Eduard Bauerfeind

Pelagic processes and vertical flux of particles: an overview of a long-term comparative study in the Norwegian Sea and Greenland Sea

Pelagic processes and their relation to vertical flux have been studied in the Norwegian and Greenland Seas since 1986. Results of long-term sediment trap deployments and adjoining process studies are presented, and the underlying methodological and conceptional background is discussed. Recent extension of these investigations at the Barents Sea continental slope are also presented. With similar conditions of input irradiation and nutrient conditions, the Norwegian and Greenland Seas exhibit comparable mean annual rates of new and total production. Major differences can be found between these regions, however, in the hydrographic conditions constraining primary production and in the composition and seasonal development of the plankton. This is reflected in differences in the temporal patterns of vertical particle flux in relation to new production in the euphotic zone, the composition of particles exported and in different processes leading to their modification in the mid-water layers.In the Norwegian Sea heavy grazing pressure during early spring retards the accumulation of phytoplankton stocks and thus a mass sedimentation of diatoms that is often associated with spring blooms. This, in conjunction with the further seasonal development of zooplankton populations, serves to delay the annual peak in sedimentation to summer or autumn. Carbonate sedimentation in the Norwegian Sea, however, is significantly higher than in the Greenland Sea, where physical factors exert a greater control on phytoplankton development and the sedimentation of opal is of greater importance. In addition to these comparative long-term studies a case study has been carried out at the continental slope of the Barents Sea, where an emphasis was laid on the influence of resuspension and across-slope lateral transport with an analysis of suspended and sedimented material.

Seasonal variability of sediment trap collections in the Northeast Water Polynya. Part 2. Biochemical and microscopic composition of sedimenting matter

The annual pattern of vertical particle flux in the Northeast Water (NEW) Polynya was recorded from August 1992 to July 1993 by means of moored time-series sediment traps. A distinct seasonal pattern in sedimentation was observed, with highest flux rates during August–October 1992. During this time 40–70% of the annual total sedimented matter (dry weight, DW) and the components, carbonate, particulate organic carbon and nitrogen (POC and PON), particulate biogenic silica (bPSi) and biogenic matter were recorded: 9.83, 2.04, 1.03, 0.69, 0.14 and 5.55 g m−2, respectively. Microscopic analysis of the particles revealed that diatoms contributed about 10% of the POC flux, but up to 40% of the POC flux originated from the houses and faeces of appendicularians during the period of highest flux rates. In contrast, faecal pellets were only a minor component of sedimenting POC after the opening of the polynya in June 1993. During this period a sedimentation event of Melosira arctica dominated the microscopically recognizable fraction of the POC. Following the low winter values a significant deviation in POC flux in March documented an early onset of plankton growth and a rapid response to the formation of a winter polynya paralleled by a local change in ice conditions. This was supported by the stable nitrogen isotope signature of the sedimented matter, also indicating an early onset of plankton production in the NEW Polynya. However, the overall amplitude of the Δ15N signal in the sinking particles showed only small variations (<4‰) and was significantly below the amplitude observed in sedimented material from the Northern North Atlantic (≌ 8‰). The composition of the sedimented matter, comprising mainly fast sinking particles (appendicularian houses, faecal peliets and Melosira aggregates) lead us to conclude that sedimentation in the NEW Polynya was spatially heterogeneous.

Sea‐ice impact on long‐term particle flux in the Greenland Sea's Is Odden‐Nordbukta region, 1985–1996

Five sediment traps deployed in the Greenland Sea at a depth of 500 m between 72°N and 75°N by the Sonderforschungsbereich 313, Kiel, Germany, provide the necessary data to compare particle flux with ambient ice regimes. Sedimentation in this seasonally ice-covered region is dependent upon the following three basic parameters: (1) ice concentration, (2) duration of ice cover, and (3) distance from the ice edge. These factors vary significantly with time and space. We develop algorithms that provide annual sedimentation amounts for the area contained by 71°N to 76°N, the Greenland coast, and 10°E. For a severe ice year the area of seasonal ice cover and an 80-km-wide band extending along the maximum extent of the ice edge, the Biological Marginal Ice Zone (BMIZ), combine to provide 92% of the total sedimentation. For particulate organic carbon and silica this zone accounts for 89% each of the total sedimentation. In a light ice year the respective percentages are 84% for dry weight, 87% for particulate organic carbon, and 81% for biogenic particulate silica. These figures are slightly less than sedimentation for a severe ice year. If the Is Odden-Nordbukta region is replaced by open ocean for purposes of comparison, the BMIZ out produces the open ocean for POC by a factor of 3.2. Projecting the algorithms for the Is Odden-Nordbukta region to the rest of the Greenland Sea, we conclude that the Is Odden-Nordbukta region is a substantial producer of sedimentation.

