Impacts and uncertainties of climate-induced changes in watershed inputs on estuarine hypoxia

Abstract

Abstract. Multiple climate-driven stressors, including warming and increased nutrient delivery, are exacerbating hypoxia in coastal marine environments. Within coastal watersheds, environmental managers are particularly interested in climate impacts on terrestrial processes, which may undermine the efficacy of management actions designed to reduce eutrophication and consequent low-oxygen conditions in receiving coastal waters. However, substantial uncertainty accompanies the application of Earth System Model (ESM) projections to a regional modeling framework when quantifying future changes to estuarine hypoxia due to climate change. In this study, two downscaling methods are applied to multiple ESMs and used to force two independent watershed models for Chesapeake Bay, a large coastal-plain estuary of the eastern United States. The projected watershed changes are then used to force a coupled 3-D hydrodynamic-biogeochemical estuarine model to project climate impacts on hypoxia, with particular emphasis on projection uncertainties. Results indicate that all three factors (ESM, downscaling method, and watershed model) are found to contribute significantly to the uncertainty associated with future hypoxia, with the choice of ESM being the largest contributor. Overall, in the absence of management actions, there is a high likelihood that climate change impacts on the watershed will expand low-oxygen conditions by 2050, relative to a 1990s baseline period; however, the projected increase in hypoxia is quite small (4 %) because only climate-induced changes in watershed inputs are considered and not those on the estuary itself. Results also demonstrate that the attainment of established nutrient reduction targets will reduce annual hypoxia by about 50 % compared to the 1990s. Given these estimates, it is virtually certain that fully implemented management actions reducing excess nutrient loadings will outweigh hypoxia increases driven by climate-induced changes in terrestrial runoff.

Design and redesign of a bottom trawl survey in Chesapeake Bay, USA

“…Fishing activities have also had major effects on both resident and seasonally available natural resources in the bay, including cases of stock collapse (Richards and Rago, 1999;Wilberg et al, 2011). Climate change has impacted the bay ecosystem through warming (Ding and Elmore, 2015;Hinson et al, 2022), altered timing of spring phenological events (Thomas et al, 2017), spatiotemporal extent of hypoxic volume (Irby et al, 2018;Tian et al, 2022), and relative habitat utilization among the bay and coastal areas by several taxa (Schonfeld et al, 2022). Additional climate change related effects on the physical, chemical, and biological processes of the bay are expected in the future (Najjar et al, 2010).…”

Is Hot Drought a Risk in the US Mid-Atlantic? - a Potomac Basin Case Study

“…The Potomac River basin is in the United States' Mid-Atlantic region and situated within the larger Chesapeake Bay watershed. Projections of future precipitation for the Chesapeake Bay region have varied widely (Najjar et al, 2009;Pyke & Najjar, n.d.), with more recent projections indicating that long-term average precipitation will increase (Shenk et al, 2021) while there continues to be uncertainty regarding the sign of change in future stream flows (Hinson et al, 2022). Analyses of historical data indicate that the Potomac basin has been in a transition region with respect to changes in climate and hydrology over the past century.…”

Impacts of Marine Heatwaves on Subsurface Temperatures and Dissolved Oxygen in the Chesapeake Bay

“…Eutrophication, and subsequently hypoxia, in the CB, are primarily driven by the Susquehanna River nutrient load during the peak spring discharge (Kemp et al., 2005; M. Li et al., 2016; Malone & Newton, 2020). Climate change could exacerbate hypoxic conditions in the CB by a combination of (a) increasing early summer stratification due to sea level rise and increased river flow (though sea level rise could also oxygenate the Bay by increasing exchange flow, and a consensus on river discharge trends has not been reached); (b) reducing the solubility of DO due to warming waters (e.g., deoxygenation); and (c) increasing oxygen demand due to the combined effects of rising temperatures and higher nutrient loads from increased river flow (Cai et al., 2021; Hinson et al., 2023; Irby et al., 2018; Murphy et al., 2011; Ni et al., 2019). These long‐term changes in CB water quality are well documented, but little is known about the impact of MHWs on estuarine water quality, particularly DO, and how these extreme events may compound underlying water quality issues.…”