EFFECTS OF PROPELLER SCARRING ON ORGANIC MATTER DECOMPOSITION IN THALASSIA TESTUDINUM SEAGRASS MEADOWS

Abstract

Coastal seagrass meadows provide numerous ecosystem services, including carbon sequestration. High organic matter burial rates, coupled with slow decomposition within anoxic sediments, create large carbon stocks within meadows. Despite their ecological importance, seagrass meadows are highly susceptible to anthropogenic disturbances that can accelerate decomposition and carbon loss. Propeller scarring is a disturbance that occurs when boats navigate into shallow meadows and the propellers damage plant biomass and disturb the sediment environment. Propeller scars negatively affect seagrass ecosystems, such as by reducing faunal abundance and sediment organic carbon content. While these disturbances are widespread in shallow meadows, their effects on ecosystem processes, such as organic matter decomposition, are poorly understood. To investigate how propeller scarring affects decomposition, we conducted an in situ experiment across three Thalassia testudinum meadows in the Florida Keys. Litterbags filled with green or rooibos tea were buried in surface sediments within scars and adjacent undisturbed seagrass and collected after 2, 4, 7, and 26 weeks to estimate decomposition rates for both labile (green) and recalcitrant (rooibos) materials. Seagrass and sediment characteristics were also measured to evaluate their influence on decomposition rates. Decomposition rates of labile material were most influenced by sediment grain size but were not affected by propeller scarring. Decomposition of recalcitrant material, however, was slightly faster within propeller scars than undisturbed seagrass. This effect did not extend beyond the directly scarred area, as rates for rooibos tea were most influenced by T. testudinum aboveground biomass. Our results demonstrate that propeller scars can affect rates of organic matter decomposition in seagrass meadows and add important information to our understanding of how coastal habitats respond to this widespread anthropogenic disturbance.