Antarctic krill, Euphausia superba, is an important species in the Southern Ocean and is vulnerable to climate change. We studied the carbon sources for krill-specifically pelagic and ice-associated (sympagic) particulate organic matter (POM), representing phytoplankton and sea-ice algae, respectively-during March 2019 across three distinct areas of the Kong H & aring;kon VII Sea (KHS) in the eastern Weddell Gyre. Analyses of delta 13C and delta 15N composition revealed large geographic differences in pelagic POM that were reflected in krill across life stages. At Astrid Ridge (the eastern study area), elevated pelagic POM delta 13C suggested that sea-ice algae seeding influenced phytoplankton development, in line with the more extensive sea-ice cover in this area and with the dominance of typically ice-associated pennate diatoms. This was also reflected in the elevated krill delta 13C compositions at Astrid Ridge as compared to the other two areas. In contrast, in the western areas (Maud Rise and 6 degrees East), lower delta 13C values in both pelagic POM and krill suggested limited influence of sea-ice algae on phytoplankton communities and krill feeding. Moreover, no significant difference in the delta 13C and delta 15N compositions was found among krill genders and life stages throughout the study area. These geographical, rather than ontogenetic, differences suggest that krill of all life stages feed opportunistically on the most available algal sources in early autumn in the KHS. Considering the ongoing reduction in the seasonal sea-ice cover, continued research on the dietary plasticity of krill is essential to understand the resilience and health of krill stocks in a changing Southern Ocean.

Although sea-ice ridges are prominent features of the Arctic Ocean, very little is known about their role as habitats and in biogeochemical cycles. Here, we show that ridges provide complex sea-ice habitats which host unique and diverse biological communities. Seasonally, ridges appear to transition from a biological repository in winter to biological hotspots in summer, surpassing algal biomass in level ice and surface waters by up to eight-fold. In summer, ridges can contain up to 80% of the total area integrated sea-ice algal biomass, emphasizing their importance in the Arctic sea-ice ecosystem. However, environmental shifts, such as meltwater infiltration and freezing inside the ridge in late summer, alter microbial communities from being predominantly autotrophic to heterotrophic. Our work provides evidence of contrasting roles of sea-ice ridges for Arctic carbon cycling in summer and shows that the habitats in the ridge interior harbor unique microbial communities, adding complexity to Arctic biodiversity.

Abstract There is no light for photosynthesis by phytoplankton and sea-ice algae during the polar night, but microbial grazers remain active through the dark winter months in the Arctic Ocean. Where the energy to sustain these organisms comes from is unknown. Here we observed active tintinnid ciliates during the polar night, heterotrophic protists known to feed on phytoplankton and smaller heterotrophic protists. Our calculations indicate that the pelagic microbial loop transferring energy from bacterial production through microbial grazers was not sufficient to sustain the observed tintinnid biomass. However, the sea ice contained frozen-in particulate organic carbon produced during the previous growth seasons. We show that enough food particles can be released by mechanical break-up during sea-ice ridging, which together with bacterial production, sustained the observed tintinnid biomass in the water column. This is an important but overlooked mechanism for winter survival of plankton in the Arctic Ocean.

The projected transition of the central Arctic Ocean (CAO) into a warmer, seasonally ice-free ocean requires more knowledge of this environment to predict changes in the structure and dynamics of its ecosystems. We aimed to compare the state and underlying processes of Nansen Basin and Amundsen Basin ecosystems observed in August–September 2021 and assess impacts of Atlantic Water inflow and fresher Transpolar Drift waters, respectively, on these ecosystems. The basins differed in features of sea ice, hydrography, and chemical and biological compositions. The near-slope open water in western Nansen Basin showed a clear fingerprint of warm, saline Atlantic Water, with larger vertical turbulent fluxes facilitating nutrient transport across the pycnocline and supporting larger standing stocks of bacteria, protists, and zooplankton. Pelagic primary production and microbial and faunal stocks decreased northward and into Amundsen Basin, likely due to lower nutrient concentrations, stronger stratification, and reduced light through the more continuous and thicker ice and snow cover in Amundsen Basin, possibly also impacted by seasonally declining light levels. Transpolar Drift signals included lower salinity, stronger stratification, and higher silicate concentrations in Amundsen Basin surface waters. Similarities to earlier observations included the increase in small-sized algae from Nansen Basin into Amundsen Basin and overall low faunal abundances in the CAO, suggesting that overarching patterns remained unchanged over past decades. Examples of species range extensions and notable taxon absences relative to earlier studies, however, could be due to borealization and changes in sea-ice conditions, respectively. Higher density ecosystem sampling and consistent time series are recommended to confirm such conclusions. The distinct basin differences call for a regional approach to future management of the CAO. We especially caution against using the area of strong Atlantic Water inflow in southern Nansen Basin as representative of the entire basin, let alone Amundsen Basin or the CAO.

