This study provides a baseline analysis of sediment pollution in Kalmar Guest Harbor, Sweden, focusing on metals and microplastics. The study site, a bustling coastal area, was chosen to investigate the connections between anthropogenic activities, bioturbation, and environmental contaminants. The results revealed that the pollution extends beyond 30 cm of depth below the seafloor, with elevated levels of copper (Cu), tungsten (W), cadmium (Cd), zinc (Zn), lead (Pb), and microplastics. Significantly, there is no previous publication alarming the W contamination in the Baltic Sea sediment, and therefore, our findings highlight the need for further investigation into tungsten contamination in the region. Furthermore, we explored the distribution patterns, potential sources and relationships of different contaminants. The K-means cluster analysis revealed that bioturbation is speculated to influences the depth concentration of pollutants, particularly at shallow depths (0‬–18 cm). Metal contaminants do not appear to be predominantly bound to MP particles. 

Zooplankton are crucial for food webs and biogeochemical cycles. However, warming associated with climatechange may alter their seasonal timing and reproductive strategies. This study investigated how long-termwarming impacted zooplankton (mainly copepods) phenology and overwintering strategies by comparing a Bal-tic Sea bay, heated by warm water discharge for more than 50 yr, with an unaffected control bay. Field observa-tions showed that copepod and phytoplankton population growth began earlier in the heated bay than in thecontrol bay, suggesting that copepod abundance was driven by both temperature and food availability in theheated bay and by a stronger temperature dependence in the control bay. Resting eggs are normally producedas a life-history strategy to survive unfavorable environmental conditions. Our laboratory incubation experi-ment showed fewer dormant resting eggs hatched from the heated bay sediment compared with the controlbay, supporting an evolutionary change in overwintering strategy. In conclusion, the results seemed to suggestthat copepods adjusted their life-history in elevated temperatures by relying less on the strategy of usingsediment-stored dormant eggs and instead started their spring development earlier, when phytoplankton foodwas available. Hence, this study suggests that climate change can shift copepod overwintering strategies, leadingto potential cascading effects in the food web and affecting overall biodiversity and productivity.

The Baltic Sea, a brackish basin characterised by significant organic matter deposition, presents a crucial area for climate change research. This study examines the long-term evolution of methane geochemistry in sediments from four sites within the Gotland basins of the Baltic Sea, spanning the past 14,000 years since deglaciation. This timescale enables us to capture the full transition from lacustrine to marine conditions and link past organic matter accumulation with present-day methane dynamics. Using a transport-reaction model, we also explore future scenarios (2020−2100), aligned with climate projections, to assess how changes in bottom water temperature, organic matter loading, and freshwater input may influence methane production and emission from sediments. Our findings reveal that a 2 °C rise in bottom water temperature could increase free gas formation, though without directly impacting methane release into the sea. However, elevated organic matter loading significantly influences methane diffusion through the seafloor. Additionally, anticipated freshwater influx and subsequent reductions in sulphate concentrations will substantially enhance methane diffusion into seawater. The model projects that rising temperatures, eutrophication, and freshwater input will together drive increased methane emissions into the Baltic Sea, with potential consequences for climate change amplification.

Climate change driven ocean warming is a worldwide environmental issue that can impact cycling of greenhouse gases. However, how methane production in marine sediments as a potential contributor to atmospheric greenhouse gases versus its consumption at the sulfate–methane transition zone will be affected by climate change related warming is still not well constrained. In this study, sediments from two Baltic Sea bays with long-term temperature differences were collected during summer and winter. The primary difference between the two bays was that one had been heated by a nearby power plant for 50 years, resulting in a 5.1 °C increase in annual average temperature compared to an unheated control bay. The results showed that near-seafloor sediment methane concentrations were 50 times higher compared to present-day conditions. Furthermore, the sediment fluxes along with microbial community composition changes suggested that long-term warming may thin the sulfate reduction zone, such that methanotrophic archaea and sulfate reducing bacteria peaked at shallower sediment depths in the heated bay. Overall, the results from long-term warming in natural sediment environment indicated that future climate change warming may increase the risk of methane release to the water and eventually the atmosphere.

Deeply fractured rocks of meteorite impact craters are suggested as prime niches for subsurface microbial colonization. Methane can be a product of such microbial communities and seeps of methane from impact craters on Earth are of strong interest as they act as analogs for Mars. Previous studies report signs of ancient microbial methanogenesis in the Devonian Siljan meteorite impact structure in Sweden, but the proportion of microbial methane, metabolic pathways, and potential modern activity remain elusive. In this study, gas composition, hydrochemistry, oil organic geochemistry, and microbial community analyses are reported in 400 m deep fractures of the Siljan impact structure. The results showed a dominantly microbial origin for methane, which was supported by highly negative delta 13CCH4 and positive delta 13CCO2 values along with multiply substituted isotopologues (Delta 13CH3D) that indicated disequilibrium fractionation due to microbial kinetic isotope effects. The presence of C2 to C5 hydrocarbons suggested a minor thermogenic input in the gas mix. Characterization of the microbial community via 16S rRNA gene amplicon sequencing and real-time PCR indicated a low abundance of several methanogenic archaeal populations, which is common for settings with active methanogenesis. Evidence of oil biodegradation suggested that secondary microbial hydrocarbon utilization was involved in the methanogenesis. Low sulfate and high alkalinity in the groundwaters also suggested a dominantly microbial methane formation driven by infiltration of freshwater that was coupled to sulfate reduction and secondary utilization of early mature thermogenic hydrocarbons.

