Sea ice bio-optical measurements

Field-based sampling: As part of Australian Antarctic Science project # 4298, a total number of 44 sea ice sites were sampled for bio-optical measurements along 4 transects on land-fast sea ice off Davis Station (Antarctica) during November – December 2015. Measurements included simultaneous hyperspectral down-welling (ice surface) irradiance (triplicate) and under-ice radiance (triplicate) measurements (320 – 900 nm, 3.3 nm resolution) with a TriOS ACC and Trios ARC radiometer, respectively. The radiance measurements were conducted with the TriOS ARC radiometer mounted onto an L-shaped arm (for deployment details see Melbourne-Thomas et al. 2015). Subsequently, snow thickness was measured with a ruler and an ice core was collected directly above the radiometer location. Sea-ice freeboard (tape measure) and ice thickness (ice core length) were also recorded. Ice cores (9 cm internal diameter) were cut into sections, and these were melted in the dark at +4 degrees C, filtered onto GFF filters and then used to measure ice algal pigment content (using High Performance Liquid Chromatography (HPLC) and spectral ice algal absorption coefficients (ap, ad, aph) for entire vertical profiles or for the lower-most 0.1 m of ice cores. The location of the sampling grid had its origin (x=0, y=0) at GPS position: -68.568904, 77.945439. Transects (128m – 512 m in length) started at x=60, x=70, x=80 and x=90 m and were sampled at y-positions of 0m, 0.5m, 1m, 2m, 4m, 8m, 16m, 32m, 64m, 128m, (256m, and 512m) on 19/11/2015, 23/11/2015, 29/11/2015 and 02/12/2015, respectively.Analysis of ice algal chlorophyll a concentration:For pigment analysis, 0.25 to 1.0 litres of melted ice core subsamples were passed through 25 mm diameter glass-fiber (Whatman GF/F) filters. The filters were then frozen and stored below −80 degrees C prior to analysis using HPLC. Samples were extracted over 15 to 18 hours in acetone before analysis by HPLC using a modified C8 column and binary gradient system with an elevated column temperature [Van Heukelem and Thomas, 2001]. Pigments were identified by retention time and absorption spectra from a photo-diode array (PDA) detector, and concentrations were determined from commercial and international standards (Sigma; DHI, Denmark). Analysis of particulate (algal and non-algal) absorption:The optical density (OD) spectra of the particulate material on these filters (see section above) were measured over the 350 to 750 nm spectral range in 0.9 nm increments, using a Cintra 404 UV/VIS dual-beam spectrophotometer equipped with an integrating sphere. The pigments on the sample filter were then extracted using the method of Kishino et al. [1985]'s method to determine the OD of the non-algal particles in a second scan. The OD due to ice algae was then obtained by calculating the difference between the optical density of the total particulate and non-algal