Open Research Online The Open University’s repository of research publications and other research outputs Primary production export flux in Marguerite Bay (Antarctic Peninsula): linking upper water-column production to sediment trap flux Journal Item How to cite: Weston, Keith; Jickells, Timothy D.; Carson, Damien S.; Clarke, Andrew; Meredith, Michael P.; Brandon, Mark A.; Wallace, Margaret I.; Ussher, Simon J. and Hendry, Katherine R. (2013). Primary production export flux in Marguerite Bay (Antarctic Peninsula): linking upper water-column production to sediment trap flux. Deep-Sea Research Part I: Oceanographic Research Papers, 75 pp. 52–66. For guidance on citations see FAQs. (cid:13)c 2013 Elsevier Ltd. Version: Accepted Manuscript Link(s) to article on publisher’s website: http://dx.doi.org/doi:10.1016/j.dsr.2013.02.001 Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk Author’s Accepted Manuscript Primary production export flux in Marguerite Bay (Antarctic Peninsula): Linking upper water-column production to sediment trap flux. Keith Weston, Timothy D. Jickells, Damien S. Carson, Andrew Clarke, Michael P. Meredith, Mark A. Brandon, Margaret I. Wallace, Simon J. Ussher, Katharine R. Hendry www.elsevier.com/locate/dsri PII: S0967-0637(13)00041-1 DOI: http://dx.doi.org/10.1016/j.dsr.2013.02.001 Reference: DSRI2197 To appear in: Deep-Sea Research I Received date: 16 May 2012 Revised date: 29 January 2013 Accepted date: 7 February 2013 Cite this article as: Keith Weston, Timothy D. Jickells, Damien S. Carson, Andrew Clarke, Michael P. Meredith, Mark A. Brandon, Margaret I. Wallace, Simon J. Ussher and Katharine R. Hendry, Primary production export flux in Marguerite Bay (Antarctic Peninsula): Linking upper water-column production to sediment trap flux., Deep-Sea Research I, http://dx.doi.org/10.1016/j.dsr.2013.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errorsmaybediscoveredwhichcouldaffectthecontent,andall legal disclaimersthatapply to the journal pertain. Primary production export flux in Marguerite Bay (Antarctic Peninsula): linking upper water-column production to sediment trap flux. Keith Westona, ,b*, Timothy D. Jickellsa, Damien S. Carsonc, Andrew Clarked, Michael P. Meredithd, Mark A. Brandone, Margaret I. Wallaced,e, 1, Simon J. Ussherf,2, Katharine R. Hendryg,3 aLaboratory of Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ UK bCentre for Environment, Aquaculture and Fisheries Science, Lowestoft, Suffolk NR33 0HT UK cSchool of GeoSciences, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JW UK dBritish Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge. CB3 0ET UK eEarth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK fSchool of Geography, Earth and Environmental Science, University of Plymouth, Plymouth, PL4 8AA UK gDepartment of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN UK 1Present address: Scottish Environment Protection Agency, Bremner House, The Castle Business Park, Stirling, UK, FK9 4TF;2Present address: Bermuda Institute of Ocean Sciences, Ferry Reach, St. Georges, GE01, Bermuda;3Present address: School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, UK CF10 3AT, UK 1 *Corresponding author: [email protected] Tel +44-(0)1502-524210 Fax +44- (0)1502 513865 Abstract A study was carried out to assess primary production and associated export flux in the coastal waters of the western Antarctic Peninsula at an oceanographic time-series site. New, i.e. exportable, primary production in the upper water-column was estimated in two ways; by nutrient deficit measurements, and by primary production rate measurements using separate 14C-labelled radioisotope and 15N-labelled stable isotope uptake incubations. The resulting average annual exportable primary production estimates at the time-series site from nutrient deficit and primary production rates were 13 and 16 mol C m-2 respectively. Regenerated primary production was measured using 15N-labelled ammonium and urea uptake, and was low throughout the sampling period. The exportable primary production measurements were compared with sediment trap flux measurements from 2 locations; the time-series site and at a site 40 km away in deeper water. Results showed ~1% of the upper mixed layer exportable primary production was exported to traps at 200m depth at the time-series site (total water column depth 520m). The maximum particle flux rate to sediment traps at the deeper offshore site (total water column depth 820m) was lower than the flux at the coastal time-series site. Flux of particulate organic carbon was similar throughout the spring-summer high flux period for both sites. Remineralisation of particulate organic matter predominantly 2 occurred in the upper water-column (<200m depth), with minimal remineralisation below 200m, at both sites. This highly productive region on the Western Antarctic Peninsula is therefore best characterised as ‘high recycling, low export’. Keywords: primary