International Journal of Marine Science, 2024, Vol.14, No.4, 285-294 http://www.aquapublisher.com/index.php/ijms 287 3.2 Formation of hypoxic and anoxic zones The formation of hypoxic (DO < 2 mg L-1) and anoxic (DO < 0.5 mg L-1) zones is a direct consequence of severe DO depletion. These zones are characterized by the absence of oxygen, which severely impacts marine ecosystems. In the Baltic Sea, the extent of these zones has been linked to both eutrophication and climate change, with significant regime shifts towards more anoxic conditions observed over the past decades. The stratification of water columns, particularly the presence of a halocline, plays a crucial role in maintaining these hypoxic and anoxic conditions by preventing the mixing of oxygen-rich surface waters with deeper layers (Almroth‐Rosell et al., 2021). In eutrophic estuaries, the dynamics of hypoxia are influenced by various factors, including algal blooms and community respiration (Figure 1). For example, in western Long Island Sound and Jamaica Bay, ephemeral algal blooms can cause brief periods of oxygen supersaturation, followed by prolonged hypoxia as the organic matter decomposes. Additionally, processes such as nitrification, driven by sewage discharge, further contribute to the acidification and hypoxia in these estuaries (Wallace and Gobler, 2021). Figure 1 High-resolution continuous surface water sampling for nitrate (μM; SUNA UV nitrate sensor), normalized chlorophyll a fluorescence (μg L-1), dissolved oxygen (DO; mg L-1), pHT (total H+ scale), and the partial pressure of CO2 (pCO2; μatm) around Long Island, New York during September 2014 (Adopted from Wallace and Gobler, 2021) The study by Wallace and Gobler (2021) demonstrated the spatial distribution of nitrate, chlorophyll a fluorescence, dissolved oxygen (DO), and pH in high-resolution continuous surface water samples. The figure shows that eutrophication-induced algal blooms significantly increased dissolved oxygen and pH levels in the surface waters. However, as algae die and decompose, oxygen is rapidly consumed, leading to hypoxia or even
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