Paul Wassmann

Retention versus export food chains: processes controlling sinking loss from marine pelagic systems

The role of export and retention food chains forpelagic-benthic coupling is considered by evaluatingdifferent food chain scenarios and processes such asaggregation, grazing and zooplankton-mediated fluxes.The consequences of grazing of primary production bydifferent zooplankton for the vertical export ofparticulate organic matter from the euphotic zone arediscussed. Reference is made to existing data andalgorithms regarding primary production and verticalexport of carbon from the euphotic zone, both onannual and daily time scales. Examples regarding therole of nutrient addition, removal of pelagiccarnivores and zooplankton grazing for vertical fluxare presented. It is speculated how variable grazingimpact of micro- and mesozooplankton, as well asherbivorous, omnivorous and carnivorous feedingstrategies of mesozooplankton could compete withaggregation during phytoplankton blooms and influenceexport fluxes. It is concluded that the transport ofparticulate organic matter to depth not only dependson bottom-up regulation as determined by physicalforcing, but also on the structure and function of theprevailing planktonic food web. Scenarios arepresented which indicate that top-down regulationplays a pivotal role for the regulation of verticalflux. This conclusion may have crucial consequencesfor future biogeochemical programmes investigatingpelagic-benthic coupling in the ocean. The endeavoursof many research programmes are dominated by lines ofthought where straightforward biogeochemistry andbottom-up regulation is the focus. Phyto- andzooplankton as well as process-oriented researchactivities have to be the focal point of futureresearch if the current comprehension of export fromand retention in the upper layers is going to makedistinct progress.

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.

Seasonal variation in vertical flux of biogenic matter in the marginal ice zone and the central Barents Sea

The spatial and seasonal variations in the vertical flux of particulate biogenic matter were investigated in the Barents Sea in winter and spring 1998 and summer 1999. Arrays of simple cylindrical sediment traps were moored for 24 h between 30 and 200 m along a transect from the ice-free Atlantic water to Arctic water with up to 80% ice cover. Large gradients in the quantity and composition of the sinking particles were observed in the south–north direction, and in relation to water column structure and stability, which depend on the processes of ice retreat. The magnitude of the vertical flux of particulate organic carbon (POC) out of the upper mixed layer ranged from background winter values (30–70 mg C m 2 day 1 ) to 150–300 mg C m 2 day 1 in summer and 500–1500 mg C m 2 day 1 in spring. Vertical flux of chlorophyll a (CHL) was negligible in winter, generally <1 mg m 2 day 1 in summer, and up to 38 mg m 2 day 1 in spring. In spring, the proportion of phytoplankton carbon (dominated by Phaeocystis pouchetii in the Atlantic water and Thalassiosira antarctica in the Arctic water) in the sinking POC was up to 50%. Both colonial and single-celled forms of P. pouchetii were equally abundant in the water column and sediment traps. In contrast to the spring season, the vertical flux of phytoplankton during summer was dominated by a variety of flagellates (e.g. small unidentified flagellates, Ochromonas crenata, Dinobryon balticum and single-celled P. pouchetii). The magnitude of the vertical flux to the bottom in spring was comparable in the Arctic and Atlantic waters (ca. 200 mg C m 2 day 1 ), but the composition and C/N ratio of the particles were different. The regulation of biogenic particle sedimentation took place in the upper layers and over very short vertical distances, and varied with season and water mass. The vertical flux was mainly shaped by the water column stratification (strong salinity stratification in the Arctic water; no stratification in the Atlantic water) and also by the activity of plankton organisms. Zooplankton faecal pellets were an important constituent of the vertical flux (up to 250 mg C m 2 day 1 ), but their significance varied widely between stations. The daily sedimentation loss rates of POC in spring exceeded the loss rates in summer on the average of 1.7 times. The complexity of the planktonic community during summer suggested the prevalence of a retention food chain with a higher capacity of resource recycling compared to spring. D 2002 Elsevier Science B.V. All rights reserved.

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.

Seasonal and spatial changes in biomass, structure, and development progress of the zooplankton community in the Barents Sea

Abstract During three cruises, in March and May 1998 and July 1999, seasonal and regional variations in biomass and vertical distribution of mesozooplankton as well as cohort development in Calanus spp. were investigated along a transect across the central Barents Sea and marginal ice zone. There were no considerable changes in zooplankton biomass between the seasons. Throughout the investigation, the average biomass for the entire region approximated to ca. 5 g dry weight (DW) m −2 while station-to-station variation ranged with an order of magnitude (1–14 g DW m −2 ). Biomass of nauplii and small copepods (200–500 μm in body length) obtained from water bottles samples exceeded that from WP-2 net samples 1.5–6.6 times. The maximum abundance of this group reached 16×10 5 ind. m −2 in the upper 100-m layer, suggesting a significant grazing pressure on phytoplankton. Spatial distribution of Calanus species and some selected species suggests that the zooplankton community composition was primarily affected by water mass circulation and bottom topography. Both the depth distribution of mesozooplankton and cohort progress in Calanus finmarchicus and Calanus glacialis revealed two waves of spring events. The first started in the southernmost area of the Barents Sea and the second nearby the Polar Front. Both developed towards the north.