In this study, we document layers of incorporated platelet ice up to 0.81 m thick in broken-out landfast ice for the first time in the Kong Hakon VII Hav, Southern Ocean. Sampling occurred in January 2022, when sea ice was highly porous (average brine volume > 10%) and actively melting. Chlorophyll a (chl a) concentrations were significantly higher (Mann-Whitney U-test, U = 154, p = 0.072) in bottom-incorporated platelet ice layers in comparison to neighboring layers of other textures. The maximum chl a concentration (191.80 mu g l(-1)) was observed in bottom-incorporated platelet ice and dominated by the pennate diatom Berkeleya adeliensis. Due to biological uptake, mean (+/- SD) bulk concentrations of nitrate (0.48 +/- 0.26 mu mol l(-1)) and phosphate (0.15 +/- 0.2 mu mol l(-1)) were low for summertime sea ice in the Southern Ocean, but biological enrichment of nitrite was observed throughout the ice column. Sea-ice texture was likely a relevant driver of the biological and biogeochemical properties observed in this study due to its impact on habitable pore space and brine-scale nutrient dynamics; however, specifically differentiating between effects due to depth versus those due to texture was beyond the scope of this study. The ice floes sampled here were some of the last to melt in the summer, suggesting that bottom-incorporated platelet ice layers may serve as a final refugium for sea-ice algae before the seasonal minimum in sea-ice extent. This refugium effect could become increasingly important under future warming scenarios, in which summertime conditions are expected to occur earlier and/or persist longer.

Present study is the first quantitative and coherent presentation of the submerged marine vegetation along the Greenland east coast, from 65.5°N to 76.8°N, based on data obtained from 286 underwater video transects. Based on cluster analysis, four different marine submerged vegetation community figurations were identified: a southern and deeper kelp forest including Laminaria solidungula and Agarum clathratum , the marine vegetation along the Blosseville coast, seaweed meadows characterized by, e.g., submerged Fucus distichus , and high‐arctic kelp forest. The habitat figurations were related to and potentially explained by drivers considered to be key for their spatial distribution. The drivers considered were latitude as a proxy for light conditions with stronger seasonality and receding light conditions toward the north, suitable substratum for the marine vegetation to establish and grow, and the sea ice conditions with respect to light attenuation and scouring. Two of the vegetation types were explained by latitude, whereas the two vegetation types identified for the mid segment of the surveyed coastline were considered to be more correlated to local/regional conditions such as the presence of dynamic sea ice and glaciers as well as smaller‐sized hard substratum. Some degree of marine vegetation/kelp forest pauperization was observed with increasing latitude, expressed as a decrease in coverage and depth distribution. The vegetation belt was declining from a depth of 34 to 18 m within the northward latitudinal gradient surveyed, although for some species, no change in species‐specific maximal depth limits could be observed.

The Gulf of Gdańsk belongs to the best-known marine areas in the Baltic, with regular environmental observations since mid-20th century. It covers the widest array of marine habitats in Polish Maritime areas (from large river mouth to Gdańsk Deep), shallow vegetated lagoon and stony reefs and the highest resources of species diversity (about 400 Metazoa and over 300 Protista species). The area was also important as a fishing ground as well as a key site for the marine industry, shipping and tourism. The review of the changes in the Gulf of Gdańsk over last 40 years shows that it follows some of the global trends (increase in temperature, storminess, sea level rise, decrease in ice and oxygen), while the specific local phenomena like eutrophication and contamination are more difficult to assess (e.g., strong reduction in nutrient discharge did not change the levels of P and N in the system). After recovery from the environmental crisis in the 1980s, the toxic compounds in sediment and seawater are below the accepted thresholds. The reduction in some toxins resulting from better management (e.g., Mercury or chlorinated compounds) is blurred by the negative effects of climate warming (expansion of anoxic sediments) and contamination connected with its biogeochemical activity. Formerly degraded coastal habitats are recovering (especially seagrass), while the commercial fish catch collapsed, likely caused by the large-scale phenomena (climate warming), not directly connected with the local conditions of the Gulf. The societal use of the Gulf changed from industrial/fishery to largescale tourism and service, with fast growing pressure for coastline urbanization. The key phenomena (drivers of events) of the area include eutrophication, industrialization and biodiversity recovery.

