Christian Wexels Riser

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 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.

Production, retention and export of zooplankton faecal pellets on and off the Iberian shelf, north-west Spain

Vertical distribution of faecal pellets (FP), their sedimentation and the production rates of FP by mesozooplankton were studied during a cruise on and off the Iberian shelf in August 1998. The cruise was divided into two legs, each of them a short-term Lagrangian drift experiment. FP were collected with water bottles, with drifting sediment traps and during experiments carried out onboard the ship. The pellets were enumerated and their biovolumes and carbon contents (FPC) were calculated. The standing stock of FP in the upper 50 m was on average three times higher during the first on-shelf experiment than during the second off-shelf experiment. There were large diurnal variations, but no clear pattern emerged between day and night sampling. The vertical export of FPC from the upper, productive layer was on average one order of magnitude greater on the shelf (range 6–160 mg.m−2.d−1) compared to the off-shelf experiment (range 1–30 mg.m−2.d−1). FPC sedimentation explained ∼20% of the total POC export from the euphotic layer on the shelf, but <5% off the shelf. FP sedimentation was dominated by medium-sized cylindrical pellets (40–60 μm in diameter), but larger cylindrical pellets (60–100 μm in diameter) also played an important role. The smaller FP size fractions were never of any significance, in spite of the high abundance of smaller calanoid and cyclopoid copepods. The community production of FPs by mesozooplankton were calculated for the off shelf stations, and the average retention potential of FP in the upper 200 m was estimated to be ∼98%. Thus retention processes are clearly important for cross-shelf advection of FPs, their injection into the deep ocean and in the regulation of pelagic benthic coupling.

Winter-spring feeding and metabolism of Arctic copepods : insights from faecal pellet production and respiration measurements in the southeastern Beaufort Sea

Faecal pellet production (FPP) and respiration rates of Calanus glacialis, C. hyperboreus and Metridia longa were measured under land-fast ice in the southeastern Beaufort Sea during the winter–spring transition (March–May 2004) prior to the phytoplankton spring bloom. Despite different overwintering and life cycle strategies and remaining low concentrations of suspended chlorophyll a and particulate organic matter, all species showed increasing FPP rates in spring. A corresponding increase in respiration was only observed in C. glacialis, while respiration remained constant in C. hyperboreus and M. longa. In C. glacialis and C. hyperboreus calculated ingestion covered respiratory expenditures. The constancy of the oil sac volume in M. longa suggests that the animals fed during winter-spring. Pre-bloom grazing as shown here seems to acclimate the copepod populations physiologically for the upcoming high feeding season, so that they are able to resume maximum grazing and reproduction as soon as the phytoplankton bloom is initiated.

Vertical flux of biogenic matter during a Lagrangian study off the NW Spanish continental margin

Lagrangian experiments with short-term, drifting sediment traps were conducted during a cruise on RRS Charles Darwin to the NW coast of Spain to study the vertical flux and composition of settling biogenic matter. The cruise was split into two legs corresponding to (i) a period of increased production following an upwelling event on the continental shelf (3–10 August 1998) and (ii) an evolution of a cold water filament originating from the upwelled water off the shelf (14–19 August). The export of particulate organic carbon (POC) from the upper layer (0–60m) on the shelf was 90–240mgC.m−2.d−1 and off the shelf was 60–180mgC.m−2.d−1. Off shelf the POC flux at 200m was 50–60mg.m−2.d−1. A modest sedimentation of diatoms (15–30mgC.m−2.d−1) after the upwelling was associated with increased vertical flux of chlorophyll a (1.8–2.1mg.m−2.d−1) and a decrease of the POC:PON molar ratio of the settled material from 9 to 6.4. Most of the pico-, nano-, and microplankton in the settled material were flagellates; diatoms were significant during the on shelf and dinoflagellates during the off shelf leg. Off shelf, the exponential attenuation of POC flux indicated a strong retention capacity of the plankton community between 40 and 75m. POC:PON ratio of the settled particulate matter decreased with depth and the relative portion of flagellates increased, suggesting a novel, flagellate and aggregate mediated particulate flux in these waters. Export of POC from the euphotic layer comprised 14–26% of the integrated primary production per day during the on shelf leg and 25–42% during the off shelf leg, which characterises the importance of sedimentation in the organic carbon budget of these waters.

