Svein Kristiansen

The freshwater composition of the Fram Strait outflow derived from a decade of tracer measurements

The composition of the Fram Strait freshwater outflow is investigated by comparing 10 sections of concurrent salinity, ?18O, nitrate and phosphate measurements collected between 1997 and 2011. The largest inventories of net sea ice meltwater are found in 2009, 2010 and 2011. The 2009–2011 sections are also the first to show positive fractions of sea ice meltwater at the surface near the core of the EGC. Sections from September 2009–2011 show an increased input of sea ice meltwater at the surface relative to older September sections. This suggests that more sea ice now melts back into the surface in late summer than previously. Comparison of April, July and September sections reveals seasonal variations in the inventory of positive sea ice meltwater, with maximum inventories in September sections. The time series of sections reveals a strong anti-correlation between meteoric water and net sea ice meltwater inventories, suggesting that meteoric water and brine may be delivered to Fram Strait together from a common source. We find that the freshwater outflow at Fram Strait exhibits a similar meteoric water to net sea ice meltwater ratio as the central Arctic Ocean and Siberian shelves, suggesting that much of the sea ice meltwater and meteoric water at Fram Strait may originate from these regions. However, we also find that the ratio of meteoric water to sea ice meltwater inventories at Fram Strait is decreasing with time, due to an increased surface input of sea ice meltwater in recent sections.

The hydrography and biology of a bloom of the coccolithophorid Emiliania huxleyi in the northern north sea

In June=July 1994 a study was made of a small bloom of the coccolithophorid Emiliania huxleyi in an area of the North Sea to the east of the Shetland Islands. Observations on the hydrography of the study area indicated the bloom was associated with Atlantic water and was confined to an area in which a stable shallow mixed layer had formed. There was no evidence to suggest association of horizontal physical structure with the bloom development. High cell densities of >1‐6 10 6 cells dm 3 , together with low concentrations of PIC (<50 m gd m 3 ) and detached liths (2‐3 10 4 liths cm 3 ) indicated that the bloom was studied at an early stage of development. Biochemical and physiological observations indicated active growth was taking place. The results presented are discussed in comparison with previous studies carried out in both oceanic and shelf seas.

Base-line variations in stable isotope values in an Arctic marine ecosystem: effects of carbon and nitrogen uptake by phytoplankton

Stable isotope values are useful for elucidating C and N cycling and pathways in marine and aquatic ecosystems. Variations in the base-line isotope values, the δ13C and δ15N values of phytoplankton, put constraints on their usefulness as tracers for trophic interactions and sources of organic matter in food web studies, however. We investigated the C and N stable isotope values of suspended particulate organic matter in relation to uptake of total dissolved inorganic carbon and nitrate, chlorophyll a concentration and the isotope composition of dissolved inorganic carbon in an Arctic marine environment (northern Barents Sea) in order to improve the understanding of factors regulating the variation in stable isotope values at the base of the marine food web. The stable isotope values of water-column suspended particulate organic carbon (δ13Corg) and nitrogen (δ15Norg) varied from −28.3‰ to −20.2‰ and 2.9‰ to 8.3‰, respectively, among stations sampled during spring and summer. δ13Corg was not linearly related to carbon uptake, but the values were on average 3‰ higher at stations in a late-bloom stage, characterised by higher carbon uptake compared to early-bloom stations. Accumulation of phytoplankton biomass had a strong impact on δ13Corg values, reflected in a positive relationship between δ13Corg and chlorophyll a concentration. δ15Norg was positively related to the percentage of nitrate taken up from initial (winter) concentrations. These results indicate a strong relationship between bloom progression and isotope composition of particulate organic C and N pools. Synoptic data on stable isotope compositions, nutrient concentrations and phytoplankton biomass therefore improve the interpretation of isotope values when these are compared across pools with different turnover times, such as phytoplankton and consumers or suspended and sedimentary organic matter.

The importance of silicon for marine production

A review of silicon, with emphasis on its forms, uptake, dissolution and role in marine primary production, is given. The importance of silicon in marine food webs is discussed, as well as the concentrations of silicon in various areas and the importance of changing N:Si:P ratios. The methodology for measuring silicate transformations has recently been improved by the introduction of the highly enriched 32Si isotope. Results from uptake experiments using 32Si in nutrient rich coastal water and in open ocean are presented. The uptake kinetic experiments showed that the silicate uptake usually is unsaturated. We propose that closer attention in the future should be paid to the importance of balanced nutrient composition as well as nutrient supply dynamics for the development of eutrophication versus efficient trophic transfer and fish production in nutrient enriched systems. Close attention should also be paid to the mechanisms that reduce the inputs of silicate to coastal waters.

Floating ice-algal aggregates below melting arctic sea ice.

During two consecutive cruises to the Eastern Central Arctic in late summer 2012, we observed floating algal aggregates in the melt-water layer below and between melting ice floes of first-year pack ice. The macroscopic (1-15 cm in diameter) aggregates had a mucous consistency and were dominated by typical ice-associated pennate diatoms embedded within the mucous matrix. Aggregates maintained buoyancy and accumulated just above a strong pycnocline that separated meltwater and seawater layers. We were able, for the first time, to obtain quantitative abundance and biomass estimates of these aggregates. Although their biomass and production on a square metre basis was small compared to ice-algal blooms, the floating ice-algal aggregates supported high levels of biological activity on the scale of the individual aggregate. In addition they constituted a food source for the ice-associated fauna as revealed by pigments indicative of zooplankton grazing, high abundance of naked ciliates, and ice amphipods associated with them. During the Arctic melt season, these floating aggregates likely play an important ecological role in an otherwise impoverished near-surface sea ice environment. Our findings provide important observations and measurements of a unique aggregate-based habitat during the 2012 record sea ice minimum year.