Sea ice microalgae are an important source of energy for the polar marine food web, representing the primary carbon source prior to pelagic phytoplankton blooms. Here we investigate community dynamics of sea ice microalgal communities in land-fast sea ice across six different fjords in high-Arctic Svalbard, Norway, during Spring (April – May). We found that light (0.1 – 23% incoming PAR / 0.1 – 193 μmol photons m-2s-1) played a central role in determining community composition, with more diverse assemblages observed in sites with more light transmitted to the bottom ice community. In April, microalgal assemblages were similar when under-ice light transmittance was similar, independent of geographical location, however this light-derived separation of community structure was not evident in May. At all sites, assemblages were dominated by pennate diatoms, with the most abundant taxon being Nitzschia frigida. However, with increasing under-ice light transmittance, we saw an increase in the relative abundance of Dinophyceae, Navicula spp. and Thalassiosira spp.. A positive relationship between light and δ13C enrichment and C:N ratios in the ice algal biomass demonstrated the effect of light on the biochemical composition of ice algae. Light did not correlate with cell abundance or chlorophyll a concentration. With anticipated changes to Arctic sea ice extent and snow cover as a result of climate change, we will see shifts in the light transmitted to the bottom ice community. These shifts, whether caused by reduced light transmittance from increased snow cover or increased light transmittance from thinning ice, snow depth or increased rainfall, will likely alter sea ice microalgal community composition, which in turn, may influence the success of secondary production and biogeochemical cycling in polar waters.

This comprehensive study examines primary production (PP) within the Spitsbergen fjords, Hornsund, and Kongsfjord, over a 25-year period (1994-2019), across 45 stations and 348 incubation levels at various depths. PP and hydrological parameters were measured at 28 sampling stations in Kongsfjorden and 17 in Hornsund, with the locations of "Glacier," "Inner," and "Outer" zones defined to reflect the varying influence of glacial meltwater. Our study revealed spatial and temporal variability in PP, both at the surface and within the water column with very high depth resolution. The highest PP values were observed in the Glacier and Inner zones of Hornsund, particularly in the water layer up to 3 m depth, exceeding 20 mgC m(-3) h(-1). A notable decline in PP with increasing depth was observed in both fjords, with the Glacier zones displaying the highest productivity at the surface. The study also highlights the influence of glacial meltwater on surface water conditions, affecting the PP in the upper layers of both fjords. The observed gradient in the depth of maximum PP toward the mouth of the fjord varied between the two fjords, with Kongsjord displaying more dynamic variations. The spatial distribution of integrated primary production (Pi) suggested lower productivity in the glacial regions, likely due to light limitation caused by high concentrations of mineral particulate matter. The values of Pi were considerably higher in Hornsund, approximately twice as high overall, with specific emphasis on the Glacier and Inner zones where Pi values were about 6.5 and 2.5 times higher, respectively, when compared to those observed in Kongsfjord.

The underwater meadows of the Puck Bay, once thriving with eelgrass Zostera marina, bladderwrack Fucus vesiculosus and black carrageen Furcellaria lumbricalis, experienced a decline in water quality during the 1960s and 1970s due to untreated sewage pollution. This, together with commercial exploitation, led to the disappearance of bladderwrack in 1977, with unsuccessful attempts at reintroduction in the early 2000s. In December 2023, a SCUBA survey near Rzucewo revealed a numerous bladderwrack in a benthic free-living form after 46 years of absence. The algae were found between 1.7 and 2.7 m depth, loosely positioned on the seabed, often within Z. marina beds, and with blue mussels Mytilus edulis attached. This reappearance suggests a positive trend in seawater quality and overall state of the Puck Bay, especially when combined with recent recovery of other algae species. The apparent return of bladderwrack could enhance ecosystem functionality, benefiting fish recruitment, grazer and algal biomass. Further investigations on bladderwrack's reappearance are needed in order to verify whether this is the only ecotype currently present in the Puck Bay.

