By 2100AD, it is predicted that approximately 16% of lakes worldwide will experience less frequent mixing and become permanently stratified (meromictic) as a result of climate change. Within the Arctic and subarctic, it is anticipated that increases in global air temperature will be magnified leading to strong feedback effects on the climate system, drastic changes in ecologically sensitive aquatic systems and increasing carbon emissions from lakes. Finland contains a dense network of lakes of differing mixing regimes, including meromictic lakes, enabling the opportunity to understand their response to warming temperatures. Hydroclimatic reconstructions have been undertaken, however they have not been focused on mixing regime changes and they have not been combined with modelling efforts to fully understand changes across past, present, and future timescales. Here, we present preliminary results from reconstructions of lake mixing regimes from sediment cores and initial modelling results from five lakes in the Evo National Park, southern Finland. We take a multi-proxy approach to reconstruct lake mixing regimes, including analysis of geochemistry (micro-XRF), chironomids, bacterial pigments, radiocarbon dating and varve counting. Ultimately understanding how lakes have responded to past changes in climate will enable baselines to be established against which to assess any future changes and will also enable models projecting future mixing regime changes to have appropriate boundary conditions set. The General Lake Model, forced with field data provided by Lammi Biological Research Station and from two fieldwork seasons in 2023, is used to understand contemporary and predict future mixing regimes. This research provides a full chronology of lake mixing regimes from the mid Holocene until 2100 linking climatic events with mixing regime changes through combining palaeoenvironmental reconstructions and modelling efforts.

Rapidly rising temperatures in the Arctic cause thaw of permafrost and increase coastal erosion that mobilizes permafrost-derived organic carbon (OC) into coastal waters. In the water column, permafrost-OC may either degrade and thus, enhance climate warming by adding greenhouse gases to the atmosphere or settle on the seabed and be buried in the sediments. In this study, we focused on the composition and degradation of particulate OC (POC) within the flocculation (nepheloid) layer - a turbulent layer close to seabed that holds a high amount of suspended sediments/particles and transports them across the vast Siberian Arctic shelves. More importantly, previous studies have shown that permafrost-OC, exported to the Arctic Ocean via coastal erosion, is largely carried in the POC fraction of the flocculation layer. To study flocculation layer dynamics, sediment cores were collected using a multicorer device from the East Siberian Sea, Laptev Sea, and Kara Sea onboard R/V Akademik Mstistlav Keldysh in 2020. The overlying water of the sediment cores was stirred under controlled conditions to mimic sediment resuspension. The entrained suspended sediments were collected and incubated for two weeks (in the dark) to assess their susceptibility to degradation. During the incubation, dissolved O2, POC, dissolved OC (DOC), dissolved inorganic carbon and δ13C of each carbon pool were measured at set time points. Additionally, to better understand sediment entrainment and degradation, sediment physical properties, including grain size and mineral-specific surface area, and macromolecular composition were determined. Our preliminary results show that stirring largely entrains the smallest sediment particles, while it seems not to influence sediment macromolecular composition suggesting that none of the compound classes such as polysaccharides or aromatic compounds are preferentially entrained. Our incubation data show losses in dissolved O2 suggesting microbial degradation, however, instead of decreases in the OC pools, especially POC shows increases combined with increases or decreases in DOC. These carbon dynamics likely result from interactions between different carbon pools such as adsorption of DOC to particles and/or leaching of POC to the DOC pool. With accelerated coastal erosion and increase in storminess in the Arctic Ocean due to sea ice loss, understanding dynamics of the flocculation layer and degradation of permafrost-OC on the Arctic sea shelves is becoming even more important to better constrain their potential climate impact.

Arctic rivers deliver ≈11% of global river discharge into the Arctic Ocean, while this ocean represents only ≈1% of the global ocean volume. Ongoing climate warming across the Arctic, and specifically Siberia, has led to regional-scale changes in precipitation patterns, greater rates of permafrost thaw and active layer deepening, as well as enhanced riverbank and coastal erosion. Combined, these climatic and cryospheric perturbations have already resulted in increased freshwater discharge and changes to constituent loads (e.g. dissolved organic carbon - OC) supplied from land to the Arctic Ocean.To date, the majority of studies examining terrestrial organic matter (OM) delivery to the Arctic Ocean have focused almost entirely on freshwater (riverine) or fully-marine environments and been conducted during late summer seasons – often due to logistical constraints. Despite this, an improved understanding of how OC is transformed, mineralised and released during transit through the highly reactive nearshore estuarine environment is critical for examining the fate and influence of terrestrial OM on the Arctic Ocean. Capturing seasonality over the open water period is also necessary to identify current OM fluxes to the ocean vs the atmosphere, and aid in constraining how future changes may modify them.Here we focus upon carbon dioxide (CO2) and methane (CH4) measurements collected during six repeated transects of the Kolyma River and nearshore zone (covering ~120 km) from 2019. Transects spanned almost the entirety of the riverine open water season (June to September). We use these results, in parallel with gas concentrations derived from prior studies, to develop and validate a simple box-model of gas emissions from the nearshore zone.Observations and model‐derived output data reveal that more than 50% of the cumulative gross delivery of CH4 and CO2 to the coastal ocean occurred during the freshet period with dissolved CH4 concentrations in surface water reaching 660 Nanomole per liter [nmol/l]. These results demonstrate the relevance of seasonal dynamics and its spatial variability which are needed in order to estimate greenhouse gas fluxes on an annual basis.More accurate understanding of land-ocean carbon fluxes in the Arctic is therefore crucial to mitigate the effects of climate change and to support the decisions of policy makers.

