High Times and Dry Times In a Salt Marsh: Budgeting For Supplemental Freshwater Needs at the Big Boggy National Wildlife Refuge, USA

Salt marshes can be vulnerable to reduced freshwater input. Reduced freshwater inflows, particularly during a hot or dry summer season, can be catastrophic for vegetation productivity, organic accretion and inorganic sedimentation, and the ability of a marsh to maintain a sustainable elevation facing relative sea level rise (RSLR). Unfortunately, it is challenging for scientists to obtain inflow records for ungauged watersheds and link them with historical trends of salt marsh loss. We sought to address this challenge in Big Boggy National Wildlife Refuge (NWR), a small watershed in East Matagorda Bay, Texas. Our objective was to link the quantity of freshwater inflow with salt marsh sustainability and recommend management actions for the NWR. We first explored land cover trends and found that this watershed lost more than one-third of its low marsh since 1953. We then measured the streamflow into and out of the watershed, created a water budget, and modeled historical and future inflows from 1953 to 2100. Freshwater inflows have been increasing on average since 1953, but a combination of RSLR, sediment starvation, and punctuated seasonal droughts are likely responsible for the loss of salt marsh. We also estimated supplemental water needs during potential droughts out to 2100. We conclude that managers cannot fundamentally alter the accretion versus RSLR balance in this basin except by modifying freshwater input. Thus, during droughts, they should focus on providing these inputs and avoiding vegetation loss. Our work points to both water purchases and land management options that can achieve this goal.