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

Abstract

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 NH 4 + -N for whole lake is ca. 10,000 ton/a, and PO 4 3--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 phos-

The Fluctuation of Arsenic Levels in Lake Taihu

“…Usually these lakes had an average depth greater than 7 m, which is much greater than that in Taihu [34]. In this study, however, stratification in Taihu was not seen ( Table 2), which is in agreement with the previous reports in Taihu, mainly due to its much less depth (1-3 m) [26,27].…”

“…Phosphorus in the water of Taihu has been reported to be very heterogeneous, which may result in wrong judgment on the entrophication status due to improper utilization of eutrophication formula using phosphorus levels [25]. The heterogeneous phosphorus levels in Taihu was regarded as been the effects of strong turbulence due to the winding wave and the very shallow depth in Taihu, causing the release of non-trapped phosphorus from sediments to upper water column [23,26]. Arsenic is a chemical analogue of phosphorus; it is usually present in the product of insecticides, acts as supplements in industrial products [1].…”

Spatiotemporal Distribution of Eutrophication in Lake Tai as Affected by Wind

“…Sediment resuspension induced by wind-induced waves commences once bottom stresses exceed the critical shear strength [43,44]. In this study, once the daily mean wind speed exceeded 2.1 m·s −1 , both the nitrogen and phosphorus concentrations increased probably because of the nutrient release effect of sediment.…”

Changes in phytoplankton communities along nutrient gradients in Lake Taihu: evidence for nutrient reduction strategies

“…Internal phosphorus release was the main phosphorus source in Lake Taihu after P reduction strategies were applied in the 1990s, contributing about six times the external loading (Qin et al, 2006). The dominant northeast wind in winter and southeast wind in summer led to the difference in internal phosphorus release in the two sampling areas.…”

“…However TDP, especially SRP (soluble reactive phosphorus), would decrease owing to re-oxygenation in the water column with the continued disturbance. Although TDP and SRP had no signifi cant change with suspended particulates sinking during one entire disturbance process, the continual wind-induced disturbance in Lake Taihu could still release a large quantity of TDP and SRP from sediment supported phytoplankton growth (Zhu et al, 2005;Qin et al, 2006). To succeed in eutrophication management and algal bloom control, internal P release should be given much more attention.…”