Abstract
To evaluate the performance of ocean-colour retrievals of total chlorophyll-a concentration requires direct comparison with concomitant and co-located in situ data. For global comparisons, these in situ match-ups should be ideally representative of the distribution of total chlorophyll-a concentration in the global ocean. The oligotrophic gyres constitute the majority of oceanic water, yet are under-sampled due to their inaccessibility and under-represented in global in situ databases. The Atlantic Meridional Transect (AMT) is one of only a few programmes that consistently sample oligotrophic waters. In this paper, we used a spectrophotometer on two AMT cruises (AMT19 and AMT22) to continuously measure absorption by particles in the water of the ship's flow-through system. From these optical data continuous total chlorophyll-a concentrations were estimated with high precision and accuracy along each cruise and used to evaluate the performance of ocean-colour algorithms. We conducted the evaluation using level 3 binned ocean-colour products, and used the high spatial and temporal resolution of the underway system to maximise the number of match-ups on each cruise. Statistical comparisons show a significant improvement in the performance of satellite chlorophyll algorithms over previous studies, with root mean square errors on average less than half (~ 0.16 in log10 space) that reported previously using global datasets (~ 0.34 in log10 space). This improved performance is likely due to the use of continuous absorption-based chlorophyll estimates, that are highly accurate, sample spatial scales more comparable with satellite pixels, and minimise human errors. Previous comparisons might have reported higher errors due to regional biases in datasets and methodological inconsistencies between investigators. Furthermore, our comparison showed an underestimate in satellite chlorophyll at low concentrations in 2012 (AMT22), likely due to a small bias in satellite remote-sensing reflectance data. Our results highlight the benefits of using underway spectrophotometric systems for evaluating satellite ocean-colour data and underline the importance of maintaining in situ observatories that sample the oligotrophic gyres.
Highlights
Phytoplankton are an essential component of the ocean, modifying its biological, chemical and physical environment
Notwithstanding the remarkable efforts by NASA to produce this dataset, when comparing the normalised frequency distribution of in situ chlorophyll samples in NASA bio-Optical Marine Algorithm Data set (NOMAD) (Fig. 3a, Version 2.0 ALPHA) with that from the global ocean (Fig. 3b, estimated from an annual 2005 Ocean Colour Climate Change Initiative (OC-CCI) composite of chlorophyll), it is clear that oligotrophic waters are under-represented
The chlorophyll distribution of the two resulting in situ datasets (AMT19 and AMT22) were found to be similar to that observed in the global ocean, with a slight bias towards the under-sampled oligotrophic gyres
Summary
Phytoplankton are an essential component of the ocean, modifying its biological, chemical and physical environment. The majority of light absorbed by phytoplankton is transferred to heat, which can modify the temperature and physical structure of the water column (Sathyendranath, Gouveia, Shetye, Ravindran, & Platt, 1991; Zhai, Tang, Platt, & Sathyendranath, 2011), with a smaller component used in photosynthesis, the conversion of inorganic carbon (carbon dioxide) to organic carbon. Phytoplankton contribute to the biogeochemical cycling of a variety of climatically-important elements, such as silica, nitrate and phosphate. It is for these reasons phytoplankton are recognised as an Essential Climate Variable in the implementation plan of the Global Climate Observing System (GCOS, 2011)
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