Supply‐controlled calcium carbonate dissolution decouples the seasonal dissolved oxygen andpHminima in Chesapeake Bay

Abstract

Acidification can present a stress on organisms and habitats in estuaries in addition to hypoxia. Although oxygen and pH decreases are generally coupled due to aerobic respiration, pH dynamics may be more complex given the multiple modes of buffering in the carbonate system. We studied the seasonal cycle of dissolved oxygen (DO), pH, dissolved inorganic carbon, total alkalinity, and calcium ion (Ca 2+ ) along the main channel of Chesapeake Bay from May to October in 2016. Contrary to the expected co-occurrence of seasonal DO and pH declines in subsurface water, we found that the pH decline ended in June while the DO decline continued until August in mid-Chesapeake Bay. We discovered that aerobic respiration was strong from May to August, but carbonate dissolution was minor in May and June and became substantial in August, which buffered further pH declines and caused the seasonal DO and pH minima mismatch. The rate of calcium carbonate (CaCO 3 ) dissolution was not primarily controlled by the saturation state in bottom water, but was instead likely controlled by the supply of CaCO 3 particles. The seasonal variability of Ca 2+ addition in the mid-bay was connected to Ca 2+ removal in the upper bay, and the timing of high carbonate dissolution coincided with peak seasonal biomass of upper Bay submerged aquatic vegetation. This study suggests a mechanism for a novel decoupling of DO and pH in estuarine waters associated with CaCO 3 , but future studies are needed to fully investigate the seasonality of physical transport and cycling of CaCO 3 .

Decoupling of Estuarine Hypoxia and Acidification as Revealed by Historical Water Quality Data

“…It has to be mentioned that the empirically modeled TA we used in this analysis was relatively conservative to salinity, but recent observational studies have shown non-conservative behavior of TA associated with processes beyond aerobic respiration. 26,30 We used limited 1 year observed, non-conservative TA in the box model and found ∼15% higher rates of P DIC,non-aerobic , suggesting that our approach may be biased. Future work with a new set of TA observations could help constrain this source of uncertainty.…”

“…The decoupling of eutrophication-induced deoxygenation and acidification in late summer in Chesapeake Bay is consistent with recent hypotheses. 30 A possible mechanism is that nutrient inputs stimulate the generation and transport of CaCO 3 that later supports carbonate dissolution and alkalinity generation in hypoxic bottom waters. Recent reports have suggested that CaCO 3 dissolution is a dominant contributor of alkalinity in Chesapeake Bay in late summer, which can elevate pH, 30 but there is no evidence that this dissolution occurs at higher rates in years of elevated nutrient loading.…”

“…30 A possible mechanism is that nutrient inputs stimulate the generation and transport of CaCO 3 that later supports carbonate dissolution and alkalinity generation in hypoxic bottom waters. Recent reports have suggested that CaCO 3 dissolution is a dominant contributor of alkalinity in Chesapeake Bay in late summer, which can elevate pH, 30 but there is no evidence that this dissolution occurs at higher rates in years of elevated nutrient loading. Su et al 25 suggested that submerged macrophyte beds stimulate CaCO 3 production, but these beds are usually less productive (and thus likely generate less CaCO 3 ) in years of high riverine inflows.…”

Stressing over the Complexities of Multiple Stressors in Marine and Estuarine Systems

“…The counterbalance between OA and river basification [161] results in the most variability in the pH and carbonate chemistry in the autotrophic midbay [162], with strong pH fluctuations driven by phytoplankton photosynthesis. Additionally, seasonally, calcium carbonate dissolution is an important buffering mechanism for bottom water pH changes in late summer in the midbay, leading to higher pH values in August than in June despite persistent hypoxic conditions during the summer [146,163,164]. Wind-driven lateral upwelling can advect deep water with low pH onto shallow shoals and reduce the aragonite saturation state, potentially exposing a large fraction of oyster beds in the main stem of the bay to undersaturated carbonate mineral conditions during wind events [165,166].…”

Coupled Carbonate Chemistry - Harmful Algae Bloom Models for Studying Effects of Ocean Acidification on Prorocentrum minimum Blooms in a Eutrophic Estuary

“…Due to the counter-balance between OA and river alkalinization (Shen et al, 2020), pH in the autotrophic mid-bay has shown no significant long term trends but displays strong short-term fluctuations likely associated with phytoplankton photosynthesis. Also, seasonally, calcium carbonate dissolution is an important buffering mechanism for pH changes in late summer in the mid-bay, leading to higher pH values in August than in June, despite persistent hypoxic conditions during the summer (Su et al, 2020;Su et al, 2021). How these long term pH trends and seasonal variations affect seasonal development of HABs and how the increasing atmospheric pCO 2 in a warming climate influence HABs are largely unknown but are of critical importance for managing coastal resources.…”