Bottom hypoxia has emerged as one of the key environmental consequences of eutrophication in coastal areas worldwide. Its formation, severity, frequency and duration are governed not only by anthropogenic nutrient inputs, but also by a combination of physical, chemical and biological processes. Hong Kong waters have natural physical and geomorphological characteristics that prevent the occurrence of persistent seasonal bottom hypoxia. This study focused mainly on physical parameters such as light penetration, spring versus neap tidal effect, effect of water depth, and the influence of benthic resuspension on bottom dissolved oxygen (DO) dynamics near the sediment–water interface....[ Read more ]

Bottom hypoxia has emerged as one of the key environmental consequences of eutrophication in coastal areas worldwide. Its formation, severity, frequency and duration are governed not only by anthropogenic nutrient inputs, but also by a combination of physical, chemical and biological processes. Hong Kong waters have natural physical and geomorphological characteristics that prevent the occurrence of persistent seasonal bottom hypoxia. This study focused mainly on physical parameters such as light penetration, spring versus neap tidal effect, effect of water depth, and the influence of benthic resuspension on bottom dissolved oxygen (DO) dynamics near the sediment–water interface.

Two stations located in southern Hong Kong waters, were chosen as a representative of shallow and deep areas that provide a steep topographical gradient where the effect of tidal shoaling on nutrient and DO dynamics could be assessed. Although this area has received treated sewage discharge from the Stonecutters Island sewage treatment plant since 2001, bottom hypoxia over time scales of weeks has rarely occurred. Light penetration and tidal–induced vertical mixing due to shallow water depths were the primary factors suppressing the formation of bottom hypoxia in the shallow coastal area (<10 m).

The concept of the bottom critical height model was developed, in which the bottom critical height (BCH) is the height above the sediment where the depth–integrated photosynthetic oxygen production equals oxygen consumption in the water column, and the bottom height (BH) is the layer below the pycnocline. When the ratio of BCH to BH is ≤1, photosynthetic production of oxygen is higher than respiration consumption of DO and vise versa when the ratio is >1. Shallowness increases the lighted portion of the bottom height and allows bottom photosynthesis to occur and offset DO consumption. The ratio of BCH/BH was clearly <1 in the shallow coastal area, indicating that there was a low likelihood for the occurrence of bottom hypoxia due to photosynthetic oxygen production. Conversely, in the deep area (~20 m), this ratio was >1, which indicated a favourable condition for the occurrence of bottom hypoxia due to no or little lighted portion of the bottom height and hence, the lack of bottom photosynthesis, which resulted in net DO consumption. Besides light penetration, tidal–induced vertical mixing produced direct physical reareation for the bottom water in the shallow coastal area, particularly during spring ebbs. Although horizontal advection of deeper seawater did occur, spring tidal forcing was not strong enough to bring the entire low oxygen water mass to the shallow station. During spring ebbs, lower (but non–hypoxic) bottom DO water was associated with more sediment resuspension and porewater was redistributed into the overlying water at the shallow station. In contrast, episodic (less than a week) bottom hypoxia did occurred at the deep station when the spring tide brought in a large amount of low oxygen deep water from the continental shelf. The formation of bottom hypoxia was likely to be further fueled by continuous algal bloom sedimentation and subsequent DO consumption in the already low oxygen deep water from the shelf. By the time the water mass reached the deep station during floods, most of the POC and DO might have already been consumed and eventually resulted in a hypoxic water mass with an extremely low respiration rate, but high DOC.

The benthic flux of nutrients is often associated with low bottom DO, and this correlation was observed at the deep station particularly during spring ebbs, although the shallow water column was more nutrient–rich compared to the deep station. Smaller nutrient effluxes (particularly for nitrate) observed at the deep station suggested that nutrient recycling was not as vigorous as it was at the shallow station. Hence, the biogeochemical coupling seemed to be stronger at the shallow than the deep station. Ammonium and nitrate were fluxed out of the sediment at the shallow station during ebbs. The nutrient release might be associated with the redistribution of nutrient–rich porewater mechanically and/ or lateral tidal transport of nutrient–rich inshore water (possibly from the Stonecutters Island sewage treatment plant). At the deep station, the bottom 2 m was less nutrient–rich and bottom nutrient homogeneity was observed. This implied horizontal lateral advection did dominate the bottom nutrient distribution. At the deep station, relatively higher bottom ammonium and phosphate during ebbs was attributed by lateral tidal transport from inshore areas. Higher bottom phosphate was closely associated with hypoxic water mass and phosphate desorption under low oxygen tension might have occurred at the deep station and possibly help to fuel algal blooms during periods of episodic P limitation.