Primary production in the Sub-Antarctic and Polar Frontal Zones south of Tasmania, Australia; SAZ-Sense survey, 2007

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Abstract

The Sub-Antarctic Zone (SAZ) in the Southern Ocean provides a significant sink for atmospheric CO2 and quantification of this sink is therefore important in models of climate change. During the SAZ-Sense (Sub-Antarctic Sensitivity to Environmental Change) survey conducted during austral summer 2007, we examined CO2 sequestration through measurement of gross primary production rates using 14C. Sampling was conducted in the SAZ to the south-west and south-east of Tasmania, and in the Polar Frontal Zone (PFZ) directly south of Tasmania. Despite higher chlorophyll biomass off the south-east of Tasmania, production measurements were similar to the south-west with rates of 986.2±500.4 and 1304.3±300.1 mg C m−2 d−1, respectively. Assimilation numbers suggested the onset of cell senescence by the time of sampling in the south-east, with healthy phytoplankton populations to the south-west sampled three weeks earlier. Production in the PFZ (475.4±168.7 mg C m−2 d−1) was lower than the SAZ, though not significantly. The PFZ was characterised by a defined deep chlorophyll maximum near the euphotic depth (75 m) with low production due to significant light limitation. A healthy and less light-limited phytoplankton population occupied the mixed layer of the PFZ, allowing more notable production there despite lower chlorophyll. A hypothesis that iron availability would enhance gross primary production in the SAZ was not supported due to the seasonal effect that masked possible responses. However, highest production (2572.5 mg C m−2 d−1) was measured nearby in the Sub-Tropical Zone off south-east Tasmania in a region where iron was likely to be non-limiting (Bowie et al., 2009. Biogeochemical iron budgets of the Southern Ocean south of Australia: decoupling of iron and nutrient cycles in the subantarctic zone by the summertime supply. Global Biogeochemical Cycles 23(4), doi:10.1029/2009GB003500).

Introduction

The Southern Ocean plays a significant role in global climate by acting as both a source and sink for atmospheric CO2 depending on region and season (Poisson et al., 1993, Sabine et al., 2004). The region south of the Polar Frontal Zone (PFZ) is considered a carbon source due to the upwelling of nutrient-rich Circumpolar Deep Water at the Antarctic Convergence. Upwelling is caused through westerly winds that dominate the Southern Ocean, which cause surface waters to move northward due to Ekman transport. The northward moving waters are cold and dense, and subsequently sink to form Antarctic Intermediate Water (Lumpkin and Speer, 2007). A proportion also reaches the more northerly Sub-Antarctic Front (SAF) where it sinks to form Sub-Antarctic Mode water. These sinking waters take up large amounts of CO2 at the surface due to high gas transfer associated with fast wind speeds, combined with low initial carbon content of the seawater (Sabine et al., 2004, Takahashi et al., 2009). The Sub-Antarctic Zone (SAZ) in proximity to the SAF, is considered to be an efficient region for carbon sequestration (Metzl et al., 1999, McNeil et al., 2001). The PFZ may also act as a CO2 sink, although there is high variability and sequestration is not as pronounced as in the SAZ (Poisson et al., 1993).

Overlying these physical circulation patterns are those of the biological system that are affected by seasonal change (Chisholm, 2000). Sequestration of carbon from the atmosphere depends primarily on initial fixation rates during photosynthesis (Griffiths et al., 1999, Sedwick et al., 2002, Westwood et al., 2010), with subsequent carbon transfer to the deep ocean dependent on biological recycling, remineralisation rates (Cardinal et al., 2005b), and the sinking/export rates of organic matter (Cardinal et al., 2001, Trull et al., 2001a, Richardson and Jackson, 2007). Optimal gross primary production generally occurs during spring and early summer, following the replenishment of nutrients to surface waters over winter, and coinciding with high light availability and the onset of stratification (Bradford-Grieve et al., 1997, Boyd et al., 1999, Lourey and Trull, 2001, Hiscock et al., 2003). The biological system can be directly influenced by the physical system, e.g. through vertical mixing that affects light and nutrient availability (Marra, 1980, Mitchell et al., 1991, Nelson and Smith, 1991, Franck et al., 2000). In the SAZ, mixed layer depths are >400 m during winter and shoal to 75–100 m during summer (Rintoul and Trull, 2001). However, summer mixed layers may still be deep enough for phytoplankton cells to be circulated below the euphotic zone.

There are strong gradients of silicate and nitrate from south to north in the Southern Ocean, with limiting silicate concentrations common in the SAZ throughout the year (Dugdale et al., 1995, Trull et al., 2001b, Cardinal et al., 2005a). Nitrate concentrations are generally higher than silicate, with non-limitation previously demonstrated during summer (Sedwick et al., 2002). Possible reasons for high silicate depletion relative to nitrate include less efficient remineralisation (Cardinal et al., 2005a), or high silicate:nitrate uptake by phytoplankton due to iron limitation (Franck et al., 2000, Watson et al., 2000, Hutchins et al., 2001). In the PFZ, silicate and nitrate concentrations are higher than the SAZ at the start of the growth season due to the presence of upwelled nutrient-rich Circumpolar Deep Water (Franck et al., 2000). However, biological activity may deplete silicate resources by early to late summer (Hiscock et al., 2003).

