The Importance of Freshwater to Spatial Variability of Aragonite Saturation State in the Gulf of Alaska

Abstract

High‐latitude and subpolar regions like the Gulf of Alaska (GOA) are more vulnerable than equatorial regions to rising carbon dioxide (CO2) levels, in part due to local processes that amplify the global signal. Recent field observations have shown that the shelf of the GOA is currently experiencing seasonal corrosive events (carbonate mineral saturation states Ω, Ω < 1), including suppressed Ω in response to ocean acidification as well as local processes like increased low‐alkalinity glacial meltwater discharge. While the glacial discharge mainly influences the inner shelf, on the outer shelf, upwelling brings corrosive waters from the deep GOA. In this work, we develop a high‐resolution model for carbon dynamics in the GOA, identify regions of high variability of Ω, and test the sensitivity of those regions to changes in the chemistry of glacial meltwater discharge. Results indicate the importance of this climatically sensitive and relatively unconstrained regional freshwater forcing for Ω variability in the nearshore. The increase was nearly linear at 0.002 Ω per 100 µmol/kg increase in alkalinity in the freshwater runoff. We find that the local winds, biological processes, and freshwater forcing all contribute to the spatial distribution of Ω and identify which of these three is highly correlated to the variability in Ω. Given that the timing and magnitude of these processes will likely change during the next few decades, it is critical to elucidate the effect of local processes on the background ocean acidification signal using robust models, such as the one described here.

Simulated Impact of Glacial Runoff on CO2 Uptake in the Gulf of Alaska

“…We speculate that our results may be representative of high‐latitude glacial regions outside of the Gulf of Alaska and recommend further sampling of carbon chemistry variables in glacial outflow. The effects suggested here have a significant impact on the regional carbon budget as well as implications for regional fisheries sensitive to ocean acidification (Siedlecki et al, ). This mechanism may therefore be important for projecting coastal ecosystem impacts associated with the continued oceanic uptake of atmospheric CO 2 .…”

“…The larger Northeast Pacific Model (NEP) (Curchitser et al, 2005) supplies the boundary conditions and the initial conditions of all model state variables except dissolved inorganic carbon (DIC) and TA. Inclusion of DIC and TA is based on previous implementations within the ROMS framework (Fennel et al, 2008), with modifications for ammonium cycling impacts on the carbon cycle and freshwater influence (Siedlecki et al, 2017).…”

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

“…As this region continuous to rapidly deglaciate (Arendt et al, 2002;Larsen et al, 2007;O'Neel et al, 2005), not only the amount of freshwater discharge will increase (Beamer et al, 2017), but most of the glaciers surrounding the GOA will recede away 15 from the ocean and into higher elevations (Huss and Hock, 2015), resulting in a change of the biogeochemical composition of freshwater. This and previous studies have shown that freshwater discharge is the largest driver of the nearshore inorganic carbon dynamics in summer and fall, and is known to exacerbate the effects of ocean acidification Siedlecki et al, 2017;Pilcher et al, 2018). Understanding the composition of freshwater sources is, therefore, particularly important for the study of long-term trends of ocean acidification in the GOA-LME.…”

“…Overall, freshwater fluxes used in this study are more realistic than in previous studies because they are derived from a temporally and spatially highly resolved hindcast simulation forced with meteorological reanalysis data 5 sets. Compared to previous studies (Royer, 1982;Wang et al, 2004;Hill et al, 2015;Siedlecki et al, 2017), this current model configuration is better able to reproduce low salinity levels in coastal areas across the GOA and can thereby more adequately reproduce the Alaska Coastal Current (Danielson et al, in review).…”

“…However, neither of these models simulate the influence of spatially and temporally highly variable freshwater input along the coast. Siedlecki et al (2017) used the monthly riverine input climatology by Royer (1982) and applied it equally to the top most vertical cell along the land mask, whereas Xiu and Chai (2014) did not include riverine 5 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, 2) the carbon cycle, 3) the large spatial and temporal variability of freshwater input, 4) historical simulation long enough to be able to distinguish natural variability from the anthopogenic longterm trend, 5) multiple phytoplankton groups, and 6) iron limitation to reproduce the highly productive nearshore and high-nutrient low-chlorophyll offshore regions.…”