The role of algal blooms and community respiration in controlling the temporal and spatial dynamics of hypoxia and acidification in eutrophic estuaries.

While hypoxia and acidification can be common occurrences in eutrophic coastal zones, the precise, coupled temporal and spatial dynamics of these conditions are poorly described. Here, continuous measurements of water column pH, pCO ₂ , carbonate chemistry, and dissolved oxygen (DO) concentrations were made from spring through fall across two, temperate eutrophic estuaries, western Long Island Sound (LIS) and Jamaica Bay, NY, USA. Vertical dynamics were resolved using an underway towing profiler and an automated stationary profiling unit. During the study, high rates of respiration in surface and bottom waters (> -0.2 mg O ₂ L ^-1  h ^-1 ) yielded strongly negative rates of net ecosystem metabolism during the summer (-4 to -8 g O ₂ m ^-2 d ^-1 ). Ephemeral surface algal blooms caused brief periods (< one week) of basification and supersaturation of DO that were succeeded by longer periods of acidification and hypoxia. In deeper regions, hypoxia (< 2 mg L ^-1 DO) and acidic water (pH < 7; total scale; pCO ₂ levels >2000 μatm) that persisted continuously for >40 days in both estuaries was often overlain by water with higher DO and pH. Diurnal vertical profiles demonstrated that oxic surface waters saturated with respect to calcium carbonate and DO during the day transitioned to unsaturated and hypoxic at night. Evidence is presented that, beyond respiration, nitrification in surface water promoted by sewage discharge and oxidation processes in sediments also contribute to acidification in these estuaries. Collectively, this study demonstrates the pervasive, persistent, and dynamic nature of hypoxia and acidification in eutrophic estuaries are likely to shape marine food webs.
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