MOBY is a fixed mooring system operated by the National Oceanic and Atmospheric Administration (NOAA) that provides a continuous time series of water-leaving radiance and surface irradiance in the visible region of the spectra since 1997. The site is located a few kilometres west of the Hawaiian Island of Lanai where the water depth is about 1200 m. Since its deployment, MOBY measurements have been the primary basis for the on-orbit vicarious calibrations of the SeaWiFS and MODIS ocean colour sensors. A full description of the MOBY system and processing is provided in Clark et al. (2003). Data are freely available for scientific use at the MOBY Gold directory. The products of interest are the “scientific time series” files, which refer to MOBY data averaged over sensor-specific wavelengths and particular hours of the day (around 20:00–23:00 UTC). For this work, the satellite band-average products for SeaWiFS, MODIS AQUA, MERIS, VIIRS-SNPP, VIIRS-JPSS (also known as NOAA-20 VIIRS), OLCI-S3A, and OLCI-S3B were compiled from the “R2017 reprocessing”. The “inband” average subproduct was used, and to maintain the highest quality, only data determined from the upper two arms (“Lw1”) and flagged “good” quality were acquired. Data from the MOBY203 deployment were discarded due to the absence of surface irradiance data. The compiled variable was the remote-sensing reflectance, “rrs”, which was computed from the original water-leaving radiance (“Lw”) and surface irradiance (“Es”). The water-leaving radiances were corrected for the bidirectional nature of the light field (Morel and Gentili, 1996; Morel et al., 2002) using the same lookup table and method as that used in the SeaWiFS Data Analysis System (SeaDAS) processing code. The MOBY data were reprocessed in 2017 (“MOBY R2017 reprocessing”) to include various improvements in the calibration of the instrument and post-processing, which include: (1) a new method to extrapolate the upwelling radiance attenuation coefficient to the surface (Voss et al., 2017); (2) an increase in arm depth by 0.234 m; and (3) a single pixel shift in the data for the red spectrograph collected at a bin factor of 384. Only the last two changes were included in the present compilation. The first change uses model results to improve Lw at wavelengths above 575 nm, by correcting the diffuse upwelling radiance attenuation coefficient for inelastic effects. Thus for wavelengths above 575 nm, the Lw21 product in the Gold directory should be investigated. As mentioned before, the MOBY data compiled in this work are sensor-specific. Therefore, attention is necessary to use the correct MOBY data when validating a particular sensor. The way MOBY data are stored in the final merged table is consistent with the original wavelengths; however, these wavelengths can differ from what is sometimes expected to be the central wavelength of a given band and sensor. Irrespective of the wavelength where MOBY data are stored in the final table, for validation of bands 1–6 of SeaWiFS, MOBY data stored in the final merged table at 412, 443, 490, 510, 555, and 670 nm, respectively, should be used. For validation of bands 1–7 of MODIS AQUA, MOBY data stored in the final merged table at 416, 442, 489, 530, 547, 665, and 677 nm, respectively, should be used. For validation of bands 1-10 of MERIS, MOBY data stored in the final merged table at 410.5, 440.4, 487.8, 507.7, 557.6, 617.5, 662.4, 679.9, 706.2, and 752.5 nm, respectively, are the appropriate data. For validation of bands 1–12 of OLCI-S3A, MOBY data stored in the final merged table at 400.3032, 411.8453, 442.9626, 490.493, 510.4676, 560.4503, 620.4092, 665.2744, 674.0251, 681.5705, 709.1149, and 754.1813, respectively, are the appropriate data. For validation of bands 1–12 of OLCI-S3B, MOBY data stored in the final merged table at 400.5947, 411.9509, 442.9882, 490.3991, 510.4022, 560.3664, 620.284, 665.1312, 673.8682, 681.3856, 708.9821, and 754.0284, respectively, are the appropriate data. For validation of bands 1–5 of VIIRS-SNPP, MOBY data stored in the final merged table at 412.9, 444.5, 481.2, 556.3, and 674.6 nm, respectively, are the appropriate data. Finally, for validation of bands 1–5 of VIIRS-JSPP, MOBY data stored in the final merged table at 411, 445, 489.01, 556, and 667 nm, respectively, are the appropriate data. For the latter sensor, the original value was 489 nm, but it was changed to 489.01 nm to differentiate from the 489 nm of MODIS AQUA. The lookup table to fully normalize “rrs” only covers the range 413–660 nm; compared to the previous versions of the compilation, in the present version, the “rrs” MOBY at wavelengths outside this range were not discarded and fully normalized using the closest entry of the lookup table (i.e. at 413 or 660 nm).

