A novel method of identifying estuary high-nutrient zones for water quality management.

Coastal shallow waters are highly vulnerable to pollution, often leading to the development of intricate eutrophication zones. However, accurately determining these areas poses a significant challenge due to the complex interplay of estuarine hydrodynamics and nutrient transformation. To address such issue, a novel method was proposed to identify high-nutrient zones through calculating the continuous zonation of released tracers when their instantaneous concentrations declined to 1/e of their initial values. The method was well tested using idealized estuary models with varying shape parameters, water depths and river discharges. The results consistently revealed that the boundaries of high-nutrient zones fell within the mixed zone, characterized by salinity levels of 10- 20 psu. In Shenzhen Bay, a typical shallow bay, distinct differences were observed in the concentrations of dissolved inorganic nitrogen (DIN) and PO43-. Both the 20 psu isohaline and the proposed method effectively identified the partition boundary of high DIN and PO43- in 2001-2010, but only the newly proposed method demonstrated accuracy in delineating the actual high-nutrient zone during the continuous nutrient reduction period from 2010 to 2020. This study provides a practical and feasible approach that can serve as an auxiliary decision-making tool for managing estuarine water environments, and it has potential to facilitate the implementation of timely and effective measures for pollution control.

Why do red tides occur frequently in some oligotrophic waters? Analysis of red tide evolution history in Mirs Bay, China and its implications.

The process and management of red tide in oligotrophic waters are poorly understood as most studies on red tide were focused on eutrophic areas. In this study, 404 red tide events together with the historical water quality dynamics during 1991-2020 were investigated in an anthropogenically influenced bay in China - Mirs Bay, whose most region is oligotrophic except small inshore areas. Red tides of oligotrophic offshore accounted for 20 % of all. With the effective governmental management on inshore areas, concentration of PO4 and DIN has been decreased to a low level (PO4 <0.01 mg/L while DIN <0.1 mg/L) in the bay since about 2000. However, the reduction of nutrients was still accompanied by the frequent outbreaks of red tides, as well as a shift of dominant algae from diatoms to dinoflagellates, which might be due to the unbalanced nutrient reduction, such as N:P ratio fluctuation and organic nutrient increase. This shift might trigger more red tide events and even some super ones (long-duration or large-scale) in oligotrophic areas. Detailed analysis on red tide events combined with model simulation proved that the outbreak of red tide in Mirs Bay was caused by the joint contribution of nutrients and hydrodynamics. Nutrients of inshore area supported the red tides there, and with the help of physical conditions, red tides inshore could be transferred to offshore areas and then were likely to bloom again or be preyed to support blooms of other organisms. This study acknowledged that the reduction of both N and P either inorganic or organic nutrients was essential to control red tides, even in oligotrophic waters, but a balanced strategy considering the dual reduction of both nitrogen and phosphorus was of pivotal role to restore the health of coastal water systems disturbed by human.

Save reservoirs of humid subtropical cities from eutrophication threat.

Reservoir water is the most important freshwater resource for many cities, especially in densely populated humid subtropical areas. Economic growth, population increase, and urbanization have been putting reservoir water of Shenzhen (China), a humid subtropical city, under severe threat of eutrophication and water supply shortage. In this study, we focused on an upstream reservoir of Shenzhen and established a 3-dimensional hydrodynamic-ecological model to investigate the water dynamics and nutrient budget. Tributaries to the reservoir were identified as the greatest contributors to nitrogen and phosphorus loads. Zones with weak flows and high nutrient concentration have high risks of causing blooms. Several mitigation measures were proposed, including improving flow by adding additional water exit locations in the reservoir, reducing nutrients in tributaries, and enhancing algal predation, and were evaluated with the established model. The strategies combining hydrodynamic improvement and phosphorus reduction were suggested to decision makers and government managers for short-term management. However, for future water safety, excessive nitrogen is a potential danger. This study provides a modeling framework that can be applied to anthropogenic-influenced reservoirs elsewhere.

Predominant factors of disaster caused by tropical cyclones in South China coast and implications for early warning systems.

Predicting disastrous wind and rainfall associated with tropical cyclones (TCs) is critical to prevent and mitigate the casualties and damage of TCs. The studied warning area was chosen with a radius of 800 km centered on Hong Kong in which the tracks of TCs making landfall in China are concentrated. In general, the number of TCs making landfall decreased but landfall locations and intensities of TCs increased since 1990. Our results suggested minimum sea level pressure (MSLP) in TC affected areas was the predominant disaster-warning factor and indicator for the resulting risks and damages of TCs in 1975-2017. The MSLP of 990 hPa monitored in a TC affected area was a threshold for severe impacts and prediction of strong wind and heavy rainfall. Early warning using a combination of MSLP and the nearest approach distance of TCs (MSLP of 990 hPa for distance of 100 km) outperformed the current warning system based on wind speed, often providing more timely warning and reducing the false warnings.

Eutrophication control strategies for highly anthropogenic influenced coastal waters.

