Moutier W, Thomalla S.J., Dr Stewart Bernard, Galina Wind, Ryan-Keogh T J, Marie Smith

The Southern Ocean (SO) is highly sensitive to climate change. Therefore, an accurate estimate of phytoplankton biomass is key to being able to predict the climate trajectory of the 21st century. In this study, MODerate resolution Imaging Spectroradiometer (MODIS), on board EOS Aqua spacecraft, Level 2 (nominal 1 km 1 km resolution) chlorophyll-a (CSat) and Particulate Organic Carbon (POCsat) products are evaluated by comparison with an in situ dataset from 11 research cruises (2008–2017) to the SO, across multiple seasons, which includes measurements of POC and chlorophyll-a (Cin situ) from both High Performance Liquid Chromatography (CHPLC) and fluorometry (CFluo). Contrary to a number of previous studies, results highlighted good performance of the algorithm in the SO when comparing estimations with HPLC measurements. Using a time window of 12 h and a mean satellite chlorophyll from a 5 5 pixel box centered on the in situ location, the median CSat:Cin situ ratios were 0.89 (N = 46) and 0.49 (N = 73) for CHPLC and CFluo respectively. Differences between CHPLC and CFluo were associated with the presence of diatoms containing chlorophyll-c pigments, which induced an overestimation of chlorophyll-a when measured fluorometrically due to a potential overlap of the chlorophyll-a and chlorophyll-c emission spectra. An underestimation of 0.13 mg m-3 was observed for the global POC algorithm. This error was likely due to an overestimate of in situ POCin situ measurements from the impact of dissolved organic carbon not accounted for in the blank correction. These results highlight the important implications of different in situ methodologies when validating ocean colour products.

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(a) Comparison between measured (Cin situ) and retrieved (CSat) chlorophyll-a for box with a time window of 12 h. Blue and orange dots indicate samples measured from HPLC (CHPLC) and fluorometry (CFluo) respectively, while the dashed line shows the 1:1 relationship. (b) Probability density function of the logarithm base 10 of the ratio between CSat and Cin situ from fluorometry (blue line) and HPLC (orange line).