Freshwater reservoirs are regarded as an important anthropogenic source of methane (CH4) emissions. The temporal and spatial variability of CH4 emissions from different reservoirs results in uncertainty in the estimation of the global CH4 budget. In this study, surface water CH4 concentrations were measured and diffusive CH4 fluxes were estimated via a thin boundary layer model in a temperate river-reservoir system in North China, using spatial (33 sites) and temporal (four seasons) monitoring; the system has experienced intensive aquaculture disturbance. Our results indicated that the dissolved CH4 concentration in the reservoir ranged from 0.07 to 0.58 µmol/L, with an annual average of 0.13 ± 0.10 µmol/L, and the diffusive CH4 flux across the water-air interface ranged from 0.66 to 3.61 μmol/(m2•hr), with an annual average of 1.67 ± 0.75 μmol/(m2•hr). During the study period, the dissolved CH4 concentration was supersaturated and was a net source of atmospheric CH4. Notably, CH4 concentration and diffusive flux portrayed large temporal and spatial heterogeneity. The river inflow zone was determined to be a hotspot for CH4 emissions, and its flux was significantly higher than that of the tributary and main basin; the CH4 flux in autumn was greater than that in other seasons. We also deduced that the CH4 concentration/diffusive flux was co-regulated mainly by water temperature, water depth, and water productivity (Chla, trophic status). Our results highlight the importance of considering the spatiotemporal variability of diffusive CH4 flux from temperate reservoirs to estimate the CH4 budget at regional and global scales.

Internal nutrient loadings pose a high risk of being an additional N and P source, exacerbating eutrophication and deteriorating water quality. In this study, we selected the Daheiting Reservoir (DHTR) in North China, with a pronounced water level gradient, to investigate internal N and P loadings, estimate N and P fluxes across the sediment‒water interface based on the pore water profiles, and reveal the potential effects of water discharge from an upstream reservoir and high-intensity cage aquaculture on the risks of internal N and P release. The results indicated that DHTR presented with severe internal nutrient loadings, and the N and P fluxes showed significant spatiotemporal variations. NH4+-N and soluble reactive phosphorus (SRP) fluxes were higher in deep areas (averages of 26.14 and 9.9 mgm-2d-1, respectively) than in shallow areas near inflows (averages of 5.0 and 1.24 mgm-2d-1, respectively). Unexpectedly, the estimated NH4+-N and SRP fluxes were the lowest in summer (averages of 3.94 and 0.33 mgm-2d-1, respectively), which may have been influenced by seasonal thermal stratification and copious discharge from the hypolimnion of the upstream reservoir (Panjiakou Reservoir). Comparison of annual internal and external N and P loadings revealed that water discharge from the upstream Panjiakou Reservoir was the dominant source of N and P to the reservoir, contributing up to 83.6% of N input and 55.4% of P input. The internal P loading also contributed to water eutrophication to a great extent, accounting for 34.7% of the total P input. Our results highlight the impact of upstream reservoir discharge operation on downstream reservoir water quality and the importance of controlling the internal nutrient loading in cascade reservoirs, and further provide theoretical and practical foundations for the development of policies and strategies to conserve reservoir ecosystems.

