Distribution of dinoflagellate cysts in surface sediments of the Mackenzie Shelf and Amundsen Gulf, Beaufort Sea (Canada)

In order to document long-term climate cycles and predict future climate trends for the Arctic, we need to look at the geological records to establish the link between historical and pre-historical sea-surface parameters. Dinoflagellate cysts (dinocysts) are used as proxy indicators of sea-surface parameters (temperature, salinity, sea-ice cover, primary productivity) jointly with transfer functions and a modern dinocyst reference database, to reconstruct the evolution of sea-surface conditions at decadal and millennial timescales. Here we present the surface distribution of recent dinocyst assemblages from 34 surface sediment samples collected on the Mackenzie Slope/Amundsen Gulf during the 2004 CASES (Canadian Arctic Shelf Exchange Study) cruise. Dinocyst concentrations in surface sediments are relatively high outside the Mackenzie plume area and increase gradually eastward toward Amundsen Gulf. The cysts of autotrophic dinoflagellates are dominant throughout the study area, while the maximum abundance of heterotrophic taxa is found within the Mackenzie plume. Hierarchical clustering analyses allowed defining two dinocyst assemblages. Assemblage I is located on the Mackenzie Slope and southern Amundsen Gulf, while Assemblage II is located within the Cape Bathurst Polynya area in northern Amundsen Gulf. Both assemblages are dominated by Operculodinium centrocarpum, but are distinguished on the basis of the relative abundance of Islandinium minutum, a taxon generally associated with sea ice. I. minutum is found in lower abundance in the Cape Bathurst Polynya.     

Physical and biological correlates of virus dynamics in the southern Beaufort Sea and Amundsen Gulf

As part of the Canadian Arctic Shelf Exchange Study (CASES), we investigated the spatial and seasonal distributions of viruses in relation to biotic (bacteria, chlorophyll-a (chl a)) and abiotic variables (temperature, salinity and depth). Sampling occurred in the southern Beaufort Sea Shelf in the region of the Amundsen Gulf and Mackenzie Shelf, between November 2003 and August 2004. Bacterial and viral abundances estimated by epifluorescence microscopy (EFM) and flow cytometry (FC) were highly correlated (r² = 0.89 and r² = 0.87, respectively), although estimates by EFM were slightly higher (FC = 1.08 × EFM + 0.12 and FC = 1.07 × EFM + 0.43, respectively). Viral abundances ranged from 0.13 × 10⁶ to 23 × 10⁶ ml^− 1, and in surface waters were ~ 2-fold higher during the spring bloom in May and June and ~ 1.5-fold higher during July and August, relative to winter abundances. These increases were coincident with a ~ 6-fold increase in chl a during spring and a ~ 4-fold increase in bacteria during summer. Surface viral abundances near the Mackenzie River were ~ 2-fold higher than in the Mackenzie Shelf and Amundsen Gulf regions during the peak summer discharge, concomitant with a ~ 5.5-fold increase in chl a (up to 2.4 μg l^− 1) and a ~ 2-fold increase in bacterial abundance (up to 22 × 10⁵ ml^− 1). Using FC, two subgroups of viruses and heterotrophic bacteria were defined. A low SYBR-green fluorescence virus subgroup (V2) representing ~ 71% of the total viral abundance, was linked to the abundance of high nucleic acid fluorescence (HNA) bacteria (a proxy for bacterial activity), which represented 42 to 72% of the bacteria in surface layers. A high SYBR-green fluorescence viral subgroup (V1) was more related to high chl a concentrations that occurred in surface waters during spring and at stations near the Mackenzie River plume during the summer discharge. These results suggest that V1 infect phytoplankton, while most V2 are bacteriophages. On the Beaufort Sea shelf, viral abundance displayed seasonal and spatial variations in conjunction with chl a concentration, bacterial abundance and composition, temperature, salinity and depth. The highly dynamic nature of viral abundance and its correlation with increases in chl a concentration and bacterial abundance implies that viruses are important agents of microbial mortality in Arctic shelf waters.     