Gas migration and seafloor exudation are common phenomena in both deep and shallow water settings. However, the formation mechanisms and the relationships between different gas migration-related structures remain only partially understood. We constructed a reduced physical model of a submarine slope with two layers of varying permeabilities, subjected to a punctuated air injection to simulate gas migration and seafloor exudation. The air passage resulted in various structures, including mounds, pockmarks, chimneys, and different types of fractures (tensile, shear, irregular, radial, dendritic, and semicircular). Their processes, evolution, and interconnections were recorded and analyzed using image processing techniques. The results reveal the development of different stages of gas migration leading to seafloor exudation, from the initial fracturing stage to gas release into the water column, emphasizing the crucial role of impermeable layer thickness, the distribution of structures along the slope, and the impact of local topographic features. Our model provides robust insights into sediment deformation and the formation of structures associated with gas migration and exudation on the seafloor.

Climate change related warming is a serious environmental problem attributed to anthropogenic activities, causing ocean water temperatures to rise in the coastal marine ecosystem since the last century. This particularly affects benthic microbial communities, which are crucial for biogeochemical cycles. While bacterial communities have received considerable scientific attention, the benthic eukaryotic community response to climate change remains relatively overlooked. In this study, sediments were sampled from a heated (average 5°C increase over the whole year for over 50 years) and a control (contemporary conditions) Baltic Sea bay during four different seasons across a year. RNA transcript counts were then used to investigate eukaryotic community changes under long-term warming. The composition of active species in the heated and control bay sediment eukaryotic communities differed, which was mainly attributed to salinity and temperature. The family level RNA transcript alpha diversity in the heated bay was higher during May but lower in November, compared with the control bay, suggesting altered seasonal activity patterns and dynamics. In addition, structures of the active eukaryotic communities varied between the two bays during the same season. Hence, this study revealed that long-term warming can change seasonality in eukaryotic diversity patterns. Relative abundances and transcript expression comparisons between bays suggested that some taxa that now have lower mRNA transcripts numbers could be favored by future warming. Furthermore, long-term warming can lead to a more active metabolism in these communities throughout the year, such as higher transcript numbers associated with diatom energy production and protein synthesis in the heated bay during winter. In all, these data can help predict how future global warming will affect the ecology and metabolism of eukaryotic community in coastal sediments.

The world's oceans are challenged by climate change linked warming with typically highly populated coastal areas being particularly susceptible to these effects. Many studies of climate change on the marine environment use large, short-term temperature manipulations that neglect factors such as long-term adaptation and seasonal cycles. In this study, a Baltic Sea 'heated' bay influenced by thermal discharge since the 1970s from a nuclear reactor (in relation to an unaffected nearby 'control' bay) was used to investigate how elevated temperature impacts surface water microbial communities and activities. 16S rRNA gene amplicon based microbial diversity and population structure showed no difference in alpha diversity in surface water microbial communities, while the beta diversity showed a dissimilarity between the bays. Amplicon sequencing variant relative abundances between the bays showed statistically higher values for, e.g., Ilumatobacteraceae and Burkholderiaceae in the heated and control bays, respectively. RNA transcript-derived activities followed a similar pattern in alpha and beta diversity with no effect on Shannon's H diversity but a significant difference in the beta diversity between the bays. The RNA data further showed more elevated transcript counts assigned to stress related genes in the heated bay that included heat shock protein genes dnaKJ, the co-chaperonin groS, and the nucleotide exchange factor heat shock protein grpE. The RNA data also showed elevated oxidative phosphorylation transcripts in the heated (e.g., atpHG) compared to control (e.g., atpAEFB) bay. Furthermore, genes related to photosynthesis had generally higher transcript numbers in the control bay, such as photosystem I (psaAC) and II genes (psbABCEH). These increased stress gene responses in the heated bay will likely have additional cascading effects on marine carbon cycling and ecosystem services.

A recently acquired multidisciplinary dataset comprising acoustic surveys (high-resolution sub- bottom profiles, multi-beam bathymetry, and broad band mid-water echo sounder), geochemistry (gas chemical and isotopic composition, porewater chemistry), and sedimentology (core lithology and X-ray CT) in the area of the Landsort deep (450 m of depth), south of Stockholm Archipelago, revealed the existence of an extensive (20 km2) region of the seafloor where massive gas release is occurring in the form of multiple bubble streams. This new discovery represents a major seafloor methane release site in Europe and is comparable in area to other large sites worldwide such as the ones in Svalbard and in the South Atlantic Ocean associated with gas hydrate provinces. The gas is formed mostly by methane of microbial origin. Surprisingly, bubbles rise 100’s of meters above the seafloor and reach surface waters above the halocline/oxycline at around 80 m of depth. Some bubbles appear to reach the sea-air interface and their potential methane contribution to the atmosphere is under investigation. Another surprising observation is the absence of major seafloor features like pockmarks in the gas release area. The reasons for the seafloor methane release in the Landsort deep are still not entirely clear, but our preliminary acoustic and sedimentological data suggest that bottom currents may have acted to facilitate the accumulation of organic-rich sediments in a thick drift deposit during the Holocene and the modern warm period (latest 100 years). Our data further suggest that the high sedimentation rate in the drift deposit continuously supplies fresh organic matter that is quickly buried below a thin sulphate reduction zone, fueling vigorous methanogenesis and abundant methane formation. Similar methane release sites might be discovered in other known large drift deposits in the Baltic Sea.