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

Highlights

• 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.

Abstract

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 (K_na). Multi-metric water pollution index (WPI_KR) 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. K_na 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 that cropland region reservoirs strongly linked to urban reservoirs in terms of deteriorating water quality. Based on WPI_KR results, cropland and urban region reservoirs reflected as ‘poor’, estuarine as ‘fair’ condition while forest region indicated ‘excellent to good’ chemical health status. According to CTSI, forest and estuarine reservoirs were in the oligotrophic state, urban in mesotrophic and cropland region reservoirs from mesotrophic to eutrophic state. This study supports a reduction in phosphorus yielding fertilizers in the Korean Peninsula that could lead to eutrophication of lentic ecosystems in the future along with mitigation of anthropogenic impacts in the reservoir's watershed.

Introduction

Reservoirs are principally manmade waterbodies that are prevalent in diverse settings (Znachor et al., 2018) and implicitly support the different human activities ranging from irrigation to ecological functions making them integral components in the Korean Peninsula. Generally, the reservoirs are purposefully constructed to accomplish wide-ranging functions including ecological, drinking water supplies, irrigational services, fisheries, shipping routes, industrial water supplies and cooling sinks, hydropower generation, flood mitigations, recreation etc. (Wetzel, 2001, Li et al., 2018, Znachor et al., 2018, Atique and An, 2019). A variety of factors including land use pattern, changing climate, water level management, catchment-wide socio-economic activities as well as water abstraction from the reservoirs affect the ecological functions of reservoirs (Jones et al., 2011, Hayes et al., 2017, Kopáček et al., 2017). Artificial reservoirs digress from natural lakes in numerous fundamental aspects such as outstretched morphology, reduced water residence time (WRT) coupled with conspicuous water level fluctuations and intermittent water abstraction events (Wetzel, 2001, Hayes et al., 2017, Znachor et al., 2018). Proportionately significant, reservoirs and lakes sequester carbon, mitigate inundations, sustain biodiversity, foster fish populations and game birds, and revive aquifers. Anthropogenic actions compounded by intensive rainfall in the watershed have greater potential to disrupt reservoir functions and degrade water quality, thereby jeopardizing the ecosystem services they deliver.

Water quality has utmost implications to fisheries, domestic water supplies, aesthetics and sustainable management of water resources (Haider et al., 2016, Mehboob et al., 2017, Iqbal et al., 2017, Vollmer et al., 2018) as well as to other predictable practices and ecosystem services (Batool et al., 2018, Haider et al., 2018). Owing to rapid urbanization and surmounting global populations, the quality, as well as quantity of accessible water resources have become grave concerns of mankind (Sun et al., 2016, Atique and An, 2018, Kim et al., 2019). Diverse ecological hiccups, triggered by the extreme waste discharges from the multiplying industrial units, agricultural activities, bourgeoning population size, intensive urbanization, etc., all are portending and deteriorating the surface water quality by posing serious water challenges in developing nations (Liu et al., 2012, Hutchins et al., 2018, Atique et al., 2019). Water quality is a dynamic phenomenon usually predicted by approximation/deviation or degradation in the significant water chemistry factors (Logan and Taffs, 2013, Hutchins et al., 2018). Such fluctuations of significant features such as total phosphorus (TP) and total nitrogen (TN) levels are essential when range within their optimum limits, however, upon deviation pose to be detrimental to aquatic biodiversity (Atique and An, 2018, Moon et al., 2020). Such deviations could be used for decision-making as well as the suitable labeling of water resources for the intended purposes (Liu et al., 2012, Logan and Taffs, 2013, Trochine et al., 2017).

Elevated nutrient value is largely recognized as a significant problem poignant to the ecological framework and functioning of aquatic ecosystems (Trochine et al., 2017). The nutrient loadings are influenced by several factors e.g. precipitation (da Rocha Junior et al., 2018) and intensified by environmental impacts driven by anthropogenic activities (Meehl et al., 2007, Jung et al., 2016, Khan et al., 2018, Bae et al., 2020) and climate change (IPCC, 2014). Yet further intensified because of soil erosion and aggravated by the multiple land use patterns: cropland, livestock, industrial, urban, forest and estuarine (Moss et al., 2011). In lakes and reservoirs, a decline in redox potential at the water–sediment interface results in the nutrient loadings stimulated by anoxic conditions at the bottom that may trigger the release of phosphorus (Lake et al., 2007). Significant variations in nutrient cycling gravely compromised lakes and reservoirs water quality in several ways due to the increase in eutrophication. For instance, intensive degradation of organic matter results into oxygen depletion and the visual depth is obscured by phytoplankton biomass in water (Bonilla et al., 2012, Jeppesen et al., 2015, Brasil et al., 2016). Whatsoever be the case, reservoir productivity and clarity are extremely affected by the change in nutrient loadings leading to severe effects on biotic and abiotic factors (Frey et al., 2011, Schindler, 2012). Enrichment of reservoirs with nutrients stimulates the transformations into mesotrophic, eutrophic, and in some cases hypertrophic (Schindler, 2012, Jeppesen et al., 2015). Eutrophication has become a widespread ecological problem reported across the globe. It is consequent of either natural or artificial processes instigated by the excessive nutrients, mainly phosphorus (TP) and nitrogen (TN), leading to intensified primary productivity and eventually blooms, with subsequent disequilibrium in ecosystem with gradual degradation (An et al., 2003), besides intense modifications in reservoir trophic food-webs (Smith, 2003, Park et al., 2015, Jung et al., 2016).

Land use patterns define the variabilities of the associated ecosystem services, functions, and relationships (Wang et al., 2017, Yang et al., 2018). Anthropological and natural land use patterns have compounded effect on geomorphology, biogeochemical cycles, connected and nested ecosystems, sediment characteristics, hydrological processes, nutrient loads and water chemistry at the universal, regional and local scales (Wang et al., 2017, Shrestha et al., 2018). Precipitation acts as a connecting tool between different landscape patterns, point and non-point sources, and the existing aquatic ecosystems in the catchment area by transporting the pollutants and nutrients into the rivers and reservoirs. For instance, TN:TP ratios are closely linked to various land uses directly influencing the biodiversity of an aquatic ecosystem (Choi et al., 2015).

There are roughly 18,000 artificial reservoirs in South Korea predominantly used for agricultural purposes. Roughly 70% of these agricultural reservoirs have been constructed approximately five or more decades ago (Lee and Oh, 2018). Furthermore, the water depth of these reservoirs remains around 10 m, making them a good study target for water quality, water level, and retention as well as pollutants and nutrient-related researches. Moreover, the impact of Asian monsoon becomes significantly important factor linking the land use and nutrient dynamics (An and Park, 2002, Jung et al., 2016). Long-term intensive monitoring of agricultural reservoirs involving hydrological, chemical and physical modifications, diverse land use patterns largely supported by anthropogenic activities in basin continuously compounded by Asian monsoon and climate change makes an interesting case study. Although reservoirs are the predominant lentic ecosystems in Korea, it is only recently that their unique features are considered for research and management.

Recognizing the grander importance of agricultural reservoirs, we planned on detection of inter-annual and spatio-seasonal trends in water chemistry, divulging links between nutrients and land use patterns and detection of links between Chlorophyll a and total organic carbon (TOC). Further, we characterized water pollution status by modified multi-metric water pollution index (WPI_KR), along with light regime andtrophic state index deviation (TSID) in relation to the land use patterns.