Picophytoplankton and nanophytoplankton abundance and distribution in the southeastern Beaufort Sea (Mackenzie Shelf and Amundsen Gulf) during Fall 2002

The distribution of picophytoplankton (0.2–2 µm) and nanophytoplankton (2–20 µm) in the Beaufort Sea–Mackenzie Shelf and Amundsen Gulf regions during autumn, 2002 is examined relative to their ambient water mass properties (salinity, temperature and nutrients: nitrate + nitrite, phosphate, and silicate) and to the ratio of variable to maximum fluorescence, Fv/Fm. Total phytoplankton and cell abundances (< 20 µm) were mainly correlated with salinity. Significant differences in picophytoplankton cell numbers were found among waters near the mouth of the Mackenzie River, ice melt waters and the underlying halocline water masses of Pacific origin. Picophytoplankton was the most abundant phytoplankton fraction during the autumnal season, probably reflecting low nitrate concentrations (surface waters average ~ 0.65 µM). The ratio Fv/Fm averaged 0.44, indicating that cells were still physiologically active, even though their concentrations were low (max Chl a = 0.9 mg m^− 3). No significant differences in Fv/Fm were evident in the different water masses, indicating that rate limiting conditions for photosynthesis and growth were uniform across the whole system, which was in a pre-winter stage, and was probably already experiencing light limitation as a result of shortening day lengths.     

Suspended sediment and erosion dynamics in Kugmallit Bay and Beaufort Sea during ice-free conditions

The Mackenzie River is the largest river on the North American side of the Arctic and its huge freshwater and sediment load impacts the Canadian Beaufort Shelf. Huge quantities of sediment and associated organic carbon are transported in the Mackenzie plume into the interior of the Arctic Ocean mainly during the freshet (May to September). Changing climate scenarios portend increased coastal erosion and resuspension that lead to altered river-shelf-slope particle budgets. We measured sedimentation rates, suspended particulate matter (SPM), particle size and settling rates during ice-free conditions in Kugmallit Bay (3–5 m depth). Additionally, measurements of erosion rate, critical shear stress, particle size distribution and resuspension threshold of bottom sediments were examined at four regionally contrasting sites (33–523 m depth) on the Canadian Beaufort Shelf using a new method for assessing sediment erosion. Wind induced resuspension was evidenced by a strong relationship between SPM and wind speed in Kugmallit Bay. Deployment of sediment traps showed decreasing sedimentation rates at sites along an inshore–offshore transect ranging from 5400 to 3700 g m^− 2 day^− 1. Particle settling rates and size distributions measured using a Perspex settling chamber showed strong relationships between equivalent spherical diameter (ESD) and particle settling rates (r² = 0.91). Mean settling rates were 0.72 cm s^− 1 with corresponding ESD values of 0.9 mm. Undisturbed sediment cores were exposed to shear stress in an attempt to compare differences in sediment stability across the shelf during September to October 2003. Shear was generated by vertically oscillating a perforated disc at controlled frequencies corresponding to calibrated shear velocity using a piston grid erosion device. Critical (Type I) erosion thresholds (u) varied between 1.1 and 1.3 cm s^− 1 with no obvious differences in location. Sediments at the deepest site Amundsen Gulf displayed the highest erosion rates (22–54 g m^− 2 min^− 1) with resuspended particle sizes ranging from 100 to 930 µm for all sites. There was no indication of biotic influence on sediment stability, although our cores did not display a fluff layer of unconsolidated sediment. Concurrent studies in the delta and shelf region suggest the importance of a nepheloid layer which transports suspended particles to the slope. Continuous cycles of resuspension, deposition, and horizontal advection may intensify with reduction of sea ice in this region. Our measurements coupled with studies of circulation and cross-shelf exchange allow parameterization and modeling of particle dynamics and carbon fluxes under various climate change scenarios.     

