Community Structure of the Bacteria Associated with Nodularia sp. (Cyanobacteria) Aggregates in the Baltic Sea

The community structure of the bacteria associated with Nodularia spumigena (Mertens) cyanobacterial aggregates in the Baltic Sea was studied with temperature gradient gel electrophoresis (TGGE), using a 16S rRNA gene fragment as a target. Various developmental stages of the aggregates and free-floating cyanobacterial filaments were sampled to reveal possible changes in associated microbial community structure during development and senescence of the aggregates. The microbial community structures of all samples differed, and the communities of young and decaying aggregates were separated by cluster analysis of the TGGE fingerprint data. Sequencing of the TGGE fragments indicated the presence of bacteria from the α-, β-, and γ-proteobacterial groups, as well as members of Cytophaga–Flexibacter–Bacteroides lineages and gram-positive Actinobacteria spp. The majority of the Nodularia-associated sequences were not closely related to previously reported 16S rDNA sequences from the Baltic Sea or any other environment. The structure of the bacterial assemblage reflects the environmental changes associated with the succession and decay of the cyanobacterial aggregates. In addition, the sequence data suggest that the N. spumigena (Mertens) blooms in the Baltic Sea may host thus far uncharacterized bacterial species.     

Evidence for active microbial nitrogen transformations in sea ice (Gulf of Bothnia, Baltic Sea) in midwinter

Nutrient concentrations, chlorophyll-a, bacterial biomass and relative activity of denitrifying organisms were investigated from ice-core, brine and underlying water samples in February 1998 in the Gulf of Bothnia, Baltic Sea. Examined sea ice was typical for the Baltic Sea; ice bulk salinity varied from 0.1 to 1.6 psu, and in underlying water salinity was from 4.2 to 4.7 psu. In 2- to 3-months-old sea ice (thickness 0.4–0.6 m), sea-ice communities were at the winter stage; chl-a concentrations were generally below 1 mg m⁻³ and heterotrophic organisms composed 7–20% of organism assemblage. In 1-month-old ice (thickness 0.2–0.25 m), an ice spring bloom was already developing and chl-a concentrations were up to 5.6 mg m⁻³. In relation to low salinity, high concentrations of NH⁺ ₄, NO⁻ ₂, PO³⁺ ₄ and SiOH₄ were found in the ice column. The results suggest that the upper part of ice accumulates atmospheric nutrient load during the ice season, and nutrients in the upper 10–20 cm of ice are mainly of atmospheric origin. The most important biological processes controlling the sea-ice nutrient status are nutrient regeneration, nutrient uptake and nitrogen transformations. Nutrient regeneration is specially active in the middle parts of the 50- to 60-cm-thick ice and subsequent accumulation of nutrients probably enhances the ice spring bloom. Nitrite accumulation and denitrifying activity were located in the same ice layers with nutrient regeneration, which together with the observed significant correlation between the concentrations of nitrogenous nutrients points to active nitrogen transformations occurring in the interior layers of sea ice in the Baltic Sea. Accepted: 12 June 2000     

Experimental Evidence on Nutrient and Substrate Limitation of Baltic Sea sea-ice Algae and Bacteria

The effect of nutrient limitation on Baltic Sea ice algae, and substrate and nutrient limitation on ice bacteria, was studied in a series of in situ -experiments conducted during the winter of 2002 in northern Baltic Sea. Community level changes in algal biomass (chlorophyll a) and productivity, and bacterial thymidine and leucine incorporation were followed for one week after the addition of nutrient and/or organic carbon rich filtered seawater to the experimental units. The results showed the major contribution of snow cover to the algal responses during the beginning of the ice-covered season. Algal communities were able to grow even in January if no snow was present. Nutrient addition did occasionally have an effect on algal biomass and productivity in the ice. Surprisingly, seeding effect from the ice to the underlying water was negatively affected by the nutrient availability in March. Bacterial limitation varied between nutrient (phosphorus) and substrate limitations. The results showed, that limitation in both algal and bacterial communities changed periodically in the northern Baltic Sea ice.     

