Long-Term Interannual and Seasonal Links between the Nutrient Regime, Sestonic Chlorophyll and Dominant Bluegreen Algae under the Varying Intensity of Monsoon Precipitation in a Drinking Water Reservoir

Abstract

Long-term variations in reservoir water chemistry could provide essential data in making sustainable water quality management decisions. Here, we analyzed the spatiotemporal variabilities of nutrients, sestonic chlorophyll-a (CHL-a), nutrient enrichment, dominant algal species, and overall chemical water health of the third-largest drinking water reservoir in South Korea during 2000–2020. Our results distinctly explained the strong influence of monsoon rainfall on spatial and annual water chemistry variations. We observed a consistent increase in the chemical oxygen demand alluding to organic matter pollutants, while a steady declining trend in the sestonic CHL-a. The long-term total phosphorus (TP) level showed a steady reduction from the riverine zone to the lacustrine area. However, a higher total coliform bacteria (TCB) was observed at the water intake tower sites. TP displayed a strong link to algal CHL-a and ambient nitrogen phosphorus ratios, suggesting a robust phosphorus-limitation state. The severe phosphorus-limitation was also corroborated by the findings of trophic state index deviation. The high and low flow dynamics exhibited the strong influence of intensive rainfall carrying many nutrients and sediments and flushing out the sestonic CHL-a. Successive eutrophic conditions prevailed along with dominating blue-green algae species (Microcystis and Anabaena). We observed a strong positive correlation (r = 0.62) between water temperature and CHL-a and between total suspended solids and TP (r = 0.65). The multi-metric water pollution index characterized the overall water quality as ‘good’ at all the study sites. In conclusion, the long-term spatiotemporal variabilities of the ecological functions based on the nutrient-CHL-a empirical models are regulated mainly by the intensive monsoon precipitation. The drinking water could become hazardous under the recurrent eutrophication events and chemical degradations due to uncontrolled and untreated inflow of sewage and wastewater treatment plant effluents. Therefore, we strongly advocate stringent criteria to mitigate phosphorus and organic pollutant influx for sustainable management of Daecheong Reservoir.

Seasonal and Long-Term Connections between Trophic Status, Sestonic Chlorophyll, Nutrients, Organic Matter, and Monsoon Rainfall in a Multipurpose Reservoir

“…The drinking water supply crises tremendously challenge sustainable human society expansion due to global anthropogenic pollution [1][2][3]. Reports abound on the human-caused disturbances at the catchment scale (e.g., by extensive industrialization, urbanization, intensive agriculture) in the lentic ecosystems, resulting in various water quality issues [4][5][6]. One of the most conspicuous and long-term issues is consistent nutrient enrichment (eutrophication), causing significant water quality degradations in the lentic ecosystems [7][8][9].…”

Effects of Long-Term Increases in Water Temperature and Stratification on Large Artificial Water-Source Lakes in South Korea

“…The retention time of each lake can vary according to its hydrologic management. Physical factors of lakes, such as differences in water temperature, upstream river inflow rate, and retention time, significantly affect the delivery and accumulation of thermal energy of water bodies [10,[52][53][54]. Particularly for PD, the average water temperature change in the surface layer and water depth is considered to be complex because mixing occurs inside the lake due to the flow of the Namhan River and the Bukhan River, which have different water temperatures during various periods [55].…”

Inter-Annual and Seasonal Variations of Water Quality and Trophic Status of a Reservoir with Fluctuating Monsoon Precipitation

“…The equations used for calculation of TSIs were: As parameters used in Carlson's TSI are to represent algal biomass in water, TSIs of different variables should be identical if biological productivity is limited by p and turbidity of water results from phytoplankton particles. In practice, however, TSIs of different parameters are frequently different from each other, and it was proposed that differences in numerical magnitudes of TSI Chl , TSI TP , and TSI SD can be used to identify different limiting conditions in an aquatic ecosystem [7,41,42]. A simple two-dimensional graphical method in which TSI Chl -TSI SD values are plotted against TSI Chl -TSI TP was proposed for identification of relationships between the trophic state variables [41,43]; this method was utilized in this study.…”

“…The most prominent feature of temporal distributions of the organic parameters was a significant increase toward the end of the study period, with the gradient higher than the first two-thirds. The fact that BOD and COD maintained mostly increasing trends without strong association with precipitation change may indicate that their increases might have been caused by the changes in anthropogenic activities occurring in the surrounding watershed [7,51]. Indeed, a minor increase in industrial pollution sources was reported over the period from 2012 to 2016, with a concurrent decrease in farming activities [52].…”

“…Despite the importance of maintaining good water quality, the innate physical characteristics and their variabilities of reservoirs complicate efficient reservoir management [7][8][9][10][11]. Artificial reservoirs are known to have large spatiotemporal variation of water quality parameters, such as nutrients (total nitrogen (TN) and total phosphorus (TP)), chlorophyll-a (Chl-a), suspended solids (SS), and water clarity represented by Secchi depth (SD) due to the fluctuating traits of hydraulic characteristics in space and time [12].…”