| Lake Tai is the third largest freshwater lake in China with an area of over 36,500 km-. The lake is a source of drinking water for 5 million people in Wuxi, and contributes to 12.5% of gross domestic product (GDP) of China. The water of Lake Tai is highly eutrophied due to rapid development of both industry and agriculture in the area around the lake in recent 30 years. Cynobacterial bloom has occurred in Lake Tai for many years causing a serious deterioration of water quality, and the intensive bloom led to a public water crisis in May 2007.Atmospheric deposition may be an important source for nutrients entering Lake Tai. We sampled aerosols using two mid-flow samplers (TSP and PM2.5 samplers) and an 8-stage cascade impactor in Meiliang bay of lake Tai between July 8th and 21st, 2009, and collected total 53 aerosol samples for analysis of soluble ions and total elements.A particle deposition model and cascade sample measurements were used to estimate dry deposition fluxes of nutrients to the Lake Tai. Rainwater samples were used for the calculation of wet deposition flux of nitrogen. Air-mass backtrajectories were calculated from the National Oceanic and Atmospheric Administration (NOAA) GDAS data base using HYSPLIT program, and showed the synoptic situation and general sources of air masses sampled at Lake Tai. This study indicated the potential effects of atmospheric dry deposition on the N/P ratio and phytoplankton growth rate in the Lake Tai water, and provided useful information to policy-makers on how to reduce the eutrophication and cynobacterial bloom of the lake.High concentrations of dissolved inorganic nitrogen (DIN) were observed in fine particles with an average of 1.8μg m-3, about twice as much as that in coarse particles. However coarse particles had much higher deposition velocities compared to fine particles, and the maximum difference could be 4 orders of magnitude. Thus coarse particles were the main contributor to the dry deposition flux of DIN with the highest flux of 833μg N m-2 d-1, over 500 times than fine particles. The average daily dry flux of DIN was about 34 ton with 58% of NH4+-N over Lake Tai in July 2009. Assuming deposition flux of dissolved organic nitrogen (DON) was 16% of the atmospheric flux of total nitrogen (TN), the average annual atmospheric flux of TN would be 8410 t, which accounted for 29%-38% of total nitrogen input to the Lake Tai and became an important source of nitrogen to the lake. There was significantly positive relationship between NH4+and SO42- (R2=0.94). High concentrations of NH4+may be derived from the transport of non-local sources according to air mass backtrajectories. Local agricultural activities were not a significant source of NH3 during our sampling period.The average ratio of DIN to total phosphorus was approximately 58 in the aerosols sampled over Lake Tai, which was higher than the N/P ratio of 23 in the lake water. It was suggested that the increasing nitrogen may lead to an increase of phytoplankton growth in a freshwater lake as the water N/P ratio is greater than 25[1]. Atmospheric dry deposition not only adds DIN but also increases the N/P ratio towards 25 in Lake Tai, which may be one of important factors for inducing cynobacterial bloom. |
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