Chlorophyll-a concentration (chl-a) is important to assess the health and state of ocean ecosystems. With the availability of global ocean-colour chl-a estimates that now span 25 years, there has been a concerted effort to produce merged data products from different satellite sensors to assess changes in chl-a over the global ocean for long periods of time. However, to date, the performance of these merged chl-a products has not been thoroughly assessed. To perform such an assessment, we assembled a large global in-situ dataset of quasi-autonomous spectrophotometrically-derived chl-a that resulted in >13000 satellite match-ups and then filtered them to produce the highest quality data. A suite of merged ocean-colour chl-a products were then assessed using the in-situ chl-a. All of these satellite products (except the gap-filled ones) exhibited similarly consistent results with the in-situ chl-a data with a mean relative percentage difference of 30%, with the Ocean Colour – Climate Change Initiative (OC-CCI) products showing a slightly improved performance.
The OC-CCI Chl-a product was then used to compute net primary production (NPP) using the wavelength resolving model of Morel (1991) modified by Smyth et al. (2005) to create a 21-year time-series. The NPP time-series was assessed in the North Atlantic in areas of similar phenology, climatology, and annual NPP in the north-east Atlantic Ocean. Over the entire area, there was an increase in NPP from 1998 to 2003, followed by a significant decline until 2018. The pattern was predominant in north-western European coastal waters and specific areas of the English Channel, Irish Sea, North Sea and Norwegian Sea and altered the timing and phenology of NPP. The implications for altering the energy transfer to higher trophic levels and altering the uptake of CO₂ by regional seas will be explored.
 

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