Soil temperatures in permafrost (i.e. perennially frozen ground) are rising globally. The increasing temperatures accelerate permafrost thaw and release of organic carbon, that has been locked in permafrost soils since the last glacial period, to the contemporary carbon cycle. The potential remineralisation of organic carbon to greenhouse gases can contribute to further climate warming. Particulate organic carbon (POC) in the Kolyma River is older than dissolved organic carbon (DOC) thus serves as a good tracer for abrupt permafrost thaw (i.e. river bank erosion and thermokarst) that dominantly releases old POC. While dissolved organic carbon (DOC) mobilised from the old Yedoma outcrops on the banks of the Kolyma River is shown to be highly labile, vulnerability of POC to biodegradation is not yet known. In this study we aim to constrain degradation rates for POC in the Kolyma River. To capture seasonal variability of the POC pool and its degradation rate the incubation was conducted both during the spring freshet and in late summer (2019 and 2018, respectively). We incubated whole-water samples over 9 to 15 days and quantified POC (and DOC) loss over time, as well as dissolved inorganic carbon (DIC). The incubation was carried out in the dark. We also tracked changes in POC composition and age with carbon isotopes (d13C-OC, d13C-DIC, ∆14C). Preliminary results from 2018 suggest a decrease in POC concentrations of up to 30 % while those of DOC decrease by up to 11 %. The rate of POC degradation is nearly three times faster than DOC though the absolute amounts of DOC are in turn higher than those of POC (< 1 mg L-1 for POC and ~3 mg L-1 for DOC). Furthermore, the changes in d13C of POC, DOC and DIC suggest ongoing microbial degradation and conversion of organic carbon into inorganic carbon. These first estimates show that POC degrades fairly rapidly while transported in the Kolyma River. A better understanding of POC degradation along lateral flow paths is critical for improving our knowledge of permafrost thaw and its possible climate impacts in the future.

The Arctic ocean receives 11% of the global river discharge and the Arctic rivers drain large permafrost rich catchments. Where these rivers outflow into the marginal shelf seas of the Arctic ocean the terrestrial dissolved organic matter (tDOM) which they transport has an important role to play in the coastal ecosystem. This tDom is derived from inland permafrost and as it thaws under future climate scenarios there are expected to be changes to both the composition and quantity of riverine tDOM. At the same time there will be changes to the seasonality and magnitude of river discharge, due to increased precipitation and earlier snow melt, and to the light availability, due to reduced seasonal sea ice. To understand the possible impact of these changes on the coastal ecosystem it is important to understand the present role of permafrost derived tDOM and the possible changes to the nearshore circulation. We model the hydrodynamics of the extensive shallow shelf of the Laptev sea, into which drains the Lena river – the 13th largest in the world by discharge. The output from the hydrodynamic model is used to drive the ecosystem model ERSEM which has been adapted to explicitly include a permafrost tDOM input. This coupled model system allows us to investigate both the role of present day tDOM in an Arctic coastal ecosystem and to hypothesise on the impact of increases in future. In particular we attempt to quantify the efficacy of the microbial carbon pump under different tDOM inputs.

Freshwater ecosystems are known to substantially contribute to the methane budget of arctic and subarctic regions, however small lakes and ponds have generally been overlooked due to difficulties identifying them with satellite imagery because of their small areal extent. Recent pan-arctic and global syntheses show an inverse relationship between dissolved methane and lake surface area, suggesting small ponds and lakes may contribute disproportionately to Arctic methane emissions. The work presented here investigates the relationship between lake surface area and methane emissions, in order to better constrain freshwater methane emissions for the Yukon-Kuskokwim Delta, Alaska. We measured methane flux from 13 lakes and ponds of a range of size classes using floating chambers deployed on each lake for 5-24 h. Additionally, spatial and temporal variations in methane emissions were examined …