Marit Reigstad

On the trophic fate of Phaeocystis pouchetii (Hariot). VI: Significance of Phaeocystis-derived mucus for vertical flux

The development and decline of a phytoplankton spring bloom dominated by the prymnesiophyte Phaeocystis pouchetii were studied in Balsfjord, northern Norway between 30 March and 27 May 1992. At a fixed station, the concentration and composition of suspended particulate matter was monitored and compared to the particulate matter collected in sediment traps at six different depths. Direct sedimentation of phytoplankton contributed a minor fraction to particle flux and was confined to a few diatom genera. No evidence was found for pronounced aggregation of Phaeocystis colonies during bloom decline or direct sedimentation of either Phaeocystis colonies or single cells, Particle flux was dominated by faecal-pellet sedimentation during most of the study period, suggesting zooplankton grazing to be a main loss factor. Despite an abrupt decrease in faecal-pellet sedimentation after the decline of the bloom, particulate-carbon sedimentation rates remained high. High post-bloom sedimentation rates were characterized by elevated C/N and C/Chl a ratios of largely amorphous sedimented material. Post-bloom sedimentation coincided with a decrease in transparent exopolymeric particles (TEP) in the surface layer, suggesting that this change resulted from aggregation and sedimentation of carbon-rich exopolymeric material accumulated in the surface layer in the course of the bloom. While organic-carbon accumulation indicates the significance of disintegration of Phaeocystis colonies, post-bloom mucilage sedimentation could be a secondary pathway for the vertical flux of Phaeocystis-derived organic matter.

Variations in hydrography, nutrients and chlorophyll a in the marginal ice-zone and the central Barents Sea

The project “Climatic variability and vertical carbon flux in the marginal ice zone in the central Barents Sea” was initiated to fill some of the gaps in our knowledge on the biological processes related to the dynamic hydrography in the Barents Sea. A previously modelled transect from the Atlantic waters, crossing the Polar Front into the Arctic waters and the MIZ in the central Barents Sea, was investigated to cover the zonal structure and different water masses. The present paper describes the hydrography, nutrients and Chl a distribution in March, May 1998 and July 1999 along this transect. Based on the nutrient consumption, the new production is estimated and discussed as related to topography, water masses and climate change. Atlantic water dominated in south with a Polar Front shaped by the bank topography, and water with more Arctic characteristics in north. A high, uniform nutrient regime in March was depleted giving a spring bloom in May with Chl a accumulation <100 m in the Atlantic dominated region. The phytoplankton biomass was concentrated in the upper 30 m in the strongly stratified MIZ. The new production estimates for the period ranged 30–80 g C m−2 (0.5–1.4 g C m−2 day−1). New production rates were closely related to the mixing depth with highest rates in the deeper mixed Atlantic region and trenches where the Polar Front was located. Non-Si demanding species were more important for new production in the deeper mixed regions. Seasonal changes from May to July was most likely masked by interannual variations as the July cruise took place the following year, characterised as a warmer year than 1998 in the Barents Sea due to increased Atlantic inflow in 1999 A locally produced cold but saline water mass observed on Sentralbanken in March and May resulting from the freezing process in the waters above the bank was replaced by warmer waters in July and the strongly stratified MIZ was pushed further north. Interannually variable hydrographic regimes in different regions influence the new production and the biological community in the Barents Sea.

Experimental evaluation of planktonic respiration response to warming in the European Arctic Sector

The Arctic Ocean is the region on Earth supporting the steepest warming rate and is also particularly vulnerable due to the vanishing ice cover. Intense warming in the Arctic has strong implications for biological activity and the functioning of an Arctic Ocean deprived of ice cover in summer. We evaluated the impact of increasing temperature on respiration rates of surface marine planktonic communities in the European Arctic sector, a property constraining the future role of the Arctic Ocean in the CO2 balance of the atmosphere. We performed experiments under four different temperature elevation regimes (in situ, +2, +4 and +6°C above the temperature of the sampled water) during cruises conducted in the Fram Strait region and off Svalbard during late fall–early winter, spring and summer. During late fall–early winter, where only three different temperatures were used, no response to warming was observed, whereas respiration rates increased in response to warming in spring and summer, although with variable strength.

