Dynamics of Oxygen and the Physicochemical Response in Two Comparative Hypoxia Regions

The oxygen depletion in worldwide oceans and inland waters is becoming an increasingly prevalent problem. Here, two comparative study sites, Baltic Proper (BP) and Shuikou Reservoir (SR), were selected to identify the dynamics of dissolved oxygen (DO) and the related physicochemical response by conducting five field investigations over one year period. The DO concentrations were 0–9.9 mg L−1 and 0.3–8.7 mg L−1 in BP and SR, with average oxygen change rates of −0.016~0.014 mg L−1 d−1 and −0.022~0.018 mg L−1 d−1, respectively. Such oxygen dynamics were highly related to salinity, temperature, turbidity, and chlorophyll-a than nitrogen and phosphorus. The persistent hypoxia (i.e., DO < 2 mg L−1) emerged below 63 m in BP during all sampling months where the reversal points of Brunt–Väisälä frequency N expressed in the form log10[N2 (s−2)] were −4. The seasonal hypoxia covered the downstream of SR at depths below 11.5 m in September with the highest log10[N2 (s−2)] between −3.95 and −3.64. The log10[N2 (s−2)] ≈ −4 may indicate the occurrence and development of hypoxia in both regions. In the case of the persistent and seasonal hypoxic conditions in BP and SR, the impact of temperature and turbidity on DO was opposite. Low oxygen levels in SR occurred under elevated temperature and turbidity. Additionally, under hypoxic conditions in both regions, NO3− and NH4+ concentrations significantly depended on DO changes. This study contributes to the understanding of seasonal changes in oxygen dynamics in different water bodies, and highlights different oxygen conditions and physicochemical responses to the oxygen changes.