Liancong Luo

Environmental issues of Lake Taihu, China

Lake Taihu is characterized by its shallowness (mean depth = 1.9 m) and large surface area (2,338 km2). Runoff sources are mostly from the mountainous west and southwest, and outflows are located throughout East Taihu. This causes shorter retention times in the south. In contrast, urban pollutants discharge into northern Taihu and result in poor water quality. Non-point pollution from rural areas and sewage wastewater is the primary pollution source. Water current velocity ranges from 10–30 cm s−1, and surface currents normally follow wind direction. Bottom currents appear to be a compensation flow. Most wave heights are less than 40 cm, and underwater irradiance correlates to seston in the water column. Lacustrine sediment is distributed in littoral zones, mostly along the western shoreline, with almost no accumulation in the lake center. Intensive aquaculture in East Taihu caused eutrophication and hampered water supply in surrounding areas. In addition, development of marshiness in the eastern littoral zones and East Taihu has occurred. The function of flood discharging of East Taihu has been limited by flourishing macrophytes. The problems facing in Lake Taihu will be alleviated by improving the management of nutrient sources into the lake.

Dynamics of sediment resuspension and the conceptual schema of nutrient release in the large shallow Lake Taihu, China

On the basis of investigationsin situ, it was found that mass exchange on the water-sediment interface occurred chiefly on the superficial sediment within 5–10 cm. The spatial physicochemical character of sediment was distributed uniformly. The observation of lake currents and waves indicated that the dynamic sources, which act on the interface of water and sediment, came mainly from waves under strong wind forcing, while the critical shear stresses due to the waves and currents were of the same magnitude under weak wind forcing. The critical shear stress that leads to extensive sediment resuspension was about 0.03 – 0.04N/m2, equivalent to a wind speedin situ up to 4 m/s. If a dynamic intensity exceeded the critical shear stress, such as a wind velocity up to 6.5 m/s, massive sediment re-suspension would be observed in the lake. Furthermore, field investigations revealed that the nutrient concentration of pore water within the sediment was far greater than that of overlaying water, which provides objective conditions for the nutrient release from sediment. According to nutrient analyses in the pore water from the superficial 5–10 cm sediments, a severe dynamic process in the Taihu Lake would bring out a peak nutrient release, i.e. a 0.12 mg/L increase of TN, and 0.005 mg/L increase of TP in the lake. In the end, a general scheme of nutrient release from sediment in large shallow lakes was put forward: when the wind-driven forcing imposes on the lake, it will make the sediment resuspension. At the same time, the nutrition from the pore water will follow the sediment resuspension release to overlaying water. Because of oxidation of solid particulates when it resuspends from sediment, the disturbance of hydrodynamics will enhance the suspension particulates absorbing nutrition. After the withdrawal of wind forcing, the suspended mass would deposit and bring part of the released nutrients back into sediment. The degraded organic particulate would be separated to the pore water within the sediment under the condition of deposition, and wait for the next wind forcing.

Total inputs of phosphorus and nitrogen by wet deposition into Lake Taihu, China

Lake Taihu suffers from eutrophication caused by riverine nutrient inputs and air deposition. To characterize wet deposition of phosphorus (P) and nitrogen (N) to the lake, precipitation collection and measurements of total phosphorus (TP) and total nitrogen (TN) and other components at five cities around Lake Taihu were made from July 2002 to June 2003. TP and TN concentrations and deposition rates exhibited strong spatial variation in the whole catchment. An inverse correlation between station-averaged TP and TN concentrations and precipitation amount was found. Maximal TP concentration in rainfall was found in Suzhou, and maximal TN in Wuxi. However, highest wet deposition rates of TP and TN were found in Suzhou, which suggests that atmospheric nutrients are mostly from the east and northwest area of Lake Taihu. Mean TP and TN deposition rates were 0.03 and 2.0 t km−2 year−1 respectively in Lake Taihu, which are greater than reported values in other areas by comparision. Total N and P contributed to the lake by wet deposition were 75 and 4720 t per year, respectively, which represent about 7.3% and 16.5% of total annual N and P inputs via inflow rivers. Wet deposition, especially N, could have significant effects on eutrophication in the lake, which shows that air deposition should be taken into account while reducing the external nutrients in the lake.

