Tingfeng Wu

Cyanobacterial bloom management through integrated monitoring and forecasting in large shallow eutrophic Lake Taihu (China)

The large shallow eutrophic Lake Taihu in China has long suffered from eutrophication and toxic cyanobacterial blooms. Despite considerable efforts to divert effluents from the watershed, the cyanobacterial blooms still reoccur and persist throughout summer. To mitigate cyanobacterial bloom pollution risk, a large scale integrated monitoring and forecasting system was developed, and a series of emergency response measures were instigated based on early warning. This system has been in place for 2009-2012. With this integrated monitoring system, it was found that the detectable maximum and average cyanobacterial bloom area were similar to that before drinking water crisis, indicating that poor eutrophic status and cyanobacterial bloom had persisted without significant alleviation. It also revealed that cyanobacterial bloom would occur after the intense storm, which may be associated with the increase in buoyance of cyanobacterial colonies. Although the cyanobacterial blooms had persisted during the monitoring period, there had been a reduction in frequency and intensity of the cyanobacterial bloom induced black water agglomerates (a phenomenon of algal bloom death decay to release a large amount black dissolved organic matter), and there have been no further drinking water crises. This monitoring and response strategy can reduce the cyanobacterial bloom pollution risk, but cannot reduce eutrophication and cyanobacterial blooms, problems which will take decades to resolve.

The influence of changes in wind patterns on the areal extension of surface cyanobacterial blooms in a large shallow lake in China.

It has been hypothesized that climate change will induce the areal extension of cyanobacterial blooms. However, this hypothesis lacks field-based observation. In the present study both long-term historical data and short-term field measurement were used to identify the importance of changes in wind patterns on the cyanobacterial bloom in Lake Taihu (China), a large, shallow, eutrophic lake located in a subtropical zone. The cyanobacterial bloom mainly composed of Microcystis spp. recurred frequently throughout the year. The regression analysis of multi-year satellite image data extracted by the Floating Algae Index revealed that both the annual mean monthly maximum cyanobacterial bloom area (MMCBA) increased year by year from 2000 to 2011, while the contemporaneous cyanobacterial biomass showed no significant change. However, the correlation analysis shows that MMCBA was negatively correlated with wind speed. Our short-term field measurements indicated that the influence of wind on surface cyanobacterial blooms is that the Chlorophyll-a (Chla) concentration is fully mixing throughout the water column when the wind speed exceed 7 m s(-1). At lower wind speeds, there was vertical stratification of Chla with high surface concentrations and an increase in bloom area. The regression analysis of wind speed indicates that the climate has changed over the last decade. Lake Taihu has become increasingly calm, with the decrease of strong wind frequency between 2000 and 2011, corresponding to the increase in the MMCBA over time. Therefore, we conclude that changes in wind patterns related to climate change have favored the increase of cyanobacterial blooms in Lake Taihu.

The Influence of Macrophytes on Sediment Resuspension and the Effect of Associated Nutrients in a Shallow and Large Lake (Lake Taihu, China)

A yearlong campaign to examine sediment resuspension was conducted in large, shallow and eutrophic Lake Taihu, China, to investigate the influence of vegetation on sediment resuspension and its nutrient effects. The study was conducted at 6 sites located in both phytoplankton-dominated zone and macrophyte-dominated zone of the lake, lasting for a total of 13 months, with collections made at two-week intervals. Sediment resuspension in Taihu, with a two-week high average rate of 1771 g·m-2·d-1 and a yearly average rate of 377 g·m-2·d-1, is much stronger than in many other lakes worldwide, as Taihu is quite shallow and contains a long fetch. The occurrence of macrophytes, however, provided quite strong abatement of sediment resuspension, which may reduce the sediment resuspension rate up to 29-fold. The contribution of nitrogen and phosphorus to the water column from sediment resuspension was estimated as 0.34 mg·L-1 and 0.051 mg·L-1 in the phytoplankton-dominated zone. Sediment resuspension also largely reduced transparency and then stimulated phytoplankton growth. Therefore, sediment resuspension may be one of the most important factors delaying the recovery of eutrophic Lake Taihu, and the influence of sediment resuspension on water quality must also be taken into account by the lake managers when they determine the restoration target.

