Luo Liancong

Estimation of internal nutrient release in large shallow Lake Taihu, China

Based on field investigation of wave, sediment suspension and the changes in nutrient concentration of the water column in Lake Taihu, China, we proposed two release models to quantify nutrient release under static and dynamic conditions, respectively. Under static conditions, nutrient release from sediments to the overlying water mainly depends on chemical diffusion induced by concentration gradient, in which the nutrient release is controlled by the temperature, dissolved oxygen concentration in the sediment-water interface, oxidation-reduction potential and the concentration difference between porewater and overlying water. Under dynamic condition (or disturbed condition), both dissolved and particulate nutrients in sediments are released into the water column because of wind-induced sediment suspension. The amount of nutrient release under dynamic conditions is larger than that under the static condition. The release of dissolved nutrients, however, does not increase because the wind induced turbulence made oxidation of metallic elements such as Fe (ferric iron), Mn which are capable of precipitating soluble reactive phosphate (SRP). Under dynamic conditions, therefore, the release of total phosphorus (TP) increases dramatically but the release of SRP is close to those under static conditions. In sediments of Lake Taihu, high Fe content leads to a high ratio of Fe to P contents in sediments (Fe:P ratio). Under dynamic conditions, therefore, nutrient release is controlled by the intensity of disturbance, sediment consolidation and nutrient content in sediments. As for dissolved nutrients, especially SRP, the release is also controlled by the intensity of dynamic re-oxidation, Fe content in sediments and nutrient concentration gradient between porewater and overlying water. Based on these two release modes, the release flux in Lake Taihu has been estimated. In the static condition (i.e. laboratory experimental condition), total release of NH4+-N for whole lake is ca. 10,000 ton/a, and PO43−-P is ca. 900 ton/a. In the dynamic condition, nutrient release following sediment suspension was estimated according to three different intensities of wind forcing which were defined as “calm” (wind speed is less than 2 m/s), “gentle” (wind speed is greater than 2 m/s and less than 6 m/s) and “gust” (wind speed is greater than 6 m/s). The release rate in the condition of “calm” was estimated in terms of the nutrient release in the laboratory experimental static condition; whereas the release rate in conditions of “gentle” and “gust” was estimated in terms of measurement during sediment resuspension conducted in flume experiments. With the observation of wind velocity and frequency in 2001, each type of wind forcing took the frequency of 12%, 82% and 6% for “calm”, “gentle” and “gust”, respectively. The yearly release of nitrogen was 81,000 ton and phosphorus was 21,000 ton, which is about 2–6 folds of annual external loading, respectively.

Effect of sediment resuspension on underwater light field in shallow lakes in the middle and lower reaches of the Yangtze River: A case study in Longgan Lake and Taihu Lake

Based on three continuous in situ underwater light field measurement under different wind waves conditions in Longgan Lake, Meiliang Bay of Taihu Lake in July 2003 and littoral zone near TLLER in July 2004, respectively, the effects of sediment resuspension caused by wind waves on PAR diffuse attenuation, absorption coefficients and euphotic depths are analyzed. In Longgan Lake, PAR diffuse attenuation coefficients during small, middle and large wind waves were 1.74, 2.02 and 2.45 m−1, respectively, and the corresponding PAR spectral diffuse attenuations ranged from 0.98 to 2.97, 1.34 to 3.95 and 1.80 to 5.40 m−1, respectively. In Meiliang Bay, PAR diffuse attenuation coefficients were 2.63, 3.72, 4.37 m−1 during small, middle and large wind waves. PAR diffuse attenuation coefficients increased by 41% and 66% from small to middle, large wind waves, respectively. Absorption coefficients integrated over the range of PAR of CDOM, phytoplankton were 0.26, 0.28 m−1; 0.76, 0.49 m−1, respectively during middle and large wind waves. Absorption coefficients integrated over the range of PAR of non-algal particulate matter and total suspended particulate matter increased from 0.94 to 1.73 m−1, and from 1.70 to 2.22 m−1, respectively during middle and large wind waves. Relative contributions of absorption coefficients of non-algal particulate matter to total absorption coefficient integrated over the range of PAR were 44.14%, 65.05%, respectively, during middle and large wind waves. PAR euphotic depths decreased by 0.40, 0.19, 0.20 m from middle to large wind waves in Longganhu Lake, Meliang Bay and littoral zone near TLLER. Significant correlations were found between transparency, PAR diffuse attenuation coefficients, euphotic depths and total suspended particulate matter, wind velocity, wave height. Most significant correlations were found between transparency, PAR diffuse attenuation coefficients, euphotic depths and inorganic suspended particulate matter but low correlations for chlorophyll a, dissolved organic carbon. Increase of total suspended particulate matter, especially inorganic suspended particulate matter caused by wind waves was the dominant factor affecting underwater light field in shallow lakes in the middle and lower reaches of the Yangtze River based on observations at three stations.

