Abstract Ocean colour data are commonly used to quantify primary production, study phytoplankton dynamics and calibrate marine models, thus understanding the origin of errors in the retrieved chlorophyll-a (Chl-a) product is critical. One source of uncertainty in retrieved Chl-a products can be related to large photosynthetic cells, characterised by lower mass-specific absorption coefficients due to increased packaging effect. Here, we explore the relationship between phytoplankton size structure and an ocean colour product using optical simulations and in situ observations. Specifically, we use an optical model to explore how phytoplankton cell size and abundance influence phytoplankton absorption and backscattering coefficients and the implication this has for water leaving radiance and the estimated Chl-a derived from satellite ocean colour. The optical model simulations show phytoplankton cell size has a significant impact on the remote-sensing reflectance, with Chl-a packaged in 5 to 10 μm cells resulting in about 54 to 76% the simulated ocean colour Chl-a compared to 1 μm cells, as determined by an algorithm that converts reflectances to Chl-a. To support optical simulations, size-fractionated Chl-a samples were collected from several water masses to investigate the phytoplankton size contribution (i.e., 10 μm) to the total Chl-a. We focused on the offshore eastern Australian ocean region, largely characterised by oligotrophic waters in which phytoplankton dominate the optical properties of the water column. Of the 22 stations sampled, a total of ten in situ size fractionated Chl-a measurements were matched-up with the corresponding clear-sky satellite Chl-a product. The matched-up points revealed a systematic underestimation of in situ Chl-a. With the low amount of data, it was not possible to statistically relate the satellite underestimation to a specific phytoplankton size class, but the observations showed that the largest satellite Chl-a underestimates were found when phytoplankton larger than 10 μm represented more than 50% of the phytoplankton community. Additional measurements that combine in situ optical measurements with phytoplankton size distribution and carbon/Chl-a ratio would help to clarify the relationship between phytoplankton size structures and remotely sensed Chl-a product.

中文翻译：

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模拟浮游植物细胞大小和丰度对固有光学特性 (IOP) 和遥感叶绿素 a 产品的影响

摘要 海洋颜色数据通常用于量化初级生产、研究浮游植物动力学和校准海洋模型，因此了解检索到的叶绿素 a (Chl-a) 产品中的误差来源至关重要。回收的 Chl-a 产品的一个不确定性来源可能与大型光合细胞有关，其特征是由于包装效应增加而导致的质量特异性吸收系数较低。在这里，我们使用光学模拟和原位观察探索浮游植物大小结构与海洋颜色产品之间的关系。具体来说，我们使用光学模型来探索浮游植物细胞大小和丰度如何影响浮游植物吸收和后向散射系数，以及这对水离开辐射和从卫星海洋颜色得出的估计 Chl-a 的影响。光学模型模拟显示浮游植物细胞大小对遥感反射率有显着影响，与 1 μm 细胞相比，Chl-a 封装在 5 到 10 μm 细胞中，导致模拟海洋颜色 Chl-a 大约为 54% 到 76%，由将反射率转换为 Chl-a 的算法确定。为了支持光学模拟，从几个水团中收集了大小分级的 Chl-a 样品，以研究浮游植物大小对总 Chl-a 的贡献（即 10 μm）。我们专注于近海的澳大利亚东部海洋区域，主要以贫营养水域为特征，其中浮游植物在水柱的光学特性中占主导地位。在采样的 22 个站点中，共有 10 个原位尺寸分级 Chl-a 测量结果与相应的晴空卫星 Chl-a 产品相匹配。匹配的点揭示了对原位 Chl-a 的系统性低估。由于数据量少，无法将卫星低估与特定浮游植物大小类别从统计上联系起来，但观察表明，当浮游植物大于 10 微米时，发现最大的卫星 Chl-a 低估了浮游植物群落。将原位光学测量与浮游植物尺寸分布和碳/Chl-a 比率相结合的其他测量将有助于阐明浮游植物尺寸结构与遥感 Chl-a 产品之间的关系。但观察结果表明，当浮游植物大于 10 μm 代表浮游植物群落的 50% 以上时，发现最大的卫星 Chl-a 低估。将原位光学测量与浮游植物尺寸分布和碳/Chl-a 比率相结合的其他测量将有助于阐明浮游植物尺寸结构与遥感 Chl-a 产品之间的关系。但观察结果表明，当浮游植物大于 10 μm 代表浮游植物群落的 50% 以上时，发现最大的卫星 Chl-a 低估。将原位光学测量与浮游植物尺寸分布和碳/Chl-a 比率相结合的其他测量将有助于阐明浮游植物尺寸结构与遥感 Chl-a 产品之间的关系。