Maija Heikkilä

Climate change is amplified in the Arctic, which is warming four times faster than the globe. Lakes are abundant across Arctic landscapes, integral to hydrological cycles and associated ecosystem functions and services, and act as sentinels of environmental change in the region. Across the terrestrial Arctic, widespread but spatially heterogeneous greening trends have been documented through remote sensing, linked to field-observed increases in vegetation growth, range expansion, and altered community composition. Concurrently, increases in terrestrial organic matter loading have been reported in some Arctic lakes, associated with browning, while other lakes are greening, linked to enhanced algal growth under shifting nutrient and thermal conditions.While the theoretical basis for recent catchment vegetation and lake-water quality shifts is clear, circum-Arctic evidence linking the two phenomena remains scarce. Here, we assess coupled greening and browning trends of terrestrial vegetation and aquatic indicators (total organic carbon, TOC; chlorophyll-a, ChlA) in ~100 circum-Arctic lake-catchment systems across Alaska, Canada, Greenland, Fennoscandia, and Russia. TOC and ChlA are reconstructed from sediment records using visible–near infrared spectroscopy (VNIRS)-based inference. Catchment vegetation change is quantified based on annual peak greenness and growing-season length, using spectral vegetation indices (NDVI, EVI2, and NIRv) from Landsat, AVHRR, and MODIS satellites over 1984–2025. Patterns in vegetation trends are described and analyzed using a custom land-cover reclassification, aboveground biomass, and vegetation height datasets. Our remote-sensing results indicate widespread greening of catchments since the 1980s, at heterogeneous rates across Arctic regions and vegetation zones. Early sediment-based reconstructions indicate TOC increases in numerous lakes over the same period; ChlA is generally increasing but not consistently coupled to TOC. The greening-browning relationship will be evaluated through multivariate association analyses, accounting for physiographic and bioclimatic setting (e.g., latitude, topography, temperature/precipitation, vegetation type, hydrological connectivity). Our presentation will summarize catchment vegetation and lake-water TOC and ChlA trajectories across the Arctic, and identify the conditions under which they are linked most strongly.

Diatoms are excellent environmental proxies due to their often species-specific, narrow environmental tolerances, but the annual succession of diatom communities in coastal seas remains deficiently known. This knowledge gap constrains our ability to reconstruct past seasonal changes in marine systems, as variation in seasonal conditions strongly impacts the composition of diatom species assemblages, and consequently assemblages preserved in the sediments. In particular species dynamics related to sea ice are still underexplored, as ice-cover restricts undisturbed access. Automated sequencing sediment traps offer an effective solution to overcome these challenges, even during the ice-cover period. In this PhD project, the seasonal succession of coastal diatom communities, as well as their contribution to vertical particulate organic matter fluxes and sedimentation are studied over multiple years in Tvärminne Storfjärden, Gulf of Finland. The aim is to enhance our understanding of the seasonal patterns of diatom species succession and sedimentation at a high temporal resolution, with a focus on understanding seasonal environmental drivers of species assemblage composition and the development of paleoenvironmental reconstruction methods. The data is collected with automated sequencing sediment traps, moored to the sea floor to continuously collect vertical material fluxes settling from the sea surface at a two-week temporal resolution. Two sediment traps are deployed at depths of 15 m and 27m (approximately 3 meters above the seabed) to assess how processes like decomposition and predation impact the vertical sediment flux. Diatom assemblages are analysed by microscopic identification and compared to simultaneous environmental measurements of e.g., sea-surface temperature, salinity and sea-ice cover to assess species-specific seasonal ecologies and deposition patterns. In addition, bulk organic geochemical analysis renders information about carbon flux and sources to the seafloor. Enhanced seasonal ecological information will improve diatom-based methods, enabling more accurate reconstructions of past and predictions of future coastal environments. Also, the advancement provides valuable insights into the impacts of ongoing environmental change and anthropogenic pressure on aquatic systems and, ultimately human well-being.

