Bernt Zeitzschel

Primary production and sedimentation during spring in the Antarctic Peninsula region

Phytoplankton biomass and composition, primary productivity (in situ simulated and in vitro incubations) and sedimentation rates (measured with free-drifting sediment traps suspended at 100 m depth) were recorded in the Bransfield Strait area of the Antarctic Peninsula during November to December 1980. Three distinct and persistent zones were encountered: low biomass comprising flagellates and diatoms in the Drake Passage and Scotia Sea (zone I): high to moderate biomass of Phaeocystis and diatoms in the northern and central Bransfield Strait (zone II); and moderate biomass (Thalassiosira spp. in the process of forming resting spores) in the vertically homogeneous water on the northern Antarctic Peninsula shelf (zone III). Nutrient concentrations were high throughout; zooplankton grazing relative to phytoplankton biomass and production was heavy in zone I but negligible in the other 2 zones. Rates of primary production in zones I, II, and III averaged 230, 1660 and 830 mg C m−2 d−1, respectively. Assimilation numbers were low throughout (< 1 mg Chl a)−1 h−1) and growth physiology of the zonal phytoplankton assemblages was basically similar. Sedimentation rates recorded by 2 traps in zone II were low (97 and 138 mg C m−2 d−1) and higher (546 mg C m−2 d−1) in a third trap which collected mostly euphausiid faeces. Sedimentation was heaviest in zone III (450 to 1400 mg C m−2 d−1) where collections of the 3 traps deployed were dominated by intact diatom frustules (Thalassiosira spp.). Spore formation and heavy sedimentation of diatoms thus also occurs at the end of Antarctic blooms in spite of high ambient nutrients. As approximately two-thirds of the diatoms in traps were resting spores, we suggest that sinking of cells represents a seeding strategy which ensures regional persistence of neritic assemblages. Species-specific differences in seeding strategies may well be important in determining spatial and temporal patterns of Antarctic phytoplankton abundance. This aspect of phytoplankton biology is likely to have far-reaching implications, not previously considered, for the structure of Antarctic food webs.

Basin‐wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration

Particle flux data from 27 sites in the Atlantic Ocean have been compiled in order to determine regional variations in the strength and efficiency of the biological pump and to quantify carbon fluxes over the ocean basin, thus estimating the potential oceanic sequestration of atmospheric CO2. An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the export ratio (ER = POC flux/primary production) at 125 m of between 0.08 and 0.38 over the range of production from 50 to 400 g C m−2 yr−1. Significant regional differences in changes of the export ratio with depth are related to the temporal stability of flux. Sites with more pulsed export have higher export ratios at 125 m but show more rapid decreases of POC flux with depth, resulting in little geographic variation in fluxes below ∼3000 m. The opposing effects of organic carbon production and calcification on ΔpCO2 of surface seawater are considered to calculate an “effective carbon flux” at the depth of the euphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone integrated over the Atlantic Ocean between 65°N and 65°S amounts to 3.14 Gt C yr−1. Of this, 5.7% is remineralized above the winter mixed layer and thus does not contribute to CO2 sequestration on climatically relevant timescales. The effective carbon flux, termed Jeff, amounts to 2.47 Gt C yr−1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (seen in a decrease of the opal:carbonate ratio) is seen across the entire North Atlantic, indicating a basin-wide phenomenon that may be related to large-scale changes in climatic forcing.

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.

The plankton tower. II. Release of nutrients from sediments due to changes in the density of bottom water

Results of an enclosure experiment carried out in Kiel Bight are presented. A water column of about 30 m3, extending from the surface to the bottom over a 3 m2 patch of sediment (coarse sand), was isolated and observed over a period of 33 days. Considerable water exchange took place with the surroundings partly because of near-surface openings that appeared in the enclosure but mainly because of density changes due to salinity fluctuations in the surrounding water. This denser water entered the enclosure through the sediments, displacing the lighter water through the near-surface openings. Very high nutrient and low oxygen values were measured in the bottom water immediately following this higher-density water influx. The same effect was observed outside the enclosure, but this phenomenon was not marked here, probably due to greater turbulent mixing obscuring this effect. It is postulated that interstitial, water is flushed out of coarse-grained sediments by gravity displacement due to changes in the density of bottom water. In certain areas this mechanism of nutrient release from the sediments is presumably of great ecological importance, both for the phytoplankton and the benthos.

