Seasonal and regional variations in precipitation chemistry in the Lake Taihu Basin, China

To evaluate seasonal and regional variations in precipitation chemistry in Lake Taihu Basin, 5 cities around the lake were selected for monthly collection of rainfall samples from July 2002 to June 2003. The analytical parameters included pH, cations (K+, Na+, Ca2+, Mg2+, NH4+), anions (F−, Cl−, SO42-,NO3-), total nitrogen (TN) and total phosphorus (TP). In descending order, concentrations (in meq L−1) of the ions in wet deposition can be ordered as SO42->Ca2+>NH4+>NO3->Cl−>Na+>Mg2+>K+>F− with concentratin percentages of 48.7%, 19.1%, 11.6%, 8.1%, 5.6%, 2.6%, 1.7%, 1.6% and 1%, respectively.Rainfall was seasonally variable, with high rates in spring and summer but low rates in autumn and winter, while ion concentrations followed the reverse seasonal cycle. To evaluate deposition rates (DR) of different elements, average DR of each ion was calculated from a simple model. Most of the highest DRs of these substances occurred in Suzhou and Wuxi, located east and north of Lake Taihu, which shows there was considerable variation over the 5 stations. This regional pattern reflects local emissions from industry and agriculture, and suggests that steps need to be taken by local governments for particle emission reductions. The estimated annual inputs of TN and TP over the lake are 4722 and 75 t, respectively, which accounts for around 16.5% and 7.3% of the annual total external inputs of these nutrients. Primary productivity in Lake Taihu associated solely with wet deposition of nitrogen to the lake was estimated to be 7.03×106 mol C day−1. These results suggest that wet deposition can have significant impacts on the trophic status of Lake Taihu and that attention to nutrients in precipitation should be included in the management of external nutrient loads to the lake.