Changes in Phytoplankton Biomass and Nutrient Quantities in Sea Ice as Responses to Light/Dark Manipulations during different Phases of the Baltic Winter 2003

The response of Baltic Sea ice communities to changing light climate was studied in three subsequent 3 week in situ experiments on the SW coast of Finland. The investigation covered three different winter periods, short day with low solar angles leading to limited light in the ice, late winter with deep snow cover and early spring with melting snow and increasing light availability. The experimental setup consisted of transparent (no snow) and completely darkened (heavy snow cover) plexiglass tubes in which the ice cores were incubated in situ from 1 to 2 weeks. Changes in the concentrations of inorganic nutrients (NO₃⁻-–N, PO₄³⁻-–P, SiO₄⁻-–Si) and chlorophyll-a concentration in the phytoplankton community composition were recorded as responses to different light manipulations. Changes in inner ice light intensity in untreated ice as well as the temperature both in air and ice were recorded over the entire study period. Increased irradiance in late winter/early spring and during meltdown affected the chlorophyll-a amount in the sea ice. During these periods the phytoplankton community in the top layers decreased possibly as a consequence of photo-acclimation. Closer to the bottom of the ice, however, the increased inner ice light intensity induced algal growth. Complete exclusion of light stopped the algal growth in the whole ice column. Darkening the ice cores also slowed down the ice melting opposite to accelerated melting caused by increased light. The significant differences found in nutrient concentrations between the light and dark treatments were mostly explicable by changes in algal biomass. No obvious changes were observed in the phytoplankton community composition due to light manipulation, diatoms and heterotrophic flagellates dominating throughout the study period.     

Algal and bacterial processes in platelet ice during late austral summer

 The biota inhabiting layers of platelet ice were investigated in the Weddell Sea during late austral summer. Due to meltwater release, the salinity of the interstitial water between platelets was reduced. Algae and bacteria accumulated within this ice environment attaining concentrations of up to 500 μg in total pigments (chlorophyll a plus phaeopigments) and 2 mg in bacterial biomass per liter. Pennate diatoms of the genus Fragilariopsis were most common in the platelet layer, while ice-free water was dominated by autotrophic nanoflagellates. Protozoa contributed only 5% or less to the total protistan (microalgae plus protozoa) cell concentration in the ice, compared to about 10% in open water, thus suggesting a low grazing pressure within the platelet habitat. The bulk of bacterial biomass occurred within the dense assemblages of pennate diatoms that grew attached to the ice platelets. Algal and bacterial concentrations in the interstitial water between platelets were much lower. Measurements of bacterial growth showed that substantial heterotrophic potential can be established within assemblages inhabiting late summer platelet ice. Small-scale analyses of bacterial activity patterns revealed that those bacteria that were closely associated with ice and/or algae showed considerably less biomass-specific substrate uptake than cells that occurred freely suspended in the interstitial water, indicating that their physiological state differed. Received: 21 October 1995/Accepted: 27 January 1996     

Biomass,composition and activity of organism assemblages along a salinity gradient in sea ice subjected to river discharge in the Baltic Sea

A study was undertaken to examine the activity and composition of the seasonal Baltic Sea land-fast sea-ice biota along a salinity gradient in March 2003 in a coastal location in the SW coast of Finland. Using a multi-variable data set, the less well-known algal and protozoan communities, and algal and bacterial production in relation to the physical and chemical environment were investigated. Also, the first coincident measurements of bacterial production and dissolved organic matter (DOM) in a sea-ice system are reported. Communities in sea ice were clearly autotrophy-dominated with algal biomass representing 79% of the total biomass. Protozoa and rotifers made up 18% of biomass in the ice and bacteria only 3%. Highest biomasses were found in mid-transect bottom ice. Water column assemblages were clearly more heterotrophic: 39% algae, 12% bacteria and 49% for rotifers and protozoa. Few significant correlations existed between DOM and bacterial variables, reflecting the complex origin of ice DOM. Dynamics of dissolved organic carbon, nitrogen and phosphorus (DOC, DON and DOP) were also uncoupled. A functional microbial loop is likely to be present in the studied ice. Existence of an under-ice freshwater plume affects the ecosystem functioning: Under-ice water communities are influenced directly by river-water mixing, whereas the ice system seems to be more independent—the interaction mainly taking place through the formation of active bottom communities.     