Geochemical forms of phosphorus in sediments of three large, shallow lakes of China

ABSTRACT A sequential extraction method for the fractionation of phosphorus (P) in lake sediments was used to analyze phosphorus fractions of sediments taken from three large, shallow, eutrophic freshwater lakes of China-Taihu Lake, Chaohu Lake, and Longgan Lake. All three lakes are located in the lower reaches of the Changjiang River (Yangtze River). In Taihu Lake and Chaohu Lake, algae blooms occurred every year, while Longgan Lake was a macrophyte-dominated lake. Results showed that exchangeable phosphorus fractions were much higher in the eutrophic lake sediments than in the macrophyte-flourishing lake sediment. Also, the ratio of Fe:P in the sediments of the algae-predominant lakes was generally much lower than that in the macrophyte-predominant lakes. Thus, the geochemical fractions of phosphorus in sediments had a closer relationship with the type of aquatic vegetation.

Experimental study on phosphorus release from sediments of shallow lake in wave flume

Influence of wave on sediment resuspension and nutrients release from sediments, collected from Lake Taihu and Lake Chaohu, was studied in flume experiments. Under strong-wave conditions, concentrations of suspended solids (SS), total phosphorus (TP) and dissolved total phosphorus (DTP) in overlying water were increased significantly following the sediments re-suspension. During the experiments on sediments of Lake Taihu and Lake Chaohu, TP concentrations increased 6 times and 3 times, and DTP concentration increased 100% and 70% more than it in presuspension, respectively. Concentration of soluble reactive phosphorus (SRP) of experiment on sediment of Lake Taihu increased 25%. During the massive sediment suspension, the dissolved phosphorus in pore water and much of the phosphorus adsorbed by the sediment particles were released into overlying water. The phenomena in this wave flume experiment are quite similar to the situation observed in situ of Lake Taihu. The critical wave stresses of sediment re-suspension are nearly equal. The change of concentrations of SS, TP, and SRP was the same as that in situ situation. This study showed that concentrations of TP and SRP in lake water could be increased significantly by wave disturbance. Phosphorus release was significantly enhanced by wave disturbance at the beginning of massive sediment re-suspension, but decreased later.

Wind-wave affected phosphate loading variations and their relationship to redox condition in Lake Taihu

Variation of wind speed and the physico-chemical parameters, such as dissolved phosphate, ferrous and manganese in lake water were observed on site. Together with the chemistry analysis and simulated experiment in lab, the change of phosphate concentration in lake water was analyzed. The variation of ferrous/phosphate ratio explained that along with the enhancement of wind-wave effect and the oxidation ability of lake water, the effects of co-precipitation and removal of dissolved phosphate and iron in the lake water were reinforced. The ferrous/phosphate ratio in pore water was less than 2.0, demonstrating that the dissolved phosphate can be released into the overlying water. But, in the lake water, the stability of phosphate was controlled by the water dynamics. The phosphate release experiment showed that molecular release was only part of the whole and the direct discharge of phosphate in the pore water was also a part. The mineralization and biological process of suspended particulates in the water may be another important reason for the whole phosphate loadings.