BOUSSOLE project started in 2001 with the objective of establishing a time series of bio-optical properties in oceanic waters to support the calibration and validation of ocean colour satellite sensors (Antoine et al., 2006). The project consists of a monthly cruise program and a permanent optical mooring (Antoine et al., 2008). The mooring collects radiometry and inherent optical properties (IOPs) in continuous mode every 15 min at two depths (4 and 9 m nominally). The monthly cruises are devoted to the mooring servicing, to the collection of vertical profiles of radiometry and IOPs, and to water sampling at 11 depths from the surface down to 200 m, for subsequent analyses including phytoplankton pigments, particulate absorption, CDOM absorption, and suspended particulate matter load. The BOUSSOLE mooring is in the western Mediterranean Sea at a water depth of 2400 m. All pigment (2001–2019) and radiometric (two subsets: 2003–2012 and 2015–2019) data were provided by the Principal Investigators. The first radiometric subset was obtained from measurements made with multispectral Satlantic OCI-200 radiometers; the second radiometric subset was obtained from measurements made with hyperspectral Satlantic OCR radiometers, convolved with spectral response function of Sentinel3 OLCI-A bands. The compiled variables were “rrs” and “chla_hplc”. Remote-sensing reflectance was computed from the original “fully-normalized” water-leaving radiance (“nLw_ex”), which is the “normalized” water-leaving radiance (“nLw” previously described), with a correction for the bidirectional nature of the light field (Morel and Gentili, 1996; Morel et al., 2002). The solar irradiance (“Fo”) was computed from two available variables in the original set of data: the normalized water-leaving radiance (“nLw”) and the remote-sensing reflectance (“rrs”), using the equation “Fo = nLw/rrs”. Only radiometric observations that meet the following criteria were used: (1) tilt of the buoy was less than 10∘; (2) the buoy was not lowered by more than 2 m as compared to its nominal water line (to ensure the Es reference sensor is above water and exempt from sea spray); and (3) the solar irradiance was within 10 % of its theoretical clear-sky value (determined from Gregg and Carder, 1990). The latter criterion was used to select clear skies only. An additional quality control was to remove observations that were 50 % higher or lower than the daily average. This removed a small number of “spikes” in the time series. The final quality control step was to remove days where the standard deviation was more than half of the daily average. This was meant to identify days with high variability. Very few days (N=2) were removed with this test. These quality control criteria were applied per wavelength, which resulted in some observations with an incomplete spectrum.

