Can reductions in water residence time be used to disrupt seasonal stratification and control internal loading in a eutrophic monomictic lake?

Assessing alternative lake management actions for climate change adaptation

Lake management actions are required to protect lake ecosystems that are being threatened by climate change. Freshwater lakes in semiarid regions are of upmost importance to their region. Simulations of the subtropical Lake Kinneret project that rising temperatures will cause change to phytoplankton species composition, including increased cyanobacteria blooms, endangering lake ecosystem services. Using lake ecosystem models, we examined several management actions under climate change, including two alternatives of desalinated water introduction into the lake, hypolimnetic water withdrawal, watershed management changes and low versus high lake water level. To account for prediction uncertainty, we utilized an ensemble of two 1D hydrodynamic-biogeochemical lake models along with 500 realizations of meteorological conditions. Results suggest that supplying desalinated water for local use, thus releasing more natural waters through the Jordan River, increasing nutrient flow, may reduce cyanobacteria blooms, mitigating climate change effects. However, these results are accompanied by considerable uncertainty.

Interacting impacts of hydrological changes and air temperature warming on lake temperatures highlight the potential for adaptive management

Globally, climate warming is increasing air temperatures and changing river flows, but few studies have explicitly considered the consequences for lake temperatures of these dual effects, or the potential to manage lake inflows to mitigate climate warming impacts. Using a one-dimensional model, we tested the sensitivity of lake temperatures to the separate and interacting effects of changes in air temperature and inflow on a small, short-residence time (annual average ≈ 20 days), temperate lake. Reducing inflow by 70% increased summer lake surface temperatures 1.0-1.2 °C and water column stability by 11-19%, equivalent to the effect of 1.2 °C air temperature warming. Conversely, similar increases in inflow could result in lake summer cooling, sufficient to mitigate 0.75 °C air temperature rise, increasing to more than 1.1 °C if inflow temperature does not rise. We discuss how altering lake inflow volume and temperature could be added to the suite of adaptation measures for lakes.

Elevated internal phosphorus loading from shallow areas of eutrophic boreal lakes: Insights from porewater geochemistry

Internal phosphorus (P) loading is widely recognized as a major cause of lake eutrophication. One conventional paradigm states that the magnitude of internal loading through P diffusion is constrained by the presence of iron (Fe) oxides in surface sediments under oxic conditions near the sediment-water interface (SWI). However, biogeochemical P dynamics in Fe-rich sedimentary systems are still not fully understood, especially in eutrophic lakes where intensively coupled organic matter (OM) remineralization and reductive dissolution of Fe-bound P (Fe-P) exist concurrently. Here, we assess the diagenetic processes that govern sedimentary P cycling in two eutrophic Fe-rich lakes in southern Finland, Lake Hiidenvesi and Lake Kytäjärvi, using a combination of porewater and solid-phase analyses. Coupled reductive dissolution of Fe-P and OM remineralization controlled P regeneration in both lakes, with Fe-P acting as the dominant source for porewater P. Vivianite formation likely immobilized sedimentary P in the deepest basin of Hiidenvesi. Elevated P diffusion rates were observed at shallow sites under oxic bottom water conditions in summer in both lakes, stimulated by enhanced remineralization of both freshly- (mostly phytoplankton-origin) and earlier-deposited OM under elevated temperatures. Areas overlain by oxic bottom water contributed more benthic P fluxes to the water column compared to anoxic/hypoxic areas in both lakes during all sampling seasons. Our study suggests that in shallow eutrophic settings with high OM deposition and elevated temperatures, remineralization in upper sediments regenerates P efficiently enough to support a significant amount of P release to the water column even under sedimentary molar Fe/P ratios >20. We also discuss the implication of our findings for lake restoration strategies.

Pulses outweigh cumulative effects of water diversion from river to lake on lacustrine phytoplankton communities

Due to the allochthonous input of nutrients and species, the cumulative effects of water diversion on water-receiving lakes deserve attention. Taking the water diversion project from the Yangtze River to Lake Taihu (WDYT) as an example, we explored the temporal effects of WDYT on the phytoplankton community and physicochemical habitat of Lake Taihu in autumn and winter from 2013 to 2018. Although the short-term diversion significantly increased the risk of importing nutrients, the relatively high quality of the diversion water compared with other inflow rivers had improved the water quality of the water-receiving lake region. The seasonal water diversion significantly increased phytoplankton diversity and community network complexity and reshaped the lacustrine community to be diatom-dominated with their relative proportions of 24.1-64.9% during water diversion periods. The contributions of physicochemical habitat changes induced by water diversion to variations in phytoplankton communities were 24.0-28.0%. The differences in phytoplankton diversity, community composition and physicochemical habitat in the water-receiving lake region between the diversion and non-diversion years were more evident than those between the non-diversion years in the same season, when comparing the multivariate dispersion indices among them. However, the lacustrine phytoplankton community during non-diversion periods still has not been essentially altered after several years of diversion, so the pulse effects of short-term water diversion were more obvious than the long-term cumulative impacts. Better control of allochthonous nutrients, appropriate increase in inflow water, adhering to the long-term operation, should be effective to enhance ecological benefits of such water diversion projects.