Influence of the Mackenzie River plume on the sinking export of particulate material on the shelf

We examined the influence of the Mackenzie River plume on sinking fluxes of particulate organic and inorganic material on the Mackenzie Shelf, Canadian Arctic. Short-term particle interceptor traps were deployed under the halocline at 3 stations across the shelf during fall 2002 and at 3 stations along the shelf edge during summer 2004. During the two sampling periods, the horizontal patterns in sinking fluxes of particulate organic carbon (POC) and chlorophyll a (chl a) paralleled those in chl a biomass within the plume. Highest sinking fluxes of particulate organic material occurred at stations strongly influenced by the river plume (maximum POC sinking fluxes at 25 m of 98 mg C m^− 2 d^− 1 and 197 mg C m^− 2 d^− 1 in 2002 and 2004, respectively). The biogeochemical composition of the sinking material varied seasonally with phytoplankton and fecal pellets contributing considerably to the sinking flux in summer, while amorphous detritus dominated in the fall. Also, the sinking phytoplankton assemblage showed a seasonal succession from a dominance of diatoms in summer to flagellates and dinoflagellates in the fall. The presence of the freshwater diatom Eunotia sp. in the sinking assemblage directly underneath the river plume indicates the contribution of a phytoplankton community carried by the plume to the sinking export of organic material. Yet, increasing chl a and BioSi sinking fluxes with depth indicated an export of phytoplankton from the water column below the river plume during summer and fall. Grazing activity, mostly by copepods, and to a lesser extent by appendicularians, appeared to occur in a well-defined stratum underneath the river plume, particularly during summer. These results show that the Mackenzie River influences the magnitude and composition of the sinking material on the shelf in summer and fall, but does not constitute the only source of material sinking to depth at stations influenced by the river plume.     

Measuring the thicknesses of the freshwater-layer plume and sea ice in the land-fast ice region of the Mackenzie Delta using helicopter-borne sensors

Helicopter-borne sensors have been used since the early 1990s to monitor ice properties in support of winter marine transportation along the east coast of Canada. The observations are used in ice chart production and to validate ice hazard identification algorithms using satellite advanced synthetic aperture radar (ASAR) imagery. In this study we evaluated the sensors' additional capability to monitor the freshwater plume characteristic beneath land-fast ice. During the Canadian Arctic Shelf Exchange Study (CASES) data were collected over the Mackenzie Delta in the southern Beaufort Sea where a buoyant river plume exists. Results showed that the electromagnetic–laser system could describe not only the ice properties but also the horizontal distribution of the freshwater plume depths that decreased in depth stepwise offshore as the flow of the buoyant plume was restricted by a series of ridge-rubble fields running parallel to the coast. Relative to the 2 m mean ice thickness, the plume layer depth varied from zero under mobile offshore pack ice to 3 m inshore of the third set of ridge-rubble fields.     

Benthic foraminifera in the surface sediments of the Beaufort Shelf and slope, Beaufort Sea, Canada: Applications and implications for past sea-ice conditions

This paper presents new data on distribution patterns of modern benthic foraminifera and other microfossils from the Canadian Arctic, specifically the Beaufort Shelf and slope. The material was collected in June to August of 2004 and is the first of its kind in this area to be collected since 1970. We examined the smaller sizes (45–63µm) as well as > 63µm and discovered that many species had been severely underrepresented in previous studies. Deep sea forms, that had been overlooked previously, were common on the shelf; two species (Elphidiella arctica and Ammotium cassis) appeared in preliminary results to be indicators of methane seepage; and it was possible to make determinations of sea-ice coverage using a combination of foraminifera and tintinnids (planktic ciliates). Our data indicated the presence of many of the same species as previous studies from this area, but improved techniques of sample processing greatly increased the number of specimens and species found (particularly the small deep sea arctic species Buliminella hensoni and Bolivina arctica) which provide much more reliable data for paleoceanographic determinations. One of the primary objectives for this work was to provide baseline data to help determine paleo-ice cover; these data cover a broad range of conditions on the Beaufort Shelf that make it possible to achieve this objective as well as improving what it is known about the assemblages on this shelf as compared to other arctic shelf areas, such as the Siberian Shelf).     