Whilst silicate availability may control the growth of diatoms and silicoflagellates that require this nutrient, the availability of iron may be more important for other phytoplankton groups in the Southern Ocean (Trull et al., 2001b). Iron is mainly derived from continental sources (e.g. through run-off and/or dust inputs) with limitation likely in regions such as the SAZ and PFZ zones, which are remote from land masses (Martin et al., 1990, Blain et al., 2007). Other less well-defined sources of iron may be through upwelling or lateral advection (Martin et al., 1990, de Baar et al., 1999, Hiscock et al., 2003, Bowie et al., 2009). The stimulation of phytoplankton production with iron additions has previously been demonstrated in the SAZ and PFZ zones during both spring and summer (Gervais et al., 2002, Sedwick et al., 2002). Low iron combined with low silicate availability in the SAZ has been shown to favour the growth of non-silicious, iron-efficient phytoplankton species such as eukaryotic picoplankton and cyanobacteria (Hutchins et al., 2001). Light availability may also influence the extent of iron limitation with light-limited more affected than light-saturated populations (Hiscock et al., 2008). In the Sub-Tropical Zone (STZ) to the north of the SAZ, iron limitation may be less severe given that these waters are close to continental land masses and mixed layer depths are shallow, allowing high light availability (Sedwick et al., 1999).

The SAZ-Sense survey was conducted in January–February 2007 to characterise key components and processes of planktonic ecosystems in the Southern Ocean, their influence on CO2 exchange with the atmosphere and deep ocean, and their sensitivity to anthropogenic CO2 emissions and associated climate change. One of the main predictions from climate change models is for a reduction in global overturning circulation and therefore a decrease in the uptake of CO2 in the SAZ (Sarmiento and LeQuere, 1996, Matear and Hirst, 1999). However, the relative contributions of physical versus biological processes of CO2 removal in this region are currently unclear (Trull et al., 2001c). Fundamental to this understanding is the measurement of gross primary production and the factors responsible for its control. The SAZ region south of Tasmania, Australia, provides an ideal site to examine the effects of iron availability in particular. Examination of SeaWiFS and MODIS data over the past decade has shown an increase in surface chlorophyll to the south-east of Tasmania, whereas no similar increase has occurred to the south-west. A current theory for this difference is that iron availability to the south-east has increased due to a southward shift in the prevailing westerly winds (Lyne et al., 2005). This may increase dust inputs from the continent (Mahowald et al., 1999, Jickells et al., 2005), as well as to intensify eddies from the East Australian Current (EAC) that may penetrate the SAZ (Clementson et al., 1998, Ridgway, 2007). Variability of iron availability in surface waters has previously been related to mesoscale eddy activity (Measures and Vink, 2001). The aim of this study was therefore to compare gross primary production rates and photosynthetic parameters in the SAZ to the south-east and south-west of Tasmania in relation to iron availability in particular, and to contrast this with production rates in the PFZ where CO2 sequestration is variable (Poisson et al., 1993). Results are discussed within the context of light and macronutrient availability, which may also play an important role in the control of production.

Section snippets

Survey area

The survey was conducted south of Tasmania, Australia, during austral summer from 19 January to 19 February 2007. It consisted of three process stations (occupied for 6–7 days) connected by two transects (south-west and south-east) conducted between approximately 43.5 and 54.1°S latitude (Fig. 1). The south-west transect was located between 140.6 and 146.3°E longitude (stations 2–48). The south-east transect was located between 146.3 and 153.3°E longitude (stations 39–107). Both transects

Results

In the Australian sector of the Southern Ocean the Polar Front (PF) divides into northern (PF-N, Fig. 1) and southern (PF-S) branches (Rintoul and Bullister, 1999, Trull et al., 2001b). The zone between the two branches is defined as the IPFZ, with the PFZ to the north of the PF-N but south of the closely aligned SAF. For the purposes of the results the IPFZ is grouped with the PFZ. The SAZ occurs between the SAF and STF, with the STZ to the north of the STF. The STF is defined as the region

Process Stations 1 and 3 (SAZ)

Iron availability in the SAZ to the south-east of Tasmania (P3) was higher than the P1 region to the south-west (Bowie et al., 2009, Lannuzel et al., 2011), yet primary production during the SAZ-Sense survey was similar at both sites. This suggested that iron availability may not have had a direct effect on primary production in the SAZ. However, high chlorophyll concentrations, low assimilation numbers and the presence of Phaeophytin a at P3 suggested that there had been increased primary

Acknowledgments

We would like to thank Captain Scott Laughlin and the crew of RSV Aurora Australis for their expertise in supporting this work. Thanks also to Rick van den Enden, Dr. Imojen Pearce, and Dr. Martina Doblin for HPLC sampling and analysis, Dr. Bronte Tilbrook, Kate Berry and Kristina Paterson for DIC analysis, Dr. Andrew Bowie for dissolved iron measurements, and Mark Rosenberg and the CTD team for temperature and salinity data. For technical support we would like to thank Allan Poole and Aaron

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