Sea-ice microalgae are a key source of energy and nutrient supply to polar marine food webs, particularly during spring, prior to open-water phytoplankton blooms. The nutritional quality of microalgae as a food source depends on their biomolecular (lipid:protein:carbohydrate) composition. In this study, we used synchrotron-based Fourier transform infra-red microspectroscopy (s-FTIR) to measure the biomolecular content of a dominant sea-ice taxa, Nitzschia frigida, from natural land-fast ice communities throughout the Arctic spring season. Repeated sampling over six weeks from an inner (relatively stable) and an outer (relatively dynamic) fjord site revealed high intra-specific variability in biomolecular content, elucidating the plasticity of N. frigida to adjust to the dynamic sea ice and water conditions. Environmental triggers indicating the end of productivity in the ice and onset of ice melt, including nitrogen limitation and increased water temperature, drove an increase in lipid and fatty acids stores, and a decline in protein and carbohydrate content. In the context of climate change and the predicted Atlantification of the Arctic, dynamic mixing and abrupt warmer water advection could truncate these important end-of-season environmental shifts, causing the algae to be released from the ice prior to adequate lipid storage, influencing carbon transfer through the polar marine system.

The existence of ice-edge phytoplankton blooms in the Southern Ocean is well described, yet direct observations of the mechanisms of phytoplankton bloom development following seasonal sea-ice melt remain scarce. This study constrains such responses using biological and biogeochemical datasets collected along a coastal-to-offshore transect that bisects the receding sea-ice zone in the Kong Håkon VII Hav (off the coast of Dronning Maud Land). We documented that the biogeochemical growing conditions for phytoplankton vary on a latitudinal gradient of sea-ice concentration, where increased sea-ice melting creates optimal conditions for growth with increased light availability and potentially increased iron supply. The zones of the study area with the least ice cover were associated with diatom dominance, the greatest chlorophyll a concentrations, net community production, and dissolved inorganic carbon drawdown, as well as lower sea surface fugacity of CO2. Together, these associations imply higher potential for an oceanic CO2 sink due, at least in part, to more advanced bloom phase and/or larger bloom magnitude stemming from a relatively longer period of light exposure, as compared to the more ice-covered zones in the study area. From stable oxygen isotope fractions, sea-ice meltwater fractions were highest in the open ocean zone and meteoric meltwater fractions were highest in the coastal and polynya zones, suggesting that potential iron sources may also change on a latitudinal gradient across the study area. Variable phytoplankton community compositions were related to changing sea-ice concentrations, with a typical species succession from sympagic flagellate species (Pyramimonas sp. and Phaeocystis antarctica) to pelagic diatoms (e.g., Dactyliosolen tenuijunctus) observed across the study area. These results fill a spatiotemporal gap in the Southern Ocean, as sea-ice melting plays a larger role in governing phytoplankton bloom dynamics in the future Southern Ocean due to changing sea-ice conditions caused by anthropogenic global warming.

The decrease in Arctic sea-ice extent and thickness as a result of global warming will impact the timing, duration, magnitude and composition of phytoplankton production with cascading effects on Arctic marine food-webs and biogeochemical cycles. Here, we elucidate the environmental drivers shaping the composition, abundance, biomass, trophic state and vertical flux of protists (unicellular eukaryotes), including phytoplankton, in the Barents Sea in late August 2018 and 2019. The two years were characterized by contrasting sea-ice conditions. In August 2018, the sea-ice edge had retreated well beyond the shelf break into the Nansen Basin (>82 degrees N), while in 2019, extensive areas of the northwestern Barents Sea shelf (>79 degrees N) were still ice-covered. These contrasting sea-ice conditions resulted in marked interannual differences in the pelagic protist community structure in this area. In August 2018, the protist community was in a post-bloom stage of seasonal succession characterized by oligotrophic surface waters and dominance of small-sized phytoplankton and heterotrophic protists (predomi-nantly flagellates and ciliates) at most stations. In 2019, a higher contribution of autotrophs and large-celled phytoplankton, particularly diatoms, to total protist biomass compared to 2018 was reflected in higher chlo-rophyll a concentrations and suggested that the protist community was still in a late bloom stage at some sta-tions. It is noteworthy that particularly diatoms contributed a considerably higher proportion to the protist biomass at the ice-covered stations in both years compared to the open-water stations. This pattern was also evident in the higher vertical protist biomass flux in 2019, dominated by dinoflagellates and diatoms, compared to 2018. Our results suggest that the predicted transition toward an ice-free Barents Sea will lengthen the oligotrophic summer period with low algal biomass and associated low vertical flux.

