagrass habitats. Responses to grazing vary among meadows, but the drivers of this variability are not well understood. Hydrogen sulfide (H₂S) is a phytotoxin produced via sulfate reduction that can limit plant growth at high concentrations and may be mediated by grazing-induced changes in seagrass metabolism. To investigate if grazing-induced differences in sediment sulfur (S) dynamics could explain variability in floral species composition, we surveyed seagrass meadows with and without active green turtle grazing along Florida’s west coast. Across sites, there was a trend for higher seagrass and algal species richness in areas grazed by turtles. However, while concentrations of both sediment porewater H₂S (range 1-1388 uM) and reducible inorganic S (range 0.9-15.3 umol S g^-1 sediment) varied across sites, neither differed consistently between grazed and ungrazed meadows, suggesting that factors other than, or in addition to, sediment S dynamics may be important in driving responses of the seagrass floral community to grazing. Given increasing green turtle abundance with successful conservation efforts in recent decades, our findings highlight the need for a better understanding of the drivers of variability in seagrass and algal communities across meadows grazed by green turtles.

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)