Cross-ecosystem nutrient subsidies in Arctic and alpine lakes: implications of global change for remote lakes

Glacially-fed lakes of West Greenland have elevated metal and nutrient concentrations and serve as regional repositories of these materials

The fate of heavy metals and nutrients melting out of the cryosphere into aquatic systems is not well understood. To address this, we measured heavy metals and nutrients in the water and sediment of four glacially-fed (GF) and four snow and groundwater-fed (SF) lakes near Kangerlussuaq, West Greenland during the summer of 2023. Average nutrient concentrations - total phosphorus (TP), NO3-, and NH4+ - in the water of GF lakes were 86 % higher and average total concentrations of some metals - Cd, Pb, Cr, Co, Ni, Al, Fe, Cu, Zn - were 137 % higher compared to nearby SF lakes. This pattern was also reflected in the sediment, where metal concentrations were generally higher in GF lakes compared to SF lakes. However, a few metals, including Hg, As, Cu, and Pb, were higher in SF compared to GF sediment. Our results suggest metals may be increasing over the past few years to decades in GF lakes, and certain metals have increased in SF lakes, notably Hg has substantially increased (298 %), as well as Pb (52.8 %), in SF lakes over the past century. The increase in Hg and Pb in SF, but not GF lakes, is likely due to the higher organic carbon and longer residence times of SF compared to GF lakes. In one GF lake, we quantified inputs and outputs of metals and nutrients, and we found that loads declined by an average of 71 % for metals and 68 % for nutrients from the lake inlet to outlet, suggesting the lake is a sink for these materials. SF lakes also appear to be reservoirs of some metals, specifically atmospherically deposited metals (Hg, Pb, As, and Cu). Our results highlight that GF lakes in West Greenland are elevated in nutrients and some metals compared to nearby SF lakes, indicating that the source of these materials is likely meltwater from the glacial system. We found that GF lakes can sequester a high percentage of the nutrients and metals flowing into them; however, as meltwater fluxes increase due to climate change, the ability of these lakes to remain sinks is an open question.

Water quality and geochemical facie of high-altitude lakes in Tawang, Eastern Himalaya, India

High-altitude lakes (HALs) can be used as a supplement or alternative source of water in areas where there is a water shortage. When these lakes are efficiently managed, they can supply more water resources to fulfil the increasing demand. Water quality assessment aids in the identification of adequate and safe drinking water sources. It minimizes threats to the public's health by making sure that lake water extraction fulfills safety and health regulations. Water quality and hydrogeochemical study was conducted on six HALs of the Tawang district of Arunachal Pradesh during the year 2022. The water quality index (WQI) values varied from excellent to poor (33.87 to 101.95). Lake 6 stands out with its exceptional water quality as it had the minimum average WQI value of 52.98. In contrast, Lake 5 had the lowest water quality among the studied lakes with the maximum average WQI value of 95.31. However, the water might not be safe to drink due to the elevated levels of fluoride in these lakes. It is crucial to address and minimize the high fluoride levels to ensure the safety and acceptability of the water for consumption. The Piper diagram showed that Ca2+  > Mg2+  > Na+  > K+ and HCO3-  > Cl-  > SO42-, respectively, were the primary cations and anions present in these lakes. The Gibbs diagram also demonstrated the effect of rock weathering and precipitation dominance on the water chemistry in the research area. These results provide insightful information about the water quality of HALs, which is essential information for concerned government departments and agencies to manage water issues more efficiently. Based on current research, the HALs in this region have a lot of potential to meet the growing demand for drinking water.

Dominance of net autotrophy in arid landscape low relief polar lakes, Nunavut, Canada

The Arctic is the fastest warming biome on the planet, and environmental changes are having striking effects on freshwater ecosystems that may impact the regional carbon cycle. The metabolic state of Arctic lakes is often considered net heterotrophic, due to an assumed supply of allochthonous organic matter that supports ecosystem respiration and carbon mineralization in excess of rates of primary production. However, lake metabolic patterns vary according to regional climatic characteristics, hydrological connectivity, organic matter sources and intrinsic lake properties, and the metabolism of most Arctic lakes is unknown. We sampled 35 waterbodies along a connectivity gradient from headwater to downstream lakes, on southern Victoria Island, Nunavut, in an area characterized by low precipitation, organic-poor soils, and high evaporation rates. We evaluated whether lakes were net autotrophic or heterotrophic during the open water period using an oxygen isotopic mass balance approach. Most of the waterbodies were autotrophic and sites of net organic matter production or close to metabolic equilibrium. Autotrophy was associated with higher benthic primary production, as compared to its pelagic counterpart, due to the high irradiance reaching the bottom and efficient internal carbon and nutrient cycling. Highly connected midstream and downstream lakes showed efficient organic matter cycling, as evidenced by the strong coupling between gross primary production (GPP) and ecosystem respiration, while decoupling was observed in some headwater lakes with significantly higher GPP. The shallow nature of lakes in the flat, arid region of southern Victoria Island supports net autotrophy in most lakes during the open water season. Ongoing climate changes that lengthen the ice-free irradiance period and increase rates of nutrient evapoconcentration may further promote net autotrophy, with uncertain long-term effects for lake functioning.

Landscape Controls on Nutrient Stoichiometry Regulate Lake Primary Production at the Margin of the Greenland Ice Sheet

Global change is reshaping the physical environment and altering nutrient dynamics across the Arctic. These changes can affect the structure and function of biological communities and influence important climate-related feedbacks (for example, carbon (C) sequestration) in biogeochemical processing hot spots such as lakes. To understand how these ecosystems will respond in the future, this study examined recent (< 10 y) and long-term (1000 y) shifts in autotrophic production across paraglacial environmental gradients in SW Greenland. Contemporary lake temperatures and light levels increased with distance from the ice sheet, along with dissolved organic C (DOC) concentrations and total nitrogen:total phosphorus (TN:TP) ratios. Diatom production measured as biogenic silica accumulation rates (BSiARs) and diatom contribution to microbial communities declined across these gradients, while total production estimated using C accumulation rates and δ13C increased, indicating that autochthonous production and C burial are controlled by microbial competition and competitive displacement across physiochemical gradients in the region. Diatom production was generally low across lakes prior to the 1800’s AD but has risen 1.5–3× above background levels starting between 1750 and 1880 AD. These increases predate contemporary regional warming by 115–250 years, and temperature stimulation of primary production was inconsistent with paleorecords for ~ 90% of the last millennium. Instead, primary production appeared to be more strongly related to N and P availability, which differs considerably across the region due to lake landscape position, glacial activity and degree of atmospheric nutrient deposition. These results suggest that biological responses to enhanced nutrient supply could serve as important negative feedbacks to global change.