Sub-Antarctic zone mooring study of interannual variability in particulate carbon export - Phytoplankton Data

Papers arising from phytoplankton experiments associated with the SAZ (Subantarctic Zone) project.This work was complete as part of ASAC (AAS) project 1156.Taken from the abstracts of the referenced papers:Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterised by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe, ~0.05 nM) and silicic acid (Si, less than 1 micro M). During the Subantarctic Zone AU9706 cruise in March 1998 we investigated the relative importance of Fe and Si in controlling phytoplankton growth and species composition at a station within the subantarctic water mass (46.8 degrees S, 142 degrees E) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 micro M added silicic acid (+Si); and 1.9 nM added iron plus 9 micro M added silicic acid (+Fe+Si). We followed a detailed set of biological and biogeochemical parameters over 8 days. Fe added alone clearly increased community growth rates and nitrate drawdown and altered algal community composition relative to control treatments. Surprisingly, small, lightly silicified pennate diatoms grew when Fe was added either with or without Si, despite the extremely low ambient silicic acid concentrations. Pigment analyses suggest that lightly silicified chrysophytes (type 4 haptophytes) may have preferentially responded to Si added either with or without Fe. However, for many of the parameters measured the +Fe+Si treatments showed large increases relative to both the +Fe and +Si treatments. Our results suggest that iron is the proximate limiting nutrient for chlorophyll production, photosynthetic efficiency, nitrate drawdown, and diatom growth, but that Si also exerts considerable control over algal growth response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community.The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47 degrees S, 142 degrees E) and south (54 degrees S, 142 degrees E) of the subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress. Shipboard perturbation experiments examined two light (mean in situ and elevated) and two Fe (nominally 0.5 and 3 nM) treatments under silicic acid-replete conditions. Mean in situ light levels (derived from incident irradiances, mixed layer depths (MLDs), wind stress, and a published vertical mixing model) differed at the two sites, 25% of incident irradiance I0 at 47 degrees S and 9% I0 at 54 degrees S because of MLDs of 40 (47S) and 90 m (54S), when these stations were occupied. The greater MLD at 54S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47S experiment, chlorophyll a levels increased to greater than 1 micro gram per litre only in the high-Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54S experiment there were increases in chlorophyll a (to greater than 2 micro grams per litre), cell abundances, silica production, and carbon fixation only in the high-light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54S (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47 and 54S sites appear to represent minimum and maximum MLDs, where Fe and Fe/ irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed.

本数据集涵盖与SAZ（Subantarctic Zone，亚南极带）项目相关的浮游植物实验产出论文。本研究为ASAC（AAS）项目1156的组成部分。
数据源自所引用论文的摘要：南大洋亚南极带夏季和秋季表层海水具有高浓度硝酸盐与磷酸盐，但溶解态铁（Fe, ~0.05 nM）与硅酸（Si, 低于1 μM）浓度极低。1998年3月，我们在AU9706亚南极带科考航次中，于亚南极水团内的站位（南纬46.8°，东经142°）开展船基瓶式培养实验，探究铁与硅在调控浮游植物生长及物种组成中的相对重要性。实验设置包括未添加营养盐的对照组；添加1.9 nM铁的处理组（+Fe）；添加9 μM硅酸的处理组（+Si）；以及同时添加1.9 nM铁与9 μM硅酸的处理组（+Fe+Si）。我们在8天的培养周期内追踪了一系列详细的生物学与生物地球化学参数。单独添加铁的处理组相较对照组，显著提升了群落生长速率与硝酸盐消耗率，并改变了藻类群落组成。出乎意料的是，尽管环境中硅酸浓度极低，但无论是否添加硅，添加铁的处理组中均出现了小型、轻度硅化的羽纹硅藻。色素分析结果显示，轻度硅化的金藻（4型定鞭藻类，haptophytes）可能优先响应了单独或与铁共同添加的硅。不过，在多数测定参数中，+Fe+Si处理组相较+Fe与+Si处理组均出现了显著提升。我们的研究结果表明，铁是叶绿素生成、光合效率、硝酸盐消耗以及硅藻生长的直接限制性营养盐，但硅也对藻类生长响应具有显著调控作用，这提示铁与硅在塑造亚南极浮游植物群落结构中均发挥着重要作用。
1998年南半球秋季，我们在亚极锋南北两侧的站位（北侧：南纬47°，东经142°；南侧：南纬54°，东经142°）开展实验，探究光照与铁（Fe）供给对浮游植物过程的影响。两个站位的土著藻类均表现出营养盐胁迫特征。船基调控实验在硅酸充足的条件下设置了两种光照水平（原位平均光照与增强光照）与两种铁水平（名义浓度0.5 nM与3 nM）。两个站位的原位平均光照水平由入射辐照度、混合层深度（mixed layer depths, MLDs）、风应力以及已发表的垂直混合模型推导得出：当科考船抵达站位时，47°S站位的混合层深度为40 m，其原位光照为入射辐照度I0的25%；而54°S站位的混合层深度为90 m，其原位光照仅为I0的9%。54°S站位更大的混合层深度体现在其土著浮游植物的细胞叶绿素a浓度是47°S站位的10倍。在47°S站位的实验中，仅高铁处理组的叶绿素a浓度提升至1 μg/L以上，且不受光照水平影响，这表明该站位存在铁限制现象。细胞丰度、硅生成速率与固碳速率也呈现出相同的趋势。与之相反，在54°S站位的实验中，仅在添加了铁的增强光照处理组中，叶绿素a浓度（提升至2 μg/L以上）、细胞丰度、硅生成速率与固碳速率才出现显著提升，这提示铁与光照共同限制了藻类生长速率。在54°S站位（硅酸浓度非限制性条件下），光照通过调控藻类铁配额成为藻类生长速率的关键决定因素；然而，由于铁与光照的共限制特性以及当前数据集的局限性，我们无法明确区分铁与光照在调控浮游植物生长中的相对贡献。基于澳大利亚南部亚南极（SA）海域夏季平均混合层深度的气候学数据，47°S与54°S站位分别代表了混合层深度的最小值与最大值，其中铁限制与铁/光照共限制分别可能调控藻类生长。本文还讨论了亚南极海域中，随季节变化的藻类生长限制因子改变所带来的生态学启示。