Effects of hydrodynamics on phosphorus concentrations in water of Lake Taihu, a large, shallow, eutrophic lake of China

To understand the effect of hydrodynamical process on water phosphorus concentration, wind, wave, and several water quality indices were observed in Meiliang Bay, a shallow and eutrophic bay locates in north of Lake Taihu. During the 7 day observation period, wind speed and significant wave height were recorded more than 3 h per day, and water samples were collected in five water-depth layers once a day. Hydrodynamical disturbance had no significant correlationship with the water quality at the top layer when the significant wave height was smaller than 30 cm, but it significantly increased suspended solids (SS) concentration of the bottom water layer. Concentrations of nutrients showed no positive correlationship with SS concentration in the water body. Intensive sediment resuspension may not have occurred when the hydrodynamic stress on sediment was only a little higher than the critical stress for sediment resuspension. A new method for confirming the critical stress for intensive sediment resuspension and nutrient release still needs to be developed. The range of the water quality indices was quite high during the seven days of observation. High variation seems to be a common character of large shallow lakes like Taihu.

Modeling the response of phytoplankton to reduced external nutrient load in a subtropical Chinese reservoir using DYRESM-CAEDYM

Abstract Cui Y, Zhu G, Li H, Luo L, Cheng X, Jin Y, and Trolle D. Modeling the response of phytoplankton to reduced external nutrient load in a subtropical Chinese reservoir using DYRESM-CAEDYM. Lake Reserv Manage. 32: 146--157. The DYRESM-CAEDYM model was used to quantify the potential effects of reductions in external nutrient loads to the Shahe Reservoir in China, with an emphasis on effects on phytoplankton biomass. DYRESM-CAEDYM was calibrated using field data for a 2-year period and validated against a separate 1-year period. Simulated values for water temperature, dissolved oxygen, total nitrogen, total phosphorus, and chlorophyll a showed good agreement with field measurements. The model tended to underestimate peak values of inorganic nutrients such as ammonium, nitrate, and phosphate, with phosphate levels close to detection limits in both observed and simulated values. The model successfully captured seasonal phytoplankton succession, indicating spring peaks of diatom biomass and summer peaks of cyanophytes. Scenarios in which different reductions in external loading were applied suggest that simultaneous reductions in both external nitrogen and external phosphorus loading is more effective in controlling phytoplankton biomass than reducing input of either nutrient alone. Diatoms responded more strongly to load reductions in phosphorus input than nitrogen load, whereas cyanophytes showed the opposite pattern. A combined 25% reduction in the external loading of both nutrients could significantly decrease phytoplankton biomass in the reservoir below the 7 μg/L chlorophyll a required by drinking water standards, and the study indicated that DYRESM-CAEDYM can be a useful management tool for eutrophication control.

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.

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.

Study on turbulence characteristics and sensitivity of quadrant analysis to threshold level in Lake Taihu

The identification of coherent structures is very important in investigating the sediment transport mechanism and controlling the eutrophication in shallow lakes. This study analyzed the turbulence characteristics and the sensitivity of quadrant analysis to threshold level. Simultaneous in situ measurements of velocities and suspended sediment concentration (SSC) were conducted in Lake Taihu with acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) instruments. The results show that the increase in hole size makes the difference between dominant and non-dominant events more distinct. Wind velocity determines the frequency of occurrence of sweep and ejection events, which provide dominant contributions to the Reynolds stress. The increase of wind velocity enlarges the magnitude of coherent events but has little impact on the events frequency with the same hole size. The events occurring within short periods provide large contributions to the momentum flux. Transportation and diffusion of sediment are in control of the intermittent coherent events to a large extent.