Winter–spring dynamics in sea-ice carbon cycling in the coastal Arctic Ocean

An understanding of microbial interactions in first-year sea ice on Arctic shelves is essential for identifying potential responses of the Arctic Ocean carbon cycle to changing sea-ice conditions. This study assessed dissolved and particulate organic carbon (DOC, POC), exopolymeric substances (EPS), chlorophyll a, bacteria and protists, in a seasonal (24 February to 20 June 2004) investigation of first-year sea ice and associated surface waters on the Mackenzie Shelf. The dynamics of and relationships between different sea-ice carbon pools were investigated for the periods prior to, during and following the sea-ice-algal bloom, under high and low snow cover. A predominantly heterotrophic sea-ice community was observed prior to the ice-algal bloom under high snow cover only. However, the heterotrophic community persisted throughout the study with bacteria accounting for, on average, 44% of the non-diatom particulate carbon biomass overall the study period. There was an extensive accumulation of sea-ice organic carbon following the onset of the ice-algal bloom, with diatoms driving seasonal and spatial trends in particulate sea-ice biomass. DOC and EPS were also significant sea-ice carbon contributors such that sea-ice DOC concentrations were higher than, or equivalent to, sea-ice-algal carbon concentrations prior to and following the algal bloom, respectively. Sea-ice-algal carbon, DOC and EPS-carbon concentrations were significantly interrelated under high and low snow cover during the algal bloom (r values ≥ 0.74, p < 0.01). These relationships suggest that algae are primarily responsible for the large pools of DOC and EPS-carbon and that similar stressors and/or processes could be involved in regulating their release. This study demonstrates that DOC can play a major role in organic carbon cycling on Arctic shelves.     

Bacterial production and microbial food web structure in a large arctic river and the coastal Arctic Ocean

Globally significant quantities of organic carbon are stored in northern permafrost soils, but little is known about how this carbon is processed by microbial communities once it enters rivers and is transported to the coastal Arctic Ocean. As part of the Arctic River-Delta Experiment (ARDEX), we measured environmental and microbiological variables along a 300 km transect in the Mackenzie River and coastal Beaufort Sea, in July–August 2004. Surface bacterial concentrations averaged 6.7 × 10⁵ cells mL^− 1 with no significant differences between sampling zones. Picocyanobacteria were abundant in the river, and mostly observed as cell colonies. Their concentrations in the surface waters decreased across the salinity gradient, dropping from 51,000 (river) to 30 (sea) cells mL^− 1. There were accompanying shifts in protist community structure, from diatoms, cryptophytes, heterotrophic protists and chrysophytes in the river, to dinoflagellates, prymnesiophytes, chrysophytes, prasinophytes, diatoms and heterotrophic protists in the Beaufort Sea.Size-fractionated bacterial production, as measured by ³H–leucine uptake, varied from 76 to 416 ng C L^− 1 h^− 1. The contribution of particle-attached bacteria (> 3 µm fraction) to total bacterial production decreased from > 90% at the Mackenzie River stations to < 20% at an offshore marine site, and the relative importance of this particle-based fraction was inversely correlated with salinity and positively correlated with particulate organic carbon concentrations. Glucose enrichment experiments indicated that bacterial metabolism was carbon limited in the Mackenzie River but not in the coastal ocean. Prior exposure of water samples to full sunlight increased the biolability of dissolved organic carbon (DOC) in the Mackenzie River but decreased it in the Beaufort Sea.Estimated depth-integrated bacterial respiration rates in the Mackenzie River were higher than depth-integrated primary production rates, while at the marine stations bacterial respiration rates were near or below the integrated primary production rates. Consistent with these results, P_CO2 measurements showed surface water supersaturation in the river (mean of 146% of air equilibrium values) and subsaturation or near-saturation in the coastal sea. These results show a well-developed microbial food web in the Mackenzie River system that will likely convert tundra carbon to atmospheric CO₂ at increasing rates as the arctic climate continues to warm.     

