Primary Production in the Sub-Antarctic and Polar Frontal Zones; SAZ-Sense Voyage, 2007

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 plus or minus 500.4 and 1304.3 plus or minus 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 week earlier. Production in the PFZ (475.4 plus or minus 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).Table 1:Gross primary production at each CTD station and associated data; Mixed layer depth (Zm, m), incoming PAR (mol m-2 d-1), vertical light attenuation (Kd, m-1), euphotic depth (Zeu, m), differences between euphotic depth and mixed layer depth (Zeu-Zm, m), column-integrated chlorophyll a (0 to 150 m, mg m-2), column-integrated production (0 to 150 m, mg C m-2 d-1), production within the mixed layer (mg C m-2 d-1), production below the mixed layer (mg C m-2 d-1), production within the euphotic zone (1% PAR, mg C m-2 d-1), production below the euphotic zone (mg C m-2 d-1). Kd values that were calculated from chlorophyll a v PAR regressions are marked with an asterisk. At some stations there was a surface mixed layer as well as a secondary mixed layer and both depths are indicated.Table 2:Photosynthetic attributes of phytoplankton with depth at each CTD station; Mixed layer depth (m), euphotic depth (Zeu, m), maximum photosynthetic rate [Pmax, mg C (mg chl a)-1 h-1], maximum photosynthetic rate corrected for photoinhibition [Pmaxb, mg C (mg chl a)-1 h-1], initial slope of the light-limited section of the P-I curve [alpha, mg C (mg chl a)-1 h-1 (micro-mol m-2 s-1)-1], rate of photoinhibition [beta, mg C (mg chl a)-1 h-1 (micro-mol m-2 s-1)-1], intercept of the P-I curve with the carbon uptake axis [c, mg C (mg chl a)-1 h-1], light intensity at which carbon-uptake became saturated (Ek, micro-mol m-2 s-1), and chlorophyll a measured using HPLC (mg m-3).

南大洋副极地海区（Sub-Antarctic Zone, SAZ）是大气二氧化碳（atmospheric CO2）的重要碳汇，因此对该碳汇的量化研究在气候变化模型中具有关键意义。在2007年南半球夏季开展的副极地环境变化敏感性（SAZ-Sense）科考航次中，研究团队通过碳-14（14C）标记法测定总初级生产力（gross primary production）速率，对碳封存过程展开了系统研究。采样区域覆盖塔斯马尼亚岛西南、东南侧的副极地海区，以及塔斯马尼亚岛正南侧的极锋区（Polar Frontal Zone, PFZ）。
尽管塔斯马尼亚东南侧海域的叶绿素生物量更高，但两处的生产力测定结果相近，分别为986.2±500.4 mg C m⁻² d⁻¹与1304.3±300.1 mg C m⁻² d⁻¹。光合同化数（assimilation numbers）结果表明，东南侧采样时浮游植物（phytoplankton）已出现细胞衰老迹象，而三周前在西南侧采集的浮游植物种群仍处于健康状态。极锋区的总初级生产力为475.4±168.7 mg C m⁻² d⁻¹，低于副极地海区，但差异未达统计学显著水平。极锋区的典型特征为：在真光层深度（约75米）附近存在清晰的深层叶绿素最大值，但由于光照限制显著，生产力水平较低；而混合层内的浮游植物种群健康且受光照限制较弱，尽管叶绿素含量更低，仍实现了更为可观的生产力。
研究团队曾提出“铁有效性（iron availability）可提升副极地海区总初级生产力”的假说，但该假说未得到支持，因为季节效应掩盖了潜在的响应信号。不过，在塔斯马尼亚东南侧邻近海域的亚热带海区（Sub-Tropical Zone），研究测得最高的总初级生产力（2572.5 mg C m⁻² d⁻¹），该区域的铁供应大概率未成为限制因子（Bowie等，2009）。
表1：各温盐深仪（CTD）站位的总初级生产力及相关参数，包括：混合层深度（Zm，单位：米）、入射光合有效辐射（PAR，单位：mol m⁻² d⁻¹）、垂直光衰减系数（Kd，单位：m⁻¹）、真光层深度（Zeu，单位：米）、真光层深度与混合层深度的差值（Zeu-Zm，单位：米）、0~150米柱积分叶绿素a（单位：mg m⁻²）、0~150米柱积分初级生产力（单位：mg C m⁻² d⁻¹）、混合层内初级生产力（单位：mg C m⁻² d⁻¹）、混合层以下初级生产力（单位：mg C m⁻² d⁻¹）、真光层内（光合有效辐射为表层1%处）初级生产力（单位：mg C m⁻² d⁻¹）、真光层以下初级生产力（单位：mg C m⁻² d⁻¹）。通过叶绿素a与PAR回归分析计算得到的Kd值标注有星号。部分站位同时存在表层混合层与次表层混合层，已一并标注两种混合层深度。
表2：各CTD站位不同深度浮游植物的光合特性参数，包括：混合层深度（单位：米）、真光层深度（Zeu，单位：米）、最大光合速率[Pmax，单位：mg C (mg chl a)⁻¹ h⁻¹]、校正光抑制（photoinhibition）后的最大光合速率[Pmaxb，单位：mg C (mg chl a)⁻¹ h⁻¹]、光限制阶段P-I（光合速率-光强）曲线的初始斜率[α，单位：mg C (mg chl a)⁻¹ h⁻¹ (μmol m⁻² s⁻¹)⁻¹]、光抑制速率[β，单位：mg C (mg chl a)⁻¹ h⁻¹ (μmol m⁻² s⁻¹)⁻¹]、P-I曲线与碳摄取轴的截距[c，单位：mg C (mg chl a)⁻¹ h⁻¹]、碳摄取达到饱和时的光强（Ek，单位：μmol m⁻² s⁻¹），以及高效液相色谱法（High Performance Liquid Chromatography, HPLC）测定的叶绿素a浓度（单位：mg m⁻³）。