AERONET-OC is a component of AERONET, including sites where sun photometers operate with a modified measurement protocol leading to the determination of the fully-normalized water-leaving radiance (Zibordi et al., 2006, 2009). As a result of a collaboration between the Joint Research Centre (JRC) and NASA to develop (Hooker et al., 2000) and exploit (Zibordi et al., 2002) the technology, this component has been specifically developed for the validation of ocean colour radiometric products. The strength of AERONET-OC is “the production of standardized measurements that are performed at different sites with identical measuring systems and protocols, calibrated using a single reference source and method, and processed with the same codes” (Zibordi et al., 2006, 2009). All high quality data (“Level-2”) were acquired from the project website for 11 sites: Abu_Al_Bukhoosh (∼25∘ N, ∼53∘ E), COVE_SEAPRISM (∼36∘ N, ∼75∘ W), Gloria (∼44∘ N, ∼29∘ E), Gustav_Dalen_Tower (∼58∘ N, ∼17∘ E), Helsinki Lighthouse (∼59∘ N, ∼24∘ E), LISCO (∼40∘ N, ∼73∘ W), Lucinda (∼18∘ S, ∼146∘ E), MVCO (∼41∘ N, ∼70∘ W), Palgrunden (∼58∘ N, ∼13∘ E; Philipson et al., 2016), Venice (∼45∘ N, ∼12∘ E), and WaveCIS_Site_ CSI_6 (∼28∘ N, ∼90∘ W). The compiled variable was “rrs”. Remote-sensing reflectance was computed from the original “fully-normalized” water-leaving radiance (see Sect. 2.2.2 for definition). The solar irradiance (“Fo”), which is not part of the AERONET-OC data, was computed from the Thuillier et al. (2003) solar spectrum irradiance by averaging “Fo” over a wavelength-centred 10 nm window. Data were compiled for the exact wavelengths of each record, which can change over time for a given site depending on the specific instrument deployed. In comparison with the previous version of the compilation, the present OC-CCI data set version 3, now uses the “version 3” reprocessing of AERONET-OC data (Zibordi et al., 2021).

SeaBASS is one of the largest archives of in situ marine bio-optical data (Werdell et al., 2003) with a long-established inventory (Hooker et al., 1994). It is maintained by NASA's Ocean Biology Processing Group (OBPG) and includes measurements of optical properties, phytoplankton pigment concentrations, and other related oceanographic and atmospheric data. The SeaBASS database consists of in situ data from multiple contributors, collected using a variety of measurement instruments with consistent, community-vetted protocols, from several marine platforms such as fixed buoys, handheld radiometers, and profiling instruments. Quality control of the received data includes a rigorous series of protocols that range from file format verification to inspection of the geophysical data values (Werdell et al., 2003). Radiometric data were mostly acquired through the “validation” search tool, which provided in situ data with matchups for particular ocean colour sensors (Bailey and Werdell, 2006). The criterion in the search–query was defined to have the minimal flag conditions in the satellite data, to retrieve a greater number of matchups, and therefore in situ data. Regarding phytoplankton pigment data, the majority were acquired through the “pigment” search tool, which provided pigment data directly from the archives. As was stated in the SeaBASS website, the “pigment” search tool was originally designed to return only in vitro fluorometric measurements, which is consistent with our approach, but over time chlorophyll-a measurements made using other methods (e.g. in vivo fluorometry) were included in the retrieved pigment data. In the pigment data used in this work, a large number of in situ fluorometric measurements from continuous underway instruments were identified and discarded. These data were initially identified from cruises with more than 50 observations per day and then re-checked in the SeaBASS website to confirm whether indeed they were continuous underway measurements. A total of 120 412 such measurements were identified and discarded. Given the large volume of this group of data, it is possible that some chlorophyll-a observations from in vivo methods may have escaped the scrutiny and persisted into the final merged data set. The “pigment” search tool was recently discontinued, and instead the “file” search tool can be used, which was also used here to acquire chlorophyll, as well as radiometric observations, for more recent years. The remote sensing reflectance acquired from the “file” search tool was corrected for the bidirectional effects (Morel and Gentili, 1996; Morel et al., 2002). The compiled variables from SeaBASS data were “rrs”, “chla_hplc”, “chla_fluor”, “aph”, “adg”, “bbp”, and “kd”.