Mesozooplankton species distribution in the NW and N Iberian shelf during spring 2004: Relationship with frontal structures

We have analysed the mesozooplankton community structure in the southern Bay of Biscay shelf and its relationship with the hydrographic conditions during spring 2004. According to thermohaline characteristics, we observed two frontal zones of distinct origin along the shelf (around 7° and 3°W), that allowed us to differentiate three different hydrographic domains. The westernmost part of the shelf (WC), defined by the presence of relatively warm and salty water related to the presence of the Iberian Poleward Current (IPC), the easternmost region (EC), characterized by colder and fresher water and subject to the influence of freshwater inputs from the Adour river in the French coast, and a region in the Central Cantabrian Sea (CC), where thermohaline characteristics were intermediate between these two extremes. The mixing layer depth (MLD) regime in these areas was also different: the WC region was characterized by a mixed water column, whereas in the EC region the river discharges produces stratification of the upper meters of the water column (< 10 m); in the CC region, we found a distinct vertical mixing regime that separated coastal (stratification) from shelf (mixed water column) stations, giving rise to a notorious across-shelf front. We found a good match between the aforesaid hydrographic regions and the distribution of mesozooplankton species composition and community assemblages: the Mantel correlation between physical variables and mesozooplankton distribution was highly significant (n = 63, r = 0.70, α < 0.001). In the WC region, the community was dominated by Paracalanus parvus, Oithona helgolandica, Acartia clausi and Clausocalanus pergens, while in the EC region the most dominant species were Noctiluca scintillans, Oncaea media and Temora longicornis. The CC region showed similar composition of copepods than the WC region, but larvaceans (Oikopleura spp. and Fritillaria spp.) were more abundant in the CC region than in the WC region. Within each zone, the relative abundances of the dominant species differed between coastal and shelf locations.     

Variability of mesozooplankton spatial distribution in the North Aegean Sea, as influenced by the Black Sea waters outflow

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m^− 3 and 14,157 ind m^− 3) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi, Centropages typicus, Paracalanus parvus. The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera, Oithona copepodites, Oncaea media, Ctenocalanus vanus, Farranula rostrata. During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.     

Linking mercury exposure to habitat and feeding behaviour in Beaufort Sea beluga whales

Mercury (Hg) levels in the Beaufort Sea beluga population have been increasing since the 1990's. Ultimately, it is the Hg content of prey that determines beluga Hg levels. However, the Beaufort Sea beluga diet is not understood, and little is known about the diet Hg sources in their summer habitat. During the summer, they segregate into social groups based on habitat use leading to the hypothesis that they may feed in different food webs explaining Hg dietary sources. Methyl mercury (MeHg) and total mercury (THg) levels were measured in the estuarine-shelf, Amundsen Gulf and epibenthic food webs in the western Canadian Arctic collected during the Canadian Arctic Shelf Exchange Study (CASES) to assess their dietary Hg contribution. To our knowledge, this is the first study to report MeHg levels in estuarine fish and epibenthic invertebrates from the Arctic Ocean. Although the Mackenzie River is a large source of Hg, the estuarine-shelf prey items had the lowest MeHg levels, ranging from 0.1 to 0.27 μg/g dry weight (dw) in arctic cisco (Coregonus autumnalis) and saffron cod (Eleginus gracilis) respectively. Highest MeHg levels occurred in fourhorn sculpin (Myoxocephalus quadricornis) (0.5 μg/g dw) from the epibenthic food web. Beluga hypothesized to feed in the epibenthic and Amundsen Gulf food webs had the highest Hg levels matching with high Hg levels in associated food webs, and estuarine-shelf belugas had the lowest Hg levels (2.6 μg/g dw), corresponding with the low food web Hg levels, supporting the variation in dietary Hg uptake. The trophic level transfer of Hg was similar among the food webs, highlighting the importance of Hg sources at the bottom of the food web as well as food web length. We propose that future biomagnification studies incorporate predator behaviour with food web structure to assist in the evaluation of dietary Hg sources.     