Rapid economic development and urbanization necessitate an understanding of anthropogenic effects on coastal eutrophication. It is a significant challenge for governments to alleviate water degradation and remediate coastal ecosystems. Shenzhen Bay in China, located in a developed and populous area, was selected to analyze the decadal scale influence of anthropogenic activities on eutrophication as well as the effectiveness of governmental remediation strategies. The results showed that the contribution of nutrients from anthropogenic sources accounted for over 80% of the total loads into the bay. Beginning in 1990, increased anthropogenic activities resulted in the loss of environmental capability and resilience, exacerbated eutrophication and water quality degradation. However, the status of eutrophication has been improved since 2005, following the application of intensive management actions implemented in 2000. The practice of eutrophication control suggested that, in view of technical and engineering feasibility, coastal strategies for similar shallow bays should initially reduce phosphorus, followed by nitrogen and eco-remediation to alleviate the serious aquatic situation immediately. The recovery period of eutrophication would be at least five years after governmental actions from Shenzhen Bay's experience. Furthermore, simulated scenarios indicated that eutrophication ranking of Shenzhen Bay could reach and remains medium Moderate, following a 35% reduction in total nitrogen and a 20% reduction in total phosphorus with corresponding eco-remediation.

Human impacts and changes in the coastal waters of south China.

Human impact on the environment remains at the center of the debate on global environmental change. Using the Hong Kong-Shenzhen corridor in south China as an example, we present evidence that rapid urbanization and economic development in coastal areas were the dominant factors causing rapid changes in coastal waters. From 1990 to 2012, coastal seawater temperature increased ~0.060°C per year, sea level rose 4.4mm per year and pH decreased from 8.2 to 7.7, much faster than global averages. In the same period, there were exponential increases in the local population, gross domestic product and land fill area. Empirical analyses suggest that the large increase in the population affected local temperature, and economic development had a major impact on local pH. Results also show that pH and temperature were significantly correlated with local sea level rise, but pH had more predictive power, suggesting it could be considered a predictor for changes in local sea level. We conclude that human activities could significantly exacerbate local environmental changes which should be considered in predictive models and future development plans in coastal areas.

A novel flat plate algal bioreactor with horizontal baffles: structural optimization and cultivation performance.

A novel flat plate photobioreactor with horizontal baffles was developed. The effects of aeration intensity and aeration site were investigated using computational fluid dynamics. The effects of baffle structural parameters, including the ratio of clearance between the baffle and the bioreactor wall (d) to bioreactor width (D) and the ratio of distance between two adjacent baffles (h) to D on the flow and mixing performance were assessed. A good light/dark cycle and strong, uniform mixing performance were created at d/D=0.5 and h/D=1 when the aeration site was located at the bottom center. The cultivation performance was assessed by culturing Chlorella vulgaris 31. The maximum biomass productivity in the optimized bioreactor was 1.88 times than that of a traditional bioreactor without baffles. For a light path length of 80 mm, the optimized baffles offer a large economical advantage in improving algal productivity and reduce growth condition difference.

[Variations of annual load of TN and TP in the deep bay watershed, Shenzhen].

The empirical coefficient of sewage disposal, export coefficient model and mean concentration method were respectively used to estimate variations of annual load TN and TP from Shenzhen and Hong Kong areas in the Deep Bay Watershed from 1986 to 2011. The results showed that, the annual average loads of TN and TP were 10 388.2 t, 10 727.9 t, 10 937.3 t, and 2 694.5 t, 1 929.2 t, 1388.7 t, respectively in the whole watershed during three periods, 80s, 90s and years after 2000. With the rapid development of society, economy and the urbanization, annual pollution loading of TN and TP in Shenzhen area showed an obviously increase, 4373.6 t and 195.9 t, by 261.0% and 64.2% for point source, and 1067.2 t and 151.0 t, by 63.4% and 84.9% for non-point source, respectively. Non-point source with high pollution load was mainly caused by the expanding of land for construction and roads. The contribution ratios of TN and TP from Shenzhen area increased from 42.4% and 27.0% to 85.1% and 75.2%. Annual loads of TN and TP in Hong Kong area decreased 3 028.5 t and 1 031.5 t, by 66.3% and 79.0% reduced.

[Simulation and evaluation on TIN and PO4(3-) -P reduction in Deep Bay, China].

Environmental Fluid Dynamics Code (EFDC), three-dimensional hydrodynamic and water quality models, are employed to simulate the transport of total inorganic nitrogen (TIN) and orthophosphate (PO4(3-) -P) in Deep Bay, China under different hydrologic conditions. It shows that, the computational results of TIN and PO4(3-) -P match very well with monthly-averaged field data in dry and wet seasons. The results of TIN and PO4(3-) -P reduction scenarios show that, it is necessary to reduce TIN concentration in Pearl River to Grade III to guarantee TIN in Deep Bay comply with the marine functional zonation requirement of the standard of Grade III. TIN and PO4(3-) -P concentrations in Deep Bay could comply with the marine functional zonation requirement if 95% TIN and PO4(3-) -P loading are reduced from the inflow when degradation ignores. When the degradation is considered, TIN could comply with the marine functional zonation requirement if 83% TIN loading are reduced from the inflow.