fractions. The OD measurements were converted to absorption spectra using blank filter measurements, and by first normalizing the scans to zero at 750 nm and then correcting for the path length amplification using the coefficients of Mitchell [1990]. A detailed description of the method is given in Clementson et al. [2001], and followed SeaWiFS protocols [Muller et al., 2003]. An exponential function was fitted to all spectra of non-algal particulate material:ad(λ) = ad(350 nm) exp[−S(λ − 350 nm)] + b, (1)where ad(λ) is the residual absorption coefficient over the wavelength (λ) range 350 to 750 nm of the particles after methanol extraction, also referred to as absorption of detritus [m−1] although this may include absorption of non-extractable pigments and heterotrophic protists. A non-linear least-squares technique was used to fit Equation 1 to the untransformed data, where S and b are empirically-determined constants. The inclusion of an offset b allows for any baseline correction. In some samples, pigment extraction was incomplete, leaving small residual peaks in detritus spectra at the principal chlorophyll absorption bands. To avoid distorting the fitted detritus spectra, data at these wavelengths were omitted when all spectra were fitted. Total particulate spectra were smoothed using a running box-car filter with 10 nm width, and the fitted detritus spectra were subtracted to yield the ice algae spectra. Subtracting fitted detritus spectra minimized any artifacts due to incomplete extraction of pigments. The resulting ice algae spectra were base-corrected by subtracting absorption at 750 nm to obtain aph(λ). The following parameters were then determined: ap(λ) = absorption coefficient of particles [m−1]; aph(λ) = absorption coefficient of ice algae [m−1] calculated as the difference between ap(λ) and ad(λ).Literature cited: Clementson, L. A., J. S. Parslow, A. R. Turnbull, D. C. McKenzie, and C. E. Rathbone (2001), Optical properties of waters in the Australasian sector of the Southern Ocean, Journal of Geophysical Research: Oceans, 106(C12), 31,611–31,625, doi:10.1029/2000jc000359.Kishino, M., M. Takahashi, N. Okami, and S. Ichimura (1985), Estimation of the spectral absorption-coefficients of phytoplankton in the sea, Bulletin of Marine Science, 37(2), 634–642.Melbourne-Thomas, J., K. Meiners, C. Mundy, C. Schallenberg, K. Tattersall, and G. Dieckmann (2015), Algorithms to estimate Antarctic sea ice algal biomass from under-ice irradiance spectra at regional scales, Marine Ecology Progress Series, 536, 107–121, doi:10.3354/meps11396.Mitchell, B. G. (1990), Algorithms for determining the absorption coefficient for aquatic particulates using the quantitative filter technique, Orlando’90, 1302, 137–148, doi:10.1117/12.21440.Müller, J. L., R. R. Bidigare, C. Trees, W. M. Balch, and J. Dore (2003), Ocean Optics Protocols for Satellite Ocean Colour Sensor Validation, Revision 5, Volume V: Biogeochemical and Bio-Optical Measurements and Data, NASA Tech. Memo.Van Heukelem, L., and C. S. Thomas (2001), Computer-assisted high-performance liquid chromatography method development with applications to the isolation and analysis of phytoplankton pigments, Journal of Chromatography A, 910(1), 31–49, doi:10.1016/s0378-4347(00)00603-4.