Nutrient status of the Northeast Water Polynya

The nutrient distribution in the Northeast Water Polynya (NEW) was investigated intensively between the end of May and the beginning of August 1993 during the R/V Polarstern cruise ARK IX. The major characteristics were low initial nitrate concentrations (ca. 4 μM) in the surface mixed layer of the East Greenland Shelf Water, accompanied by high silicate values (ca. 10–14 μM). These concentrations were not reduced by phytoplankton growth. Silicate was rather homogeneously distributed in the entire water column, whereas nitrate increased continuously with depth to about 13 μM. Phosphate concentrations were about 1.1 μM and had a similar distribution to that of silicate. During the course of the summer, nutrients became depleted, and nitrate was exhausted in large parts of the NEW. Silicate was reduced to values of less than 2 μM at some stations which implies that diatom growth continued despite nitrate depletion, ammonium serving as a nitrogen source. The polynya is fertilised by water with the initial nutrient concentrations downstream of the Norske Øer Ice Shelf. This process continuously supplies nutrients to the surface throughout the year and these are transported northward by the anticyclonic surface circulation following the topography of the trough system. The northern boundary of this tongue of relatively nutrient-rich water is controlled by the uptake of nutrients by phytoplankton in summer. Its extemsion is variable due to interactions between biological processes, circulation and ice cover. In the Ob Bank region the nutrient distribution can be altered by the inflow of Polar Water from the north when strong northerly winds prevail as happened during the first part of the study.     

Protist entrapment in newly formed sea ice in the Coastal Arctic Ocean

Protist abundance and taxonomic composition were determined in four development stages of newly formed sea ice (new ice, nilas, young ice and thin first-year ice) and in the underlying surface waters of the Canadian Beaufort Sea from 30 September to 19 November 2003. Pico- and nanoalgae were counted by flow cytometry whereas photosynthetic and heterotrophic protists ≥ 4 µm were identified and counted by inverted microscopy. Protists were always present in sea ice and surface water samples throughout the study period. The most abundant protists in sea ice and surface waters were cells < 4 µm. They were less abundant in sea ice (418–3051 × 10³ cells L^− 1) than in surface waters (1393–5373 × 10³ cells L^− 1). In contrast, larger protists (≥ 4 µm) were more abundant in sea ice (59–821 × 10³ cells L^− 1) than in surface waters (22–256 × 10³ cells L^− 1). These results suggest a selective incorporation of larger cells into sea ice. The ≥ 4 µm protist assemblage was composed of a total number of 73 taxa, including 12 centric diatom species, 7 pennate diatoms, 11 dinoflagellates and 16 flagellates. The taxonomic composition in the early stage of ice formation (i.e., new ice) was very similar to that observed in surface waters and was composed of a mixed population of nanoflagellates (Prasinophyceae and Prymnesiophyceae), diatoms (mainly Chaetoceros species) and dinoflagellates. In older stages of sea ice (i.e., young ice and thin first-year ice), the taxonomic composition became markedly different from that of the surface waters. These older ice samples contained relatively fewer Prasinophyceae and more unidentified nanoflagellates than the younger ice. Diatom resting spores and dinoflagellate cysts were generally more abundant in sea ice than in surface waters. However, further studies are needed to determine the importance of this winter survival strategy in Arctic sea ice. This study clearly shows the selective incorporation of large cells (≥ 4 µm) in newly formed sea ice and the change in the taxonomic composition of protists between sea ice and surface waters as the fall season progresses.     

