Landscape heterogeneity impacts water chemistry, nutrient regime, organic matter and chlorophyll dynamics in agricultural reservoirs

[Display omitted] •Changing water chemistry in lentic ecosystems reflects landscape heterogeneity.•Phosphorus acts as the limiting nutrient in agricultural reservoirs irrespective of land use patterns.•Forests are still the least-impacted regions in the Korean peninsula.•Chemical health status of cropland and urban region reservoirs revealed as poor.•Cropland region reservoirs are the most vulnerable to frequent eutrophication events in the future. The objectives of this study were to determine the seasonal and temporal variabilities of nutrients, chlorophyll (Chl-a) and some other parameters in 95 agricultural reservoirs influenced by the Asian monsoon during 2008–2017, and elucidates the links of water chemistry dynamics to landscape heterogeneity. Using the long-term data, we analyzed seasonal and inter-annual patterns of the water quality in relation to the intensity of the summer monsoon and developed the seasonal empirical models on CHL-nutrients (N, P) in relation to the light regime by the calculation of non-algal light attenuation coefficient (Kna). Multi-metric water pollution index (WPIKR) reflected on the chemical health of reservoirs in relation to monsoon and land use patterns. The results divulged the intensity of monsoon rainfall tremendously influenced nutrients, Chl-a and TOC level in cropland and urban regions. TP showed the highest percentage of relationship with Chl-a and ambient nutrient ratios indicating it as the limiting factor particularly in cropland, urban and forest region. TOC disclosed the highest relationship with Chl-a in cropland (R2 = 0.66) and forest (R2 = 0.71) regions. Secchi disk depth (SDD) displayed a strong negative relationship with suspended solids, TP and Chl-a in cropland and estuarine regions. Kna indicated a strong relationship with TP and TOC in cropland and estuarine region reservoirs. On the other hand, trophic state index specified the majority of reservoirs in zooplankton grazing state except few in the estuarine region showing the propensity of blue-green algae presence. Further principal component analysis (PCA) designated cropland region characterized by higher TP and Chl-a level within increasing Carlson’s trophic state index (CTSI), the urban region receiving higher sediments and chemical oxygen demand. On the contrary, the forest region showed a tendency of higher organic matter (TOC) while estuarine characterized by higher electrical conductivity (EC). Hierarchical cluster analysis (HCA) showed th