Vernal sedimentation trends in north Norwegian fjords temporary anomaly in Th particulate fluxes related to Phaeocystis pouchetii proliferation

Sabine Schmidt • Sauveur Belviso • Paul Wassmann • Gerard Thouzeau • Jacqueline Stefels

Received: 23 May 2006 / Accepted: 5 October 2006 / Published online: 8 March 2007 © Springer Science+Business Media B.V. 2007

Abstract We report data of a naturally occurring radionuclide, 234Th, an in situ tracer, to investigate vertical export of biogenic matter during a vernal bloom of Phaeocystis pouchetii in the fjords of northern Norway. To optimise sampling of different stages of the bloom, three fjords with increasing oceanic influence (Balsfj-ord, Malangen fjord and Ullsfjord, respectively) were investigated in April 1997. Contrasting situations were encountered between the three fjords: the proliferation of P. pouchetii in Ullsfj-ord surface waters coincided with a drastic

EPOC, UMR 5805, Avenue des facultes, Talence

Cedex 33405, France e-mail: [email protected]

S. Belviso

LSCE, UMR 1572, Gif sur Yvette Cedex 91 198, France

P. Wassmann

Institute for Aquatic Resources and Environmental Biology, The Norwegian College of Fishery Science, University of Tromso, Tromso, Norway

G. Thouzeau

IUEM, UMR 6539, Technopole Brest-Iroise, Plouzane 29280, France

J. Stefels

Laboratory of Plant Physiology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands reduction of particulate 234Th fluxes in traps, although particulate organic carbon (POC) and dimethylsulphoniopropionate (DMSP) were exported and 234Th was available in surface waters. When large colonies make up a significant fraction of the vertical flux, as observed in Ullsfjord in April 1997, there may be a large and rapid change in the POC/234Th ratio, further complicating the use of 234Th as a tracer for POC export. The results suggest that the proliferation of Phaeocys-tis pouchetii during vernal bloom could temporary increase OC/234Th ratio of particles and delay the particulate export of 234Th, and probably of other particle-reactive species, from surface waters.

Keywords fjord • Northern Norwegian shelf waters • Particle flux • Phaeocystis • Sediment trap • 234Th

Introduction

The microalga genus Phaeocystis has a worldwide distribution and is known to develop massive blooms in polar and temperate waters (Riebesell et al. 1995). Spring blooms of Phaeocystis often follow diatom blooms, after the decline of dissolved silicate (Lancelot et al. 2005; Wassman et al. 2005). This colony-forming alga is an important source of the volatile organic sulphur compound dimethyl sulphide (DMS) and its dense blooms can act as a carbon sink (Rousseau et al. 2000; Belviso et al. 2006). Both sulfur and carbon cycles are relevant to climate change studies, sulphur as an important source of cloud condensation nuclei, and carbon being the main contributor to the greenhouse effect. Quantifying the role of Phaeocystis blooms in particle fluxes and export is therefore important if we want to understand their potential control on the escape of carbon from the photic zone of coastal waters. Indeed, the community structure determines how much of the primary production settles via the classical food web, via large phytoplankton and their grazers, and how much nutrients are regenerated via the microbial food web (Eppley and Peterson 1979; Verity 2000; Stelfox-Widdicombe et al. 2004). Within the EU funded program Escape (Entangled Sulphur and Carbon cycles in Phaeocystis dominated Ecosystems), this study focused upon the vertical export of biogenic material in relation to the plankton community dynamics and structures in north Norwegian fjords.

Thorium-234 (234Th, t1/2 = 24.1 days), a naturally occurring radionuclide, was used in this work to explore particle dynamics during the development of a Phaeocystis bloom. 234Th is produced in seawater via the decay of its long-lived and highly soluble parent, uranium-238 (238U). Since 234Th is highly particle-reactive, and hence sticks to all particle surfaces, the clearance of 234Th from surface waters is a direct indication of the removal rate of material on sinking particles from the upper ocean. Review papers clearly show the interest in this tracer as a tool for estimating particulate organic carbon export (Buesseler et al. 2006 and references herein). In surface waters, biological activity is the main source of particles and can vary considerably on short time scales. Previous studies have shown the close coupling between dissolved 234Th scavenging and new production in the ocean (Coale and Bruland 1985). The particulate 234Th residence time in oceanic surface waters is of the order of a few days to a few weeks (Moran and Buesseler 1992) and appears to be mainly governed by the classical food web, via the export of detritus (marine snow, aggregates, faecal pellets) (Schmidt et al. 1992; Buesseler et al. 2006).

The present paper discusses 234Th data in the water column and in settling particles, together with the determination of the stocks of Phaeocystis pouchetii and DMSP in surface waters of north Norwegian fjords. The aims of this study are: (i) to describe temporal variability of 234Th activities and fluxes during the progress of a vernal bloom, (ii) to calibrate trap collection efficiencies using 234Th data, and (iii) to assess the impact of Phaeocystis proliferation on vertical export of particles.