production; new production; regenerated production; f ratio; Antarctic Peninsula; Southern Ocean 1. Introduction Biogeochemical processes in Antarctic coastal waters can exert important influences on both benthic and planktonic ecological phenomena, e.g. Karl et al. (1991); Barnes and Clarke (1994); Prezelin et al. (2000); Smith et al. (2008). These processes also have the potential to exert larger scale impacts on the chemistry of the globally important water masses that form on the Antarctic shelf (Broecker and Peng, 1982; Falkowski et al., 1998). The short phytoplankton production season drives many of the biogeochemical processes involved (Clarke et al., 2008; Ducklow et al., 2008). Despite a short phytoplankton growth period in these Antarctic shelf regions, the overall annual primary productivity is high relative to the Southern Ocean (Arrigo et al., 1998; Vernet et al., 2008). This is in part due to the presence of sufficient trace nutrients, e.g. dissolved Fe, in coastal Antarctic waters (Ardelan et al., 2010) in contrast to the Southern Ocean (Martin et al., 1990). This high primary production can however only be exported if it is in part ‘new’ production (sensu Dugdale and Goering (1967)). In order to calculate this ‘new’ or exportable primary production, concurrent measurements of rates of new, regenerated and 3 total primary production were carried out at the Rothera Oceanographic and Biological Time Series (RaTS) site in Marguerite Bay. The RaTS was initiated at this site in 1997 to investigate the impacts of climatic change on the physical, biogeochemical and biological aspects of the Antarctic marine environment (Clarke et al., 2008). Climate change is particularly pronounced in polar regions because of the number of feedbacks in the ice- atmosphere-ocean system (Smith et al., 1998; Meredith and King, 2005; Clarke et al., 2007, Montes-Hugo et al., 2009). Concurrent estimates of new primary production from nutrient profiles were also carried out using the approach of Serebrennikova and Fanning (2004). Combining the short-term direct rate measurements of phytoplankton growth with the nutrient drawdown method, which integrates to some extent over space and time, allows an improved estimation of the areal annual exportable primary production for the Marguerite Bay region. The exportable primary production in the upper water-column may in turn result in export of organic matter to deeper waters, if it is not remineralised during export. The primary production that is exported is important both for understanding C and other macronutrient cycles, and for the delivery organic material to support benthic faunal communities (Smith et al., 2008). In order to quantify this exported flux, there have been many studies using sediment traps along the Antarctic Peninsula, e.g. von Bodungen et al. (1986); Palanques et al. (2002); Baldwin and Smith (2003); Kim et al. (2004). The introduction of sediment trapping studies lasting many years on the WAP has in particular highlighted the extreme annual variability of these fluxes and high flux rates during summer (Ducklow, 2007). In the southern region of the western Antarctic Peninsula (WAP), diatom dominance of summer primary production (Huang et al., 2012) may result 4 in higher rates of biogenic opal flux to deep water (Nelson et al., 1996; Pondaven et al., 2000), in addition to delivering organic matter to the benthos (Smith et al., 2008). The deployment of sediment traps at 2 sites in this study provides the most southerly export flux measurement for the WAP. The data from the sediment traps are used to investigate how upper water-column exportable primary production (<100m) relates to exported particle flux to deeper waters. Overall this study constructs an annual upper water-column primary production budget at the RaTS site. This budget is then compared to nutrient drawdown-derived primary production, and particle export measurements for Marguerite Bay. The work presented here tests whether Antarctic coastal waters in the southern WAP act as a globally important CO sink, as postulated by Arrigo et al. (2008a), due to high primary 2 productivity resulting in high export flux to the benthos. 2. Methods and materials 2.1 Study sites Marguerite Bay is a fjordic region with relatively slow water advection compared to biogeochemical process rates, making the region suitable for biogeochemical budgeting (Fig. 1a). The RaTS site is ~4km offshore in Ryder Bay (Fig. 1b) over a water depth of 520m (Clarke et al., 2008). This site was the location for the collection of conductivity- temperature-depth (CTD) profiles and water samples for primary production and nutrient uptake experiments (Table 1). When the main RaTS site could not be reached a secondary 5 station was used (Fig. 1b) as in Clarke et al. (2008) and Venables et al. (in press). The secondary site is assumed to be representative of the main RaTS site, consistent with various considerations of