Sustainable Seagrass Restoration in the Virginia Coastal Bays

Abstract

1Seagrass depth limits are important to consider when thinking about the future of coastal 2 ecosystems through climate change and nutrient loading scenarios. Seagrasses provide many 3 ecosystem services to coastal areas worldwide, including providing habitat for many 4 economically important species, acting as a significant carbon sink, and improving water quality. 5Because seagrasses are declining globally, it is important to be able to identify areas for 6 restoration where seagrasses could be successful in order to maximize use of time and money. 7Current models for the Virginia Coast Reserve (VCR) only consider effects of light and point 8 measurements of temperature on the maximum depth limit for eelgrass. However, it has been 9 shown that multiple factors can affect light requirements in plants such as sediment 10 characteristics and pore-water chemistry. Sediment characteristics, such as grain size and organic 11 matter content increase light requirements in plants by physically blocking light and decreasing 12 oxygen concentrations in the sediments allowing for the intrusion of phytotoxins such as pore-13 water sulfide and ammonium. With climate change causing a rise in global temperatures, 14 seagrasses will become even more sensitive to changes in their light environment, such as those 15 caused by coastal eutrophication, and will need to increase light requirements further to maintain 16 a positive carbon balance. This may affect depth limits in seagrasses by limiting their range for 17 growth at the minimum depth limit due to increases in temperature and at the maximum depth 18 limit due to declining light conditions with depth. Because the persistence of restoration projects 19 is dependent on the feedbacks between hydrodynamics, light attenuation, and temperature at the 20 meadow scale, it is important to consider the effects of light and temperature measurements over 21ii time in terms of other stressors such as pore-water chemistry and sediment characteristics to 22 accurately find the maximum and minimum depths for eelgrass growth. 23This thesis addresses how maximum and minimum depth limits change over an 24 environmental gradient of sediment grain size and organic matter content in the Virginia coastal 25 bays. The impact of changes in light attenuation in terms of water quality and temperature on 26 maximum and minimum depth limits was investigated through spatial analysis of field and 27 bathymetry data. The predicted depth ranges were compared to ranges of transplanted plants 28 along a depth gradient from 0.4 m to 2.0 m MSL (mean sea level) bracketing the known range 29 for eelgrass growth in Hog Island Bay, 0.8 m to 1.6 m MSL. I found that the maximum depth 30 limit for eelgrass growth can be predicted by light levels in areas with low pore-water sulfide 31 concentrations; however, in areas with high sediment pore-water sulfide concentrations there 32 may be a more complex interaction occurring where light requirements increase due to sulfide 33 intrusion. Predicting the minimum...