Human actions were responsible for both initiation and termination of varve preservation in Lake Vesijärvi, southern Finland

The influence of lake restoration efforts on lake bottom-water conditions and varve preservation is not well known. We studied varved sediments deposited during the last 80 years along a water-depth transect in the Enonsaari Deep, a deep-water area of the southernmost Enonselkä Basin, Lake Vesijärvi, southern Finland. For the last few decades, the Enonselkä Basin has been subject to ongoing restoration efforts. Varve, elemental, and diatom analyses were undertaken to explore how these actions and other human activities affected varve preservation in the Enonsaari Deep. In contrast to most varved Finnish lakes, whose water columns have a natural tendency to stratify, and possess varve records that span thousands of years, varve formation and preservation in Lake Vesijärvi was triggered by relatively recent anthropogenic stressors. The multi-core varve analysis revealed that sediment in the Enonsaari Deep was initially non-varved, but became fully varved in the late 1930s, a time of increasing anthropogenic influence on the lake. The largest spatial extent of varves occurred in the 1970s, which was followed by a period of less distinguishable varves, which coincided with diversion of sewage from the lake. Varve preservation weakened during subsequent decades and was terminated completely by lake aeration in the 2010s. Despite improvements in water quality, hypolimnetic oxygen depletion and varve preservation persisted beyond the reduction in sewage loading, initial aeration, and biomanipulation. These restoration efforts, however, along with other human actions such as harbor construction and dredging, did influence varve characteristics. Varves were also influenced by diatom responses to anthropogenic forcing, because diatoms form a substantial part of the varve structure. Of all the restoration efforts, a second episode of aeration seems to have had the single most dramatic impact on profundal conditions in the basin, resulting in replacement of a sediment accumulation zone by a transport or erosional zone in the Enonsaari Deep. We conclude that human activities in a lake and its catchment can alter lake hypolimnetic conditions, leading to shifts in lake bottom dynamics and changes in varve preservation.

Projecting thermal stratification and hypolimnetic oxygen conditions by coupling paleolimnological and 1D lake model approaches

<p>Climate change raises many questions about the future of lakes’ thermal regime and hypolimnetic oxygen conditions. One dimensional models have been widely implemented over that last years <sup>1–3</sup>, but most of these models are calibrated against very few years of limnological records, potentially limiting the robustness in long-term reconstructions and preventing inclusion of future scenarios. To analysis the variability and the effects of climate change on thermal regime and oxygen conditions of deep hard-water lakes, we relayed on paleolimnological records and 1D thermal lake model calibrated against time series of limnological data collected by the French Observatoire des LAcs (OLA). Continuous sediment records on four peri-alpine lakes (Lake Geneva, Lake Annecy, Lake Bourget and Lake Aiguebelette) were analysed using micro-XRF Mn-Fe ratio as proxy to infer near-annual trends of oxygen conditions for the past 300 years<sup>4</sup>. Past hypoxia dynamics were further inferred from varved records preserved in sediment cores<sup>5</sup>. General Lake Model (GLM), i.e. a 1-D modelling tool, has been constrained by climate data derived from meteorological observations and CMIP6 simulations in order to reconstruct and forcast stratification regims for the next century. Our paleolimnological results show that fluctuations in hypoxic volumes since the 1950s were great and that these fluctuations were essentially driven by climatic factors, legitimating the use of thermal model approaches for future projections of hypolimnetic oxygen conditions. In this line, thermal regime simulations based on GLM forecast an intensification in thermal stratification and an increase in volumes of water warmer than 9°C over the period 1850-2100 with potential consequence for hypolimnetic oxygen conditions and ecological habitats. Coupling model and paleolimnological approaches seem a promising way to examine the evolution of lakes in the past, and to realistically anticipate the future of lakes for the next decades.</p>

Effects of phosphorus control on primary productivity and deep-water oxygenation: insights from Lake Lugano (Switzerland and Italy)

Attempts to restore Lake Lugano, Switzerland and Italy, from eutrophication have produced weak responses in the target variables (primary productivity and hypolimnetic oxygen concentrations), indicating shortcomings in the underlying eutrophication model. An analysis of monitoring data showed that the decrease in phosphorus concentration, although nearly compliant with restoration targets, produced only slight decreases in primary productivity and no change in hypolimnetic oxygen conditions. These target variables were equally or more sensitive to factors external to trophic state, including plankton structure, which influenced primary productivity, and the depth of mixing during turnovers, which influenced hypolimnetic oxygen. To improve the chance of success, the restoration approach should revise the phosphorus concentration target and explicitly account for the influence of external variation, especially mixing depth.

Dynamic carbon-oxygen interactions over minute to annual time scales in an experimentally-oxygenated reservoir

<p>Oxygen dynamics in lakes and reservoirs are changing worldwide due to human activities. Changing hypolimnetic oxygen conditions will substantially alter carbon cycling in aquatic ecosystems, as oxygen dynamics near the sediment-water interface regulate whether carbon inputs will be buried, respired as carbon dioxide, or respired as methane. At the decadal scale, warming temperatures and increased nutrient loads are increasing the prevalence and duration of anoxia. Conversely, at the daily scale, mixing due to more powerful storms may periodically increase hypolimnetic oxygen availability. It remains unclear, however, how carbon quantity and quality will respond to these changes in oxygen at different time scales. Our team used unprecedented whole-ecosystem manipulations of hypolimnetic oxygen concentrations in a eutrophic reservoir to identify how changes in oxygen at different time scales (i.e., weeks to months) alter freshwater carbon processing, burial, and greenhouse gas emissions. Against the backdrop of multiple-week shifts between oxic and anoxic conditions in the bottom waters of the experimental reservoir over multiple years, we observed that the dominant scale of variability in dissolved organic matter (DOM) concentrations was predominantly at the daily scale in the summer and monthly scale in the winter. At the monthly time scale, dissolved oxygen concentrations controlled DOM; at the daily time scale, water temperature and photooxidation controlled DOM. Modeling and field results show that intermittent week-long oxic conditions mineralized “legacy” carbon that had accumulated over years of sedimentation and changed the dominant terminal electron acceptor pathways used for mineralization on the daily scale. Building off of this work, future oxygenation experiments will examine the role of alternate electron acceptors in carbon release from sediments on the daily scale, the impact of carbon quality on carbon processing under varying oxygen conditions at the daily to week scale, and the effects of future oxygen scenarios on carbon cycling in lakes and reservoirs around the world on the annual to decadal scale.</p><p><br><br></p>

Long-term trends in hypolimnetic volumes and dissolved oxygen concentrations in Boreal Shield lakes of south-central Ontario, Canada

Temperature–oxygen profiles, collected biweekly to monthly for ∼40 years, were used to calculate end-of-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in six small lakes located in south-central Ontario, Canada. Coherent decreases in thermocline depth and increases in hypolimnetic volume, mean hypolimnetic dissolved oxygen (DO) concentration, and VWHO were observed in five of the six study lakes. All lakes underwent an abrupt increase in VWHO and mean hypolimnetic DO after 2010. In four of the six study lakes, the highest hypolimnetic DO concentrations were observed in years where chlorophyll a concentrations were low, whereas at five of the six study lakes the highest hypolimnetic volumes were observed when dissolved organic carbon concentrations were relatively high. Warmer spring or winter air temperatures were associated with higher hypolimnetic DO concentrations at two sites, and longer ice-free periods were associated with smaller hypolimnetic volumes at two sites. These results suggest that the recent VWHO increases in the studied south-central Ontario lakes may be a function of multiple drivers that include changes in primary production, lake water transparency, and regional climatic factors.