The prediction of algal chlorophyll-a and water clarity in lentic ecosystems is a hot issue due to rapid deteriorations of drinking water quality and eutrophication processes. Our key objectives of the study were to predict long-term algal chlorophyll-a and transparency (water clarity), measured as Secchi depth, in spatially heterogeneous and temporally dynamic reservoirs largely influenced by the Asian monsoon during 2000–2017 and then determine the reservoir trophic state using a multiple linear regression (MLR), support vector machine (SVM) and artificial neural network (ANN). We tested the models to analyze the spatial patterns of the riverine zone (Rz), transitional zone (Tz) and lacustrine zone (Lz) and temporal variations of premonsoon, monsoon and postmonsoon. Monthly physicochemical parameters and precipitation data (2000–2017) were used to build up the models of MLR, SVM and ANN and then were confirmed by cross-validation processes. The model of SVM showed better predictive performance than the models of MLR and ANN, in both before validation and after validation. Values of root mean square error (RMSE) and mean absolute error (MAE) were lower in the SVM model, compared to the models of MLR and ANN, indicating that the SVM model has better performance than the MLR and ANN models. The coefficient of determination was higher in the SVM model, compared to the MLR and ANN models. The mean and maximum total suspended solids (TSS), nutrients (total nitrogen (TN) and total phosphorus (TP)), water temperature (WT), conductivity and algal chlorophyll (CHL-a) were in higher concentrations in the riverine zone compared to transitional and lacustrine zone due to surface run-off from the watershed. During the premonsoon and postmonsoon, the average annual rainfall was 59.50 mm and 54.73 mm whereas it was 236.66 mm during the monsoon period. From 2013 to 2017, the trophic state of the reservoir on the basis of CHL-a and SD was from mesotrophic to oligotrophic. Analysis of the importance of input variables indicated that WT, TP, TSS, TN, NP ratios and the rainfall influenced the chlorophyll-a and transparency directly in the reservoir. These findings of the algal chlorophyll-a predictions and Secchi depth may provide key clues for better management strategy in the reservoir.

We studied the chemical water quality, physical habitat and biotic integrity of an urban stream subjected to restoration measures in South Korea. We used the water pollution index (WPI), qualitative habitat evaluation index (QHEI) and index of biotic integrity (IBI) on the water quality, physical habitat and fish assemblage data respectively, during 2007–2016 in Gap Stream to evaluate the ecological health before and after restoration measures. The results revealed annual mean total phosphorus (TP) dramatically decreased by 13-fold for 10 years and the values of biological oxygen demand (BOD) and chemical oxygen demand (COD) also decreased by >50% over the time, indicating decrease in nutrient enrichment and organic matter pollution after restoration measures. In the meantime, biological health analyses using IBI indicated no major transformation in fish assemblages under restoration impact. However, the proportion of sensitive species increased, and tolerant species decreased after restoration. Gap stream is home to 50 distinct fish species. QHEI proposed that the physical habitat health was in ‘good’ condition during the study period. WPI showed the chemical water quality status remained ‘poor’ during and before restoration but improved to ‘fair-good’ condition in the ensuing years after restoration. The IBI results, however, indicated ‘very poor-poor’ biotic integrity irrespective of restoration measures. CHL-a and TP showed strong (r < 0.7) to moderately strong (r = 0.5–0.7) correlation with significantly important water quality factors. Spatially significant pattern change in TN and TP was obvious as measured levels were significantly higher (p < 0.01) in downstream than upstream. Principal component analysis successfully indicated the placement of water quality factors and indices used as in three distinct stream compartments. The higher pollutant levels in the downstream mainly linked to the nutrient-rich effluents from emerging from the wastewater treatment plants (WWTPs) and industrial complexes operative in the stream catchment. Overall, restoration measures indicated minor impact on fish assemblages and physical habitat due to slow and steady improvement, however, water quality improved due to a decline of nutrients and chemicals downstream. This indicated a positive tendency of improvements in physical habitat and richness of fish assemblages in Gap Stream.

Invasive alien fish species (IAFS) influence recipient ecosystems in multiple ways, from altered native fish communities to poor ecological health and higher economic losses to control and eradication. We tested key drivers and connections between an IAFS (Micropterus salmoides) presence, absence, geomorphological, hydro-chemical, physical habitat, native fish assemblages, and large river basins biotic integrity during 2016–2019. A total number of 62,121 individuals (TNI) representing 74 fish species were observed, out of which 68 species (45,677 TNI) belonged to the Geum River (GR) basin, while 63 species (16,444 TNI) were from the Mankyong/Dongjin River (MDR) basin. The results illustrated a significant contrast based on stream order, catchment area, altitude, stream gradient, and width among the sites with and without largemouth bass. However, fluctuations in nutrients (nitrogen and phosphorus) were not affected by variations in pH, dissolved oxygen, conductivity, suspended solids, and river flow. The largemouth bass has emerged as the 8th largest fish population in the GR and swiftly occupies the MDR basin with a constancy value of 45.5. Native sensitive (r = −0.38), insectivore (r = −0.252), carnivores (r = −0.085), and TNI (r = −0.512) displayed a negative correlation with increasing largemouth bass abundance. Largemouth bass induced significant regime transformation in the carnivore species. A significant difference (p < 0.01) of biotic integrity was illustrated among the largemouth bass population sites. A conspicuous interplay between ‘poor’ ecological health (r = 0.33, p = 0.038, n = 41) sites and ‘fair–excellent’ (r = −0.38, p < 0.001, n = 622) sites as compared to the overall ecological health linked with largemouth bass abundance (r = −0.397, p < 0.001, n = 663) alluded to stronger impact of the IAFS. In conclusion, the largemouth bass has potentially altered the native fish assemblage and ecological health. Further, we conclude that rapidly shifting flow patterns supported by the expanding anthropogenic interventions (weirs and dam) are the most approving factors of impending fish invasions.