Nutrient fluxes induced by disturbance in Meiliang Bay of Lake Taihu

A wave flume experiment was conducted to study nutrient fluxes at water-sediment interface of Meiliang Bay under different hydrodynamic conditions. The results reveal that hydrodynamics has remarkable effects on nutrient fluxes in this area. With a bottom wave stress of 0.019 N m−2 (equivalent to disturbance caused by wind SE 5–7 m s−1 at the sediment sample site of Meiliang Bay), the fluxes of TN, TDN and NH4+-N were separately 1.92 × 10−3, −1.81 × 10−4 and 5.28 × 10−4 mg m−2 s−1 (positive for upward and negative for downward), but for TP, TDP and SRP, the fluxes were 5.69 × 10−4, 1.68 × 10−4 and −1.29 × 10−4 mg m−2 s−1. In order to calculate the released amount of nutrients based on these results, statistic analysis on the long-term meteorological data was conducted. The result shows that the maximum lasting time for wind SE 5–7 m s−1 in this area is about 15 h in summer. Further calculation shows that 111 t TN, 32 t NH4+-N, 34 t TP and 10 t TDP can be released into water (the sediment area was 47.45% of the whole surface area), resulting in concentration increase of 0.025, 0.007, 0.007 and 0.002 mg L−1 separately. With stronger disturbance (bottom wave stress is 0.217 N m−2 which is equivalent to disturbance caused by wind SE 10–11 m s−1 at the same site), there has been significant increase of nutrient fluxes (1.16 × 10−2, 6.76 × 10−3, 1.14 × 10−2 and 2.14 × 10−3 mg m−2 s−1 for TN, DTN and NH4+-N and TP). The exceptions were TDP with flux having a decrease (measured to be 9.54 × 10−5 mg m−2 s−1) and SRP with flux having a small increase (measured to be 5.42 × 10−5 mg m−2 s−1). The same statistic analysis on meteorological data reveal that the maximum lasting time for wind SE 10–11 m s−1 is no more than 5 h. Based on the nutrient fluxes and the wind lasting-time, similar calculations were also made suggesting that 232 t TN, 134.9 t TDN, 228 t NH4+-N, 42.7 t TP, 2.0 t TDP and 1.1 t SRP will be released from sediment at this hydrodynamic condition resulting in the concentration increases of 0.050, 0.029, 0.049, 0.009, 0.0004 and 0.0002 mg L−1. Therefore in shallow lakes, surface disturbance can lead to significant increase of nutrient concentrations although some components in water column had negative flux with weak disturbance (e.g. TDN and SRP in this experiment). In this case, sediment looks to be a source of nutrients. These nutrients deposited in sediment can be carried or released into water with sediment resuspension or changes of environmental conditions at water-sediment interface, which can have great effects on aquatic ecosystem and is also the characteristics of shallow lakes.

Study on the simulation of transparency of Lake Taihu under different hydrodynamic conditions

It was indicated in this study that there were negative relations between the concentrations of suspended solid (SS) and transparency according to the analysis of measured data of Lake Taihu. Their relations in pervious studies were reviewed, which showed that the changes of transparency in Lake Taihu could be reflected by simulating suspended solid concentration (SSC). Measured data showed that the changes of SSC with wind speed were similar at different water depths. SSC increased with the increasing of wind speed. Both wave and lake current of Lake Taihu had positive relations with SSC. However, wave was the main factor affecting sediment suspension, while flow took the second place. In this study, a numerical model coupling lake current, wave and SSC of Lake Taihu was developed. In the SS model, the combined effects of wave and current were included. The amounts of suspended and deposited sediments near the lake bed surface layer were treated separately. The stochastic characteristics of turbulent flow pulsation near lake beds were also considered, and the start-up conditions of sediment suspension were introduced to the model. The model elucidated the mutual exchange processes between sediment particles in SS and active sediments within and on the bed surface layer. Simulated results showed that lake current had relatively significant effects on the SSC at littoral areas of Lake Taihu, while SSC at the central area of the lake was mainly influenced by wave. The changes of transparency with SSC were simulated for Lake Taihu using this model. Calculated results were validated by measured data with good fitness, which indicated that the model is basically suitable for the simulation and prediction of transparency of Lake Taihu.