The Arctic is the fastest‐warming region globally. Lake ice is a sentinel indicator of Arctic change, with wide impacts on hydrological regimes, biodiversity, and ecosystem services. While small lakes are ubiquitous across northern boreal and tundra zones, ice observations remain biased toward large lakes with distinct freezing and melting dynamics. We present high‐resolution, field camera‐based ice phenology records of 10 small lakes (2–20 ha) in northwest Finnish Lapland spanning low (LE, ∼300 m) and high (HE, 770–1,010 m) elevations over two consecutive seasons. Ice‐on timing was uniform across elevations, occurring at cumulative degree‐day sums of −10°C·d on shallow LE lakes, and −30°C·d on deeper HE lakes. Thawing from groundwater upwelling was observed across many LE lakes. While the recorded melting process at LE occurred over several days during late May, at HE it extended over weeks into late June to early July, due to long‐lasting continuous snow cover, cold meltwater inflow, and increased sublimation. Our thermodynamic lake ice model accurately predicted total ice thickness ( R = 0.99, RMSE ≤ 5.8 cm), reaching ∼80 cm at LE and ∼100 cm at HE, but the snow‐ice fraction was underpredicted. Freezing and melting were strongly modulated by snow, highlighting the impact of future precipitation changes on ice thickness, quality, and ice‐off timing. The rapid and spatially uniform freezing suggests a direct response of small lake phenology to Arctic warming—unlike large lakes, where the summer heat storage, depth, and turbulent mixing are important modulators.

The Arctic is the fastest-warming region globally. Lake ice is a sentinel indicator of Arctic change, with wide impacts on hydrological regimes, biodiversity, and ecosystem services. While small lakes are ubiquitous across northern boreal and tundra zones, ice observations remain biased towards large lakes with distinct freezing and melting dynamics. We present high-resolution, field camera-based ice phenology records of 10 small lakes (2–20 ha) in northwest Finnish Lapland spanning low (LE, ~300 m) and high (HE, 770–1010 m) elevations over two consecutive seasons. Ice-on timing was uniform across elevations, occurring at cumulative degree-day sums of -10 °C·d on shallow LE lakes, and -30 °C·d on deeper HE lakes. Thawing from groundwater upwelling was observed across many LE lakes. While the recorded melting process at LE occurred over several days during late May, at HE it extended over weeks into late June to early July, due to long-lasting continuous snow cover, cold meltwater inflow, and increased sublimation. Our thermodynamic lake ice model accurately predicted total ice thickness (R = 0.99, RMSE ≤ 5.8 cm), reaching ~80 cm at LE and ~100 cm at HE, but the snow-ice fraction was underpredicted. Freezing and melting were strongly modulated by snow, highlighting the impact of future precipitation changes on ice thickness, quality, and ice-off timing. The rapid and spatially uniform freezing suggests a direct response of small lake phenology to Arctic warming - unlike large lakes, where the summer heat storage, depth, and turbulent mixing are important modulators.

Retreating sea ice and freshening surface waters are rapidly changing Arctic coastal environments. To predict the impacts of cryosphere change, it is essential to understand ecosystem variability beyond the instrumental era. Diatoms and dinoflagellate resting cysts have been abundantly used to reconstruct past marine conditions. Our understanding of microfossil species' ecologies is predominantly based on ecophysiological studies, the spatial distribution of species in surface sediments in relation to surface‐ocean conditions and sediment‐core studies. However, little is known about their seasonal ecologies and how biotic interactions impact species abundances. Here, we present sediment trap records of diatom and dinocyst succession over 4 years in a seasonally ice‐covered Northeast Greenland fjord. We found highly seasonal diatom blooms and dinocyst production, but no clear separation in the timing between sea‐ice indicating and open‐water species fluxes. The interannual variation was not linked with the timing of sea‐ice breakup. Instead, we linked higher production of heterotrophic dinocysts with high river discharge, while the highest diatom fluxes were often recorded during relatively low river discharge, suggesting that land‐derived meltwaters controlled the blooms through stratification and light limitation. Thus, while we were not able to detect detailed species‐environment dynamics due to the resolution of the trap data, our results indicate that the limitation of light and nutrients due to river discharge may override the impacts of sea‐surface temperature and sea‐ice cycle. We highlight that land‐derived inputs should be considered alongside other parameters of sea‐surface conditions when using diatoms and dinocysts as paleoecological proxies in Arctic fjord settings.

