Quantifying the Effects of Nutrient Enrichment and Freshwater Mixing on Coastal Ocean Acidification

Abstract

The U.S. Northeast is vulnerable to ocean and coastal acidification because of low alkalinity freshwater discharge that naturally acidifies the region, and high anthropogenic nutrient loads that lead to eutrophication in many estuaries. This study describes a combined nutrient and carbonate chemistry monitoring program in five embayments of Buzzards Bay, Massachusetts to quantify the effects of nutrient loading and freshwater discharge on aragonite saturation state (Ω). Monitoring occurred monthly from June 2015 to September 2017 with higher frequency at two embayments (Quissett and West Falmouth Harbors) and across nitrogen loading and freshwater discharge gradients. The more eutrophic stations experienced seasonal aragonite undersaturation, and at one site, nearly every measurement collected was undersaturated. We present an analytical framework to decompose variability in aragonite Ω into components driven by temperature, salinity, freshwater endmember mixing, and biogeochemical processes. We observed strong correlations between apparent oxygen utilization and the portion of aragonite Ω variation that we attribute to biogeochemistry. The regression slopes were consistent with Redfield ratios of dissolved inorganic carbon and total alkalinity to dissolved oxygen. Total nitrogen and the contribution of biogeochemical processes to aragonite Ω were highly correlated, and this relationship was used to estimate the likely effects of nitrogen loading improvements on aragonite Ω. Under nitrogen loading reduction scenarios, aragonite Ω in the most eutrophic estuaries could be raised by nearly 0.6 units, potentially increasing several stations above the critical threshold of 1. This analysis provides a quantitative framework for incorporating ocean and coastal acidification impacts into regulatory and management discussions.

A regional hindcast model simulating ecosystem dynamics, inorganic carbon chemistry and ocean acidification in the Gulf of Alaska

“…Nutrient, DIC, and TA concentrations in the freshwater are based on available observations and are summarized in (Table 2). As found in other freshwater data (Rheuban et al, 2019), DIC > TA showing that the freshwater discharging into the GOA is quite acidic (Stackpoole et al, 2016(Stackpoole et al, , 2017.…”

“…in temperature and in the off-shore and freshwater end-members DIC and TA. FollowingRheuban et al (2019), we used a…”

“…Component distributions of the linear Taylor decomposition of surface a) Ωarag, b) pH, and c) pCO2 versus salinity in April (left) and August (right). FollowingRheuban et al (2019) components are total perturbation from the oceanic end-member (Salinity > 32), perturbations due to salinity, temperature, biogeochemistry, freshwater mixing, and an estimated residual term. and winds, sea surface temperature was by 1.5 • C colder in spring2006, 2007, and 2008 than average spring temperatures, and the lowest since early 1970(Janout et al, 2010).…”

The impacts of ocean acidification on marine trace gases and the implications for atmospheric chemistry and climate

“…Reductions in nutrient entrainment may be compensated for by the atmospheric deposition of anthropogenic aerosols, which itself could be countered by future improvements in air quality standards [233]. Thus, CO 2 -driven changes in seawater carbonate chemistry occur simultaneously with warming, deoxygenation, localized freshening of the ocean and changes to nutrient dynamics [234,235]. Numerous modelling studies have addressed future changes in marine ecosystems and biogeochemistry in response to these drivers, either in isolation or combined [29,49,236], yet very few have focused on trace gas emissions and OA-related feedbacks to the Earth system [42][43][44].…”

A regional hindcast model simulating ecosystem dynamics, inorganic carbon chemistry, and ocean acidification in the Gulf of Alaska

“…However, neither of these models simulate the influence of freshwater input along the coast, which exhibits high spatiotemporal variability. Siedlecki et al (2017) used the monthly riverine input climatology by Royer (1982) and applied it equally to the topmost vertical cell along the land mask, whereas Xiu and Chai (2014) did not include riverine input at all. Therefore, to fill gaps in the understanding of the inorganic carbon cycle in the GOA we need a moderately high-resolution model that includes (1) important physical processes such as downwelling, eddies, and cross-shelf fluxes;…”