Eva-Maria Nöthig

Seasonal and Regional Variation in the Pelagial and its Relationship to the Life History Cycle of Krill

The earlier concept of Antarctic pelagic seasonality has changed drastically. We now know that the characteristic pelagic community resembles the oligotrophic communities of warm, nutrient-depleted waters. Results of recent sediment trap moorings show that the Antarctic oceanic pelagial behaves as a highly efficient retention system as losses due to sinking particles are exceptionally low. We show that the distinction into “new” and “regenerating” type pelagic systems only applies to restricted regions experiencing sizeable blooms where spring sedimentation pulses have been recorded. Apparently, community biomass is built up by channelling of nitrate into the recycling pool whereby balance between auto- and heterotrophs must be maintained, presumably within time scales of weeks. Interannual variability is likely to be much less pronounced in this type of buffered pelagic system.

Tracing dissolved organic substances and nutrients from the Lena River through Laptev Sea (Arctic)

Abstract Dissolved organic substances and inorganic nutrients were determined in the central and northern Laptev Sea north of the Lena delta region in August 1995. These results are discussed with data from the Lena River from July 1994. Enhanced concentrations of dissolved organic matter (DOM) and silicate in the Lena River generate a concentration gradient extending towards the oceanic regions of the Laptev Sea. Several parameters show higher concentrations in a plume of low salinity water ( a concentrations. While inorganic nutrients increased with depth, all components of DOM decreased. Of these, carbohydrates decreased most rapidly. The distribution of DOC is largely controlled by conservative mixing. A correlation between salinity and silicate indicates its riverine input. Most of the silicate values are below the theoretical dilution line between river and ocean reflecting biological uptake. The distributions of carbohydrates and amino acids were quite variable, with the higher concentrations generally associated to low salinity and high chlorophyll values. Lignin phenols, which are unequivocal tracers for terrestrially derived organic matter, were about 23 nM in the Lena delta region, 7 nM in the lower salinity water and about 2 nM in areas less influenced by river runoff. From these values we estimate that the contribution of riverine derived freshwater to the water budget in the Laptev Sea is about 30 to 10%, decreasing towards the Eurasian Basin. Calculation based on this proportion and on the concentrations of DOC in the Lena River and the Laptev Sea indicate that about 60% of the DOC in the surface layer of the Laptev Sea is of terrigenous origin.

Phytoplankton distribution in relation to sea ice, hydrography and nutrients in the northwestern Weddell Sea in early spring 1988 during EPOS

Phytoplankton biomass and distribution of major phytoplankton groups were investigated in relation to sea ice conditions, hydrography and nutrients along three north-south transects in the north western Weddell Sea in early spring 1988 during the EPOS Study (European Polarstern Study), Leg 1. Three different zones along the transects could be distinguished: 1) the Open Water Zone (OWZ) from 58° to 60°S with high chlorophyll a concentrations up to 3.5 μg 1−1; 2) the Marginal Ice Zone (MIZ) from 60° to about 62.5° with chlorophyll a concentrations between 0.1 and 0.3 μg 1−1, and 3) the closed pack-ice zone (CPI) from 62.5° to 63.2°S with chlorophyll a concentrations below 0.1 μg 1−1. Nutrient concentrations increased towards the south showing winter values under the closed pack-ice. Centric diatoms such as Thalassiosira gravida and Chaetoceros neglectum forming large colonies dominated the phytoplankton assemblage in terms of biomass in open water together with large, long chain forming, pennate diatoms, whereas small pennate diatoms such as Nitzschia spp., and nanoflagellates prevailed in ice covered areas. Fairly low concentrations of phytoplankton cells were encountered at the southernmost stations and many empty diatom frustules were found in the samples. The enhanced phytoplankton biomass in the Weddell-Scotia-Confluence area is achieved through sea ice melting in the frontal zone of two different water masses, the Weddell and the Scotia Sea surface waters.

Genetic diversity in the marine phytoplankton: a review and a consideration of Antarctic phytoplankton

Molecular analysis of phytoplankton population structure has lagged behind other groups and has usually been inferred from physiological data determined from relatively few clones. Nearly every physiological measurement has shown that no single clone of any phytoplankton species can be considered truly representative of that species. One important reason why studies of phytoplankton population structure are perhaps 20 or more years behind those of other organisms is because of the necessity to establish clonal cultures prior to genetic analysis and the inability to perform fine-scale sampling under most conditions. Isozyme analysis, performed for a few species, has revealed heterozygosity between populations. In addition, fingerprinting analyses, such as Random Amplified Polymorphic DNAs (RAPDs) or multi-locus probes, have shown that phytoplankton blooms are not mono-clonal, are highly diverse and isolates are related by geographic origin. In the Southern Ocean, only two studies have been made of the population structure of phytoplankton. The first, based on quantitative genetic analysis of morphometric features, suggests that there is sufficient genetic variation in populations of Thalassiosira tumida to allow speciation in terms of major shifts in morphology under conditions of continued directional selection. The second, using sequence data from the noncoding regions of the internal transcribed spacer region (ITS) in the ribosomal cistron as a molecular marker, shows that populations of Phaeocystis antarctica within continental water masses are homogenous with little evidence of population structure. Populations found within the Antarctic Circumpolar Current are genetically distinct from others, suggesting the currents also play an important role in determining population structure in phytoplankton populations.