Decreasing sea-ice extent and retreating and thinning of Greenland’s glaciers are rapidly changing Arctic coastal environments by warming and freshening the sea surface and impacting light availability. In Arctic fjords, productivity is significantly influenced by the position of glacier termini, and the present retreat of the Greenland Ice Sheet will increase the number of fjords surrounded only by land-terminating glaciers in the future. This will most likely affect the productivity and ecosystem structure of coastal marine areas. To predict future cryosphere change and its impacts, it is essential to understand climate and ecosystem variability beyond the instrumental era. Here we present a high-resolution reconstruction of coastal marine ecosystem change and its linkages to terrestrial freshwater and organic matter inputs in Young Sound fjord, Northeast Greenland, over the Holocene. The reconstructions are based on marine sediment-core proxies: organic-walled palynomorphs (including e.g. dinoflagellate cysts and pollen), sympagic and pelagic biomarkers (highly branched isoprenoids and sterols) and a set of geochemical indicators (sediment organic carbon, nitrogen, their stable isotopes, and biogenic silica). The results suggest a relatively cold early Holocene with extensive sea-ice cover and low productivity. Warmer and more variable conditions take hold after approximately 9 kyr with increasing productivity, species richness and terrestrial freshwater inputs, with colder conditions seen after approximately 3.5 kyr with high productivity coupled with higher ice-algae contribution. The results also indicate that this near-shore marine ecosystem is clearly influenced by local forcings, such as terrestrial freshwater and organic matter inputs, suggesting that the continuous melting of the Greenland Ice Sheet will affect marine productivity and ecosystem structure in Greenland’s fjord systems, with potential impacts on biodiversity and sustainability of fisheries.

The Holocene epoch offers insights into past climate variability and associated environmental changes in the Arctic region, with implications for future scenarios. We present a multi-proxy study of Nuup Kangerlua (the fjord by Nuuk), southwest Greenland, covering the past ~10,500 years. Using sediment cores and a one-year sediment trap deployment, we reconstruct environmental changes, focusing on ice-sheet dynamics, oceanography, and productivity. Following the fjord deglaciation (~10-8 ka BP), the fjord was characterised by cold, low-productivity conditions with significant ice-rafted debris, transitioning to warmer, more productive conditions by ~7.5 ka BP. The mid-to-late Holocene (6.5–3 ka BP) experienced an exceptional oceanographic regime with indication of entrainment of Subpolar Mode Water (Atlantic origin) at the time of minimum extent of the Greenland Ice Sheet. At this time, dinoflagellate cyst assemblages reveal shifts from heterotrophic dominance to autotrophic taxa, signaling increased light availability and stratification. Biogeochemical proxies (TOC, δ13C, and biogenic silica) corroborate heightened productivity during this period. Contrasts with modern conditions suggest that sustained warming could alter fjord hydrography, potentially enhancing Atlantic-derived inflows. Our study provides new knowledge on the fjord's sensitivity to climate variability and it offers baselines for understanding the interplay between the Greenland ice sheet, fjord systems, and broader oceanographic processes under changing climatic conditions.

Sea ice is crucial in regulating the heat balance between the ocean and atmosphere and quintessential for supporting the prevailing Arctic food web. Due to limited and often local data availability back in time, the sensitivity of sea-ice proxies to long-term climate changes is not well constrained, which renders any comparison with palaeoclimate model simulations difficult. Here we compiled a set of marine sea-ice proxy records with a relatively high temporal resolution of at least 100 years, covering the Common Era (past 2k years) in the Greenland–North Atlantic sector of the Arctic to explore the presence of coherent long-term trends and common low-frequency variability, and we compared those data with transient climate model simulations. We used cluster analysis and empirical orthogonal functions to extract leading modes of sea-ice variability, which efficiently filtered out local variations and improved comparison between proxy records and model simulations. We find that a compilation of multiple proxy-based sea-ice reconstructions accurately reflects general long-term changes in sea-ice history, consistent with simulations from two transient climate models. Although sea-ice proxies have varying mechanistic relationships to sea-ice cover, typically differing in habitat or seasonal representation, the long-term trend recorded by proxy-based reconstructions showed a good agreement with summer minimum sea-ice area from the model simulations. The short-term variability was not as coherent between proxy-based reconstructions and model simulations. The leading mode of simulated sea ice associated with the multidecadal to centennial timescale presented a relatively low explained variance and might be explained by changes in solar radiation and/or inflow of warm Atlantic waters to the Arctic Ocean. Short variations in proxy-based reconstructions, however, are mainly associated with local factors and the ecological nature of the proxies. Therefore, a regional or large-scale view of sea-ice trends necessitates multiple spatially spread sea-ice proxy-based reconstructions, avoiding confusion between long-term regional trends and short-term local variability. Local-scale sea-ice studies, in turn, benefit from reconstructions from well-understood individual research sites.