Controls on buffering and coastal acidification in a temperate estuary

Abstract

Estuaries may be uniquely susceptible to the combined acidification pressures of atmospherically driven ocean acidification (OA), biologically driven CO2 inputs from the estuary itself, and terrestrially derived freshwater inputs. This study utilized continuous measurements of total alkalinity (TA) and the partial pressure of carbon dioxide (pCO2) from the mouth of Great Bay, a temperate northeastern U.S. estuary, to examine the potential influences of endmember mixing and biogeochemical transformation upon estuary buffering capacity (β–H). Observations were collected hourly over 28 months representing all seasons between May 2016 and December 2019. Results indicated that endmember mixing explained most of the observed variability in TA and dissolved inorganic carbon (DIC), concentrations of which varied strongly with season. For much of the year, mixing dictated the relative proportions of salinity‐normalized TA and DIC as well, but a fall season shift in these proportions indicated that aerobic respiration was observed, which would decrease β–H by decreasing TA and increasing DIC. However, fall was also the season of weakest statistical correspondence between salinity and both TA and DIC, as well as the overall highest salinity, TA and β–H. Potential biogeochemically driven β–H decreases were overshadowed by increased buffering capacity supplied by coastal ocean water. A simple modeling exercise showed that mixing processes controlled most monthly changes in TA and DIC, obscuring impacts from air–sea exchange or metabolic processes. Advective mixing contributions may be as important as biogeochemically driven changes to observe when evaluating local estuarine and coastal OA.

Interpreting biogeochemical processes through the relationship between total alkalinity and dissolved inorganic carbon: Theoretical basis and limitations

“…DIC is the sum of dissolved inorganic carbon species in seawater, including aqueous CO 2 , , and . Unlike TA, DIC is significantly influenced not only by air–sea exchange but also by processes that control TA, including biogeochemical processes such as respiration and photosynthesis, along with carbonate precipitation and dissolution (e.g., Hunt et al 2022; Van Dam et al 2021). Temperate and pressure changes affect pH and pCO 2 , but both TA and DIC are not affected by those changes.…”

“…To account for conservative mixing of water masses with different salinities on TA and DIC, a normalization scheme against a fixed salinity (nTA and nDIC) (Chen and Millero 1979; Friis et al 2003) is commonly employed to investigate the impacts of metabolic activities on seawater carbonate chemistry (e.g., Courtney et al 2021) or to differentiate metabolic effect from anthropogenic CO 2 signals (Peng et al 1998). Moreover, the slope of nTA–nDIC relationship is also widely used to infer major biogeochemical processes affecting carbonate chemistry in coastal environment (Hunt et al 2022; Szymczycha et al 2023; Xiong et al 2023; Yin et al 2023). This data interpretation method is based on the premise that different biogeochemical processes affect TA and DIC differently (Table 1).…”

“…A two endmember conservative mixing model has been widely used to study TA and DIC behaviors along a salinity gradient (e.g., Cabral et al 2021; Cai et al 2003; Friis et al 2003; Hunt et al 2022; Liu et al 2021; Yang and Byrne 2023). In this model, there are observations, with each observation comprising measurements for salinity, TA, and DIC.…”

Seasonality and response of ocean acidification and hypoxia to major environmental anomalies in the southern Salish Sea, North America (2014–2018)

“…Eastern boundary current systems accentuate this vulnerability by bringing subsurface, naturally O 2 -poor, CO 2 -rich waters toward the surface through upwelling (Feely et al, 2008;Chavez and Messié, 2009;Chavez et al, 2017). Estuarine systems such as the Salish Sea are typically lower in buffering capacity and are already rich in CO 2 due to dynamic local biological, hydrological, and geochemical processes; this natural estuarine acidification is amplified when oceanic waters acidified by the uptake of anthropogenic CO 2 are transported into the estuary via estuarine circulation (Feely et al, 2010(Feely et al, , 2018Wallace et al, 2014;Pacella et al, 2018;Cai et al, 2021;Hunt et al, 2022). Thus, estuaries connected to upwelling coastal systems, particularly in the NE Pacific, receive naturally acidified, low-oxygen marine waters relative to those in other coastal regions (e.g., Windham-Myers et al, 2018).…”

Sulfate enrichment in estuaries of the northwestern Gulf of Mexico: The potential effect of sulfide oxidation on carbonate chemistry under a changing climate

“…In addition to river-ocean mixing, biogeochemical processes including but not limited to primary production/ remineralization, nitrification/denitrification, iron reduction/ oxidation, and carbonate dissolution/precipitation all contribute to alkalinity variations in an estuary (Hunt et al 2022). Sulfate reduction and sulfide oxidation are also important reactions that contribute to coastal and estuarine alkalinity balance (Hu and Cai 2011), even though sulfate increase can also be caused by river discharge with high sulfate concentrations (Kwiecinski 1965).…”