NOMAD is a publicly-available data set compiled by the NASA OBPG at the Goddard Space Flight Center. It is a high-quality global data set of coincident radiometric and phytoplankton pigment observations for use in ocean colour algorithm development and satellite-data product-validation activities (Werdell and Bailey, 2005). The source bio-optical data is the SeaBASS archive; therefore, many dependencies exist between these two data sets, which were addressed during the merging. The current version (Version 2.0 ALPHA, 2008) includes data from 1991 to 2007 and an additional set of observations of inherent optical properties. The current version was used in this work, but with an additional set of columns of remote-sensing reflectance corrected for the bidirectional effects (Morel and Gentili, 1996; Morel et al., 2002). This additional set of columns was provided directly by the NOMAD creators. The compiled variables were “rrs”, “chla_hplc”, “chla_fluor”, “aph”, “adg”, “bbp”, and “kd”. Conversion was necessary only for “aph”, “adg”, and “bbp”, and followed the procedures described in Sect. 2.1. For the calculation of “bbp”, the variable “bb” was used with a smooth fitting to remove noise. A portion of NOMAD data were optically weighted (for methods see Werdell and Bailey, 2005). These data are not consistent with the protocols chosen in this work, but these observations were retained since NOMAD is a widely-used data set in ocean colour validation.

MERMAID provides in situ bio-optical data matched with concurrent and comparable MERIS Level 2 satellite ocean colour products (Barker, 2013a, b). The MERMAID in situ database consists of data from multiple contributors, measured using a variety of instruments and protocols, from several marine platforms such as fixed buoys, handheld radiometers, and profiling instruments. Comprehensive quality control and protocols are used by MERMAID to integrate all the data into a common and comparable format (Barker, 2013a, b). Access to MERMAID data is limited to the MERIS Validation Team, the MERIS Quality Working Group, and to the in situ data contributors. For this work, access has been granted to the MERMAID database through a signed Service Level Agreement. The MERMAID data include sub-sets of several data sets used in this compilation (MOBY, AERONET-OC, BOUSSOLE, NOMAD). These observations were removed from the MERMAID data set to avoid duplication (as discussed in Sect. 2.1). The compiled variables were “rrs”, “chla_hplc”, “chla_fluor”, “aph”, “adg”, “bbp”, “kd”, and “tsm”. Remote-sensing reflectance was calculated by dividing by π the original “fully-normalized” water-leaving reflectance (“Rw_ex”), which is the water-leaving reflectance (Rw =π Lw / Es), with a correction for the bidirectional nature of the light field (Morel and Gentili, 1996; Morel et al., 2002). Conversion was also necessary for “aph”, “adg”, and “bbp” and followed the procedures described in Sect. 2.1.

In comparison with the previous version of the compilation, a set of “chla_fluor” observations from MERMAID were considered suspicious and excluded from the compilation (N=3241, from mermaid_MAREL-carnot, mermaid_MAREL-itroise, and mermaid_MAREL-vilaine).

HOT programme provides repeated comprehensive observations of the hydrography, chemistry, and biology of the water column at a station located 100 km north of Oahu, Hawaii, since October 1988 (Karl and Michaels, 1996). This site is representative of the North Pacific subtropical gyre. Cruises are made approximately once a month to the deep-water Station ALOHA (A Long-Term Oligotrophic Habitat Assessment; 22∘45′ N, 158∘00′ W). Pigment data (“chla_hplc” and “chla_fluor”) were extracted directly from the project website. Radiometric measurements from the HOT project are also available, but observations of “rrs” and “kd” from the HOT project were acquired in this work as part of the SeaBASS data set.

GeP&CO is part of the French PROOF programme and aims to describe and understand the variability of phytoplankton populations and to assess its consequences on the geochemistry of the oceans (Dandonneau and Niang, 2007). It is based on the quarterly travels of the merchant ship Contship London from France to New Caledonia in the Pacific. A scientific observer sailed on each trip and operated the sampling for surface water, filtration, various measurements, and checking at several times of each day. The experiment started in October 1999 and finished in July 2002. Pigment data were extracted from the project website. Additional pigment data obtained during the OISO-4 cruise in the south Indian Ocean onboard R/V Marion-Dufresne (January–February 2000) were added. The samples were measured by Yves Dandonneau following the method used in the GeP&CO project. The compiled variable was “chla_hplc” and “chla_fluor”.