Seasonal shifts in ice edge phytoplankton blooms in the Barents Sea related to the water column stability

The development of the phytoplankton bloom and its relation to water column stabilisation during the transition from early to high summer (of 1991) in the seasonally ice-covered zone of the Barents Sea were studied from a meridional transect of repeated hydrographic/biological stations. The water column stabilisation is described in detail with the aid of vertical profiles of the Brunt-Väisälä frequency squared (N2). The contributions of seasonal warming and ice melting to stabilisation are elucidated by determining the effects of temperature and salinity on N2. The spring bloom in 1991 migrated poleward from June to July by about 400 km, associated with the retreat of the ice edge. The spring bloom culminated with maximum chlorophyll concentrations in the mixed layer about 100–300 km north of the centre of the meltwater lens, at its northern edge, where the ice cover was still substantial. From the distribution of N2 it becomes obvious that the bloom starts at the very beginning of stabilisation, which results solely from the release of meltwater. The increase in temperature due to the seasonal warming does not contribute to the onset of vernal blooming; temperature starts to contribute to the stratification later, when the spring bloom has ceased due to the exhaustion of nutrients in the mixed layer. By that time a deep chlorophyll maximum has formed in the seasonal pycnocline, 20–30 m below the base of the mixed layer. The effect of the seasonal ice cover on the mean areal new primary production is discussed.

Phyto- and protozooplankton biomass during austral summer in surface waters of the Weddell Sea and vicinity*

SummaryPhyto- and protozooplankton were sampled in the upper 10 m of the water column in austral summer during a cruise of RV Polarstern from January 6 to February 20 1985 in the eastern Bransfield Strait vicinity and in the northern, southeastern (off Vestkapp, twice: I and II) and southern Weddell Sea (Vahsel Bay across the Filchner Depression to Gould Bay). The plankton assemblages are discussed in relation to physical, chemical and biological factors in the different geographical areas in summer. Phytoplankton biomass (Phytoplankton carbon, PPC) ranged from 4–194 μg carbon/l and consisted on average of 65% diatoms and 35% autotrophic flagellates. Whereas in the northwest phytoplankton assemblages were dominated by small nanoflagellates (78% of PPC), higher biomass of diatoms (54–94% of PPC) occurred at the other sampling sites. In general autotrophic flagellates and small pennate diatoms dominated at oceanic stations; in neritic areas large centric diatoms prevailed. Chlorophyll a concentrations ranged from 0.25–3.14/μg chl a/l with a mean of 1.13/gmg chlorophyll a/l and an average phytoplankton carbon/chlorophyll a ratio of 39. Protozooplankton biomass (Protozooplankton carbon, PZC) ranged from 0–67 μg carbon/l and consisted of 49% ciliates, 49% heterotrophic dinoflagellates and 2% tintinnids. Heterotrophic dinoflagellates were more important in the northern investigation areas (58%–84% of PZC). Ciliates dominated the protozooplankton in the southeast and south (56%–65% of PZC); higher abundances of tintinnids were observed only in the south (11% of PZC). The most remarkable feature of the surface waters was the high protozooplankton biomass: protozooplankton amounted to 25% on an average of the combined biomass of PPC plus PZC for the entire investigation period. Protozoan biomass in the southeastern and southern Weddell Sea occasionally exceeded phytoplankton biomass. Temperature, salinity, and inorganic nutrients were generally lower in the southern regions; at most of these stations a meltwater layer occurred in the upper meters of the water column. We suggest that this physical regime allows a well developed summer system with a high proportion of heterotrophic microplankton. In the eastern Bransfield Strait, in the northern Weddell Sea and close to the coast off Vestkapp (I), however, early summer conditions occurred with less protozooplankton contribution.

Regional relationships between biological and hydrographical properties in the Weddell Gyre in late austral winter 1989

Abstract The surface layer properties of the Weddell Gyre were measured during a cruise of the R/V “Polarstern” in September and October 1989 on a transect between the tip of the Antarctic Peninsula (northwestern Weddell Sea) and Cape Norvegia (southeastern Weddell Sea). Sea ice cover, hydrography, and the distribution of inorganic nutrients and dissolved oxygen represented late winter conditions: a quasi-homogeneous Winter Water layer with near-freezing temperatures, high salinities and high levels of nitrate, and undersaturated with dissolved oxygen. The area investigated could be divided into three regions based on the physical, chemical and biological patterns: the western and eastern flanks and the gyre interior. In all areas, autotrophic biomass in sea ice was high in comparison with the underlying water column. Within the sea ice mainly diatoms and dinoflagellates were present, but the dominant autotrophic organisms in the water column were nanoflagellates. Ammonium values were relatively high in the Winter Water layer in the central region, indicating heterotrophic activity. Mesozooplankton was dominated by copepods, which can in turn be divided into two groups: overwintering, inactive Calanus acutus were found in the Warm Deep Water, whereas actively feeding Calanus propinquus were most abundant in the upper 120 m of the water column under the sea ice in the central region. These spatial differences may influence development of the respective summer pelagic communities.