[Dynamic experiment on controlling of Microcystis aeruginosa by UV-C irradiation].

A plug-flow UV-C reactor equipped with low pressure UV lamp was utilized to study the suppression effect after UV-C irradiation under the dynamic conditions on Microcystis aeruginosa, a typical cyanobacterium in algae blooms in China. The culture fluid of Microcystis aeruginosa was exposed to UV-C irradiation when pumped through the reactor. After that, the fluid was incubated under the normal culture condition, and sampled at 2 h, 1 d, 3 d, 5 d, 7 d, 9 d for determination of cell density using the inverted system fluorescence microscope. The experiments showed that, UV-C irradiation did not cause severe cell lysis, and UV-C irradiation at dose ranged from 36 to 115 mW x s x cm(-2), and 31 to 50 mW x s x cm(-2) could suppress Microcystis aeruginosa biomass growth for the 2.6 x 10(5)-2.7 x 10(5) cells x mL(-1) and 9.0 x 10(5)-1.15 x 10(6) cells x mL(-1) fluid in 9 days, respectively.

[Numerical simulation of TP transport after overflow of rainwater into urban lake].

Based on the three-dimensional advection-diffusion-reflection equation, a two-dimensional TP transport equation was deduced to simulate TP distribution and transport after overflow rainwater into urban lake from storm sewer system during rainstorm. The model has a good agreement with a group of monitor data at Lake Lichee in Shenzhen, China. The model was applied to compute the scenario in Lake Lichee under the design rainstorm, and analyse the fate of TP. It shows that TP flux into lake is 15.385 kg under city storm intensity of 28 mm/h, in which 62.3% of flux goes into water in lake and 28.1% TP flux settles surface sediment. It would take 3.0 days for the integrated treatment project operation to recover TP to the level before the rain.

[Pollutant loading analyses and TP model calculation for eutrophication in Lichee Lake in Shenzhen].

Based on 9-month consecutive in situ monitoring data, this paper investigated the pollutant sources and loadings of eutrophication in Lichee Lake in Shenzhen. The external source mainly comes from overflow of storm sewer system, which will deteriorate water quality in lake. Total phosphorus concentration was measured with a maximum of 0.347 mg/L after overflow. The sediment release experiment showed that the release rate of total nitrogen during the first week was about 0.036 8 g/(m2 x d), and less release of total phosphorus from sediment into water was measured under aerobic condition. The total phosphorus modeling of each sub-lake for Lichee Lake was developed. The model had a good agreement with 2 groups of monitoring data. And calculation results showed that, it will take 2.18 days subject to 24-hour operations of the integrated treatment project per day to improve water quality in Lake to satisfy the National Standard IV of surface water.

[Treatment effect of eutrophication restoration project for Lichee Lake in Shenzhen].

Based on 9-month monitoring field data, this paper analyzes the variation of chlorophyll-a (Chl-a), total phosphorus (TP), total nitrogen (TN) and transparency (SD) in different sub-lake districts in order to study the treatment effect of eutrophication restoration project in Lichee Lake in Shenzhen. The statistical results show that, the average of TP and TN for whole lake were below 0.1 mg x L(-1) and 1.5 mg x L(-1) respectively, the average of Chl-a of north lake district and east lake district were 16.77 microg x L(-1) and 21.45 microg x L(-1) respectively, lower than that of south lake district (35.83 microg x L(-1) and west lake district (32.69 microg x L(-1)), and the average of SD for whole lake was greater than 0.5 m. During the 9-month operation of the restoration project, the average water quality satisfied the National Standard IV for surface water and the water was improved from hypereutrophic status to eutrophic status in Lake.

[Three-dimensional fluorescence fingerprint for source determination of dissolved organic matters in polluted river].

Fluorescence fingerprint technique was used to characterize dissolved organic matter (DOM) in river from different sources. The results showed that two types of DOM fluorescence signals were observed in river water: a humic-like fluorescence with three maxima at lambda(ex)/lambda(em) = 250/460 nm(A1), 220/400 nm(A2) and 325/420 nm(C); and a protein-like fluorescence with two maxima at lambda(ex)/lambda(em) = 230/360 nm(T2) and 285/357 nm(T1). The intensity of both of protein-like fluorescence distinctly increased because of the domestic wastewater drainage in the tributary stream. After the tributary converges into the main stream, the Fe3+ concentration in the main stream is 30 times as much as that of tributary stream, and the visible blue-shift of humic-like fluorescence occurred while the others didn't occur. The intensity of all types of fluorescence decreased from source to estuary resulting from different solute chemistry. However, the intensities of humic-like fluorescence C, A1 and protein-like fluorescence T2 at the longer excitation wavelength decreased significantly because of dilution from main river stream and complexation of humic-like with Fe3+, thus the peaks disappeared at the estuary while the fluorescence peaks at the shorter excitation wave-length were relatively stable on which the river solute chemistry had little effect. Therefore, the fluorophores at the shorter excitation wavelength of 220-230 nm area potential tool to determine the sources of DOM in polluted river.