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Seminar 2022-11-09 by Dr. Chunqi Shen

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14 November 2022, 12:06 PM

Speaker: Dr. Chunqi Shen

Time: 14:30-15:45

Location: ES354


Seasonal decoupling of hypoxia and acidification in Chesapeake Bay


Estuarine aquatic ecosystems are increasingly threatened by multiple human-induced stressors associated with climate and anthropogenic changes, including eutrophication, harmful algal blooms, hypoxia and acidification. Hypoxia, a decline in water column dissolved oxygen (DO) concentrations (DO < 2 mg L-1), is well recognized as a consequence of elevated nutrient loads and the associated enhancement of phytoplankton growth. Ocean Acidification is a reduction of oceanic pH due to direct uptake of CO2 from the atmosphere, and CO2-induced acidification in estuaries is enhanced by eutrophication and intensive respiration/decomposition. Hypoxia and acidification are commonly coupled in eutrophic aquatic environments because aerobic respiration is usually dominant in bottom waters and can lower DO and pH simultaneously. However, the degree of coupling can be weakened by non-aerobic respiration and CaCO3 cycling. We applied a mass-balance box model to explore the relationship between hypoxia and acidification in Chesapeake Bay. Our results suggested stronger buffering processes (e.g., calcium carbonate dissolution) in late summer, which would elevate pH in hypoxic waters. In addition, a bay-wide pH-buffering mechanism resulting from spatially decoupled CaCO3 mineral cycling was proposed. In summer, high rates of photosynthesis by dense submerged aquatic vegetation (SAV) at the head of the bay and in shallow, nearshore areas generate high pH and CaCO3 crystals. These CaCO3 particles produced from the SAV bed are subsequently transported downstream into corrosive subsurface waters in mid Bay, where their dissolution buffers pH decreases, leading to the decoupling of hypoxia and acidification there.

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