野外采样方案：本数据集隶属于澳大利亚南极科学项目第4298号，于2015年11月至12月期间，在南极戴维斯站周边的固定冰（land-fast sea ice）上沿4条断面布设44个海冰采样点，开展生物光学测量。测量内容包括同步采集的高光谱下行辐照度（冰面处，三次重复）与冰下辐亮度（三次重复）数据，波段范围为320~900 nm，光谱分辨率3.3 nm，分别采用TriOS ACC辐射计与TriOS ARC辐射计完成采集。其中冰下辐亮度测量通过将TriOS ARC辐射计安装于L型臂上实现，部署细节详见墨尔本-托马斯等（Melbourne-Thomas et al. 2015）的研究。随后使用直尺测量雪层厚度，并在辐射计测点正上方采集冰芯。同步记录海冰干舷（卷尺测量）与冰厚（冰芯长度）。冰芯内径为9 cm，被切割为若干段后在黑暗环境中以4℃条件融化，过滤至GFF滤膜上，随后用于测定冰藻色素含量（采用高效液相色谱法（High Performance Liquid Chromatography, HPLC））以及光谱冰藻吸收系数（ap、ad、aph），测定覆盖冰芯完整垂直剖面或冰芯最下部0.1 m层段。 采样网格的原点（x=0, y=0）对应GPS坐标：-68.568904, 77.945439。四条断面长度介于128 m至512 m之间，采样起点分别为x=60、70、80、90 m，采样点y坐标依次为0 m、0.5 m、1 m、2 m、4 m、8 m、16 m、32 m、64 m、128 m、（256 m与512 m），分别于2015年11月19日、11月23日、11月29日及12月2日完成各断面采样。 冰藻叶绿素a浓度分析：针对色素分析，将0.25~1.0 L融化后的冰芯子样品通过直径25 mm的玻璃纤维滤膜（Whatman GF/F）过滤。滤膜经冷冻保存于-80℃以下，待后续采用HPLC进行分析。样品在丙酮中萃取15~18小时后，使用改性C8色谱柱与二元梯度系统，并升高柱温，通过HPLC完成分析（Van Heukelem与Thomas, 2001）。通过光电二极管阵列（Photo-Diode Array, PDA）检测器的保留时间与吸收光谱识别色素，采用商用及国际标准品（Sigma；丹麦DHI）确定色素浓度。 颗粒（藻类与非藻类）吸收分析：使用配备积分球的Cintra 404紫外-可见双光束分光光度计，在350~750 nm光谱范围内以0.9 nm的步长测定滤膜上颗粒物的光密度（OD）光谱。随后采用Kishino等（1985）的方法萃取滤膜上的色素，进行第二次扫描以测定非藻类颗粒物的光密度。冰藻对应的光密度通过总颗粒物与非藻类颗粒物光密度的差值计算得到。利用空白滤膜的测量值将光密度转换为吸收光谱，先将扫描结果在750 nm处归一化至零，再采用Mitchell（1990）提出的系数校正光程放大效应。详细方法见Clementson等（2001）的研究，并遵循SeaWiFS协议（Muller et al., 2003）。 对所有非藻类颗粒物光谱拟合如下指数函数： ad(λ) = ad(350 nm) exp[−S(λ − 350 nm)] + b, (1) 其中ad(λ)为甲醇萃取后，350~750 nm波长λ处颗粒物的残余吸收系数，也被称为碎屑吸收系数[单位：m⁻¹]，尽管其可能包含不可萃取色素及异养原生生物的吸收。采用非线性最小二乘技术将式(1)拟合至未转换的原始数据，其中S与b为经验确定的常数。引入偏移项b可实现基线校正。部分样品中色素萃取不完全，在主要叶绿素吸收波段的碎屑光谱中残留小峰。为避免拟合的碎屑光谱失真，拟合所有光谱时需剔除这些波长处的数据。对总颗粒物光谱采用宽度为10 nm的滑动盒式滤波器（box-car filter）进行平滑，随后减去拟合的碎屑光谱以得到冰藻光谱。减去拟合的碎屑光谱可最大程度降低色素萃取不完全带来的伪影。将得到的冰藻光谱在750 nm处扣除吸收值以完成基线校正，得到aph(λ)。随后确定以下参数：ap(λ)为颗粒物吸收系数[单位：m⁻¹]；aph(λ)为冰藻吸收系数[单位：m⁻¹]，通过ap(λ)与ad(λ)的差值计算得到。 参考文献： Clementson, L. A., J. S. Parslow, A. R. Turnbull, D. C. McKenzie, 与 C. E. Rathbone (2001), 《南大洋澳大拉西亚海域水体的光学特性》, 《地球物理研究杂志：海洋》, 106(C12), 31611–31625, doi:10.1029/2000jc000359. Kishino, M., M. Takahashi, N. Okami, 与 S. Ichimura (1985), 《海洋浮游植物光谱吸收系数的估算》, 《海洋科学通报》, 37(2), 634–642. Melbourne-Thomas, J., K. Meiners, C. Mundy, C. Schallenberg, K. Tattersall, 与 G. Dieckmann (2015), 《基于区域尺度冰下辐照度光谱估算南极海冰藻生物量的算法》, 《海洋生态学进展系列》, 536, 107–121, doi:10.3354/meps11396. Mitchell, B. G. (1990), 《利用定量滤膜技术测定水生颗粒物吸收系数的算法》, Orlando’90, 1302, 137–148, doi:10.1117/12.21440. Müller, J. L., R. R. Bidigare, C. Trees, W. M. Balch, 与 J. Dore (2003), 《卫星海洋颜色传感器验证的海洋光学协议（第5版）》, 第五卷：生物地球化学与生物光学测量及数据, NASA技术备忘录. Van Heukelem, L., 与 C. S. Thomas (2001), 《计算机辅助高效液相色谱法开发及其在浮游植物色素分离与分析中的应用》, 《色谱A杂志》, 910(1), 31–49, doi:10.1016/s0378-4347(00)00603-4.