Kalle Olli

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.

Decadal-scale changes of dinoflagellates and diatoms in the anomalous baltic sea spring bloom.

The algal spring bloom in the Baltic Sea represents an anomaly from the winter-spring bloom patterns worldwide in terms of frequent and recurring dominance of dinoflagellates over diatoms. Analysis of approximately 3500 spring bloom samples from the Baltic Sea monitoring programs revealed (i) that within the major basins the proportion of dinoflagellates varied from 0.1 (Kattegat) to >0.8 (central Baltic Proper), and (ii) substantial shifts (e.g. from 0.2 to 0.6 in the Gulf of Finland) in the dinoflagellate proportion over four decades. During a recent decade (1995-2004) the proportion of dinoflagellates increased relative to diatoms mostly in the northernmost basins (Gulf of Bothnia, from 0.1 to 0.4) and in the Gulf of Finland, (0.4 to 0.6) which are typically ice-covered areas. We hypothesize that in coastal areas a specific sequence of seasonal events, involving wintertime mixing and resuspension of benthic cysts, followed by proliferation in stratified thin layers under melting ice, favors successful seeding and accumulation of dense dinoflagellate populations over diatoms. This head-start of dinoflagellates by the onset of the spring bloom is decisive for successful competition with the faster growing diatoms. Massive cyst formation and spreading of cyst beds fuel the expanding and ever larger dinoflagellate blooms in the relatively shallow coastal waters. Shifts in the dominant spring bloom algal groups can have significant effects on major elemental fluxes and functioning of the Baltic Sea ecosystem, but also in the vast shelves and estuaries at high latitudes, where ice-associated cold-water dinoflagellates successfully compete with diatoms.

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.

Carbon flow patterns in the planktonic food web of the Gulf of Riga, the Baltic Sea : a reconstruction by the inverse method

We used the inverse method to reconstruct a carbon flux model for the planktonic food web of the southern part of the Gulf of Riga, the Baltic Sea. The model was based on data from three field campaigns (3–12 May 1995; 28 June–7 July 1994; 26 August–3 September 1993). The carbon flow model indicated very different channelling of gross primary production (GPP) in the three different periods. In spring 55% of GPP was channelled to detritus, while in summer 35% was exudated and channelled to dissolved organic carbon (DOC). In autumn, 42% of GPP was channelled to the zooplankton compartments. The model suggested high bacterial respiration rates and low bacterial growth efficiencies (6%) in spring and autumn. During these periods large decreases in DOC were anticipated to occur due to bacterial respiration. In summer the model suggested that bacterial respiration rate was low and that the bacterial growth efficiency was high (60%). During this period a net accumulation of DOC apparently occurred. Respiration was the most important loss of organic carbon in all seasons, corresponding on average to 152, 59 and 391% of GPP daily in spring, summer and autumn, respectively. Daily sedimentation rates of particulate organic carbon (POC) were consistently low (<3% d−1), removing 13–29% of GPP daily. The sedimented material consisted mainly of amorphous detritus. This indicates that in spite of the relatively high level of eutrophication, the planktonic system in the Gulf of Riga has a high short-term carbon retention and recycling capacity, and little POC is lost from the upper water column through sedimentation.

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.

Seasonal stages of phytoplankton community structure and sinking loss in the Gulf of Riga

In this paper, we present the biomass, species composition and sinking losses of phytoplankton and heterotrophic flagellates obtained during three seasonal stages (May 1995; June–July 1994; August 1993) from the Gulf of Riga — a eutrophied, semi-enclosed area in the Baltic Sea. The Gulf was characterised by intensive dinoflagellate (mainly Peridiniella catenata) dominated spring bloom (2700–7600 μg l−1 wet weight) while the diatom Thalassiosira baltica contributed most (80–90%) to the settling phytoplankton biomass (up to 6.5 g m−2 day−1). The mineral nutrients were abundant during the bloom and it is suggested that the differential sedimentation of species was caused by physical factors (thermal stratification of the water column). The phytoplankton biomass in summer (780–2600 μg l−1) was dominated by high abundance of filamentous cyanobacterium Aphanizomenon flos-aquae, autotrophic nanoflagellates and picoplankton. The primary vertical flux of phytoplankton (<1 g m−2 day−1) was mainly due to non-motile species and aggregate-bound picoplankton and A. flos-aquae. Below the pycnocline, resuspended dormant diatom populations caused a significant (up to 1 g m−2 day−1) secondary flux. Large heterotrophic dinoflagellates (Gyrodinium/Gymnodinium) were abundant (up to 500 μg l−1) in the middle layers and associated to ammonium regeneration. The early autumn stage was a post cyanobacterial bloom situation with relatively low phytoplankton abundance (330–860 μg l−1) dominated by A. flos-aquae and nanoflagellates. Also, the sedimentation of phytoplankton was lowest (up to 340 mg m−2 day−1). Our results indicate high seasonal differences in the phytoplankton community structure and sedimentation. However, the high within-season temporal variability overrules the variability between different areas of the southern part of the Gulf.