AMT is a multidisciplinary programme which undertakes biological, chemical, and physical oceanographic research during transects between the UK and destinations in the South Atlantic (Robinson et al., 2006). The programme was established in 1995 (e.g. Robins et al., 1996; Aiken et al., 1998) and since then has completed 29 research cruises. Pigment data between 1997 (AMT5) and 2018 (AMT28) were mostly provided by the British Oceanographic Data Centre (BODC) following a specific request for discrete observations of chlorophyll-a concentration since 1997. The AMT data were isolated by searching for the string “AMT” in the “cruise” columns and the respective Principal Investigators were then searched individually in a separated metadata file. Data not flagged with highest quality or without method of measurement were not used. For any interest in the original data, BODC is the point of contact, which ensures that if there are any updates, the most recent data are supplied. In the case of AMT 26, 27, and 28, data were provided to the OC-CCI project by Gavin Tilstone, whereas in the case of AMT 20 and 23, data were provided by Robert J. W. Brewin. The compiled variables are “chla_hplc” and “chla_fluor”.

ICES is a network of more than 4000 scientists from almost 300 institutes, with 1600 scientists participating in activities annually. The ICES Data Centre manages a number of large data set collections related to the marine environment covering the Northeast Atlantic, Baltic Sea, Greenland Sea, and Norwegian Sea. Most of data originate from national institutes that are part of the ICES network of member countries. Data were provided (on 28 April 2014) from the ICES database on the marine environment (Copenhagen, Denmark) following a specific request. The ICES data were made available under the ICES data policy, and if there is any conflict between this and the policy adopted by the users, then the ICES policy applies. The compiled variables were “chla_hplc” and “chla_fluor”.

ARCSSPP database is a synthesis of observations between 1954 and 2006 from the Arctic Ocean and northern seas (Matrai et al., 2013). The observations were acquired from data repositories, publications or provided by individual investigators. The database includes quality-controlled observations of productivity and chlorophyll-a, photosynthetically available radiation, and hydrographic parameters. This collection of data was acquired at http://www.nodc.noaa.gov/cgi-bin/OAS/prd/accession/download/63065 (last access: 18 December 2022). For the present work, only observations of chlorophyll-a concentration with known time zones were used. The compiled chlorophyll observations were from discrete samples, but the exact method (either “chla_fluor” or “chla_hplc”) was not available for all observations. Thus, the ARCSSPP chlorophyll observations were marked as “chla_fluor”, although some might have been from HPLC measurements and were flagged with “1” in a column “flag_chla_method”. The compiled variable was “chla_fluor”.

In this work, the AWI data source refers to the group of observations that were provided to the OC-CCI project by Astrid Bracher. These are bio-optical observations collected during several cruises across the globe. All data were available through the PANGAEA repository. Observations of concentration of chlorophyll-a, 1 nm spectrally resolved remote sensing reflectances, and algal pigment absorption coefficient were considered. The methods for these observations are described by Taylor et al. (2011a), Liu et al. (2018a), and Tilstone et al. (2020). For chlorophyll, data from the following cruises were used: ANT-XXIV/1, ANT-XXIV/4, ANT-XXVI/4, and MSM18/3 (Bracher et al., 2015a); SO202/2 (Zindler et al., 2013a); ANT-XXVII/2 (Bracher, 2015); ANT-XXV/1 (Taylor et al., 2011b); ANT-XXVIII/3 and SO218 (Soppa et al., 2014); ANT23-1 (Bracher et al., 2015b); MSM9-1 (Bracher et al., 2017); M91 (Hepach et al., 2016); SO234+235 (Bracher et al., 2019); SO243 (Bracher, 2019a); PS93.2 (Liu et al., 2018b); HE462 (Bracher and Wiegmann, 2019); PS99.1 (Liu et al., 2019a); PS99.2 (Liu et al., 2018c); PS103 (Bracher, 2019b); PS107 (Liu et al., 2018d); and PS113 (Bracher et al., 2020). Concerning remote sensing reflectances, the observations taken during cruises ANT-XXIV/4 and ANT-XXVI/4 (Bracher et al., 2015a), ANT-XXV/1 (Taylor et al., 2011b), and ARK26-3 (Bracher et al., 2018) were gathered. The remote sensing reflectances were corrected for the bidirectional nature of the light field (Morel and Gentili, 1996; Morel et al., 2002). The absorption coefficients were taken during cruises SO202/2 (Zindler et al., 2013), ANT_XXV/1 (Taylor et al., 2011b), ANT-XXVI/3 and ANT-XXVIII/3 (Soppa et al., 2013), ARK26-3 (Gonçalves-Araujo et al., 2018), PS93.2 (Wiegmann et al., 2019), PS99.2 (Liu et al., 2019b), and PS107 (Liu et al., 2019c). The compiled variables were “chla_hplc”, “rrs”, and “aph”.

