Virginia wetlands in the Anthropocene: Tracking wetland photosynthesis for blue carbon budgets and environmental policy

Wetlands are an invaluable ecosystem that provide many ecosystem services and play an outsized role in the global carbon cycle. Photosynthesis is one of the largest fluxes in the carbon cycle and is a foundational wetland function that underlies many wetland ecosystem services. However, salt marsh wetland gross primary production (GPP), the ecosystem-scale photosynthetic CO2 flux, is highly uncertain. Increased and improved monitoring of salt marsh GPP is needed to increase its certainty and constrain its sensitivity to climate change. To address this need, I first collect over four years of marsh-atmosphere CO2 flux measurements to examine patterns of salt marsh GPP and its sensitivity to various environmental conditions. I observe ubiquitous midday depression of GPP at daily and seasonal scales that is primarily driven by salinity and water stress during periods of reduced tidal flooding and warmer air temperatures. To my knowledge, this is the first documentation and analysis of chronic midday depression of photosynthesis in a salt marsh. I then couple the CO2 flux data with ground-based remote sensing observations to determine which remote sensing proxies best track GPP. I identify the near infrared radiation of vegetation (NIRv) index as a strong proxy for salt marsh photosynthesis, especially at longer temporal scales. In my final chapter, I analyze 30 years of wetland permit data and use satellite based NIRv to track vegetation function in wetland mitigation banks that were restored to offset permitted wetland impact activity. I find most mitigation banks have maintained vegetation function for at least 20 years after their restoration and that mitigation banking has likely upheld the ‘no net loss’ of wetland area and function required by the Clean Water Act. However, the few mitigation banks with negative long-term trends in vegetation function tended to be in coastal areas prone to sea level rise and saltwater intrusion, suggesting that further climate change may challenge ‘no net loss’ in the coming decades. Together, this dissertation furthers our understanding of the climate sensitivity of salt marsh photosynthesis and provides improved remote sensing approaches to monitor wetland vegetation function for scientific, management, and policy applications.