Particulate and phytoplankton absorption during POLARSTERN cruises ANT-XXVI/3 and ANT-XXVIII/3

The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.真光层深度（euphotic depth, Zeu）是利用卫星海洋色数据开展初级生产力（primary production, PP）模拟的关键参数。然而，当前针对卫星反演真光层深度产品的评估研究较为匮乏。本文旨在探究现有卫星反演真光层深度的方法与传感器，并评估不同真光层深度产品对南大洋（Southern Ocean, SO）初级生产力估算的影响。研究分别基于（i）表层叶绿素a（surface chlorophyll-a, Chl-a）与（ii）固有光学特性（inherent optical properties, IOP），从MODIS与SeaWiFS卫星产品中反演得到真光层深度（分别记为Zeu-Chla与Zeu-IOP），并将上述反演结果与南大洋不同区域的原位真光层深度实测数据进行对比。尽管采用固有光学特性方法结合SeaWiFS数据时反演效果最优，其平均百分比误差（average percentage of error, E）为25.43%，对数尺度下的平均绝对误差（mean absolute error, MAE）为0.10 m，但两种反演方法与所用传感器均表现出较好的稳健性。不过，研究发现在部分特定区域，两款传感器对应的Zeu-Chla与Zeu-IOP的空间分布差异可达30%，这类差异同样体现在初级生产力的估算结果中。平均而言，基于Zeu-Chla反演得到的初级生产力较基于Zeu-IOP的结果高出8%，而在南纬60°以南区域，该差值最高可达30%。此外，本文还对不同波长下通过准分析算法（Quasi-Analytical Algorithm, QAA）反演得到的卫星浮游植物吸收系数（phytoplankton absorption coefficients, aph）进行了验证，结果显示MODIS反演的aph整体稳健性优于SeaWiFS，因此在南大洋基于aph的初级生产力模型中，优先选用MODIS反演的aph数据。本研究进一步强调，需深入探究不同卫星产品间的空间差异——由于原位实测数据量不足且分布不均，仅通过原位验证可能无法发现这类差异。