Comparison of wintertime eukaryotic community from sea ice and open water in the Baltic Sea, based on sequencing of the 18S rRNA gene

The Baltic Sea is one of the world’s largest brackish water basins and is traditionally considered to be species poor. Here, we assessed the diversity of the nano-sized eukaryotic microbial wintertime community, using molecular ecological methods based on sequencing of small-subunit ribosomal RNA gene clone libraries. The results demonstrate that a rich community of small eukaryotes inhabits the Baltic Sea ice and water during winter. The community was dominated by alveolates and stramenopiles. Ciliates and cercozoans were the richest groups present, while in contrast to previous studies, diatoms showed a lower richness than expected. Furthermore, fungi and parasitic Syndiniales were present both in the water and in the sea ice. Some of the organisms in the sea-ice community were active, based on the RNA data, but a number of organisms were inactive or remnants from the freezing process. The results demonstrate that the sea-ice communities in the Baltic Sea are highly diverse and that water and ice of different ages include different protistan assemblages. Our study emphasizes the potential loss in biodiversity through diminishing ice cover as a result of climate change.     

Bacterial Community Succession in Natural River Biofilm Assemblages

Temporal bacterial community changes in river biofilms were studied using 16S rRNA gene-based polymerase chain reaction–denaturing gradient gel electrophoresis (DGGE) followed by sequence analysis. Naturally occurring biofilms were sampled in 2001 during an undisturbed 7-month low-water period in the River Garonne (SW France). During the sampling period epilithic biomass exhibited a particular pattern: two 3-month periods of accumulation that resulted in two peaks in summer and fall, each at about 25 g ash-free dry mass per square meter. Bacterial community DGGE profiles differed between the summer and fall biomass peaks and shared only 30% common operational taxonomic units (OTUs), suggesting the influence of seasonal factors on these communities. During the second biomass accrual phase, bacterial richness and the appearance of new OTUs fitted a conceptual model of bacterial biofilm succession. During succession, five OTUs (corresponding to Dechloromonas sp., Nitrospira sp., and three different Spirosoma spp.) exhibited particular patterns and were present only during clearly defined successional stages, suggesting differences in life-history strategies for epilithic bacteria. Co-inertia analysis of DGGE banding patterns and physical–chemical data showed a significant relationship between community structure and environmental conditions suggesting that bacterial communities were mainly influenced by seasonal changes (temperature, light) and hydrodynamic stability. Within the periods of stability, analysis of environmental variables and community patterns showed the dominant influence of time and maturation on bacterial community structure. Thus, succession in these naturally occurring epilithic biofilm assemblages appears to occur through a combination of allogenic (seasonal) and autogenic changes.     

Development of ice biota in a temperate sea area (Gulf of Bothnia)

A study of sea ice biota was carried out in the Gulf of Bothnia (northern Baltic Sea) during the winter of 1989–1990. Samples (ice cores) were taken at a coastal station at regular time intervals during the ice season. Chlorophyll a concentration, algal species distribution, bacterial numbers, and primary and bacterial production were measured. Colonization of the ice began in January when daylight was low. As the available light increased, the algae started to grow exponentially. The vertical chlorophyll a distribution changed and algal species composition and biomass changed during the season. During the initial and middle phase of colonization, ice-specific diatoms, Nitzschia frigida and Navicula pelagica, dominated the algal biomass. Nutrients (PO₄ ^3– and NO_3j) were found to be depleted during the time of algal exponential growth. The maximum algal biomass exceeded 800 g C 1^–1. The primary production supplied food for heterotrophic organisms. The presence of heterotrophic organisms of different trophic levels (bacteria, flagellates, ciliates and rotifers) indicated an active microbial food web.     

The Diversity of Bacterial Communities Associated with Atlantic Cod <Emphasis Type="Italic">Gadus morhua</Emphasis>

The spatial and temporal changes in the bacterial communities associated with the Atlantic cod Gadus morhua were investigated using terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S recombinant DNA (rDNA). Epidermal mucous was sampled from 366 cod caught in three harvest locations (Baltic, Icelandic, and North Seas) over three seasons (spring 2002, autumn 2002, and spring 2003), and an automated method for the high-throughput processing of environmental samples was developed using a Qiagen BioRobot. The analysis revealed that a diverse consortium of bacteria were found on fish; γ-proteobacteria and Cytophaga–Flavobacter–Bacteroides (CFB) species were dominant. T-RFLP peak profiles suggested that operational taxonomic units (OTUs) related to Photobacterium sp., Psychrobacter sp., and Bacteroides sp. were common to all sites in all three seasons, but there were intersite variations in community composition. Cod caught from different seas had distinct reproducible bacterial assemblages. Whereas communities from fish caught in the Baltic and Icelandic Seas were relatively stable over the three seasons, those from fish from the North Sea changed significantly over time.     

