Temporal-spatial features and key factors' analysis of vertical eddy diffusivities in Taihu Lake, China.

Vertical eddy diffusivity (VED) is used to quantify the vertical mixing of water column, which has a profound influence on the evolution of aquatic ecosystems. Based on half-hourly water temperature measured at -20 cm and -150 cm depths from 2015 to 2017 at stations of Pingtaishan (PTS), Dapukou (DPK), Bifenggang (BFG), and Xiaoleishan (XLS) in Taihu Lake, the daily average VED is calculated according to the phase lag of water temperature series at two depths. The temporal and spatial features and possible evolution characters of vertical turbulences are then deliberated. The results show that the VED in Taihu Lake varies by several orders of magnitude. The weak VED exhibits stronger spatial heterogeneity and high frequency characteristics and vice versa for the strong VED. The VED in the center region of the lake is stronger, in comparison to bay areas. On seasonality, the VED is the strongest in winter, moderate in spring and autumn, and the weakest in summer. Analyses show that solar radiation and wind forcing are the key meteorological factors regulating VED changes, with the solar effect somewhat stronger than wind. It is also discussed potential roles of vertical mixing in cyanobacteria becoming dominant population in Taihu Lake.

Seasonal dependency of controlling factors on the phytoplankton production in Taihu Lake, China.

In this study, 44 profiles of gross primary productivity (GPP) and sunlight, along with water temperature, Chlorophyll-a (Chla) and nutrients, were observed in Meiliang Bay of Taihu Lake, China, in the spring, summer, and fall seasons. Effects of water temperature, light, and nutrient concentration were examined in relation to the GPP-unit-Chla (GPP of algae per Chla). The results showed that the optimum temperature for the GPP of phytoplankton was 27.9°C, the optimal PNA-unit-Chla (photon number absorbed by phytoplankton per Chla) was 0.25 (mol), and the HSCN-unit-Chla and HSCP-unit-Chla (half-saturation constants of nitrogen and phosphorus of algae per Chla) were 0.005 (mg/L) and 0.0004 (mg/L), respectively. The seasonal dependency of the effect of different factors on the GPP was analyzed. Compared with temperature and nutrients, light was found to be the most important factor affecting the GPP during the three seasons. The effect of temperature and nutrients on the GPP of phytoplankton has obvious seasonal change. In spring, temperature was the secondary factor affecting the GPP of phytoplankton, and the effect of nutrients may be negligible in the eutrophic lake on account of temperature limit, which showed that the GPP of algae was only affected by the physical process. In summer and fall, temperature didn't affect the GPP of algae, and the presence of nutrients was the secondary factor affecting the GPP of phytoplankton. From summer to fall, effect of phosphorus was weakened and effect of nitrogen was enhanced.

Temporal and spatial characteristics of potential energy anomaly in Lake Taihu.

In this study, water temperature and meteorological data in Lake Taihu from June 11 to July 6, 2013, are collected to calibrate and verify the unstructured grid finite-volume community ocean model (FVCOM) coupled with a heat exchange module. The spatial and temporal variations of potential energy anomalies (PEA) in the lake, simulated by the calibrated FVCOM, are analyzed to explore the stratification and de-stratification processes in water body. The temporal variation of PEA primarily follows the diurnal cycles of solar radiation, while the spatial heterogeneity of PEA is jointly determined by solar radiation and vertical shear of horizontal velocity coupled with the topography of the lake. The maxima of PEA in the lake are not greater than 4 J/m3 in the summer and even smaller along shore regions and near the Pingtaishan station. This study is helpful to improve understanding of the effect of physical processes on the algae bloom in Lake Taihu.

Research on nutrient pollution load in Lake Taihu, China.

The eutrophication of Lake Taihu is the joint result of internal and external pollution. Research on the sources and flux loads of total nitrogen (TN) and total phosphorus (TP) has great significance for the control and management of the eutrophication issues facing Lake Taihu. We used hydrologic and water quality surveys of the surrounding rivers, precipitation investigations, and an experimental simulation of suspended sediment to analyze the sources of TN and TP to Lake Taihu and to estimate the flux loads of TN and TP. Our results indicate that (1) surrounding rivers, atmospheric deposition, and sediment suspension were the three main sources of TN and TP, with annual flux loads of 29.6 thousand t and 2.8 thousand t, respectively; (2) for the flux load of TN, influxes from surrounding rivers accounted for 61.1%, while atmospheric deposition contributed 23.5%, with both of these sources varying seasonally. Total nitrogen released from sediments contributed 15.4% of the TN load of Lake Taihu; (3) for TP flux load, surrounding rivers accounted for 42.8% and also showed seasonal variation. Atmospheric deposition and sediment release accounted for 17.9 and 39.3% with no significant seasonal variation. This article quantitatively analyzed the sources and loads of TN and TP and provided a basis for pollution control in Lake Taihu.