Particle flux, and composition of sedimenting matter, in the Greenland Sea

Vertical flux of particulate material was recorded with moored sediment traps during 1988/1989 in the Greenland Sea at 72 degrees N, 10 degrees W. This region exhibits pronounced seasonal variability in ice cover. Annual fluxes at 500 m water depth were 22.79, 8.55, 2.39, 3.81 and 0.51 g m(-2) for total flux (dry weight), carbonate, particulate biogenic silicate, particulate organic carbon and nitrogen, respectively. Fluxes increased in April, maximum rates of all compounds occurred in May-June, and consistently high total flux rates of around 100 mg m(-2)d(-1) prevailed during the summer. The increasing flux of biogenic particles measured in April is indicative of an early onset of algal growth in spring. Small pennate diatoms dominated in the trap collections during April, and were still numerous during the high flux period when Thalassiosira species were the most abundant diatoms. During May-June, up to 22% of the Thalassiosira cells collected were viable-looking cells. The faecal pellet flux increased after the May-June event. Therefore we conclude that the diatoms settled as phytodetritus, most likely in rapidly sinking aggregates. From seasonal nutrient profiles it is concluded that diatoms contribute 25% to new production during spring and 50% on an annual basis. More than 50% of newly produced silicate particles are dissolved above the 500 m horizon. High new production during spring does not lead to a pronounced sedimentation pulse of organic matter during spring but elavated vertical export is observed during the entire growth period

Particle flux variability in the polar and Atlantic biogeochemical provinces of the Nordic Seas

A decade of particle flux measurements providse the basis for a comparison of the eastern and western province s of the Nordic Seas. Ice-related physical and biological seasonality as well as pelagic settings jointly control fluxes in the western Polar Province which receive s southward flowing water of Polar origin. Sediment trap data from this realm highlight a predominantly physical flux control which leads to exports of siliceous particle s within the biological marginal ice zone as a prominent contributor. In the northward flowing waters of the eastern Atlanti c Province, feeding strategies, life histories and the succession ofdominant mesozooplankters (copepods and pteropods) are central in controlling fluxes. Furthermore, more calcareous matter is exported here with a shift in flux seasonality towards summer I autumn. Dominant pelagic processes modeled numerically as to their impact on annual organic carbon exports for both provinces confirm that interannual flux variability is related to changes in the respecti ve control mechanisms.

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.

Seasonal variability of sediment trap collections in the Northeast Water polynya. Part 1: sea-ice parameters and particle flux

Vertical particle flux was recorded in the Northeast Water polynya (NEW) by means of moored time series sediment traps from August 1992 to July 1993. Traps were deployed with USCGC Polar Sea at four positions in a trough of the polynya. The recorded annual flux varied between 4.2–9.8 g/m2 at 130 m and 26–33 g/m2 at 50 m above the sea bottom. The amount of settling particles in the NEW is controlled to a high degree by the ice regime (e.g., total ice concentration, variability, ice type fraction, snow and the melt period) due to its impact on the production of primary formed particles. By using passive microwave satellite data from the Special Sensor Microwave/Imager, it was possible to relate particle flux with total ice concentration and its changes on short (several days) and long term scales for individual sampling periods. In combination with the dry weight of the collected particles, it is possible to examine the relation of ice parameters and its impact on the sedimenting processes in the NEW. Of particular interest is the correlation of three sedimenting periods associated with ice formation in the fall, the effect of the winter polynya based on new ice formation, and the melt period.