Selected aspects of the physical oceanography and particle fluxes in fjords of northern Norway

Results regarding the physical oceanography and the dynamics of particulate fluxes in fjords of northern Norway are presented. The phytoplankton spring bloom takes usually place in April in almost homogeneous water and comes to an end before the estuarine circulation starts in late May/early June when snow and ice melting gives rise to usually one pronounced pulse of freshwater run-off. During late summer and autumn river run-off is usually small and of limited significance for the particulate dynamics. Much of the spring bloom material is apt to sink to the bottom, but overwintering herbivores give rise to decreased vertical losses from the upper layers as well as a tendency towards a decreased seasonal variability compared to more enclosed coastal systems in boreal fjords of southern Norway. While destruction and mineralisation of sedimenting matter is of significance below the euphotic zone, giving rise to a decrease in vertical flux, resuspension is of importance in the lower water column and close to the rivers. Coastal currents strongly influence the north Norwegian fjords and particulate signals from rivers are small and do not penetrate extensively into the fjords. Advection of particulate matter, phytoplankton and zooplankton along with various water masses in and out of the fjords seems to play an important role for the ecology and particulate fluxes in this area. The rapid exchange of water masses between the coastal currents and even the innermost fjords as well as the comparatively small discharge of freshwater gives rise to scenarios where particulate fluxes inside the coastal zone are to a large extent determined by external, oceanic forcing. North Norwegian fjords are, therefore, not independent entities, but in various degrees part of the Norwegian Coastal Current.

Seasonal variability and fluxes of nitrate in the surface waters over the Arctic shelf slope

Understanding the present state and possible future scenarios of Arctic Ocean primary productivity has been hampered by the scarcity of year-round nutrient measurements. Here the first yearlong moored time series of near-surface nitrate concentrations in the Eastern Arctic, together with hydrography, currents, and chlorophyll a fluorescence, is reported from the shelf slope northeast of Svalbard. Variability was dominated by the inflow of Atlantic Water (AW). Nitrate was near depleted during July–September and reached a maximum concentration of 10 μM in March. Vertical nitrate gradients were eroded by mid-December, demonstrating the importance of the AW in breaking down upper ocean stratification during fall. Upward nitrate fluxes through the nitracline in the AW inflow region during fall were 2.5 ± 0.5 mmol m−2 d−1. The spring bloom triggered extensive nitrate drawdown from June, from which an annual new production of 31 g C m−2 was estimated.

Sources of settling material: aggregation and zooplankton mediated fluxes in the Gulf of Riga

Abstract The composition of sedimenting matter from the upper mixed layer was studied in relation to aggregation dynamics and food web structure in the stratified southern Gulf of Riga, Baltic Sea. Four stations were visited during three fortnight periods in spring (1995), midsummer (1994) and late summer (1993). In spring, diatoms constituted the major part of the sedimenting matter, and their abundance in the water column declined due to the sedimentation loss. A technique which preserves the structure of the sedimenting matter was applied; it revealed that neither diatoms nor other algae were sinking as aggregates. Occasionally both motile phytoplankton and heterotrophic organisms concentrated in the sediment traps, possibly due to vertical migration. In midsummer, and especially late summer, heterotrophic biomass was large relative to autotrophic biomass; however, total heterotrophic biomass had a maximum during midsummer. During the summer periods the sedimenting matter consisted of products from the metazoa (faecal pellets, eggs, cuticles) as well as of amorphous detritus, which mainly sedimented in the form of aggregates. Physical coagulation may have produced these aggregates, but turbulent shear, which was the major force making suspended particles collide, did not correlate with aggregate sedimentation. A disintegration factor related to high turbulence may also be of importance. The sedimentation of faecal particles and other detritus was low compared to the zooplankton biomass (0.6–8% day −1 ). Despite the eutrophic conditions of the Gulf of Riga there is a high recycling relative to the sedimentation. This may be related to the importance of auto- and heterotrophic picoplankton in the carbon flow, to the generally high heterotrophic biomass and to an efficient detritus decomposition.

Importance of advection for pelagic-benthic coupling in north Norwegian fjords

Abstract Suspended biomass in the upper layer and sedimentation of organic matter were measured during spring and early summer simultaneously in Balsfjord and Malangen, northern Norway (ca 70° N) in 1992 at four different depths ranging from 30 to 170 m and 330 m, respectively. The bloom took place in almost unstratified waters in April and was dominated by diatoms and Phaeocystis pouchetii, Hariot. The biomass of the early bloom was exported from the fjord due to advection, resulting in low sedimentation rates in the fjord. Several advective episodes were observed during the course of the bloom. These were caused by large scale variation in the wind field along the coast. The bloom continued until the end of April with high POC fluxes ranging from 300-1120 mg m-2 day-1 at all depths. Suspended POC in the upper 36 m was high, varied between 16 and 31 g m-2 day-1 and was not correlated to the phytoplankton bloom. It is suggested that topographical controlled down-fjord wind stress during periods of low str...