Low light adaptation and export production in the deep chlorophyll maximum layer in the northern Indian Ocean

Abstract Phytoplankton standing stock and primary production rates were measured in the central northern Indian Ocean at 65°E and 18°N in the inter-monsoon period in May 1987. As the algal populations were dominated by minute forms difficult to assess by routine microscopic methods, main algal groups were identified by pigment patterns derived fromHPLC-analysis. Profiles of chlorophyll a showed a distinct maximum between 50 and 80 m depth, with up to 10-fold higher values than in the surface mixed layer. The main portion of this deep maximum was situated below rather than above the sharp chemocline at 50 m, which analyses of particulate C,N and P showed to be a biomass maximum as well. In the mixed surface layer cyanobacteria dominated phytoplankton biomass and primary production, whereas the deep chlorophyll maximum was composed of small successive layers of cyanobacteria, coccolithophorids, dinoflagellates and diatoms. Prochlorophytes could be detected by the presence of divinylchlorophyll a throughout the whole water column. Highest absolute rates of primary production were encountered at the nitracline at 50 m depth. Over nearly all of the photic zone, primary production rates were closely related to POC values, leaving the P/B ratio at about the same value from the 100% irradiation level at the surface down to about 1% at 60 m depth. This adaptational ability was achieved by an increased pigmentation with depth, indicated by constantly decreasing POC/Chl a ratios in the water column. It seemed to be enhanced by a strict vertical succession of different phytoplankton groups that, by means of different sets of accessory pigments, might have obtained an additional advantage in low light adaptation. The main ecological significance of the deep chlorophyll maximum layer at the time was its role as a source of export production while importing “new” nutrients from below the nitracline. HPLC and elemental analyses of sediment trap material proved this layer to be source of most of the sedimenting particles. Calculations of nitrogen fluxes suggested the import and export terms to be well balanced.

The plankton tower. I. A structure to study water/sediment interactions in enclosed water columns

A brief description is given of the plankton tower. The tower, is located in Kiel Bight and consists of a steel structure 16.5 m high, standing at a depth of 11 m. Four cylindrical plastic enclosures, each with a volume of about 30 m3, are suspended in the tower. Within each of these a column of water is isolated from the surface to the bottom, including the sediment. Results from the first experiment show that this method is suitable for investigations concerned with the study of interactions between the water column and the sediments.

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.

Storm-induced convective export of organic matter during spring in the northeast Atlantic Ocean

Observations during a spring phytoplankton bloom in the northeast Atlantic between March and May 1992 in the Biotrans region at 47°N, 20°W, are presented. During most of the observation period there was a positive heat flux into the ocean, winds were weak, and the mixed layer depth was shallow (<40 m). Phytoplankton growth conditions were favourable during this time. Phytoplankton biomass roughly doubled within the euphotic zone over the course of about 7 days during mid-April, and rapidly increased towards the end of the study until silicate was depleted. However, the stratification of the water column was transient, and the spring bloom development was repeatedly interrupted by gales. During two storms, in late March and late April, the mixed-layer depth increased to 250 and 175 m, respectively. After the storm events significant amounts of chlorophyll-a, particulate organic carbon and biogenic silica were found well below the euphotic zone. It is estimated that between 56% and 65% of the seasonal new production between winter and early May was exported from the euphotic zone by convective mixing, in particular, during the two storm events. Data from the NABE 47°N study during spring 1989 are re-evaluated. It is found that convective particle export was of importance during the early part of that bloom too, but negligible during the height of the bloom in May 1989. The overall impact of convective particle export during spring 1989 was equivalent to about 36% of new production. In view of these and previously published findings it is concluded that convective transport during spring is a significant process for the export of particulate matter from the euphotic zone in the temperate North Atlantic.

Short-term sedimentation patterns in the northern Indian Ocean

Abstract The flux of particles from the photic zone was monitored in one open ocean and two shelf stations in the northern Indian Ocean by means of drifting sediment traps in the intermonsoon period, from March to June 1987. Samples were collected over daily intervals and analysed for organic carbon, nitrogen, total phosphorus and silica. Flux rates of all elements differed by up to a factor of 10 between the Oman shelf and the open ocean area. Mean rates of carbon sedimentation were 13.6 and 1.7 mmol C m−2 day−1, respectively. On the Pakistan shelf, however, sedimentation rates were in the same low range as in the open ocean. These differences, particularly between the two shelf regions, were due to the different types of pelagic systems in the respective photic zones. The presence of nitrate in surface water of the Oman shelf permitted “new” production, which consequently led to enhanced particle export. In the open ocean and the Pakistan shelf, typical tropical recycling systems retained material by intense regeneration of nutrients in the surface layer. These differences also were reflected in the composition of the sedimenting particles. Changes in production-respiration equilibria in the photic zone lead to rapid shifts in the carbon/silica and carbon/nitrogen ratios of trapped material. Thus short term sedimentation measurements can provide valuable information on structural and functional variations in pelagic productivity.

The Plankton Tower. IV. Interactions Between Water Column and Sediment in Enclosure Experiments in Kiel Bight

Neritic ecosystems in the boreal zone generally maintain more plankton biomass over a longer period of the year than off-shore systems in the same latitude. Productivity is higher particularly during the summer stratification, between the spring and autumn phytoplankton blooms brought about by nutrients from sources other than pelagic remineralization. Plankton biomass levels maintained by recycling within a pelagic system tend to decrease with time if limiting nutrients bound in sedimenting particles are not replenished. In neritic environments, surface waters can receive nutrients from the land, but depending on water depth and local weather and geomorphology, replenishment can also come from nutrient-rich subthermocline water and sediments. In deeper bodies of water with a steep coastline, such as fjords, the sediment contribution will be less important (Takahashi et al. 1977) than in shallow water systems with more of their sediment surface within the euphotic zone (von Bodungen et al. 1975, Rowe et al. 1975).