Simulated green turtle grazing alters effects of environmental drivers on seagrass growth dynamics across seasons

Seagrasses form productive marine ecosystems that serve as important foraging grounds for grazers. Meadow productivity is vulnerable to environmental change, however, because environmental factors often strongly regulate seagrass growth. Understanding effects of grazing and environmental driver interactions on growth dynamics is therefore needed to ensure the long-term sustainability of seagrass meadow foraging habitats. We simulated natural green turtle (Chelonia mydas) grazing by experimentally clipping seagrass for 16 months in a Thalassia testudinum meadow and measured how responses in linear growth, production, the production-to-biomass ratio (P : B; compensatory growth), and leaf area index differed between clipped and unclipped seagrass in response to in situ changes in temperature and salinity. While increasing temperature and salinity had positive and negative effects, respectively, on growth rates, clipping did not alter the relationship between these abiotic drivers and seagrass growth. Simulated grazing did, however, alter effects of temperature on seagrass P : B ratio and leaf area index dynamics. Each increased significantly with temperature; however, P : B ratios only increased in experimentally clipped seagrass, whereas leaf area index only increased in unclipped seagrass. These results suggest that, given temperature-stimulated growth, grazed seagrass prioritizes increasing biomass production, whereas ungrazed seagrass prioritizes increasing photosynthetic surface area. In addition, our results demonstrate that the strength of the compensatory growth response to grazing in T. testudinum is seasonally dependent, highlighting the importance of biotic-abiotic interactions in driving growth dynamics. In a future with increasing grazer abundance and climate-driven stressors, understanding these types of interactions will be critical for long-term sustainability of seagrass ecosystems.