BATS is a long-term study by the Bermuda Institute of Ocean Sciences based on regular cruises in the western Atlantic Ocean (Sargasso Sea) since 1988. The cruises at BATS site (∼31∘40′ N, 64∘10′ W) sample ocean temperature and salinity, but are focused on biogeochemical variables such as nutrients, dissolved inorganic carbon, oxygen, HPLC of pigments, primary production, and sediment trap flux. In this work, all the phytoplankton pigment data available from the BATS website (http://bats.bios.edu/bats-data/, last access: 18 December 2022) were considered, which also included regional and transect cruises not specific to the nominal BATS site. The compiled variables were “chla_hplc” and “chla_fluor”.

The BARENTSSEA data source refers to a group of observations that were provided to OC-CCI project by Knut Yngve Børsheim. This collection was developed using data from the archives of the Institute of Marine Research (Norway). It comprises observations of temperature, salinity and chlorophyll-a routinely collected by cruises, mainly in the North Sea, the Norwegian Sea, and the Barents Sea between 1997 and 2013. The chlorophyll-a concentration was measured by filtering and extraction using Turner fluorometers. The compiled variable was “chla_fluor”.

BioChem is an archive of marine biological and chemical data maintained by Fisheries and Oceans Canada (DFO, 2018; Devine et al., 2014). The available observations are from department research initiatives and collected in areas of Canadian interest. Available parameters include pH, nutrients, chlorophyll, dissolved oxygen, and other plankton data (species and biomass). Chlorophyll measurements from in vitro fluorometric methods were acquired (from http://www.dfo-mpo.gc.ca/science/data-donnees/biochem/index-eng.html, last access: 18 December 2022) with close guidance by the BioChem helpdesk, confirming quality and methods. The used data span from 1997 to 2014 and were mainly from the Gulf of St. Lawrence (western North Atlantic). The compiled variable was “chla_fluor”.

BODC is the designated marine science data centre for the United Kingdom. The data used in this work derive from a specific request for discrete observations of chlorophyll-a concentration since 1997. Initially, this request was used to compile AMT data (see Sect. 2.2.9). The remaining data comprising observations of chlorophyll-a concentration from fluorometric and HPLC methods, mostly sampled in the North Atlantic, were analysed and added (the “dataset” string for this data source is “bodc”). Data not flagged with highest quality or without method of measurement were discarded. The compiled variables were “chla_hplc” and “chla_fluor”.

CalCOFI is a partnership of the California Department of Fish & Wildlife, National Oceanic & Atmospheric Administration Fisheries Service, and Scripps Institution of Oceanography. CalCOFI has conducted quarterly cruises off southern and central California since 1949. Data collected in the upper 500 m include temperature, salinity, oxygen, nutrients, chlorophyll, primary productivity, plankton biodiversity, and biomass. For this work, only observations of chlorophyll-a concentration derived from fluorometric methods flagged with highest quality were used. Data were acquired from the file “CalCOFI_Database_194903-201911_csv_10Jul2020.zip”. The compiled variable was “chla_fluor”.