Diel vertical migration of phytoplankton and heterotrophic flagellates in the Gulf of Riga

Abstract Vertical distribution of a phototrophic ciliate ( Mesodinium rubrum ), autotrophic dinoflagellates ( Peridiniella catenata , Heterocapsa rotundata , Dinophysis acuminata ), a cryptomonad ( Teleaulax sp.), a filamentous cyanobacterium ( Aphanizomenon flos-aquae ), a non-motile chlorophyte ( Monoraphidium sp.) and two heterotrophic dinoflagellates were studied during two diel periods in spring (May 1–2 and 27–28, 1995) and summer (June 26–27 and July 6–7, 1994) in the southern part of the Gulf of Riga, the Baltic Sea. The two seasons, corresponding to the vernal bloom and summer stage, differed considerably with respect to phytoplankton community, hydrography and vertical distribution of the species. In spring the cells stayed in the upper mixed layer above a strong thermocline at 12–15 m depth, and changes in the vertical distribution were related to surface avoidance in the middle of the day. By using correspondence analysis, the vertical distribution of all the motile species were shown to differ significantly from the non-motile Monoraphidium , but not from each other. The vertical migration was reflected in a higher variability in the depth distribution patterns of the motile species, compared to Monoraphidium . In summer 1994, Aphanizomenon , Dinophysis and Teleaulax showed similar depth distribution patterns during the first period, but different from Mesodinium and the heterotrophic dinoflagellates, while during the second period all the autotrophic species had a species specific depth distribution pattern. Me. rubrum revealed a considerably different depth distribution between the two seasons, characterised by surface accumulation in spring and relatively uniform depth distribution over the whole sampling range (0–30 m) in summer. The results demonstrate different vertical niche separation strategies between species.

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.

Ecosystem Function, Biodiversity and Vertical Flux Regulation in the Twilight Zone

The current lack of adequate investigations of the vertical export above the depth of 200-500 m where the majority of long-term sediment traps have been deployed, results in difficulties to understand and model the carbon flux. There exists a black box of several hundred metres between the surface layers where measurements and algorithms of primary production exists and where data on the carbon export to the ocean interior are available. In this black box, the twilight zone, we face a lack of basic understanding on how vertical export of biogenic matter into the oceans interior is regulated. Essential for this regulation are planktonic key organisms and the structure and dynamics of the pelagic food web. To better comprehend the pelagic carbon cycle and sequestration of CO2, it is instrumental to obtain a basic understanding how the biota determines and transforms the export production in the twilight zone. Here we discuss some of the key organisms involved in vertical flux regulation, present an idealised, conceptual model of vertical carbon export and focus upon the “pelagic mill” and vertical flux regulation in the upper 200 m. An adequate understanding of carbon cycling demands not only adequate investigations of primary production, but also concomitant research on the functional biodiversity of the pelagic zone, plankton dynamics, vertical flux and its regulation in the twilight zone.

Specific Affinity for Phosphate Uptake and Specific Alkaline Phosphatase Activity as Diagnostic Tools for Detecting Phosphorus-limited Phytoplankton and Bacteria

We analyzed responses of soluble reactive phosphorus (SRP), bioavailable phosphate (PO4), particulate phosphorus, turnover time of orthophosphate (Tt), and alkaline phosphatase activity (APA) to varying degrees of nutrient stress. The nutrient stress was evoked by different treatments in concentration and combination of inorganic nitrogen (N) and phosphorus (P), and labile organic carbon (glucose) to mesocosms in experiments carried out in eutrophic southern (Odense Fjord, Denmark) and northern (Tvärminne Archipelago, Finland) coastal zones of the Baltic Sea. Despite seasonal and geographical differences, similar responses were observed in both experiments. Low SRP (<100 nmol l−1), shortTt (<10 h), and increased levels of APA were observed in both N+P balanced and P deficient treatments, while the opposite trend was observed in P replete treatments. The shortestTt and the highest APA were found when glucose was combined with N treatment. Bioavailable PO4 was estimated usingTt and P uptake rates as derived from stoichiometric conversion of carbon based primary and bacterial production. With shorterTt, the PO4 pool declined to <1 nmol-P l−1, whereas the SRP background pool (difference between SRP and PO4) remained relatively constant (c. 50 nmol l−1). APA was inversely related to PO4 but not to SRP. Responses of specific APA and specific affinity for PO4 uptake, which are APA and PO4 uptake rates (inverse ofTt), respectively, normalized to the summed P biomass of phytoplankton and bacteria, responded consistently to the pattern and magnitude of nutrient limitation evoked in our experiments. Our results, together with a literature survey, suggest that both parameters can be useful in examining PO4 availability for the natural phytoplankton and bacteria community in P starved aquatic systems.