The Southern Ocean is a major sink of anthropogenic CO 2 and an important foraging area for top trophic level consumers. However, iron limitation sets an upper limit to primary productivity. Here we report on a considerably dense late summer phytoplankton bloom spanning 9000 km 2 in the open ocean of the eastern Weddell Gyre. Over its 2.5 months duration, the bloom accumulated up to 20 g C m −2 of organic matter, which is unusually high for Southern Ocean open waters. We show that, over 1997–2019, this open ocean bloom was likely driven by anomalies in easterly winds that push sea ice southwards and favor the upwelling of Warm Deep Water enriched in hydrothermal iron and, possibly, other iron sources. This recurring open ocean bloom likely facilitates enhanced carbon export and sustains high standing stocks of Antarctic krill, supporting feeding hot spots for marine birds and baleen whales.

The rapid decline of Arctic sea ice makes understanding sympagic (ice-associated) biology a particularly urgent task. Here we studied the poorly known seasonality of sea-ice protist and meiofauna community composition, abundance and biomass in the bottom 30 cm of sea ice in relation to ice properties and ice drift trajectories in the northwestern Barents Sea. We expected low abundances during the polar night and highest values during spring prior to ice melt. Sea ice conditions and Chlorophyll a concentrations varied strongly seasonally, while particulate organic carbon concentrations were fairly stable throughout the seasons. In December to May we sampled growing first-year ice, while in July and August melting older sea ice dominated. Low sea-ice biota abundances in March could be related to the late onset of ice formation and short time period for ice algae and uni- and multicellular grazers to establish themselves. Pennate diatoms, such as Navicula spp. and Nitzschia spp., dominated the bottom ice algal communities and were present during all seasons. Except for May, ciliates, dinoflagellates, particularly of the order Gymnodiales, and small-sized flagellates were co-dominant. Ice meiofauna (here including large ciliates and foraminifers) was comprised mainly of harpacticoid copepods, copepod nauplii, rotifers, large ciliates and occasionally acoels and foraminifers, with dominance of omnivore species throughout the seasons. Large ciliates comprised the most abundant meiofauna taxon at all ice stations and seasons (50-90 %) but did not necessarily dominate the biomass. While ice melt might have released and reduced ice algal biomass in July, meiofauna abundance remained high, indicating different annual cycles of protist versus meiofauna taxa. In May highest Chlorophyll a concentrations (29.4 mg m-2) and protist biomass (107 mg C m-2) occurred, while highest meiofauna abundance was found in August (23.9 x 103 Ind. m-2) and biomass in December (0.6 mg C m-2). The abundant December ice biota community further strengthens the emerging notion of an active biota during the dark Arctic winter. The data demonstrated a strong and partially unexpected seasonality in the Barents Sea ice biota, indicating that changes in ice formation, drift and decay will significantly impact the functioning of the ice-associated ecosystem.

Seasonal plankton time-series data are presented from Kongsfjorden from two years with contrasting environ-mental conditions. Kongsfjorden (west coast of Spitsbergen - 79 degrees N) integrates inputs from Atlantic and Arctic waters, and glacier run-off, and is thus a prime location to study impacts on ecosystem dynamics of key envi-ronmental drivers that are relevant across the Arctic. Despite extensive research in Kongsfjorden, seasonally -resolved data are scarce. From late April/early May to early September 2019 and 2020, we conducted pelagic sampling at a mid-fjord station at mostly weekly to bi-weekly resolution investigating the environmental drivers of phyto-and zooplankton community composition and phenology. During spring 2019, Atlantic water masses with temperatures > 1 degrees C were found throughout the upper 250 m of the water column, and little sea ice occurred in the fjord. Spring 2020, in turn, was characterized by the presence of local water masses with sub-zero temperatures and relatively extensive sea-ice cover. The most striking contrast between the two years was the difference in phytoplankton spring bloom composition. In 2019, the spring bloom was dominated by the colonial stage of the haptophyte Phaeocystis pouchetii and diatoms played a minor role, while the spring bloom in 2020 was dominated by diatoms of the genus Thalassiosira succeeded by P. pouchetii. Selective grazing by large co-pepods and water mass structure seem to have been the decisive factors explaining the marked difference in diatom spring bloom biomass between the years while similar spring abundances of P. pouchetii in both years indicated that this species was less impacted by those factors. Our data suggest that differences in spring bloom composition impacted trophic transfer and carbon export. Recruitment of the dominant copepods Calanus fin-marchicus and C. glacialis, Cirripedia and euphausiid larvae as well as the export of carbon to the seabed was more efficient during the diatom-dominated compared to the P. pouchetii-dominated spring bloom. In summer, the plankton composition shifted towards a flagellate-dominated community characterized by mixo-and hetero-trophic taxa adapted to a lower nutrient regime and strong top-down control by copepod grazers. However, residual silicic acid after the P. pouchetii-dominated spring bloom fueled a late summer diatom bloom in 2019. Our data provide a first glimpse into the environmental drivers of plankton phenology and underline that high -resolution monitoring over many annual cycles is required to resolve the ephemeral variations of plankton populations against the backdrop of climate change.

