Seagrass meadows fringe many temperate and tropical coasts and provide a range of ecosystem services, such as sediment stabilization, nutrient recycling, carbon sequestration, and acting as a habitat and/or nursery ground for numerous marine species, including economically important fish. Seagrass meadows are in decline worldwide as a consequence of eutrophication and pollution, physical destruction (e.g., dredging, coastal development, boat moorings), and global warming. Before arriving at BIOS, Sawall conducted research on temperate seagrass meadows in the Baltic Sea investigating the effect of heat waves and long-term (trans-seasonal) warming effects. Once at BIOS, she learned that the loss of seagrass meadows in Bermuda, in both nutrient-poor offshore and nutrient-enriched near-shore waters, substantially exceeds the global rate of decline despite their protected status (picture 3.1 in slideshow above). The main reason for their decline is increased grazing pressure by green turtles (picture 3.2), which is likely a consequence of successful green turle conservation efforts, a decline in their main predators (tiger sharks) throughout the Northern Atlantic, and an increase in sea surface temperatures (favorable for tropical turtles). At the same time, Bermuda’s tropical seagrass plants thrive under marginal subtropical conditions (low light and temperature in winter) and therefore have a low capacity to support the increasing population of grazing green turtles.
Together with the Department of Environment and Natural Resources of the Bermuda Gov. (DENR; Dr. Sarah Manual), Sawall joint a NSF-funded project (2017-2020; PI J Campbell, Smithsonian Institute) that investigates the interacting effects of grazing and nutrients on seagrass (Thalassia testudinum) along a latitudinal gradient of light and temperature in order to understand how climate change-driven shifts in species ranges (i.e. grazers) may affect seagrass habitats (picture 3.3). The so-called Thalassia Experimental Network (TEN) spans across 23 latitudes, with Panama being closest to the equator (9°) and Bermuda being the furthest north (32°). Data analysis is ongoing.
Students involved in the NSF seagrass project:
- Khalil Smith, University of Coastal Georgia, GA, USA: 12-months internship funded through the Bermuda Program, the NSF seagrass project, and the NASA COral Reef Airborne Laboratory (CORAL) mission (2018-2019)
As an extension of TEN, Sawall and Manuel (DENR) are also involved in an ongoing trans-latitudinal project that investigates the resilience and recovery potential of seagrass under varying nutrient conditions and grazing pressure (PhD project of Fee Smulders, Wageningen University, Netherlands). For this, a turtle-exclosure (cages) experiment is currently being conducted at Bailey’s Bay and seagrass recovery is being monitored under different nutrient conditions (Jan 2020 to Jan 2021; picture 3.4).
The MABEE lab, together with Sarah Manuel, is currently seeking additional support to investigate and define seagrass management strategies that mitigates further seagrass decline and promotes seagrass recovery.