Seasonal Carbonate Chemistry Covariation with Temperature, Oxygen, and Salinity in a Fjord Estuary: Implications for the Design of Ocean Acidification Experiments

Abstract

Carbonate chemistry variability is often poorly characterized in coastal regions and patterns of covariation with other biologically important variables such as temperature, oxygen concentration, and salinity are rarely evaluated. This absence of information hampers the design and interpretation of ocean acidification experiments that aim to characterize biological responses to future pCO2 levels relative to contemporary conditions. Here, we analyzed a large carbonate chemistry data set from Puget Sound, a fjord estuary on the U.S. west coast, and included measurements from three seasons (winter, summer, and fall). pCO2 exceeded the 2008–2011 mean atmospheric level (392 µatm) at all depths and seasons sampled except for the near-surface waters (< 10 m) in the summer. Further, undersaturated conditions with respect to the biogenic carbonate mineral aragonite were widespread (Ωar<1). We show that pCO2 values were relatively uniform throughout the water column and across regions in winter, enriched in subsurface waters in summer, and in the fall some values exceeded 2500 µatm in near-surface waters. Carbonate chemistry covaried to differing levels with temperature and oxygen depending primarily on season and secondarily on region. Salinity, which varied little (27 to 31), was weakly correlated with carbonate chemistry. We illustrate potential high-frequency changes in carbonate chemistry, temperature, and oxygen conditions experienced simultaneously by organisms in Puget Sound that undergo diel vertical migrations under present-day conditions. We used simple calculations to estimate future pCO2 and Ωar values experienced by diel vertical migrators based on an increase in atmospheric CO2. Given the potential for non-linear interactions between pCO2 and other abiotic variables on physiological and ecological processes, our results provide a basis for identifying control conditions in ocean acidification experiments for this region, but also highlight the wide range of carbonate chemistry conditions organisms may currently experience in this and similar coastal ecosystems.

Salish Sea Response to Global Climate Change, Sea Level Rise, and Future Nutrient Loads

“…This carbonate dilution also affects spatial and temporal distribution of pH levels. During the winter, observations show well‐mixed corrosive waters (low pH), while stratification during the summer confined corrosive waters to deeper subsurface areas (Feely et al, ; Reum et al, ). The regions of hypoxia and corrosive lower pH waters are found to occur in the same poorly flushed subbasins such as Lynch Cove in Hood Canal.…”

Seasonal variation in aragonite saturation in surface waters of Puget Sound – a pilot study

“…Reum et al (2014) presented data from five cruises between 2008. Feely et al (2010 presented the results of two cruises in 2008.…”

Locally driven interannual variability of near‐surface pH and ΩA in the Strait of Georgia

“…Those organisms using the calcium carbonate mineral form aragonite in their external hard parts (e.g., mussels, oysters, and geoducks) can be especially vulnerable since oceanic CO 2 uptake lowers the aragonite saturation state (Ω A ) of seawater, often to undersaturated levels. Numerous studies have characterized different aspects of pH and Ω A in estuarine systems around the world [e.g., de Mora , ; Barton et al ., ; Hofmann et al ., ; Reum et al ., ; Salisbury et al ., ; Cai et al ., ; Melzner et al ., ]. These studies generally find that estuarine carbonate system properties are influenced by changes in ocean carbon inventories, however, they also demonstrate high degrees of seasonality and interannual variability associated with local physics and ecosystem dynamics.…”