Distribution of diatoms in the Northeast Water Polynya, Greenland

This study formed part of the Northeast Water project (NEW project) which dealt with physical, geophysical and biological processes in the Northeast Water Polynya off Northeast Greenland. This was part of the International Arctic Polynya Programme (IAPP). The diatom composition of the water masses, sea ice and melt ponds was analysed to show the relationship between ice and the water column near the ice with regard to the origin and fate of the cells in the ice and melt ponds. Fragilariopsis oceanica, Fragiliria sp. I and Chaetoceros socialis usually dominated the phytoplankton, while the ice and melt pond samples showed a wide range of assemblages, with different single-celled pennates and two undescribed species, Navicula sp. 1 and Nitzschia sp. 1 often dominant. Planktonic algae in sea ice can be released into the water column during ice break-up and melt, thus contributing to the spring bloom in the water column, if the timing of the release and the species composition are correct. The number of different ice algal assemblages supports the theory that cells originated from the water column, the benthos and freshwater. In addition, differential growth in the sea ice or melt ponds often altered the relative abundance of species in comparison with what is usually found in their original habitat. However, many of the cells in the ice and melt ponds were dead (empty frustules), making it difficult to determine whether the cells had actually lived in these habitats.     

Number and phylogenetic affiliation of bacteria assimilating dimethylsulfoniopropionate and leucine in the ice-covered coastal Arctic Ocean

The ability of bacteria to assimilate sulfur from dimethylsulfoniopropionate (DMSP) was examined in the western Arctic Ocean by combining microautoradiography and fluorescence in situ hybridization (FISH). Assimilation of leucine was also measured for comparative purposes since leucine is considered a universal substrate for bacteria, which use it for protein synthesis. Samples were collected at 3 m depth, through a hole in the ice, in the CASES (Canadian Arctic Shelf Ecosystem Study) overwintering station in Franklin Bay (eastern Beaufort Sea) in March and May 2004 to compare two contrasting situations: winter and early spring. FISH counts indicated that the bacterial assemblage consisted of α- (up to 60% of the EUB positive cells), β- (up to 10%) and γ-proteobacteria (around 20%), and Bacteroidetes (up to 60%). The β-proteobacteria were not active with any of the two substrates tested. The remaining groups were much less efficient at assimilating DMSP-sulfur (5% of the cells) than leucine (20–35%) both in winter and in spring. Only the Roseobacter group of α-proteobacteria showed a similar assimilation of both substrates.     

Seasonal carbon distribution of copepods in the eastern Weddell Sea, Antarctica

Copepods were sampled by a multiple opening-closing net in the eastern Weddell Sea during various seasons (late winter/early spring, summer, autumn). Total copepod biomass integrated over the upper 1000 m varied seasonally between 1.7 mg C m⁻³ in late winter/early spring and 3.7 mg C m⁻³ in autumn. After the dark season the copepods were rather evenly distributed vertically and highest biomass levels were found in the mid-water layers between about 200 m and 500 m. By contrast, especially in summer but also in autumn copepod biomass concentrated in the uppermost water layer. A total of 64 calanoid species were identified in the upper 1000 m with maximum species numbers in the deepest layer. The large calanoids Calanus propinquus, Calanoides acutus, Metridia gerlachei, Euchaeta antarctica and the small calanoid Microcalanus pygmaeus prevailed and accounted for 60–70% of total copepod biomass, while the small poecilostomatoid Oncaea and the cyclopoid Oithona species comprised about 20%. Hence, the distribution pattern of the entire copepod biomass is strongly influenced by the life cycles of a few dominant species.     

100-years-changes in the phytoplankton community of Kiel Bight (Baltic Sea)

Literature data from 1905/06, 1912/13 and 1949/50 were compared with recent data (2001–2003) from Kiel Bight in order to investigate changes in phytoplankton composition and biomass, which may serve as indicators of environmental changes. In terms of biomass, diatomophyceae and dinophyceae are by far the most important groups. Their ratio is still close to unity. The share of diatomophyceae increased strongly in years with exceptionally high summer blooms (2001) or exceptionally early spring blooms (2003). The summer and autumn blooms of Chaetoceros and Skeletonema, detected in the early 20th century, are replaced by other diatoms (Cerataulina pelagica, Dactyliosolen fragilissimus, Proboscia alata, Pseudo-nitzschia spp.). Chaetoceros and Skeletonema are still important components of the spring blooms. Now as before, the autumn blooms are dominated by Ceratium spp., sometimes also by diatoms. Newly appearing bloom-forming species are mostly potentially toxic (Dictyocha speculum, Prorocentrum minimum, Pseudo-nitzschia spp.). The total phytoplankton biomass has roughly doubled in the course of the last century. The reference condition for phytoplankton biomass in Kiel Bight in the sense of the Water Framework Directive was defined at 55 mg C m^− 3 (± 10%, annual mean). The mean annual biomass of diatomophyceae and dinophyceae was 25 mg C m^− 3 (± 40%) for each, indicating that the sum of their carbon biomass amounted to 90% (± 10%) of the total phytoplankton biomass on an annual average. Diatomophyceae represented at least 80% of carbon biomass in the spring bloom peak at the beginning of the 20th century.     