Further observations on flagellates within sea ice in northern Bothnian Bay, the Baltic Sea

To improve our knowledge of flagellates inhabiting the Baltic Sea ice and water column during the winter, material was obtained from northern Bothnian Bay in March/April 1995. Light microscopical observations on live and fixed material and further transmission electron microscopy of whole mounts revealed 47 nanoflagellate taxa. In addition, detached scales of eight taxa were encountered. It is now evident that nearly all nanoflagellate classes are present within Bothnian Bay sea ice. The most common groups were cryptomonads, dinoflagellates, chrysophytes, prasinophytes, choanoflagellates and heterotrophic flagellates of unknown systematic position (Protista incertae sedis). Most flagellates in Baltic Sea ice biota apparently thrive in both the water column and the sea ice, while some, e.g. Paraphysomonas spp. (Chrysophyceae), heterotrophic euglenids, volvocalean chlorophytes, and some taxa of uncertain systematic affinity, are more frequently found within the sea ice. Received: 9 February 1997 / Accepted: 23 September 1997     

Arctic Sea ice biota: design and evaluation of a mesocosm experiment

A mesocosm experiment (enclosure volume 220 l) was designed such that sea ice inhabited by Arctic Sea ice organisms was formed and maintained under natural conditions at 66°N in Rovaniemi, Finland. The experiment was run from natural freezing in December 1994 to melting in April 1995. The ice was inhabited by diatoms, chlorophyceae, heterotrophic flagellates, ciliates, nematodes and turbellarians. Biomass in the ice, expressed as Chlorophyll a concentration, was 20–110 μg l⁻¹; total cell densities varied from 5 × 10⁶ to 35 × 10⁶ cells l⁻¹. Amongst phototrophic organisms, a succession from a flagellate-dominated community (Chlamydomonas sp.) to a multi-species diatom-dominated community was observed. Typical Arctic species such as Nitzschia frigida and Melosira arctica were present in the ice. Bacterial concentration varied between 2 × 10⁸ and 7 × 10⁸ cells l⁻¹. At least two trophic levels were present in the ice. Received: 3 April 1997 / Accepted: 9 September 1997     

The role of platelet ice microalgae in seeding phytoplankton blooms in Terra Nova Bay (Ross Sea,Antarctica): a mesocosm experiment

The aim of this study was to assess the role of platelet ice microalgal communities in seeding pelagic blooms. Nutrient dynamics, microalgal biomass, photosynthetic parameters, cell densities and species succession were studied in two mesocosm experiments, designed to simulate the transition of microalgal communities from platelet ice habitat to pelagic conditions. The microalgal assemblages were dominated by diatoms, 70% of which were benthic species such as Amphiprora kufferathii, Nitzschia stellata, and Berkeleya adeliensis. Photoacclimation of benthic species was inadequate also at relatively low irradiances. Exceptional growth capacity at different light levels was observed for pelagic species such as Fragilariopsis cylindrus and Chaetoceros spp. which may be important in seeding blooms at ice breakup. Fragilariopsis cylindrus showed high growth rates both at 65 and 10% of incident light and in nutrient replete as well as in nutrient depleted conditions. Five days after inoculation, phytoplankton biomass increased and nutrient concentrations decreased in both light conditions. Nutrient uptake rates were up to 9.10 μmol L⁻¹ d⁻¹ of TIN in the high light tank and 6.18 μmol L⁻¹ d⁻¹ in the low light tank and nutrient depletion in the high light tank occurred 3 days prior to depletion in the low light tank. At nutrient depletion, biomass concentrations were similar in both tanks, 30 and 34 μg Chla L⁻¹. This article belongs to a special topic: Five articles on Sea-ice communities in Terra Nova Bay (Ross Sea), coordinated by L. Guglielmo and V. Saggiomo, appear in this issue of Polar Biology. The studies were conducted in the frame of the National Program of Research in Antarctica (PNRA) of Italy.     

Biogeochemical conditions and ice algal photosynthetic parameters in Weddell Sea ice during early spring