Seasonal and spatial variation of suspended and sedimented nutrients (C, N, P) in the pelagic system of the Gulf of Riga

Abstract The temporal and spatial variation of suspended and sedimented C, N and P in the pelagic system in the central and southern part of the eutrophicated Gulf of Riga were investigated during late spring, summer and early autumn from 1993–1995. The temporal variability at a central station was as high or greater than the spatial variability. Thus no discernible influence of the highly eutrophicated river Daugava on the plankton dynamics and the pelagic–benthic coupling of the Gulf of Riga was observed. High sedimentation rates of carbon-rich organic material were recorded at the end of the spring bloom, while P and N were retained on average 5 and 1.2 times more effectively, respectively, than C in the pelagic system. Low sedimentation rates of P, indicating effective recycling in the mixed surface layer, were typical for most of the productive season. The low C/N ratios of the settled material during summer were probably caused by accumulation of heterotrophic dinoflagellates into the sediment traps rather than due to increased vertical export of N. It is suggested that the major supply of organic matter to the benthos is determined by the time window characterised by low heterotrophic diversity and biomass in early spring. The trophic complexity of the planktonic community in the southern and central Gulf of Riga with its continuous discharge of nutrients by large rivers such as Daugava, results in a high buffering capacity of the pelagic system towards external perturbations such as increased nutrient supply.

Partitioning of polychlorinated biphenyls between Arctic seawater and size‐fractionated zooplankton

Concentrations of hydrophobic organic contaminants in zooplankton have been hypothesized to be governed by either near-equilibrium partitioning with surrounding water, growth dilution, or biomagnification. Concentrations of 17 polychlorinated biphenyls (PCBs) were measured in size-fractionated zooplankton, in phytoplankton (> 0.7 microm), and in the dissolved water phase (< 0.7 microm) in the surface water of the northern Barents Sea marginal ice zone east and north of Spitsbergen (Norway) and in the central Arctic Ocean at 89 degrees N. The linear partition model was used to indirectly assess if PCBs were equilibrated between water and the extractable organic matter (EOM) of zooplankton. As an independent test, the relation between the EOM-normalized partition coefficient (log K(EOM)) and trophic level (TL) of the zooplankton (based on delta 15N) was investigated. All log K-log K(OW) regressions were significant (n=18, p < 0.05, r2 = 0.65-0.95), being consistent with near-equilibrium partitioning and indirectly suggesting the absence of biomagnification. No correlation was found between log K(EOM) and TL, further supporting the apparent absence of biomagnification in zooplankton. One implication of these results is a reduced uncertainty in modeling of food web uptake, in which kinetic parameterizations of biodilution or biomagnification in zooplankton may be replaced by a simpler parameterization based on equilibrium partitioning.

Strong Seasonality of Marine Microbial Eukaryotes in a High-Arctic Fjord (Isfjorden, in West Spitsbergen, Norway)

ABSTRACT The Adventfjorden time series station (IsA) in Isfjorden, West Spitsbergen, Norway, was sampled frequently from December 2011 to December 2012. The community composition of microbial eukaryotes (size, 0.45 to 10 μm) from a depth of 25 m was determined using 454 sequencing of the 18S V4 region amplified from both DNA and RNA. The compositional changes throughout the year were assessed in relation to in situ fjord environmental conditions. Size fractionation analyses of chlorophyll a showed that the photosynthetic biomass was dominated by small cells (<10 μm) most of the year but that larger cells dominated during the spring and summer. The winter and early-spring communities were more diverse than the spring and summer/autumn communities. Dinophyceae were predominant throughout the year. The Arctic Micromonas ecotype was abundant mostly in the early-bloom and fall periods, whereas heterotrophs, such as marine stramenopiles (MASTs), Picozoa, and the parasitoid marine alveolates (MALVs), displayed higher relative abundance in the winter than in other seasons. Our results emphasize the extreme seasonality of Arctic microbial eukaryotic communities driven by the light regime and nutrient availability but point to the necessity of a thorough knowledge of hydrography for full understanding of their succession and variability.