Environmental response to sewage treatment strategies: Hong Kong's experience in long term water quality monitoring

In many coastal cities around the world, marine outfalls are used for disposal of partially treated wastewater effluent. The combined use of land-based treatment and marine discharge can be a cost-effective and environmentally acceptable sewage strategy. Before 2001, screened sewage was discharged into Victoria Harbour through many small outfalls. After 2001, the Hong Kong Harbour Area Treatment Scheme (HATS) was implemented to improve the water quality in Victoria Harbour and surrounding waters. Stage I of HATS involved the construction of a 24 km long deep tunnel sewerage system to collect sewage from the densely populated urban areas of Hong Kong to a centralized sewage treatment plant at Stonecutters Island. A sewage flow of 1.4 million m³ d⁻¹ receives Chemically Enhanced Primary Treatment (CEPT) followed by discharge via a 1.2 km long outfall 2 km west of the harbor. The ecosystem recovery in Victoria Harbour and the environmental response to sewage abatement after the implementation of HATS was studied using a 21-year data set from long term monthly water quality monitoring. Overall, the pollution control scheme has achieved the intended objectives. The sewage abatement has resulted in improved water quality in terms of a significant reduction in nutrients and an increase in bottom DO levels. Furthermore, due to the efficient tidal mixing and flushing, the impact of the HATS discharge on water quality in the vicinity of the outfall location is relatively limited. However, Chl a concentrations have not been reduced in Victoria Harbour where algal growth is limited by hydrodynamic mixing and water clarity rather than nutrient concentrations. Phosphorus removal in the summer is suggested to reduce the risk of algal blooms in the more weakly-flushed and stratified southern waters, while nutrient removal is less important in other seasons due to the pronounced role played by hydrodynamic mixing. The need for disinfection of the effluent to reduce bacterial (E. coli) concentrations to acceptable levels is also confirmed and has recently been implemented.     

Net, actual, and potential sediment-water interface NH4 fluxes in the northern Gulf of Mexico (NGOMEX): Evidence for NH4 limitation of microbial dynamics

A new approach, combining ¹⁵NH₄⁺ isotope dilution and continuous-flow techniques, provided estimates of “actual” and “net” NH₄⁺ flux and sediment NH₄⁺ demand (SAD) at the sediment-water interface (SWI) of sites in the northern Gulf of Mexico (NGOMEX). The sites included a hypoxic site (C6), two sites with intermediate oxygen levels (B7 and F5), and a normoxic site (CT). Control cores without isotope addition and other cores from the same site treated with ¹⁵NH₄⁺ labeled overflowing water differentiated between net and actual regeneration flux and actual vs. potential uptake flux of NH₄⁺. Experiments were conducted in 2008 before (July) and after (September) two successive hurricanes (Gustav and Ike) and in January and August, 2009. Actual regeneration was significantly higher than net flux at most sites. Net flux did not differ significantly in most sites/dates, but the actual regeneration, and the actual and potential uptake, showed temporal and spatial variation; the flux at the hypoxic site was more active than non-hypoxic sites. SAD, the difference between potential and actual NH₄⁺ uptake flux, was higher at the hypoxic site than at non-hypoxic sites before and after the hurricanes in 2008 and during the hypoxia season in 2009. SAD related negatively to bottom water DO values. Conclusions: (1) net flux often underestimated actual regeneration, (2) hurricane activity decreased N dynamics, and (3) microbial N limitation status at the hypoxic site related to NH₄⁺ removal processes that were independent of oxygen (e.g., anaerobic heterotrophic uptake or anammox). These results indicate a rather consistent NH₄⁺ demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region.     

Radon as an indicator of limited cross-shelf mixing of submarine groundwater discharge along an open ocean beach in the South Atlantic Bight during observed hypoxia

Hypoxic conditions (dissolved oxygen (DO)<2 mg l⁻¹) have been documented in the nearshore coastal waters of Long Bay, South Carolina, United States of America, during summer months over the past several years. Hypoxia was documented in August 2009 in the nearshore (<500 m offshore) for ten consecutive days and four days in September 2009 corresponding with spring tides. This study measured radon activities of shallow beachface groundwater and nearshore bottom waters to estimate mixing rates and submarine groundwater discharge (SGD) in the nearshore waters of central Long Bay. Statistical analyses demonstrate significant correlations between high bottom water radon activities, low DO, and cooler bottom water temperatures during hypoxic conditions. Elevated radon activities during hypoxia were significantly influenced by upwelling favorable conditions which severely limited cross-shelf mixing. Model results indicate mixing of nearshore and offshore waters was limited by up to 93% (range: 43-100%) relative to non-hypoxic conditions. Data suggests previously overlooked natural phenomena including limited cross-shelf mixing and SGD can significantly influence nearshore water quality.     

Long-term divergent tidal flat benthic community recovery following hypoxia-induced mortality

Macrobenthos recovery after hypoxia-induced mass mortality was assessed in an estuarine tidal mudflat during 3 years. During the first 2 years, a Pearson-Rosenberg type of community recovery took place along with the improving bottom water oxygen conditions. After 3 months, spionid polychaetes became superabundant (i.e. opportunistic peak), followed rapidly by a steep decline (i.e. ecotone point). Subsequently, a moderate increase in species richness and a steep increase in biomass, related to the growth of long-lived species occurred (i.e. transition region). Afterwards, however, the recovering community diverged again from the ambient, undisturbed, sediments due to enhanced recruitment success of long-lived species presumably resulting from the lowered interference from bioturbation during early recovery stages in the disturbed plots. Hence, despite early community recovery may be more or less deterministic, lagged divergent community reassembling may occur at the longer-term, thereby contributing to benthos patchiness in areas which are frequently subjected to disturbances.     