Nitrogen cycling in the Barents Sea—I. Uptake of nitrogen in the water column

Abstract Uptake of ammonium, nitrate and urea by phytoplankton in the Barents Sea was measured in May–June and in August 1984 by a 15 N-technique. Ammonium was the most important nitrogen source for the phytoplankton. Nitrate was an important nitrogen source in water with large phytoplankton biomass but was of no importance in water with small phytoplankton biomass (after the spring bloom). Nitrate uptake was inhibited by ammonium concentrations > 0.8 μ g-at. N 1 −1 . Urea uptake was variable and highest in phytoplankton and nutrient-poor water. Urea uptake in the dark was 30–50% of light-saturated uptake. The concentrations of urea were ⩽1.7 μ g-at. N 1 −1 . Only 27% of the concentrations were > 0.5 μ g-at. N 1 −1 , and kinetic experiments indicated that some of these values were overestimated. No signs of severe nitrogen limitation of the phytoplankton growth were found.

Nitrogen uptake in the Weddell Sea during late winter and spring

Uptake rates of ammonium, nitrate and urea were measured during the EPOS leg 1 cruise to the Weddell Sea in October–November 1988 using the isotope 15N. Nitrate was the most important nitrogen source both for ice algae (f-ratio ≥0.88) and for phytoplankton in the water column (f-ratio ≥0.85). Indications of a gradual decrease in % new production with time were found in the outer marginal ice zone. Nitrogen uptake rates in ice algae from the sub-ice assemblage were light-limited at in situ irradiances. Significant regeneration of ammonium was found in ice algal samples only.

An Emiliania huxleyi dominated subsurface bloom in Samnangerfjorden, western Norway. Importance of hydrography and nutrients

Abstract Samnangerfjorden is a narrow and sheltered fjord with a pronounced low salinity phosphate- depleted surface layer. The coccolithophorid Emiliania huxleyi (Lohmann) Hay et Mohler bloomed in a nitrate-depleted intermediate layer below the phosphate-depleted surface layer in the inner part of the fjord in May 1992. Maximum cell density was 7 × 106 cells 1−1, and E. huxleyi accounted for 30 % of autotrophic biomass. Both cell density and the calcification rate decreased towards the mouth of the fjord. Production was P-limited in the inner part of the surface layer, while production in the intermediate layer was light-limited. The growth of E. huxleyi in the surface layer was greatly reduced by the low salinity.

Nitrogen uptake in the infiltration community, an ice algal community in Antarctic pack-ice

Abstract An infiltration community was the dominating ice algal community in pack-ice off Queen Maud Land, Southern Ocean, in January 1993. The community was dominated by autotrophic processes, and the most common species were the prymnesiophyte Phaeocystis antarctica and the diatoms Chaetoceros neglectus and Fragilariopsis cylindrus. The concentration of chlorophyll a was 1.3–47.9 μg l−1, and the inner part of the community was nitrate depleted. Uptake rates of nitrate, nitrite, ammonium, urea and amino acids were measured using 15N. Nitrate was the major nitrogen source for ice algal growth (67 ± 6% nitrate uptake). It is suggested that % nitrate uptake in the infiltration community decreases during the growth season, from 92% during spring (literature data) to 67% during summer. Scalar irradiance in the infiltration community was high and variable. It reached ca. 2000 μmol m−2 s−1 at some locations, and nitrate uptake rate was potentially photoinhibited at irradiances >500 μmol m−2 s−1. Nitrate uptake rate in an average infiltration community (0.6 m of snow cover) was lowered by 13% over a 2-week period due to photoinhibition.

Effect of glacial drainage water on the CO2 system and ocean acidification state in an Arctic tidewater‐glacier fjord during two contrasting years

In order to investigate the effect of glacial water on the CO2 system in the fjord, we studied the variability of the total alkalinity (AT), total dissolved inorganic carbon (CT), dissolved inorganic nutrients, oxygen isotopic ratio (d18O), and freshwater fractions from the glacier front to the outer Tempelfjorden on Spitsbergen in winter 2012 (January, March, and April) and 2013 (April) and summer/fall 2013 (September). The two contrasting years clearly showed that the influence of freshwater, mixing, and haline convection affected the chemical and physical characteristics of the fjord. The seasonal variability showed the lowest calcium carbonate saturation state (X) and pH values in March 2012 coinciding with the highest freshwater fractions. The highest X and pH were found in September 2013, mostly due to CO2 uptake during primary production. Overall, we found that increased freshwater supply decreased X, pH, and AT. On the other hand, we observed higher AT relative to salinity in the freshwater end-member in the mild and rainy winter of 2012 (1142 lmol kg21) compared to AT in 2013 (526 lmol kg21). Observations of calcite and dolomite crystals in the glacial ice suggested supply of carbonate-rich glacial drainage water to the fjord. This implies that winters with a large amount of glacial drainage water partly provide a lessening of further ocean acidification, which will also affect the air-sea CO2 exchange.