Heterogeneous archaeal communities in the particle-rich environment of an arctic shelf ecosystem

We evaluated the phylogenetic diversity of particle-associated and free-living archaeal assemblages from the Mackenzie River and Beaufort Sea in the western Canadian Arctic. The physico-chemical characteristics of the water separated the sampling sites into three groups: riverine, coastal and marine water, which had strikingly different archaeal communities. The riverine water was characterised by the presence of Euryarchaeota mainly belonging to the LDS and RC-V clusters. The coastal water was also dominated by Euryarchaeota but they were mostly affiliated to Group II.a. The marine waters contained most exclusively Crenarchaeota belonging to the Marine Group I.1a. The results suggest that Euryarchaeota in the coastal surface layer are associated with particle-rich waters, while Crenarchaeota are more characteristic of Arctic Ocean waters that have been less influenced by riverine inputs. The particle-associated communities were similar to the free-living ones at the riverine and marine sites but differed from each other at the coastal site in terms of the presence or absence of some taxonomic groups in one of the fractions, or differences in the proportion of the phylogenetic groups. However, there was no specific archaeal group that was exclusively restricted to the free-living or particle fraction, and the diversity of the particle-associated archaeal assemblages did not significantly differ from the diversity of the free-living communities.     

Importance of particle-associated bacterial heterotrophy in a coastal Arctic ecosystem

The large quantities of particles delivered by the Mackenzie River to the coastal Beaufort Sea (Arctic Ocean) have implications for the spatial distribution, composition and productivity of its bacterial communities. Our objectives in this study were: (1) to assess the contribution of particle-associated bacteria (fraction ≥ 3 µm) to total bacterial production and their relationships with changing environmental conditions along a surface water transect; (2) to examine how particle-based heterotrophy changes over the annual cycle (Nov 2003–Aug 2004); and (3) to determine whether particle-associated bacterial assemblages differ in composition from the free-living communities (fraction < 3 µm). Our transect results showed that particle-associated bacteria contributed a variable percentage of leucine-based (BP-Leu) and thymidine-based (BP-TdR) bacterial production, with values up to 98% at the inshore, low salinity stations. The relative contribution of particle-associated bacteria to total BP-Leu was positively correlated with temperature and particulate organic material (POM) concentration. The annual dataset showed low activities of particle-associated bacteria during late fall and most of the winter, and a period of high particle-associated activity in spring and summer, likely related to the seasonal inputs of riverine POM. Results from catalyzed reporter deposition for fluorescence in situ hybridization (CARD-FISH) confirmed the dominance of Bacteria and presence of Archaea (43–84% and 0.2–5.5% of DAPI counts, respectively), which were evenly distributed throughout the Mackenzie Shelf, and not significantly related to environmental variables. Denaturing gradient gel electrophoresis (DGGE) revealed changes in the bacterial community structure among riverine, estuarine and marine stations, with separation according to temperature and salinity. There was evidence of differences between the particle-associated and free-living bacterial assemblages at the estuarine stations with highest POM content. Particle-associated bacteria are an important functional component of this Arctic ecosystem. Under a warmer climate, they are likely to play an increasing role in coastal biogeochemistry and carbon fluxes as a result of permafrost melting and increased particle transport from the tundra to coastal waters.