Validating and Mapping Surface Water Temperatures in Lake Taihu: Results From MODIS Land Surface Temperature Products

Data from four in situ high-frequency monitoring sites in Lake Taihu, China were used to 1) assess the accuracy of MODIS land surface temperature (LST) products; 2) characterize the spatial and temporal variability in lake surface water temperature (LSWT) with Aqua MODIS thermal-infrared imagery; and 3) explore the causes of these variations. The validation showed that MODIS-derived LSWTs and in situ water temperatures were significantly correlated, with a coefficient of determination higher than 0.96 and a root mean square error between 1.2°C and 1.8°C. These results indicate MODIS LST products can be used to assess the spatial and temporal thermal behavior of Lake Taihu. A spatial analysis of daytime LSWT showed different thermal zones along the lake due to the differential heat storage capacity between the deep and shallow regions of the lake, such as the littoral and East Lake Taihu. In contrast, the nighttime LSWT showed little variation, and there was a uniform surface water temperature. Following a change in solar radiation, the annual cycle of the average LSWT (with a nearly 24°C range) started to increase in January and peaked in July. In Lake Taihu, the mean nighttime LSWT from July to September rapidly warmed over the period of 2002-2013 at an average rate of 0.059 ± 0.053°C/year. Our study promotes the use of MODIS LST products in limnology through a validation using a large amount of real-time synchronous water temperature data based on high-frequency observations, which were seldom used in previous studies.

Flume simulation of wave-induced release of internal dissolved nitrogen in Taihu Lake, China

A wave flume simulator was used to study internal nitrogen release from the surface sediment collected from Taihu Lake, China. Particulate nitrogen concentrations were positively correlated with the concentrations of suspended solids, primarily from surface erosion related to the shear stress and duration of wave action. In response to 4 cm- and 10 cm-high wave production representing waves generated in Taihu Lake by gentle and gusty winds, respectively, the mean dynamic release rate of ammonium (NH4+) from the sediment to the overlying water was 1×10−3 mg/(m2·s) and the NH4+ concentration in the overlying water increased by 0.016 mg/L, indicating that waves resulting from strong wind can induce the rapid release of dissolved nitrogen from Taihu Lake sediments. The decrease in interstitial NH4+ concentrations at all sediment depths was associated with an increase in NH4+ concentrations in the overlying water by 0.01 mg/L, showing that sediment below the eroded layer was the main source of internal nitrogen release. Changes in the interstitial dissolved oxygen and NH4+ concentrations showed that wave-induced pore water movement can greatly increase the diffusion rate, and that these effects can influence the sediment to a depth of at least 15 cm. Diffusion induced by pore water movement may be very important for the formation of an active sediment layer in Taihu Lake.

The effect of intense hydrodynamic disturbance on chromophoric dissolved organic matter in a shallow eutrophic lake

Chromophoric dissolved organic matter (CDOM) is an important component of aquatic ecosystems and its movement is a complicated process. To investigate the effect of strong wind-induced sediment suspension on CDOM concentration, we carried out an in situ observation in a large, shallow lake, which is seriously affected by eutrophication and cyanobacterial blooms, using a finely stratified sampling device and several automatic monitoring instruments. During the observation, a typhoon-induced strong wind with a mean wind speed of 6.6 m/s swept the Lake Taihu basin. The strong wind-induced hydrodynamic disturbance caused explosive sediment suspension. During the strong winds, the maximum turbidity in the overlying waters reached 175.4 NTU, which was 12.2 times the background value recorded before the strong winds. Sediment suspension resulted in the release of CDOM from within the sediment. The CDOM absorption coefficient a(355) in the overlying water showed that there was no significant difference between any two water layers for all time points during the observation (13 July to 16 July). The CDOM concentration was fully mixed along the water depth. However, the mean a(355) values recorded in all time points during the wind were higher than that before the wind (p < 0.001). The maximum value during the wind event was 2.524 ± 0.097 m−1, which was 1.4 times that of the value before the disturbance. CDOM concentrations were significantly correlated with hourly mean wind speed (p < 0.05) and turbidity (p < 0.01). Because Lake Taihu is frequently affected by strong wind processes, CDOM concentration changes caused by intense hydrodynamic disturbance are important for quantitative remote sensing of water quality and primary productivity.