Water eutrophication is a serious global issue because of excess external and internal nutrient inputs. Understanding the intensity and contribution of internal nitrogen (N) and phosphorus (P) loading in deep-water ecosystems is of great significance for water body eutrophication management. In this study, we combined intact sediment core incubation, high-resolution peeper (HR-Peeper) sampling, and analysis of N and P forms and other environmental factors in the water column and sediments to evaluate the contributions of internal N and P loading to water eutrophication by N and P fluxes across the sediment-water interface (SWI) of the Panjiakou Reservoir (PJKR), a deep-water ecosystem where eutrophication threatens the security of the local drinking water supply in North China. The results indicated that the PJKR showed obvious thermal and dissolved oxygen (DO) stratification in the warm seasons and full mixing in the cold seasons. The mean DO concentration was 9.9 and 3.55 mg/L in the epilimnion and hypolimnion, respectively, in warm seasons and 10.7 mg/L in cold seasons. The sediment acted as a source of soluble reactive phosphorus (SRP), NH4+-N, and NO2--N and a sink of NO3--N. The SRP fluxes were 5.28 +/- 4.34 and 2.30 +/- 1.93 mg m(-)(2).d(-1) in warm and cold seasons, respectively, and the dissolved inorganic nitrogen (DIN) fluxes were -0.66 +/- 47.84 and 44.04 +/- 84.05 mg m(-2).d(-1). Seasonal hypoxia accelerated the release of P rather than N from the sediments in warm seasons, which came mainly from Fe-P and Org-P under anoxic conditions. The strong negative NO3--N flux (diffusion from the water column to the sediment) implied an intensive denitrification process at the SWI, which can counteract the release flux of NH4+-N and NO2--N, resulting in the sediment acting as a weak dissolved inorganic nitrogen (DIN) source for the overlying water. We also found that internal N loading accounted for only similar to 9% of the total N loading, while internal P loading accounted for 43% of the total P loading of the reservoir. Our results highlight that efforts to manage the internal loading of deep-water ecosystems should focus on P and seasonal hypoxia.

To ensure the safety of the water supply of the Panjiakou reservoir, in 2016, the Chinese central government comprehensively banned the fishing cage culture that had lasted for almost 30 years. However, the long-term effects and retention impacts of the government's mandatory intervention on the reservoir water quality are unknown. To determine the reservoir water quality, we employed statistical methods along with the mathematical model to investigate the internal relationship since the construction of the reservoir. We applied seasonal trend decomposition using loess (STL) to explore the long-term and seasonality trend of monthly total nitrogen (TN) and total phosphorous (TP). To separate the impact of upstream water quality changes from cage culture on reservoir water quality, we employed generalized additive models (GAMs). We created a model, the LAKE2K model, to investigate the internal sources of the sediment that accumulated during the aquaculture period and its retardant effect. The results revealed that the concentration of upstream TN was more affected by non-point sources than by TP. The long-term policy of encouraging aquaculture has greatly contributed to the increase in the reservoir TP concentration rather than an increase in TN; the prohibition of cage aquaculture has resulted in a sharp drop in TP. After the ban, the sediment became the main source of TP. We suspect that the TP concentration of the reservoir and sediment will decrease gradually until a new equilibrium is reached within 10 years. This study offers lake managers an opportunity to increase their insight into the interaction of management measures with water quality and provides valuable information for the natural recovery of the eutrophic system.

This is the third paper of the series about “Dynamics of algae growth and nutrients in experimental enclosures culturing bighead carp and common carp”. In this paper, phosphorus dynamics were investigated under the condition of culturing bighead carp and common carp with added fish food (nitrogen dynamics is discussed in the second paper because their behaviors are so different from each other). Nearly fifty days’ observation results indicated that the reservoir water was typical of “phosphorus limited” water, and soluble reactive phosphorus (SRP) was the main constituent of measured total phosphorus (TP). The presence of fish food resulted in significantly higher SRP, dissolved total phosphorus (DTP) and TP concentrations in contrast with the reservoir water. Moreover, continuous supply of fish food led to the decline of total nitrogen to total phosphorus (TN:TP) from more than 100 to less than 5. Variations in the ratio of TN to TP favored the growth of blue-green algae. Fish species affected phosphorus concentrations, and culturing bait-eating common carp contributed more to reducing the SRP, DTP and TP concentrations than culturing planktivorous bighead carp. 0.5%, 4.1% and 3.1% TP can be removed in enclosures with culturing bighead carp, common carp and mixed bighead carp and common carp, respectively. Abundant phosphorus in the fish culturing activities may be present as the uneaten food, algae cells, and within the water column and sediment, which should be taken into serious consideration for the target of future water eutrophication prevention and safety of the drinking water supply.