The COP27 screened through the lens of global water security

Water security is an expression of resilience. In the recent past, scientists and public organizations have built considerable work around this concept launched in 2013 by the United Nations as "the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability". In the 27th Conference of the Parties (COP27), held in Sharm El-Sheikh (Egypt) in last November, water security was considered a priority in the climate agenda, especially in the adaption and loss and damage axes. This discussion paper represents the authors' opinion about how the conference coped with water security and what challenges remain to attend. As discussion paper, it had the purpose to stimulate further discussion in a broader scientific forum.

Regulating phytoplankton-available suspended particulate phosphorus (P) to control internal P pollution in lake: Conclusion from a short review

The necessity on controlling internal P pollution has been widely reported for lake restoration; thus far, cutting the migrations of soluble P from sediment to overlying water, especially under anoxic condition, is the main target of the internal P pollution control to achieve favorable ecological responses in lake. Here, according to the types of P directly available by phytoplankton, phytoplankton-available suspended particulate P (SPP) pollution, which mainly occurs under aerobic condition and due to sediment resuspension and soluble P adsorption by suspended particle, is found to be the other kind of internal P pollution. The SPP has long been a key index for environmental quality assessment, which could be indirectly reflected by the developed various methods for phytoplankton-available P pool analysis; also, the P has been demonstrated to be a major cause of phytoplankton breeding, typically in shallow lakes. Importantly, compared to the soluble P, SPP pollution clearly has more complicated loading pathways and P activation mechanisms and involves in different fractions of P, even part of which are with relatively high stability in sediment and suspended particle, leading to the potential control measures for the pollution being more complex. Considering the potential differences of internal P pollution among various lakes, this study is therefore calling for more research to focus on regulating phytoplankton-available SPP pollution. Recommendations are also offered to bridge knowledge gap of the regulation to design proper measures for lake restoration.

Reducing the water residence time is inadequate to limit the algal proliferation in eutrophic lakes

The eutrophication problem now threatens many lakes and reservoirs. To avoid the occurrence of algal blooms, some cities try to increase the flow rate or directly choose lakes or reservoirs with a short water residence time (WRT) as drinking water sources. However, up to now, whether such a strategy can achieve its goal is still unclear. In this study, a newly restored lake with a WRT of approximately 3 days was chosen to investigate algal growth potential as well as its responses to external nitrogen (N) and phosphorus (P) inputs. The results suggested that although the water quality of the lake could generally meet the environmental quality standards for surface water, dissolved inorganic nitrogen reached a high level with an average value of 1.58 mg/L. Meanwhile, a considerable increase in Chl-a concentration was observed across the flow direction. Especially, in July, Chl-a concentration at the site near the outlet was 8.1 times higher than that at the inlet, and cyanobacteria became the dominant species accounting for 83% of the total cell density. Nutrient enrichment experiments showed that algae could grow rapidly within 3 days with average specific growth rates (μ) of 0.36-0.42 d^-1. The addition of N and P furtherly promoted the algal growth, and μ values of the treatments with P addition were the highest at 0.67-0.83 d^-1. These results indicated that even if the WRT was reduced to 3 days, the risk of the occurrence of algal blooms still exists, and this undesirable trend would be enhanced by the short-term external nutrient input. Our findings indicated that the hydrodynamic control measures may not be entirely successful in protecting the drinking water source from algal blooms, especially when its influent has already been under eutrophication.

Patterns of internal nitrogen and phosphorus loadings in a cascade reservoir with a large water level gradient: Effects of reservoir operation and water depth

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.

Annual water residence time effects on thermal structure: A potential lake restoration measure?

Innovative methods to combat internal loading issues in eutrophic lakes are urgently needed to speed recovery and restore systems within legislative deadlines. In stratifying lakes, internal phosphorus loading is particularly problematic during the summer stratified period when anoxia persists in the hypolimnion, promoting phosphorus release from the sediment. A novel method to inhibit stratification by reducing residence times is proposed as a way of controlling the length of the hypolimnetic anoxic period, thus reducing the loading of nutrients from the sediments into the water column. However, residence time effects on stratification length in natural lakes are not well understood. We used a systematic modelling approach to investigate the viability of changes to annual water residence time in affecting lake stratification and thermal dynamics in Elterwater, a small stratifying eutrophic lake in the northwest of England. We found that reducing annual water residence times shortened and weakened summer stratification. Based on finer-scale dynamics of lake heat fluxes and water column stability we propose seasonal or sub-seasonal management of water residence time is needed for the method to be most effective at reducing stratification as a means of controlling internal nutrient loading.