Study on the Triggering Factors of Algal Bloom in Fuchunjiang Reservoir Based on a Vertically Integrated Hydrodynamic Model

A vertically integrated hydrodynamic model (2-D) was firstly used to study the mechanism of the bloom in Fuchunjiang Reservoir. It proved that: (1) the morphological characteristic could indirectly influence on the aquatic ecosystem by directly deciding the spatial distribution of the flow regime of Fuchunjiang Reservoir. The section named Sandu Bridge where has wide lateral area is the accumulating region of algal biomass because of the low flow rate and intense internal nutrient flux to the euphotic zone. Moreover, influenced by the outflow position of the reservoir, a hydrodynamic convergence zone was formed and it would increase the chance of nutrient transfer from sediment to the euphotic zone. (2) The basin hydrological process indirectly influenced on the aquatic ecosystem by directly deciding the temporal distribution of the flow regime of Fuchunjiang Reservoir. The losing of algal biomass resulted from flushing was small so that algal biomass could remain at a high level in the year when the reservoir experienced a dry flood period (such as 2004 and 2007). Then, algal bloom would occur after flood period. But, the total algal biomass is low and bacillariophyta or cryptophyta would become the dominance population in the year when the reservoir experienced a wet flood period so that Fuchunjiang Reservoir could not form bloom easily.

Spatial distribution of sediment nitrogen and phosphorus in Lake Taihu from a hydrodynamics-induced transport perspective

Hydrodynamics play an important role in sediment nutrient dynamics in large shallow eutrophic lakes. In this study, the spatial patterns of sediment nitrogen and phosphorus in Lake Taihu were compared from a hydrodynamics-induced transport perspective based on high-resolution investigation of sediment, field observations, numerical simulations and long-term ecological data analysis. The results showed that sediments were primarily distributed in the west and southeast portions of the lake. Additionally, the total nitrogen (TN) and phosphorus (TP) stored in the active sediments was 166,329 t and 67,112.4 t, respectively. The sediment TN content was 319.4-3123.8 mg kg-1, with high content areas being primarily located in the Zhushan, Meiliang and East Taihu bays. The external nitrogen-containing nutrients in the overlying water, which is mostly dissolved nitrogen, can be horizontally transported by lake currents to the water areas with high biomass levels and weak vertical hydrodynamic disturbance where sediment nitrogen enrichment primarily occurs via bio-deposition. The sediment TP content ranged between 382.6 and 1314.1 mg kg-1, and the high content areas were primarily distributed near the inflowing river mouths. Sediment phosphorus enrichment primarily occurred via physical and chemical deposition. Surface waves caused vertical phosphorus transport from sediments to the overlying water but had a limited effect on its spatial distribution. Although the horizontal transport of phosphorus was found to be weaker than that of nitrogen, short-distance vertical transport of sediment phosphorus may relieve nutrient limitations, leading to maintenance of cyanobacterial blooms found in Lake Taihu.

Spatiotemporal Changes of Cyanobacterial Bloom in Large Shallow Eutrophic Lake Taihu, China

Lake Taihu is a large shallow eutrophic lake with frequent recurrence of cyanobacterial bloom which has high variable distribution in space and time. Based on the field observations and remote sensing monitoring of cyanobacterial bloom occurrence, in conjunction with laboratory controlled experiments of mixing effects on large colony formation and colonies upward moving velocity measurements, it is found that the small or moderate wind-induced disturbance would increase the colonies size and enable it more easily to overcome the mixing and float to water surface rapidly during post-disturbance. The proposed mechanism of wind induced mixing on cyanobacterial colony enlargement is associated with the presence of the extracellular polysaccharide (EPS) which increased the size and buoyancy of cyanobacteria colonies and promote the colonies aggregate at the water surface to form bloom. Both the vertical movement and horizontal migration of cyanobacterial colonies were controlled by the wind induced hydrodynamics. Because of the high variation of wind and current coupling with the large cyanobacterial colony formation make the bloom occurrence as highly mutable in space and time. This physical factor determining cyanobacterial bloom formation in the large shallow lake differ from the previously documented light-mediated bloom formation dynamics.