Seasonal variation and spatial distribution of phyto- and protozooplankton in the central Barents Sea

Seasonal and geographical variations of suspended single-celled organisms on a transect across the western part of the Barents Sea in March and May 1998 and in June–July 1999 revealed that pico- and nanoplankton flagellates and monads ( 20 Am) prevailed in total biomass. In general, spring bloom progresses independently of the southern part of the Atlantic Water (AW) and follows the receding ice edge in the Arctic Water (ArW) to the north. The blooms started almost simultaneously and had similar composition (small diatom Chaetoceros socialis dominated total phytoplankton biomass) in both localities, so the share of resting spores, indicating the age of the bloom, differed markedly. As for underwater rise—the Sentralbanken (SBW) altered this pattern, and the spring bloom spreads from north to the south from the rise to the trench. The next stage of the bloom was dominated by the large diatoms Thalassiosira antarctica var. borealis above the Sentralbanken, in the Polar Front (PF) and in the ice-edge areas. In the southern part of transect, this stage of the spring bloom had a delay or was absent due to low stability of water column and/or due to grazing impact. The presence of ribbon-shaped forming species indicated the earlier stage of bloom in Marginal Ice Zone (MIZ). In May 1998 as well as in June/July 1999, at the ice-covered stations, early spring conditions—rather similar to the conditions in March 1998—were observed. Summer conditions at most of the stations in June–July 1999 were characterized by high species diversity of diatoms and dinoflagellates. High abundance of heterotrophic dinoflagellates and protozoans indicated the active functioning of the microbial loop in the nutritive chains. D 2002 Elsevier Science B.V. All rights reserved.

Grazing of phytoplankton by microzooplankton in the Barents Sea during early summer

Abstract Phytoplankton growth rates and grazing losses to microzooplankton were determined in surface waters of the central Barents Sea during a cruise in June/July 1999. Five stations were occupied which had been studied repeatedly over the past 15–20 years. Dilution experiments using chlorophyll a (chl a) as a tracer were used to estimate daily rates in three size fractions; image-analyzed fluorescence microscopy provided quantitative estimates of standing stocks of auto- and heterotrophic nano- and microplankton. Phytoplankton contributed the largest share of protistan biomass, followed by bacteria and microzooplankton. On average, nanophytoplankton (

Seasonal variation in production, retention and export of zooplankton faecal pellets in the marginal ice zone and central Barents Sea

Abstract Vertical distribution and sedimentation of faecal pellets (FPs) as well as the production rates of FPs by larger copepods were studied during three cruises to the Barents Sea in March and May 1998, and July 1999. Three to five 24-h stations were selected during each cruise, where at least one main station was located in Arctic water (ArW), one in the polar front region (PF) and one in Atlantic water (AW). A winter scenario was encountered in March with very low concentrations of FPs in the water column, most of the time well below 0.1 mg faecal pellet carbon (FPC) per cubic meter, and with sedimentation rates below 3 mg FPC m−2 day−1 at all depths and stations. Increased concentrations of FPs were observed in May and the maximum biomass of FPs was found in ArW (4.8 mg FPC m−3). This was reflected in high vertical flux of FPs in the ArW, just below the chlorophyll maximum (∼150 mg FPC m−2 day−1). FPC sedimentation explained ∼40% of the total particulate organic carbon (POC) export at 90 m depth at this station. Copepod FP production was moderate to high in May, reflecting favourable feeding conditions. Large spatial variation in the estimated retention potential of FPs was observed, ranging from 96% in AW to ∼40% in the PF region. The July scenario did not differ very much from that observed in May. The lowest suspended concentrations and vertical flux of FPs were again observed in AW, in spite of the high pellet-production rate. FPC explained 34% of the POC export out of the upper layer in ArW, 40% in the PF region, but only 8% in AW. The calculated retention potential of 70% of the produced copepod FPs in AW decreased to 60% and 47% in the PF region and ArW, respectively. Krill FPs comprised a significant fraction of both suspended and sedimented FPC throughout the central Barents Sea. The data show that spatial and temporal variations in the FP “retention filter” are extensive and evidently of importance for the patterns of vertical flux of organic matter and the regulation of pelagic–benthic coupling in the Barents Sea.

Tipping Elements in the Arctic Marine Ecosystem

The Arctic marine ecosystem contains multiple elements that present alternative states. The most obvious of which is an Arctic Ocean largely covered by an ice sheet in summer versus one largely devoid of such cover. Ecosystems under pressure typically shift between such alternative states in an abrupt, rather than smooth manner, with the level of forcing required for shifting this status termed threshold or tipping point. Loss of Arctic ice due to anthropogenic climate change is accelerating, with the extent of Arctic sea ice displaying increased variance at present, a leading indicator of the proximity of a possible tipping point. Reduced ice extent is expected, in turn, to trigger a number of additional tipping elements, physical, chemical, and biological, in motion, with potentially large impacts on the Arctic marine ecosystem.

Significance of sedimentation for the termination of Phaeocystis blooms

Abstract The role of sedimentation for the termination of Phaeocystis blooms is exemplified through case studies from the literature as well as from anecdotal evidence. Scenarios of high and low sedimentation following Phaeocystis blooms exist. Mass sedimentation was found in the Barents Sea and the Ross Sea, but vertical flux below the euphotic zone was insignificant in a north Norwegian fjord and the Weddell Sea. In general, no regular and recurring pattern of sedimentation events can be expected during Phaeocystis blooms. Factors influencing the fate of senescent Phaeocystis blooms are propably water depth, turbulent energy supply, aggregate formation, release of flagellated cells from colonies, microbial degradation, zooplankton grazing as well as lysis of colonies and cells. The role sedimentation plays for the termination of Phaeocystis blooms seems to be determined by the physical and biological characteristics of the specific ecosystem where the bloom occurs. In general, Phaeocystis -dominated ecosystems tend to endorse pelagic rather than benthic mineralization.