Material and methods

Study area

To optimize sampling of different stages of the recurrent vernal Phaeocystis pouchetii bloom in northern Norway, three fjords with decreasing oceanic influence were investigated in the north Norwegian coastal zone (Fig. 1). The westernmost fjord, Malangen fjord, is relatively open and exposed to shelf waters. Ullsfjord, the northernmost is the most open to coastal waters with a wide entrance. Both fjords are wide and have deep sills. Balsfjord is the least exposed: it is a tapered inland fjord, which is separated from the Malangen fjord and Ullsfjord by three narrow straits. More details on sampling areas including their physical, chemical and biological characteristics can be found in Wassman et al. (1996), Keck and Wassmann (1996), Reigstad et al. (2000) and Archer et al. (2000). The stations were visited sequentially five times, between April 7 and April 25, 1997, aboard the RV Johan Ruud.

Measurements

To sample the settling flux directly, free-floating multiple-sample programmable sediment traps (Pro-Trap) were deployed at all three fjords for about 16 h at each time (trap depth: 60 m). The Pro-Trap system consists of four polyvinyl chloride (PVC) sediment tubes, each of 0.018 square meter exposed area (cylinder height of 80 cm, trap aspect ratio A % 5.3), mounted on a stainless-steel frame. Depth and angle sensors allowed defining the position of the traps in the water

column, while light scattering was measured by a Sea Tech LS sensor. A VALEPORT 800-0 series electromagnetic current meter provided 2-axis flow velocity measurements at the trap aperture (see Belviso et al. 2006 for details). Upon recovery, swimmers (living zooplankton) were removed from the samples prior to all analyses, in order to assess properly the passive particulate fluxes. Trap samples were immediately filtered on precombusted and preweighed Whatman GF/F filters of 25 or 47 mm diameter and stored at -20°C until analysis (within six months). All carbon and nitrogen analyses were performed on a Carlo Erba NC 2500 gas chromatography analyzer. The difference-on-ignition (DOI) method was used to separate organic and inorganic carbon (Majeed 1987). Details on the determination of DMSP stocks and fluxes are given in Belviso et al. (2006).

One sediment tube, devoted to the determination of 234Th, took a single sample for the entire duration of each deployment. On recovery of the traps, trapped particles were filtered on GF/F; 234Th activities were measured with a low-background high-efficiency y detector. 234Th activities were measured from its 63.2 and 92.4-92.8 keV -gamma rays, and decay corrected to the time of sample collection (Schmidt and Reyss, 2000). The standards used for the calibration of the y detector are: (1) a mock-up of sediment and U and Th US standards from National Bureau of Standards (NBS) at 1,000 ppm, and (2) a known amount of 238U deposited on an aluminum disk checked by a-counting using a grid chamber of a known efficiency. Counting efficiencies and backgrounds for 234Th are detailed in Schmidt and Reyss (2000) and Schmidt (2006).

During the fifth (and last) sampling date, a profile of 234Th was sampled between the surface and the trap depth (60 m) at each station. Immediately after sampling, the 20 l of seawater was passed through a 0.45 im pore size filter to separate dissolved from particulate phases. Within one month after the collection, particulate

234Th (234ThP) was directly measured on the filter as trapped particles. After acidification to pH 2, pre-weighed 50 mBq 229Th chemistry yield tracer and 120 mg Fe (as FeCl3) were added to the dissolved sample. After spike equilibration, Fe(OH)3 was precipitated by adding NH4OH to pH 7. After recovery of the precipitate, the purification of 234Th was obtained by anionic exchange. After elution, Th was extracted with 1-(2thenoyl)-3,3,3-trifluoracetone in toluene at pH 3 and then evaporated onto an aluminum foil. The first a counting of this foil allowed the determination of 229Th for chemical yield (between 20% and 60%); the following y-counting allowed the measurement of 234Th. Due to the short half-life of 234Th, the separation of 234Th from its parent was done within 24 h. Calculations of dissolved 234Th activities consider ingrowth corrections for time elapsed between sampling and chemistry (Schmidt and Reyss 2000; Schmidt et al. 2002).

The U-salinity relationship (Chen et al. 1986) is appropriate for estimating dissolved 238U in the open ocean; in other regimes, i.e., continental shelves, estuaries or marginal seas, the U concentration must be measured (Schmidt and Reyss 1991; Rutgers van der Loeff et al. 2006). Seawater samples were collected at each station for 238U determination. Uranium was concentrated from 2 l seawater, as previously described for 234Th, in the presence of a known amount of 232U spike and 20 mg Fe. Then uranium activities were determined by a-counting (Schmidt 2004).

Phytoplankton abundance was determined in samples from depth profiles taken at dawn as described by Archer et al. (2000). Data on Phae-ocystis abundance provided here are means from the 1, 4 and 8 m depth samples. Phytoplankton carbon is calculated from biovolume measurements as described by Archer et al. (2000).

Irreversible scavenging model of 234Th

234Th as a tracer is widely used and critical for two tasks: to quantify fluxes and residences time of particles, and to calibrate trap efficiency by comparing estimated water-column 234Th fluxes with those measured by traps (Cochran et al. 2000). In surface waters, 234Th activities are the result of a balance between its continuous production from 238U, its decay, removal onto rapidly sinking particles, and advection/diffusion. The temporal change in total 234Th is expressed by the classical transport equation:

d ATh/ d t = kA U - kATh - P + V (1) where AU is the 238U activity, ATh is the total

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