Abstract. As an important biogeochemical indicator of aquatic ecosystem, dissolved oxygen (DO) is affected by the boundary conditions and biogeochemical processes. Biogeochemical processes can affect DO concentrations by directly consuming or generating oxygen locally, or through changing the DO fluxes from the ambient water bodies. However, the latter mechanism is still unclear. In this study, a novel method named physical modulation of biogeochemical terms is therefore proposed and coupled to a physical-biogeochemical model to investigate their contributions to the hypoxia during the summer of the Pearl River Estuary (PRE). According to the result of modulation method, re-aeration and sediment oxygen demand are the most important biogeochemical processes, and determine the distribution, the spatial extent, and the duration of hypoxia in the PRE. A DO balance analysis is conducted and reveals that although the re-aeration occurs on the air-sea interface, the reoxygenation leads to a strong DO gradient form between the surface and lower layers. As a result, the majority (89 %) of oxygen entering the surface layer from the atmosphere will be transported to the lower layer through the vertical diffusion, and 28 % eventually reach the bottom layer. Similarly, after consuming the bottom DO, sediment oxygen demand facilitates the downward DO flux of vertical diffusion and decreases the upward DO flux of vertical advection. Under the modulation of physical processes, sediment oxygen demand causes a most significant decrease in DO concentration by 4.31 mg L−1 in the bottom of the HFZ (a high frequency zone of hypoxia located off the Modaomen sub-estuary) and the west of lower estuary. However, the re-aeration supplements an average of 4.84 mg L−1 DO on the west of lower estuary, which leads to hypoxia only occur in HFZ. Numerical experiments show that turning off the re-aeration leads to an expansion of hypoxic area from 237 km2 to 2203 km2 and results in a shift of hypoxic center to the west of lower estuary. Moreover, a persistent hypoxia (hypoxic frequency > 80 %) is observed in the west of lower estuary. When compared with re-aeration and sediment oxygen demand, photosynthesis and water column respiration have fewer effects on DO conditions. In the bottom of the HFZ, photosynthesis exceeds the water column respiration and eventually supplements DO concentration by 0.98 mg L−1, causing an increase of hypoxic area to 591 km2.
Biotope Determinants of EPT Assamblages Structure – Târnava Watershed (Transylvania, Romania) Case Study
