Tatjana Ratkova

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.

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.

Sea ice algae in the White and Barents seas: composition and origin

To examine algae populations, three expeditions (in March 2001, April 2002 and February 2003) were conducted in the Guba Chupa (Chupa Estuary; north-western White Sea), and one cruise was carried out in the open part of the White Sea in April 2003 and in the northern part of the Barents Sea in July 2001. Sea ice algae and phytoplankton composition and abundance and the content of sediment traps under the land-fast ice in the White Sea and annual and multi-year pack ice in the Barents Sea were investigated. The community in land-fast sea ice was dominated by pennate diatoms and its composition was more closely related to that of the underlying sediments than was the community of the pack ice, which was dominated by flagellates, dinoflagellates and centric diatoms. Algae were far more abundant in land-fast ice: motile benthic and ice-benthic species found favourable conditions in the ice. The pack ice community was more closely related to that of the surrounding water. It originated from plankton incorporation during sea ice formation and during seawater flood events. An additional source for ice colonization may be multi-year ice. Algae may be released from the ice during brine drainage or sea ice melting. Many sea ice algae developed spores before the ice melt. These algae were observed in the above-bottom sediment traps all year around. Three possible fates of ice algae can be distinguished: 1) suspension in the water column, 2) sinking to the bottom and 3) ingestion by herbivores in the ice, at the ice-water interface or in the water column.

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.

Seasonal patterns in composition and biomass of autotrophic and heterotrophic nano- and microplankton communities on the north Norwegian shelf

Abstract From monthly transects on the shelf off northern Norway, data describe the size, abundance, and biomass of photosynthetic nano- and picoplankton, heterotrophic (aplastidic) nanoplankton, and ciliates. Samples were analyzed using a state-of-the-art color imaging system. The numerically dominant phytoplankton were pica- and nanoplankton. Highest biomasses occurred in summer, and increased earlier in the year and attained higher values in inshore stations compared to offshore stations. Mean cell sizes were small, 2-5μ.m diameter nanoplankton and 1μm eucaryotic picoplankton; cyanobacteria were comparatively unimportant contributors to biomass. The dominant herbivores appeared to be heterotrophic flagellates, which were generally similar in size, concentration, and biomass to the phototrophic forms. Their temporal and spatial distribution mimicked their putative prey, except that they often occurred in abundance deeper than the photosynthetic nanoplankton. Oligotrich ciliates were also abundant. The r...

Abundance and biomass of pico-, namo-, and microplankton on a transact across Nordvestbanken, north Norwegian shelf, in 1994

Abstract The seasonal variation of suspended microplankton on a transect across Nordvestbanken in 1994 revealed that pico- and nanoplankton flagellates and monads (< 2 fμm and 2-20 mμ, respectively) entirely determined total phytoplankton numbers and biomass. From March to May and from August to October nanoflagellates and monads comprised on average 90 % and up to 98 % of total phytoplankton biovolLrme. Only during the maximum diatom and dinoflagellate abundance in June and July, and during the appearance of Halosphaera viridis at selected stations on the mid shelf, did flagellates comprise less than 60-90% oftotal biovolume. In general, the abundances ofpicoplankton, coccolithophorids and Phaeocystis pouchetii were low, never comprising more than a few percent of total biovolume. The estimated total biomass of pico-, nano- and microplankton (from biovolume), phytoplankton (from chlorophyll and epifluorcscence estimates) and larger protozooplankton (from biovolmne) in the upper layers were on average 3-1...

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.

Seasonal variation of verticla flux of phytyoplankton and biogenic matter at Nordvestbasken, north Norwegian shelf in 1994

Abstract The seasonal magnitude and biochemical composition of vertical flux of biogenic matter and phytoplankton at the north Norwegian shelf break was investigated at 14 depths between 20 and 200 m depth. Data of vertical flux were collected for 24 hours at monthly intervals using free- drifting sediment traps. Samples were analysed for particulate organic carbon and nitrogen (POC and PON), chlorophyll a (Chl a), phaeopigments (Phaeo) and phytoplankton composition. Two main periods of POC and PON vertical flux were observed in May and July with sedimentation rates ∼300 mg POC m-2 d-1 at 200 m depth. During the other periods vertical flux was ∼100 rng POC m-2 d-1 at 200 m depth. Vertical flux of Phaeo and POC had maxima in concert and Phaeo was always larger than Chl a vertical flux. Chl a vertical flux was low. The largest vertical flux was recorded in the upper water column in May and June (0.4-0.7 mg Chl a m-2 d-1 at 20m) and decreased rapidly with depth. The highest Chl a flux at depth was recorded d...