Award: OCE-1737116

The seagrass Thalassia testudinum (turtle grass) is a valuable, habitat-forming species occurring in shallow waters of the Western Atlantic Ocean. Animal productivity associated with turtle grass is an important subsidy for fisheries, and the turtle grass provides other "ecosystem services" as well, including sediment stabilization, carbon sequestration, and water quality improvement. Turtle grass ranges from the tropics into subtropical regions, and the environmental contexts in which it grows vary widely in terms of temperature, light availability, nutrient supply, the number and species of grazers, and other factors. These aspects of environmental context are predicted to change as the global climate warms due to human activities. In some cases, changes have already been observed. For example, tropical grazers like parrotfishes are becoming more abundant in higher latitude turtle grass beds. Understanding the mechanisms and consequences of this "tropicalization" process in turtle grass beds is a scientific challenge, but will help us better understand and manage climate change threats in general. We investigated the context-dependence of climate-related turtle grass responses using coordinated, manipulative experiments at 15 turtle grass sites along a latitudinal gradient from the southern Caribbean to the northern Gulf of Mexico and Bermuda. The factors manipulated were nutrients (using fertilizer to simulate the increases in nutrient loading associated with coastal development), and grazing intensity. Grazing intensity was manipulated via cages limiting grazer access, and through simulated grazing by clipping the turtle grass. Each experimental site established 50 experimental plots; five of every possible combination of the nutrient addition, caging, and clipping treatments. Light and temperature loggers were also deployed at each site to examine how environmental parameters (known to vary across latitudes) influenced ecological responses. The experiments range for most of a year at each site. Preliminary results showed prominent latitudinal differences in turtle grass meadow structure, function, and responses to the manipulations. Seagrass cover, productivity, and morphological characteristics (leaf width) all declined with latitude, a pattern that was particularly evident during the winter seasons. Many of these characteristics strongly correlated with site-specific light and temperature measurements, suggesting that regional climatic factors likely have a strong influence on meadow characteristics. Latitudinal distinctions were also present in the responses to simulated grazing, with meadows at the northern latitudinal range limit displaying the strongest declines in areal productivity, suggesting that they may suffer the most if grazing increases. Contrary to expectations, we also found that low-latitude sites also displayed highly negative responses to simulated overgrazing, establishing a unimodal response curve of the effects of grazing across latitudes (mid-latitude sites displayed the least response to grazing). These negative responses at lower latitudes were primarily driven by one site, Panama, and were likely due to site-specific idiosyncrasies (slightly deeper waters and precipitation-driven turbidity). This has been confirmed by some of our in situ light measurements. Meadow responses to grazing become a linear function of latitude when plotted as a function of site-specific irradiance, suggesting that light-availability is a primary determinant of resilience from grazing. Nutrient responses of meadow characteristics were minor and generally site-specific. Overall, these findings suggest that climate forcing across broad latitudinal gradients may regulate ecological resilience to disturbance events associated with tropicalization-associated overgrazing. Last Modified: 06/15/2021 Submitted by: James Douglass