Physical, biogeochemical and photosynthetic parameters were measured in sea ice brine and ice core bottom samples in the north-western Weddell Sea during early spring 2006. Sea ice brines collected from sackholes were characterised by cold temperatures (range −7.4 to −3.8°C), high salinities (range 61.4–118.0), and partly elevated dissolved oxygen concentrations (range 159–413 μmol kg⁻¹) when compared to surface seawater. Nitrate (range 0.5–76.3 μmol kg⁻¹), dissolved inorganic phosphate (range 0.2–7.0 μmol kg⁻¹) and silicic acid (range 74–285 μmol kg⁻¹) concentrations in sea ice brines were depleted when compared to surface seawater. In contrast, NH₄ ⁺ (range 0.3–23.0 μmol kg⁻¹) and dissolved organic carbon (range 140–707 μmol kg⁻¹) were enriched in the sea ice brines. Ice core bottom samples exhibited moderate temperatures and brine salinities, but high algal biomass (4.9–435.5 μg Chl a l⁻¹ brine) and silicic acid depletion. Pulse amplitude modulated fluorometry was used for the determination of the photosynthetic parameters F _v/F _m, α, rETR_max and E _k. The maximum quantum yield of photosystem II, F _v/F _m, ranged from 0.101 to 0.500 (average 0.284 ± 0.132) and 0.235 to 0.595 (average 0.368 ± 0.127) in the sea ice internal and bottom communities, respectively. The fluorometric measurements indicated medium ice algal photosynthetic activity both in the internal and bottom communities of the sea ice. An observed lack of correlation between biogeochemical and photosynthetic parameters was most likely due to temporally and spatially decoupled physical and biological processes in the sea ice brine channel system, and was also influenced by the temporal and spatial resolution of applied sampling techniques.     

Bacterial Community Composition in Central European Running Waters Examined by Temperature Gradient Gel Electrophoresis and Sequence Analysis of 16S rRNA Genes

The bacterial community composition in small streams and a river in central Germany was examined by temperature gradient gel electrophoresis (TGGE) with PCR products of 16S rRNA gene fragments and sequence analysis. Complex TGGE band patterns suggested high levels of diversity of bacterial species in all habitats of these environments. Cluster analyses demonstrated distinct differences among the communities in stream and spring water, sandy sediments, biofilms on stones, degrading leaves, and soil. The differences between stream water and sediment were more significant than those between sites within the same habitat along the stretch from the stream source to the mouth. TGGE data from an entire stream course suggest that, in the upper reach of the stream, a special suspended bacterial community is already established and changes only slightly downstream. The bacterial communities in water and sediment in an acidic headwater with a pH below 5 were highly similar to each other but deviated distinctly from the communities at the other sites. As ascertained by nucleotide sequence analysis, stream water communities were dominated by Betaproteobacteria (one-third of the total bacteria), whereas sediment communities were composed mainly of Betaproteobacteria and members of the Fibrobacteres/Acidobacteria group (each accounting for about 25% of bacteria). Sequences obtained from bacteria from water samples indicated the presence of typical cosmopolitan freshwater organisms. TGGE bands shared between stream and soil samples, as well as sequences found in bacteria from stream samples that were related to those of soil bacteria, demonstrated the occurrence of some species in both stream and soil habitats. Changes in bacterial community composition were correlated with geographic distance along a stream, but in comparisons of different streams and rivers, community composition was correlated only with environmental conditions.     

Responses of Baltic Sea Ice and Open-Water Natural Bacterial Communities to Salinity Change

To investigate the responses of Baltic Sea wintertime bacterial communities to changing salinity (5 to 26 practical salinity units), an experimental study was conducted. Bacterial communities of Baltic seawater and sea ice from a coastal site in southwest Finland were used in two batch culture experiments run for 17 or 18 days at 0°C. Bacterial abundance, cell volume, and leucine and thymidine incorporation were measured during the experiments. The bacterial community structure was assessed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified partial 16S rRNA genes with sequencing of DGGE bands from initial communities and communities of day 10 or 13 of the experiment. The sea ice-derived bacterial community was metabolically more active than the open-water community at the start of the experiment. Ice-derived bacterial communities were able to adapt to salinity change with smaller effects on physiology and community structure, whereas in the open-water bacterial communities, the bacterial cell volume evolution, bacterial abundance, and community structure responses indicated the presence of salinity stress. The closest relatives for all eight partial 16S rRNA gene sequences obtained were either organisms found in polar sea ice and other cold habitats or those found in summertime Baltic seawater. All sequences except one were associated with the α- and γ-proteobacteria or the Cytophaga-Flavobacterium-Bacteroides group. The overall physiological and community structure responses were parallel in ice-derived and open-water bacterial assemblages, which points to a linkage between community structure and physiology. These results support previous assumptions of the role of salinity fluctuation as a major selective factor shaping the sea ice bacterial community structure.     

