Abstract
The variations in water quality parameters and trophic status of a multipurpose reservoir in response to changing intensity of monsoon rain was investigated by applying a trophic state index deviation (TSID) analysis and an empirical regression model to the data collected in two periods from 2014 to 2017. The reservoir in general maintained mesotrophic conditions, and Carlson’s trophic state index (TSIc) was affected most by TSITP. Nutrient concentrations, particularly phosphorus, did not show strong correlations with precipitation, particularly in the period with weak monsoon, and a significant increase in total phosphorus (TP) was observed in Spring 2015, indicating the possibility of internal phosphorus loading under decreased depth and stability of water body due to a lack of precipitation. TSIChl was higher than TSISD in most data in period 1 when a negligible increase in precipitation was observed in the monsoon season while a significant fraction in period 2 showed the opposite trend. Phytoplankton growth was not limited by nutrient limitation although nutrient ratios (N/P) of most samples were significantly higher than 20, indicating phosphorus-limited condition. TSID and regression analysis indicated that phytoplankton growth was limited by zooplankton grazing in the Spring, and that cell concentrations and community structure in the monsoon and post-monsoon season were controlled by the changing intensity of the monsoon, as evidenced by the positive and negative relationships between community size and cyanobacterial population with the amount of precipitation in the Summer, respectively. The possibility of contribution from internal loading and an increase in cyanobacterial population associated with weak monsoon, in addition to potential for nutrient enrichment in the post-monsoon season, implies a need for the application of more stringent water quality management in the reservoir that can handle all potential scenarios of eutrophication.
Highlights
The generation of reservoirs by building dams across rivers rapidly and significantly alters the shape and dimension of watersheds, as well as the physicochemical characteristics of water bodies [1,2,3]
Water temperature (T), hydrogen ion concentration, dissolved oxygen (DO), electrical conductivity (EC), and oxidationreduction potential (ORP) of the samples were measured on site with probes for these parameters attached to a portable multi meter (Hach Multimeter, HQ40D, Hach Co., Loveland, CO, USA)
Spatiotemporal variations in water quality parameters and trophic state of Yongdam Reservoir were evaluated using statistical analysis and trophic state index deviation to determine how the reservoir responds to changes in physicochemical and biological changes and to identify conditions that produced the observed transition in the phytoplankton community, under weak monsoon conditions
Summary
The generation of reservoirs by building dams across rivers rapidly and significantly alters the shape and dimension of watersheds, as well as the physicochemical characteristics of water bodies [1,2,3]. Many reservoirs are experiencing significant water quality degradation, due to excessive input and enrichment of nutrients, such as nitrogen (N) and phosphorus (P), which results in eutrophication that frequently leads to algal bloom and many associated environmental, economic, and social problems [5,6,7]. The reduction of anthropogenic input of nutrients, ones that limit algal growth, has been a major goal of water quality management of reservoirs to avoid the undesirable impact of eutrophication [8,9]. Limiting nutrients can be determined by mass or molar ratio of total N (TN) and total P (TP) [10,11], which varies widely with nutritional enrichment, geographic location, and the artificial nature of lentic water bodies [11,12,13]. It was found that algal growth was not always controlled by P even when P-limited conditions were implied by the ratio [16,17]
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