CCELTER investigates the California Current coastal pelagic ecosystem, with a focus on long-term forcing. The CCELTER data include primary and derived measurements from both Process and CalCOFI-augmented cruises, as well other time series. CCELTER data include variables from the physical environment, biogeochemistry, and biological populations/communities. For this work, chlorophyll observations measured from discrete bottle samples from CCELTER Process cruises determined by extraction and bench fluorometry (California Current Ecosystem LTER and Goericke, 2020) were used. The compiled variable was “chla_fluor”.

CIMT was a non-operational program where marine scientists from different disciplines and institutions combine their efforts on observations directed towards understanding the central California upwelling system. The CIMT archived data include coastal ocean observations from satellites, shipboard data, moorings, and large marine animal movements. For this work, pigment data from discrete bottle samples taken during CIMT monthly cruises were used. Data were acquired from the project website (https://cimt.ucsc.edu/data_portal.htm, last access: 18 December 2022). The compiled variable was “chla_fluor”.

COASTCOLOUR data sets were designed to evaluate the performance of ocean colour satellite algorithms in the retrieval of water quality parameters in coastal waters (Nechad et al., 2015a). Three types of COASTCOLOUR data sets are available: (1) a match-up data set where in situ bio-optical observations are available simultaneously with a cloud-free MERIS product; (2) an in situ reflectance data set where an in situ reflectance is available simultaneously with an in situ measurement of chlorophyll-a concentration and/or total suspended matter; and (3) a simulated data set where reflectances were generated by a radiative transfer model. This work used the match-up data set, which includes most of the in situ measurements, and is available at 10.1594/PANGAEA.841950 (Nechad et al., 2015b). The match-up data set provides optical, biogeochemical, and physical data collections at 17 sites across the globe. From this data set, observations of reflectance, chlorophyll-a, total suspended matter, and IOPs were compiled. The remote sensing reflectances were corrected for the bidirectional nature of the light field (Morel and Gentili, 1996; Morel et al., 2002). The compiled variables were “rrs”, “chla_hplc”, “chla_fluor”, “aph”, “adg”, “bbp”, and “tsm”.

ESTOC is an open-ocean monitoring site located in the eastern North Atlantic subtropical gyre. ESTOC was initiated in 1991 with particle flux measurements, and in 1994 began standard observations of the water column, in addition to the deployment of a current meter mooring. The core parameters measured at ESTOC include salinity, temperature, current speed, nutrients, chlorophyll, inorganic carbon, particulate organic carbon and nitrogen, and sinking particle flux (Neuer et al., 2007). For this work, measurements of chlorophyll-a concentration from monthly cruises from 1994 to 2011 were used. These data were provided to CCI following a specific request. The time of day was unavailable and was set to 12:00:00 (UTC). These observations were flagged with “1” in column “flag_time”. The compiled variable was “chla_fluor”.

IMOS is enabled by Australia's National Collaborative Research Infrastructure Strategy (NCRIS) funded by Australian Government. Since 2006, IMOS is operating a wide range of observing equipment throughout the coastal and open ocean around Australia, making all data openly available to the scientific community, other stakeholders, and users. In this work, the IMOS data contribution refers to two data sets. One is a data collection entitled “IMOS National Reference Station (NRS) – Phytoplankton HPLC Pigment Composition Analysis”, which was acquired from the Australian Ocean Data Network portal (https://portal.aodn.org.au, last access: 18 December 2022). This data set comprises of phytoplankton pigment composition measured by HPLC collected with small vessels on monthly basis at nine National Reference Stations as part of the IMOS National Mooring Network. The other chlorophyll-a data set measured by HPLC and fluorometry methods is a subset (2015–2021) of the IMOS Bio-optical Database also available through the AODN portal. This database comprises of a suite of bio-optical parameters from samples collected during research voyages in Australian waters and is used by the IMOS Ocean Colour Sub-Facility to assess the accuracy of satellite ocean colour products in Australian coastal and open ocean waters (Schroeder et al., 2016). The previous data compilations include an earlier subset of HPLC chlorophyll-a concentration from the IMOS Bio-optical Database that was acquired through the SeaBASS archive. These data can be found under “dataset” string “seabass” and Lesley Clementson as data contributor. The compiled variables for IMOS were “chla_hplc” and “chla_fluor”.