Photoacclimation in Antarctic bottom ice algae: an experimental approach

The aim of the study was to investigate the capacity of microalgae from the extremely low light habitat of bottom ice to acclimate to different light conditions. During austral spring 1997 the bottom layer of land-fast ice in Terra Nova Bay displayed high values of microalgal biomass up to 2,400 μg Chla L⁻¹ concentrated in a few centimetres ice layer. The algal assemblage was dominated by benthic pennate diatoms. Photoacclimation of the microalgae was addressed in terms of pigment spectra and photosynthetic parameters. Immediate and long term (minutes to days) changes in the photoprotective pigments (DD-cycle) were analysed. Severe photodamage occurred in microalgal assemblages exposed to high light. However, part of the bottom ice algal community showed a notable ability to acclimate to high irradiance levels. Changes in photosynthetic parameters preceded the sudden abrupt changes in pigment synthesis and the rapid increase in biomass and growth rates. This article belongs to a special topic: Five articles on Sea-ice communities in Terra Nova Bay (Ross Sea), coordinated by L. Guglielmo and V. Saggiomo, appear in this issue of Polar Biology. The studies were conducted in the frame of the National Program of Research in Antarctica (PNRA) of Italy.     

Small nanoplankton and bacteria in the Western Ross Sea during sea-ice retreat (spring 1994)

Spatial changes of small nanoplankton (2–10 μm) were investigated in relation to sea-ice conditions, hydrography and receding ice processes in the Ross Sea (Antarctica) during spring 1994. Abundance and biomass of heterotrophic and autotrophic nanoplankton, as well as bacterioplankton, were determined along a south-north transect from the open waters polynya towards the pack ice. Autotrophic and heterotrophic nanoplankton biomass ranged from 758 to 4570 mgC m⁻² and from 3 to 387 mgC m⁻², respectively. Heterotrophic nanoplankton accounted, on average, for about 9% of the total (i.e. autotrophic plus heterotrophic) nanoplankton biomass. The size structure of both auto- and heterotrophic nanoplankton in the Ross Sea continental shelf receding ice edge was different from that of nanoplankton associated with the shelf break and open Antarctic ice-edge area. Generally, the highest heterotrophic biomass was found in the pack-ice zone on the continental shelf, while the highest heterotrophic contribution to the total nanoplankton biomass (up to 25%) was encountered at the shelf break where phytoplankton was largely dominated by 2- to 3-μm-size cells. Accepted: 2 May 1999     

Sea ice bacterial growth rate, growth efficiency and preference for inorganic nitrogen sources in the Baltic Sea

Seasonal Baltic Sea ice is structurally similar to polar sea ice and provides habitats for diverse ice organism assemblages that are integral to the biogeochemistry and ecology of the sea during winter. Temperature and inorganic nitrogen sources have been suggested to control bacterial growth, with increasing dependence on ammonium at low temperatures. To study the bacterial growth and preference for the nitrogen source, we conducted experiments at 0 and 4°C, using ammonium and nitrate as nitrogen sources at two coastal fast-ice stations in the Gulf of Finland and in the Gulf of Bothnia during three successive winters. The two study sites differ markedly in relation to the allochthonous dissolved organic matter supply from the catchment area. High levels of bacterial growth were recorded at both study sites, with community generation times of 15–37 h. The measured bacterial growth efficiencies of 20–58% suggest that the Baltic sea ice brines provide a rich medium for bacterial growth and efficient functioning of bacteria-based food webs. Our experiments with sea ice samples showed a preference for ammonium at both temperatures and high potential growth in both types of nitrogen supplies. No major differences in phosphorus depletion rates were found at the two temperatures, but rates were always highest when ammonium was added to the experiments. These experiments point out that ice maturity, presumably through changes in bacterial community structure, impacts nitrogen processes and that these processes are pronounced prior to melting of the ice.     

Distribution of the under-ice mesozooplankton in the Kara Sea in February 2002

Mesozooplankton distribution was investigated under the sea ice in the Kara Sea at five stations in February of 2002 by Juday net hauls. Copepods dominated the mesozooplankton community, accounting for 46–88% of the total abundance and 68–99% of the biomass. Oithona similis was the most abundant species in Yenisei Bay, being present with all age stages (including egg-carrying females). For the first time, Oithona atlantica (CIII–CV copepodites, females and males) were found in the southeastern Kara Sea. In the southern part, Copepoda nauplii prevailed in terms of total abundance while the mesozooplankton in the northwestern part was entirely dominated by older stages of Pseudocalanus minutus. The mesozooplankton structure appears to be determined by available food resources and increased water temperature due to a strong influence of warm Atlantic waters.