Summertime plankton ecology in Fram Strait - a compilation of long- and short-term observations

Between Greenland and Spitsbergen, Fram Strait is a region where cold ice-covered Polar Water exits the Arctic Ocean with the East Greenland Current (EGC) and warm Atlantic Water enters the Arctic Ocean with the West Spitsbergen Current (WSC). In this compilation, we present two different data sets from plankton ecological observations in Fram Strait: (1) long-term measurements of satellite-derived (1998–2012) and in situ chlorophyll a (chl a) measurements (mainly summer cruises, 1991–2012) plus protist compositions (a station in WSC, eight summer cruises, 1998–2011); and (2) short-term measurements of a multidisciplinary approach that includes traditional plankton investigations, remote sensing, zooplankton, microbiological and molecular studies, and biogeochemical analyses carried out during two expeditions in June/July in the years 2010 and 2011. Both summer satellite-derived and in situ chl a concentrations showed slight trends towards higher values in the WSC since 1998 and 1991, respectively. In contrast, no trends were visible in the EGC. The protist composition in the WSC showed differences for the summer months: a dominance of diatoms was replaced by a dominance of Phaeocystis pouchetii and other small pico- and nanoplankton species. The observed differences in eastern Fram Strait were partially due to a warm anomaly in the WSC. Although changes associated with warmer water temperatures were observed, further long-term investigations are needed to distinguish between natural variability and climate change in Fram Strait. Results of two summer studies in 2010 and 2011 revealed the variability in plankton ecology in Fram Strait.

Biogenic Particle Sources and Vertical Flux Patterns in the Seasonally Ice-Covered Greenland Sea

Pelagic and ice-associated particle sources have been investigated to determine their contribution to vertical fluxes from upper ocean layers. Process studies were conducted from 1988 to 1997 during various seasons between 72° N and 82° N in the Marginal Ice Zone (MIZ) and open waters of the Greenland Sea. Ice-bound (in-ice and under-ice) particle production begins as early as April, prior to pelagic production, and provides material which may be set free in the course of melting or originate from food-web processes in the under-ice habitat (namely grazing by sympagic amphipods). These particles may be deposited from surface waters, degraded within pelagic food webs or, in the case of autotrophic components, may serve as a seeding population for pelagic production. Findings on transects from the open water into the pack ice stress the overall importance of the MIZ for particle export. The MIZ is characterized by highly variable physical and biological conditions which foster local phytoplankton blooms. Particle exports from the MIZ are very variable but generally high, with prominent autotrophic diatom contributions (up to 60 mg poc m−2 d−1, 30 mg Opal-Si m−2 d−1 and 107 diatoms m−2 d−1). Analyses of algal pigments and their degradation products, combined with microscopical inventories, permit the differentiation of sources of particle export. Freshly produced material from the MIZ can rapidly sediment to great depths, feeding the benthos and affecting sediment geochemistry.

Annual fluxes of particulate chemical trace compounds during the North-East water polynya experiment

Abstract Particulate fluxes of organic marker substances (alkanes, wax esters, pristane, and alkenones) and trace elements (Al, Cd, Cu, Fe, Mn, Ni, and Pb) were determined from identical samples of sediment trap material collected at North East Water Polynya (NEWP) locations E and F in the north-western Greenland Sea. Samples from the East Greenland Shelf were obtained with two sediment traps deployed at 130 m depth over a 331-day period from August 1992 to June 1993 within the NEWP experiment. Particulate fluxes of the chemical tracers showed a pronounced seasonality, which was largely consistent with ice-coverage data. Relative flux pattern exhibited initial maxima during August and September 1992. Fluxes were lowest during winter time (November 1992 to early March 1993), followed by an increase by a factor of 2–5 in mid-March 1993. In contrast, flux of n -alkanes occurred almost entirely in spring 1993. Flux of alkenones was not detectable throughout this experiment. Temporal variations of chemical tracer fluxes at trap location E were similar to those of station F during autumn 1992. Normalization calculations showed that lithogenic material was an important source for trace element fluxes, with an estimated lithogenic fraction of the bulk material flux increasing from autumn to spring. A non-lithogenic flux component was observed for Cd throughout the time of deployment. The chemical data indicate that during the peak flux periods a large proportion of the sinking particles consisted of fresh material. Differences in the patterns of the bulk material flux and the chemical tracer fluxes indicate that there must have been a substantial difference between autumn and spring in the gross chemical composition of the sinking material.