Calanus spp. grazing affects egg production and vertical carbon flux (the marginal ice zone and open Barents Sea)

Abstract Concentration of faecal pellets of Calanus finmarchicus , Calanus glacialis , and Calanus hyperboreus , as well as eggs and nauplii of the first two species in the upper 0–100 m layer, were estimated during 24-h stations in the marginal ice zone and the open Barents Sea in March, May, and July. The importance of Calanus spp. as major contributors to suspended matter and vertical flux was confirmed, as the proportion of their faecal pellets was high and rather stable in the upper 100 m layer throughout the study period, varying between 48% and 95% of total suspended pellet carbon (maximum dimension >50 μm) and comprising more than 50% of sediment matter. Feeding activity of calanoid copepods was not correlated with the seasonal changes in total phytoplankton carbon, but with assumed preferred food. Egg production was correlated with feeding activity depending on the temporal scales of the study (monthly, daily, hourly). A significant positive correlation between egg and faecal pellet concentration on a monthly scale, a weak significant ( C. finmarchicus ), or, insignificant ( C. glacialis ) positive correlation between daily egg and faecal pellet production, and a significant negative correlation between hourly egg and faecal pellet production ( C. finmarchicus ) were obtained. Significant correlations between indices of current feeding activity (or, available food), egg production and nauplii concentration obtained in the field suggest that current feeding, not lipid reserves, played a major role in supplying the energy for reproduction of C. finmarchicus and C. glacialis .

Seasonal variation in Zooplankton and suspended faecal pellets in the subarctic Norwegian Baisfjorden, in 1996

Abstract Zooplankton and the vertical distribution of suspended faecal pellets at a central station in Balsfjorden were investigated during 25 cruises between March and October 1996. Calanus finmarchicus followed by Metridia sp. dominated the biomass of large (> 500 εm) zooplankton, as is typical for subarctic fjords in northern Norway. However, small zooplankton (< 500 εm), in particular nauplii, Oithona spp., Microsetella sp., and protozooplankton, also contributed significantly to zooplankton biomass. The biomass of small zooplankton varied similarly to that of large zooplankton during the study period ranging from about 0.2 to 3.5 g C m-2, except for increased biomass of large zooplankton during an advective episode in May/June. The grazing impact of the small forms must be at least equal to that of the large zooplankton. Among the faecal pellets, cylindrical pellets of copepod origin prevailed. Filiform pellets were most probably underestimated due to the sampling procedure. Small pellets less than 80 εm in length and probably of nauplii and protozoan origin, contributed considerably to the total suspended faecal pellet biomass. They accumulated in parallel with their potential producers in the surface layers, in particular, during summer, and contributed to the typical summertime retention of nutrients and particulate biogenic matter The fate of phytoplankton-derived matter in north Norwegian coastal waters is influenced by grazing, resulting in either rapidly or slowly sinking faecal matter, depending on the size spectrum and the prevailing feeding modes of the grazer community. The importance of an adequate sampling of the entire Zooplankton community to determine the impact of grazing on the fate of phytoplankton-derived biomass is stressed.

Zooplankton-mediated carbon export: A seasonal study in a northern Norwegian fjord

Abstract In a northern Norwegian fjord throughout the productive season, sediment traps were consecutively deployed at four depths from March until October in order to study the role of zooplankton on vertical export of organic matter. The field campaign included 26 cruises to a permanent station located in the basin of Balsfjord (180 m), with the most intensive sampling conducted during spring. The zooplankton community greatly influenced the vertical flux of particulate organic carbon (POC) through production and sinking of faecal pellets (FP). Faecal pellets explained ~40% of the POC flux leaving the upper 100 m during the productive season (corresponding to ~10 g C m−2). The average monthly faecal pellet carbon (FPC) contribution ranged widely between 7 and 75% of POC. Euphausiids were the single most important contributor to the vertical FP flux and made up >90% of the FPC during certain periods. Faecal pellets produced by small and medium-sized copepods, although found in high abundance, played a minor role for the vertical flux, suggesting significant retention processes in the upper layers.