Abstract
When developing water quality improvement strategies for eutrophic lakes, questions may arise about the relative importance of point sources and nonpoint sources of phosphorus. For example, there is some skepticism regarding the effectiveness of partial reductions in phosphorus loading; because phosphorus concentrations are too high in hypertrophic lakes, in-lake phosphorus concentrations might still remain within typical range for eutrophic lakes even after the reduction of phosphorus loading. For this study, water quality and the phytoplankton and zooplankton communities were monitored in a hypertrophic reservoir (Lake Wangsong) before and after the reduction of phosphorus loading from a point source (a sewage treatment plant) by the installation of a chemical phosphorus-removal process. Before phosphorus removal, Lake Wangsong was classified as hypertrophic with a median phosphorus concentration of 0.232 mg L−1 and a median chlorophyll-a concentration of 112 mg L−1. The dominant phytoplankton were filamentous cyanobacteria for the most of the ice-free season. Following the installation of the advanced treatment process, phosphorus concentrations were reduced to 81 mg L−1, and the N/P atomic ratio increased from 42 to 102. Chlorophyll-a concentrations decreased to 42 μg L−1, and the duration of cyanobacterial dominance was confined to the summer season. Cyanobacteria in spring and autumn were replaced by diatoms and cryptomonads. Filamentous cyanobacteria in summer were replaced by colony-forming unicellular Microcystis spp. It was remarkable that zooplankton biomass increased despite the decrease in phytoplankton biomass, and especially cladoceran zooplankton which increased drastically. These responses to the reduction of point source P loading to Lake Wangsong imply that reducing the point source P loading can have a big impact even when nonpoint sources account for a large fraction of the total annual phosphorus loading. Our results also show that the phytoplankton community can shift to decreased cyanobacterial dominance and the zooplankton community can shift to higher cladoceran dominance, even when phosphorus concentrations remain within the typical range for eutrophic lakes following the reduction of phosphorus loading.
Highlights
When developing water quality improvement strategies for eutrophic lakes, questions may arise about the relative importance of point sources and nonpoint sources of phosphorus
The annual average daily P loading from nonpoint sources in the Lake Wangsong watershed was 7.5 kg P day−1, which was higher than the P loading from the major point source (STP effluent), 3.7 kg P day−1 (Gyeonggi Research Institute 2011)
Most of the annual P loading from agricultural nonpoint sources is concentrated in a few rain events during the summer monsoon season in Korea, and only limited amounts of P export occur on dry days
Summary
When developing water quality improvement strategies for eutrophic lakes, questions may arise about the relative importance of point sources and nonpoint sources of phosphorus. Water quality and the phytoplankton and zooplankton communities were monitored in a hypertrophic reservoir (Lake Wangsong) before and after the reduction of phosphorus loading from a point source (a sewage treatment plant) by the installation of a chemical phosphorus-removal process. Nonpoint P sources such as agricultural fields and forest are the major sources of the total P load. If there is significant urban development within the watershed, sewage discharge can contribute a considerable portion of P loading (Neal et al 2000; Jarvie et al 2006). In watersheds consisting of complex terrain with urban and rural land use, P loading from agricultural fields is usually larger than the P loading from sewage. P loading from point sources have uniform flow rates, and on dry days, their contribution can be larger than those of nonpoint sources
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