Spatial variability of phytoplankton, nutrients and new production estimates in the waters around Svalbard

Phytoplankton dynamics and carbon input into Arctic and sub-Arctic ecosystems were investigated around Svalbard, in summer 1991. Phytoplankton biomass, species composition and dissolved nutrient concentrations were analysed from water samples collected along seven transects. Phytoplankton biomass was low especially to the north (Chlorophyll-a mean 0.3 μg 1-1), where flagellates dominated the communities and only ice-diatoms were present. To the west, the phytoplankton composition was representative of a summer Atlantic community, in a post-bloom state. Zooplankton grazing, mainly by copepods, appeared to be the main control on biomass to the west and north of Svalbard. In the Barents Sea (east of Svalbard), an ice edge bloom was observed (Chlorophyll-a max. 6.8 μg1 -1) and the ice edge receded at a rate of approximately 11 km day-1. The phytoplankton community was represented by marginal ice species, especially Phaeocystis poucherii and Chaeroceros socialis. South of the ice edge, Deep Chlorophyll Maxima (DCM) were observed, as surface waters became progressively nutrient-depleted. In these surface waters, the phytoplankton were predominantly auto- and heterotrophic flagellates. Carbon production measurements revealed high net production (new and regenerated) to the north of the Barents Sea Polar Front (BSFW); it was especially high at the receding ice edge (reaching 1.44g C m-1 day-1). To the south, a low level of production was maintained, mainly through regenerative processes.

Protist distribution in the Western Fram Strait in summer 2010 based on 454-pyrosequencing of 18S rDNA.

In this study, we present the first comprehensive analyses of the diversity and distribution of marine protist (micro‐, nano‐, and picoeukaryotes) in the Western Fram Strait, using 454‐pyrosequencing and high‐pressure liquid chromatography (HPLC) at five stations in summer 2010. Three stations (T1; T5; T7) were influenced by Polar Water, characterized by cold water with lower salinity (<33) and different extents of ice concentrations. Atlantic Water influenced the other two stations (T6; T9). While T6 was located in the mixed water zone characterized by cold water with intermediate salinity (~33) and high ice concentrations, T9 was located in warm water with high salinity (~35) and no ice‐coverage at all. General trends in community structure according to prevailing environmental settings, observed with both methods, coincided well. At two stations, T1 and T7, characterized by lower ice concentrations, diatoms (Fragilariopsis sp., Porosira sp., Thalassiosira spp.) dominated the protist community. The third station (T5) was ice‐covered, but has been ice‐free for ~4 weeks prior to sampling. At this station, dinoflagellates (Dinophyceae 1, Woloszynskia sp. and Gyrodinium sp.) were dominant, reflecting a post‐bloom situation. At station T6 and T9, the protist communities consisted mainly of picoeukaryotes, e.g., Micromonas spp. Based on our results, 454‐pyrosequencing has proven to be an adequate tool to provide comprehensive information on the composition of protist communities. Furthermore, this study suggests that a snap‐shot of a few, but well‐chosen samples can provide an overview of community structure patterns and succession in a dynamic marine environment.

Variability in under‐ice export fluxes of biogenic matter in the Arctic Ocean

A critical question regarding the organic carbon cycle in the Arctic Ocean is whether the decline in ice extent and thickness and the associated increase in solar irradiance in the upper ocean will result in increased primary production and particulate organic carbon (POC) export. To assess spatial and temporal variability in POC export, under-ice export fluxes were measured with short-term sediment traps in the northern Laptev Sea in July-August-September 1995, north of the Fram Strait in July 1997, and in the Central Arctic in August–September 2012. Sediment traps were deployed at 2–5 m and 20–25 m under ice for periods ranging from 8.5 to 71 h. In addition to POC fluxes, total particulate matter, chlorophyll a, biogenic particulate silica, phytoplankton, and zooplankton fecal pellet fluxes were measured to evaluate the amount and composition of the material exported in the upper Arctic Ocean. Whereas elevated export fluxes observed on and near the Laptev Sea shelf were likely the combined result of high primary production, resuspension, and release of particulate matter from melting ice, low export fluxes above the central basins despite increased light availability during the record minimum ice extent of 2012 suggest that POC export was limited by nutrient supply during summer. These results suggest that the ongoing decline in ice cover affects export fluxes differently on Arctic shelves and over the deep Arctic Ocean and that POC export is likely to remain low above the central basins unless additional nutrients are supplied to surface waters.