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.

[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.

[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.