Interactive effects of hypoxia and PBDE on larval settlement of a marine benthic polychaete

Marine benthic polychaete Capitella sp. I is widely known to adapt to polluted habitats; however, its response to xenobiotics under hypoxic conditions has been rarely studied. This research aimed to test the hypothesis that interactive effects of hypoxia and congener BDE-47 of polybrominated diphenyl ethers (PBDE), which is ubiquitous in marine sediments, may alter the settlement of Capitella sp. I. Our results revealed that under hypoxic condition, settlement success and growth in body length of Capitella larvae were significantly reduced compared to those under normoxia of similar BDE-47 concentration. While no significant changes in morphology of settled larvae were noted in both exposure conditions, the presence of BDE-47 could enhance polychaete growth. The present findings demonstrated that the interactive effects of hypoxia and environmentally realistic concentrations of BDE-47 in sediments could affect polychaete settlement, which, in turn, reduce its recruitment and subsequent population size in the marine benthic ecosystem.     

Sediment-preserved diatom assemblages can distinguish a petroleum activity signal separately from the nutrient signal of the Mississippi River in coastal Louisiana

We analyzed the preserved diatom assemblages in dated sediment cores collected from five locations in the Louisiana Bight to test if there was a signature of petroleum extraction activities (hopanes and barium) distinct from the well-documented effects of nutrient loading. The results of a multi-dimensional scaling analysis indicate that the diatom assemblage changes documented throughout the 40 year record could be explained by three variables: barium and hopanes concentrations, and Mississippi River nitrogen loading. The results of a canonical correspondence analysis demonstrated that these signals could be distinguished through correlations with specific diatom species. The abundance of Actinoptychus senarius, for example, was negatively correlated with barium and the Pseudo-nitzschia delicatissima complex was positively correlated with nitrogen loading. These results provide a “proof-of-concept” demonstration that diatom assemblages preserved in the sediments can be used to study the effects of petroleum extraction activities, and that these ‘petroleum signals’ may be distinguished from other significant influences such as nutrient loading.     

低氧胁迫对菲律宾蛤仔抗氧化酶的影响

为了研究不同低氧水平对菲律宾蛤仔(Ruditapes philippinarum)的影响,在实验室条件下设置溶解氧为0.5 mg/L和2.0 mg/L的低氧海水环境,以溶氧水平为7.0 mg/L的天然海水为对照组,研究了低氧胁迫对菲律宾蛤仔抗氧化酶的影响。结果表明:在低氧胁迫的时间越长的条件下,乳酸脱氢LDH和琥珀酸脱氢酶SDH的活力逐渐降低,而碱性磷酸酶AKP活力逐渐增大;对于相同的酶,对照组的酶活力高于处理组的酶活力;在溶氧DO越低的情况下,菲律宾蛤仔的死亡率越高。     

长江口及其邻近海域表层沉积物中氧化还原敏感性微量元素的环境指示意义

对长江口及其邻近海域表层沉积物和底层悬浮体中氧化还原敏感元素分布规律和富集特征进行了分析与研究。结果表明氧化还原敏感元素在研究区具有明显的“离岸富集”特征,去除粒度效应、陆源碎屑来源组分和有机质的吸附作用等因素的影响之后,氧化还原敏感元素仍显示出在缺氧区的富集。通过同一站位底层悬浮体和沉积物中氧化还原敏感元素含量的分析比较,发现底层水缺氧是导致氧化还原敏感元素Mo、Cd、V等在沉积物中富集的主要原因。Mo、Cd、V等元素的不同富集程度可用来反映缺氧区的缺氧程度。因此,Mo, Cd, V等RSE在长江口外缺氧区及其邻近海域具有氧化还原环境指示意义,可以指示长江口外缺氧区的存在与大体范围,并可在一定程度上用来衡量缺氧区的缺氧程度。U理论上虽然也对环境的氧化还原条件敏感,但由于受陆源碎屑来源组分的影响较大,在长江口外缺氧区的富集并不明显,因此U在研究区不具有氧化还原环境指示意义。     

Assessment of marine benthic quality change in gradients of disturbance: Comparison of different Scandinavian multi-metric indices

Three multi-metric benthic macrofauna indices were used to assess marine benthic ecological quality status (EcoQS) according to the European Water Framework Directive, in seven pollution gradients mainly, western Scandinavia. The impacts included organic load, hypoxia, metals, urban effluents and physical disturbance. The indices responded in a similar threshold fashion, irrespective of impact factor identity. Usually, the border between Good and Moderate EcoQS (G/M), is determined as some deviation from a reference situation. References, however, are difficult to find. An alternative procedure is described to estimate the G/M border, not requiring reference data. Thresholds, where faunal structure deterioration commences, were identified from non-linear regressions between indices and impact factors. Index values from the less impacted side of the thresholds were assumed to come from environments of Good and High EcoQS, and the 5th percentile of these data, was defined as the G/M border. Estimated G/M borders compared well with previous studies.     

A Bayesian hierarchical modeling approach for analyzing observational data from marine ecological studies

We introduce the Bayesian hierarchical modeling approach for analyzing observational data from marine ecological studies using a data set intended for inference on the effects of bottom-water hypoxia on macrobenthic communities in the northern Gulf of Mexico off the coast of Louisiana, USA. We illustrate (1) the process of developing a model, (2) the use of the hierarchical model results for statistical inference through innovative graphical presentation, and (3) a comparison to the conventional linear modeling approach (ANOVA). Our results indicate that the Bayesian hierarchical approach is better able to detect a “treatment” effect than classical ANOVA while avoiding several arbitrary assumptions necessary for linear models, and is also more easily interpreted when presented graphically. These results suggest that the hierarchical modeling approach is a better alternative than conventional linear models and should be considered for the analysis of observational field data from marine systems.