a numerical analysis of biogeochemical controls with physical modulation on hypoxia during summer in the pearl river estuary
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2017
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Abstract
A three-dimensional (3-D) physical–biogeochemical coupled model
was applied to explore the mechanisms controlling the dissolved oxygen (DO)
dynamics and bottom hypoxia during summer in the Pearl River estuary (PRE).
By using the numerical oxygen tracers, we proposed a new method (namely the
physical modulation method) to quantify the contributions of boundary
conditions and each source and sink process occurring in local and adjacent
waters to the DO conditions. A mass balance analysis of DO based on the
physical modulation method indicated that the DO conditions at the bottom
layer were mainly controlled by the source and sink processes, among which
the sediment oxygen demand (SOD) at the water–sediment interface and the
re-aeration at the air–sea interface were the two primary processes
determining the spatial extent and duration of bottom hypoxia in the PRE.
The SOD could cause a significant decrease in the bottom DO concentrations
(averaged over July–August 2006) by over 4 mg L−1 on the shelf off the
Modaomen sub-estuary, leading to the formation of a high-frequency zone of
hypoxia (HFZ). However, the hypoxia that occurred in the HFZ was
intermittent and distributed in a small area due to the combined effects of
re-aeration and photosynthesis, which behaved as sources for DO and offset a
portion of the DO consumed by SOD. The bottom DO concentrations to the west
of the lower Lingdingyang Bay (i.e. the western shoal near Qi'ao Island)
were also largely affected by high SOD, but there was no hypoxia occurring
there because of the influence of re-aeration. Specifically, re-aeration
could lead to an increase in the bottom DO concentrations by ∼ 4.8 mg L−1 to the west of the lower Lingdingyang Bay. The re-aeration
led to a strong vertical DO gradient between the surface and the lower
layers. As a result, the majority (∼ 89 %) of DO
supplemented by re-aeration was transported to the lower layers through
vertical diffusion and ∼ 28 % reached the bottom eventually.
Additional numerical experiments showed that turning off re-aeration could
lead to an expansion of the hypoxic area from 237 to 2203 km2
and result in persistent hypoxia (hypoxic frequency > 80 %) to
the west of the lower Lingdingyang Bay. Compared to re-aeration and SOD,
photosynthesis and water column respiration had relatively small impacts on
the DO conditions; turning off these two processes increased the hypoxic
area to 591 km2. In summary, our study explicitly elucidated the
interactive impacts of physical and biogeochemical processes on the DO
dynamics in the PRE, which is critical to understanding hypoxia in this
shallow and river-dominated estuarine system.
| Reference Key |
wang2017biogeosciencesa
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|---|---|
| Authors | ;B. Wang;J. Hu;S. Li;D. Liu |
| Journal | tetrahedron letters |
| Year | 2017 |
| DOI |
10.5194/bg-14-2979-2017
|
| URL | |
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