[Retrieving for chlorophyll-a concentration and suspended substance concentration based on HJ-1A HIS image].

Based on geometry correction using ERDAS software and radiation correction using 6S model for HJ-1A hyper-spectrum image (HSI) on May 2 in 2010 and the analysis of spectrum for water data and spectral data of hyper-spectrum image, this paper processes original spectrum data of 28 sample points using method of normalization and method of first-order derivation. Single-band and band combination are selected to establish inversion models of the concentration of chlorophyll-a and solid suspensions. Choosing the model with biggest correlation coefficient, the spatial distribution map of the concentration of chlorophyll-a and solid suspensions content in Taihu Lake is acquired. The research results show: Band-73 of hyper-spectrum image which has been normalized shows the biggest correlation coefficient of the concentration of chlorophyll-a, remote sensing sediment parameter shows the biggest correlation coefficient of the concentration of solid suspensions, the result is consistent with analysis of spectral data of hyper-spectrum image. Average relative errors of predicted and measured values are within 30 percent. Spatial distribution map of water quality is consistent with the result of field surveys. Therefore, based on reference of the analysis of sensitive band of spectrum for water data, HJ-1A hyper-spectrum image can give quantitative estimation of water quality parameters in Taihu Lake.

[Effect of inherent optical parameters on average penetration depth of photon flux and the integral average cosine of underwater light field in lake Taihu during summer].

Based on the inherent optical parameters of the water and water quality data in lake Taihu from 2006-07-29 to 2006-08-01, the effect of scattering on the penetration path along the original direction of the flux and the Integral average cosine of underwater light field were study by the radiative transfer theory, and the possible mechanism was analyzed. There were increasing trend from northwest to southeast of them. There were a nonlinear relation between them and concentration of Chl-a, suspended matter, inorganism matter, organism matter. The relation was described by logarithmic function. The study was helpful for bio-optical model and the environmental effects of photosynthetic active radiation in waters.

[Analysis on the spatial characteristics of the light quantum yields in Lake Taihu in spring].

Based on the primary productions and underwater irradiance in the head of the jetty in a station of the Lake Taihu ecology network of the Chinese academia, Zhihugang estuary area, the center of Lake Taihu and Xukou Bay at April 28, May 4, May 5, May 6 in 2009, the light quantum yields of algae and P-I curve were calculated. Then the spatial characteristics were analyzed. There was significant photoinhibition in the head of the jetty in a station of the Lake Taihu ecology network of the Chinese academia; there was not appreciable photoinhibition in Zhihugang estuary area and the center of Xukou Bay; and there was only light-saturation phenomena in the center of Lake Taihu. The order of the light quantum yield per Chl-a was the center of Lake Taihu, the head of the jetty, Zhihugang estuary area and the center of Xukou Bay.

[Variation of spectrum of photosynthesis available radiation absorbed by underwater optical medium with depth in Lake Taihu in summer].

Normalized spectrum of photosynthesis available radiation (PAR), which was absorbed by all kinds of medium per unit volume of water at different depths, was calculated by the data from downward and upward irradiance and absorption coefficients of phytoplankton, non-algae particles and chromophoric dissolved organic matter (CDOM) in 41 locations in Lake Taihu from July 29 to August 1, 2006. The results showed that there were two peaks at about 450 nm and 675 nm in the normalized spectrum of PAR absorbed by phytoplankton per unit volume of water, respectively. With the increasing of the depth, the peak at about 450 nm gradually weakened, and red-shift appeared from 450 nm. The above phenomena were much more obvious in the algae-type zone and the center of Lake Taihu. The PAR absorbed by phytoplankton per unit volume of water was shift to 600-700 nm which mainly originated from 400-500 nm to 600-700 nm. The transitional speed was slower in the grass-type zone but it was faster in Meiliang Bay(algae-type zone)and the center of Lake Taihu. In the above water, the PAR absorbed by non-algae particles per unit volume of water mainly originated from 400-500 nm, but energy source gradually shifted to 500-600 nm with the increasing of depth. The variable characteristics of normalized spectrum of PAR absorbed by CDOM per unit volume of water were similar to that of non-algae particles. However, the advantage in grass-type zone which CDOM per unit volume of water absorbed PAR between 500 and 600 nm was less significant than that of non-algae particles per unit volume of water.