In-situ burning is a well-proven technology for combatting oil spills offshore. However, as a coastline oil spill response at rocky shores it is novel and investigated here for the first time. The viability and efficiency of coastline in-situ burning of oil spills and the fate of the resulting burn residue was studied through a field experiment. The experiment included a controlled release of 600 L crude oil and subsequent burn of the oil on a remote coast in Greenland. In line with other documented large-scale offshore burns, a high burn efficiency was estimated. Fate studies and chemical analysis of the burn residue diversely caught in the tidal seaweed vegetation showed varying compositions depending on the specific burn intensity. In general, however, a relative increase in high ring numbered PAHs was observed compared to fresh oil. After 4 days, waves and tidal flushing markedly reduced visible oil and burn residue from the burn area. Elevated concentrations of total hydrocarbons were found from the fresh oil in the seawater, but the impacts on the coastal water was below levels of ecotoxic concern. Overall, coastline in-situ burning proved efficient, with a high operational potential. In addition, when the oil is in place at the coast, with a possible increased operational time window compared to an offshore burn.

We studied phytoplankton and protozooplankton community composition based on light microscopy, flow cytometry, and photosynthetic pigment data in the Atlantic sector of the Southern Ocean during March 2019 (early austral autumn). Sampling was focused on the area east of the prime meridian in the Kong Håkon VII Hav, including Astrid Ridge, Maud Rise, and a south–north transect at 6∘ E. Phytoplankton community composition throughout the studied area was characterized by oceanic diatoms typical of the iron-depleted high-nutrient, low-chlorophyll (HNLC) Southern Ocean. Topography and wind-driven iron supply likely sustained blooms dominated by the centric diatom Chaetoceros dichaeta at Maud Rise and at a station north of the 6∘ E transect. For the remainder of the 6∘ E transect, diatom composition was similar to the previously mentioned bloom stations, but flagellates dominated in abundance, suggesting a post-bloom situation and likely top-down control by krill on the bloom-forming diatoms. Among flagellates, species with haptophyte-type pigments were the dominating group. At Astrid Ridge, overall abundances were lower and pennate diatoms were more numerous than centric diatoms, but the community composition was nevertheless typical of HNLC areas. The observations described here show that C. dichaeta can form blooms beyond the background biomass level and also fuels both carbon export and upper trophic levels within HNLC areas. This study is the first thorough assessment of phytoplankton communities in this region and can be compared to other seasons in future studies.

A warmer Arctic with less sea ice will likely improve macroalgae growth conditions, but observational data to support this hypothesis are scarce. In this study, we combined hydroacoustic and video inspections to compare the depth of growth, density and thickness of macroalgae (>10 cm) meadows in two contrasting climate regimes in Svalbard 1) the warm, ice free, Atlantic influenced West Spitsbergen and 2) the cold, Arctic and seasonal ice covered East Spitsbergen. Both places had similar insolation and comparable turbidity levels. Macroalgae communities at both places were similar and were formed mainly by common north Atlantic kelp species:Saccharina latissima, Alaria esculenta, Laminaria digitataandL. hyperborea. However, the density of the bottom coverage and thalli condition were strikingly different between the two sites. Algae at the warmer site were intact and fully developed and occupied most of the available hard substrate. At the colder site, only patchy macroalgae canopies were found and most thallies were physically damaged and trimmed at a uniform height due to physical ice scouring. These differences in macroalgal density and thalli condition were only found at depths down to 5 m. Deeper, no distinct differences were observed between the warm and cold sites. Sea urchins were only observed at the warm site, but in few numbers with no visible negative top-down control on macroalgae growth.