Sediment Re-suspension under Different Hydrodynamic Disturbances in Lake Taihu

Based on the collected wave and current data in July,2002 and April, 2003, Lake Taihu, the wave stress (Tw) and the current stress (Tc) were calculated for better understanding of sediment re suspension mechanism. The results show that Tc is much larger than Tw with the water surface slightly disturbed but no re suspension can be found. Under stronger disturbance, Tw is so much larger than Tc that the current effects can be ignored in the bottom stress calculation. The wave effect is more important than the current effect although Tc is slightly smaller than Tw when the water surface is forced by a moderate wind. In this case, the non linear wave current interaction was favored to be considered for the bottom stress calculation in Lake Taihu. The conclusion can benefit the coming research on nutrients release from sediment and suspended sediment concentration (SSC) response to the surface disturbance.

Current Circulation Pattern in Winter Meiliang Bay,Lake Taihu

Hydrodynamics in shallow lake has been paid more and more attention to since it has profound effect on sediment resuspension and nutrients transportation in recent years.An investigation to current circulation pat-tern was made in Meiliang Bay,Lake Taihu on19January2003and a conclusion has been drawn that the water at the mouth of Meiling Bay moves to Lake Taihu stably with a peak speed8cm/s,and the water lo-cated along the west shore moves toward the north and it is in opposite style on the east side.The current along the east shore branches off to the west to compensate for mass loss and to the south to flow into Lake Taihu.An convergence center was found in the northeast area of Meiling Bay with high nutrients concentra-tion.The water ascends in most areas and flows into Lake Taihu totally with supply fromthe rivers located a-long shore and Lake Wuli.

Study on Optical Properties of Lake Longgan

In this study, underwater light field and optical properties in Lake Longgan were studied by applying in situ measurement data from 2002 to 2003. Incident downwelling photosynthetically available irradiance (PAR, 400 -700 nm) at the water surface and at different depths were measured using a 192SA light sensor connected to a Li -1400 datalogger. Underwater spectral irradiance was measured with a high-resolution spectroradiometer ( SR -9910; Macam Photometries, Livingston, Scotland) equipped via a 4 - m light guide with a cosine-corrected light collector. The spatial distribution of light attenuation, the spectral distribution of attenuation coefficients, and the effect of different wind and wave on underwater light field were assessed. The ultraviolet light was most strongly attenuated and its attenuation decreased with the increase of wavelength. The downward attenuation coefficients were larger than upward attenuation coefficients. The ranges of spectral attenuation coefficient from 400 to 700 nm were 0. 71 - 3. 60, 1.06 -3.72 and 0.78 -2.89 m-1 at sampling station L1, L2 and L3, respectively. There were insignificant spatial differences in the optical properties, only attenuation coefficients being larger at sampling station L2 than those at sampling station station L1, L3. No significant differences of spectral irradiance ratio at sampling station L2, L3, being minimal values during blue wavelength and maximal values during550 -600 nm. In open, wind-exposed lake region with silty sediment, the increase in total suspended solids resulted from the wind and wave would increase the attenuation of light. The attenuation coefficients of PAR increased from 1. 74 to 2. 02, 2. 45 m-1 under three different wind and wave processes. The most significantly positive correlation was found between transparency, attenuation coefficient and total suspended solids. Light attenuation was predominantly correlated to the concentration of total suspended solids and dissolved organic carbon, chlorophyll a. Multiple linear regression equation at 440 nm was listed: Kd(440) =0.514-0.075SS+0. 125DOC+0. 100Chl. a