the physical and biogeochemical system (Clarke et al., 2008; Meredith et al., 2008, 2010). Two sediment traps and hydrographic moorings were deployed in Marguerite Bay from RRS James Clark Ross. Three successive mooring deployments were completed at the RaTS site (see Table 2). The second mooring was deployed 40km away in an extension of the Marguerite Trough (MT site) in 820m of water (Fig. 1a) but was lost during the second deployment (Table 2). Marguerite Bay is located on the WAP shelf, with the Antarctic Circumpolar Current (ACC) immediately adjacent to the shelf break. This puts the WAP in an unusual oceanographic position compared with other Antarctic shelf regions, which are typically separated from the ACC by subpolar gyres that inhibit the exchange of water mass properties. The WAP’s proximity to the ACC allows warm and saline Circumpolar Deep Water (CDW) from the ACC to enter onto the continental shelf along the glacially scoured troughs dissecting the shelf (Klinck et al., 2004; Martinson et al., 2008). The Antarctic Peninsula Coastal Current (APCC) flows southward along the western sides of Adelaide Island and Alexander Island (Moffat et al. (2008); Fig. 1a). Within Marguerite Bay there is a generally cyclonic flow connected to the boundary currents adjacent to Adelaide and Alexander Islands (Beardsley et al., 2004; Klinck et al., 2004). This potential connectivity has been used to argue that the marine system at the RaTS site is broadly representative of at least the inner part of the WAP shelf (Meredith et al., 2004), albeit with higher levels of chlorophyll a (chl a) compared to the WAP shelf as a whole 6 (Montes-Hugo et al., 2008). Although the path of the APCC within Ryder Bay is not well understood, the hydrography at the RaTS site is characteristic of waters inshore of the westernmost edge of the APCC (Meredith et al., 2004; Moffatt et al., 2008; Wallace, 2008). 2.2 Hydrography, water column macronutrient profiles and nutrient deficit-derived production Samples were taken for dissolved nitrate and silicate profiles at up to 12 depths during the sediment trap deployment/recovery cruises on a large research vessel at both the RaTS and MT sites (Table 2) using an array of 10L Niskin bottles attached to a rosette multisampler. A SeaBird 911plus CTD system was deployed on the same frame, with derived salinity calibrated with discrete water samples analysed on a Guildline 8400B salinometer. Water samples for nitrate and nitrite (hereafter nitrate) were filtered through ~0.7 (cid:2)m pore size pre-ashed (4 h, 400oC) glass fibre (GF/F) filters (Whatman). Samples for macronutrients were stored frozen at -200C until analysis, and analysed using a Skalar autoanalyser according to Kirkwood (1996). Accuracy of nitrate and silicate analyses was confirmed by standard reference materials, with a <5% error for analyses relative to Ocean Scientific International Ltd. (U.K.) standards. The detection limit for autoanalyser measurements of nitrate and silicate analyses was 0.1 (cid:2)M. The water column nutrient profiles were used to derive exportable primary production by nutrient deficit. Using this method, the difference between water-column integrated nitrate for the upper 100m for winter and summer sampling was calculated i.e. summer 7 nitrate drawdown (Table 3). Depth-integrated winter total (to 100m depth) was calculated using 38mmol m-3 from 0 m to 100 m. This is the average NO - concentration for all 3 depths and all CTD profiles from 200 m to bottom (see Figs. 2 and 3). The difference between integrated winter nitrate and integrated summer nitrate is assumed to arise from net nutrient uptake by phytoplankton. Summer nitrate drawdown was converted to Particulate Organic Carbon (POC) by assuming a Redfield C/N ratio of 106:16, to estimate nutrient deficit-derived exportable primary production (Serebrennikova and Fanning, 2004). The interpretation of this calculation is relatively simple in this region since there is no N fixation in this region (Karl et al., 2002), no significant riverine and 2 atmospheric N input, and restricted access to the open ocean. 2.3 Upper water-column sampling at the RaTS site At the RaTS site biogeochemical parameters and physical oceanographic data have been collected ~weekly at 15m depth since 1998 using a rigid inflatable boat (see Clarke et al. (2008) for a full description of the data set). During the study period physical oceanographic profiles were collected to >500 m depth using a SeaBird SBE19+ CTD, WetLabs fluorometer and LiCor Photosynthetically Active Radiation (PAR) sensor. RaTS site salinity data were calibrated annually with SeaBird 911+ instruments carried aboard RRS James Clark Ross and ARSV L.M. Gould, themselves calibrated against P-series standard seawater. In this study a series of additional sampling trips were made during the summer of 2005- 2006 at the RaTS site only, in order to measure primary production rates at high frequency 8