MAREDAT database is a global assemblage of pigments measured by HPLC (Peloquin et al., 2013a) from combination of 136 independent field data sets, solicited from investigators and databases. The database provides high quality measurements of taxonomic pigments including chlorophyll a and b, 19'-butanoyloxyfucoxanthin, 19'-hexanoyloxyfucoxanthin, alloxanthin, divinyl chlorophyll-a, fucoxanthin, lutein, peridinin, prasinoxanthin, violaxanthin, and zeaxanthin. The database is available through PANGAEA (10.1594/PANGAEA.793246, Peloquin et al., 2013b). For this work, only measurements of total chlorophyll-a flagged with high quality were used. The time of day was unavailable and was set to 12:00:00 (UTC). These observations were flagged with “1” in column “flag_time”. The compiled variable was “chla_hplc”.

PALMER is a monitoring station located in western Antarctic Peninsula. The Palmer station investigates the marine ecology of the Southern Ocean with focus on the pelagic marine ecosystem, including sea ice habitats, regional oceanography, and nesting sites of seabird predators. The PALMER data include measurements of meteorological, oceanographic, sea ice, predators, nutrients and biogeochemistry, pigments, primary production, zooplankton, and microbes parameters. This work used the measurements of chlorophyll analysed by HPLC and fluorometry taken at the Palmer Station (Palmer Station Antarctica LTER et al., 2020a, b) and from the annual cruises off the coast of the Western Antarctica Peninsula (Palmer Station Antarctica LTER et al., 2018, 2020c). The compiled variables were “chla_hplc”, “chla_fluor”.

SeaDataNet is a Pan-European infrastructure for ocean and marine data management. It aims to develop a standardized system for managing large and diverse data sets collected by oceanographic cruises and automatic observation systems. For this work, discrete chlorophyll-a concentration observations with an “access restriction” set to “academic” and “unrestricted” were acquired from the SeaDataNet platform with guidance from the helpdesk. Only data from the “Institute of Marine Research – Norwegian Marine Data Centre (NMD), Norway”, which comprised most of the acquired data, were used. All chlorophyll observations were from discrete samples measured by fluorometric, spectrophotometric, or HPLC methods, but the exact method was not given. Thus, the observations were marked as “chla_fluor”, although some were possibly from HPLC measurements, and were flagged with “1” in a column “flag_chla_method”. The compiled variables were “chla_fluor”.

In this work, the TPSS data source refers to a group of observations that were provided to this compilation by Trevor Platt and Shubha Sathyendranath. This is a collection of bio-optical in situ data collected during cruises predominantly in the Northwest Atlantic, but also from the Indian Ocean, South Pacific, and Central Atlantic (see Sathyendranath et al., 2009 for additional details regarding the cruises). It comprises measurements of phytoplankton pigments and algal pigment absorption coefficients. The time of day was unavailable and was set to 12:00:00 (UTC). These observations were flagged with “1” in column “flag_time”. The compiled variables were “chla_hplc”, “chla_fluor”, and “aph”.

The Tara expeditions consist of several cruises around the world, some with durations of several years, designed to study and understand the distribution of planktonic organisms in the world ocean. The discrete observations of remote sensing reflectance and chlorophyll-a concentration from HPLC measurements taken during the Tara “Oceans” (2009–2013) and “Mediterranean” (2014) expeditions were considered in this work. These data were provided to ESA OC-CCI project by Emmanuel Boss and were available in the SeaBASS archive. The remote sensing reflectances were corrected for the bidirectional nature of the light field (Morel and Gentili, 1996; Morel et al., 2002). The compiled variables were “chla_hplc” and “rrs”.