Assemblages of phytoplankton pigments along a shipping line through the North Atlantic and Tropical Pacific

International audienceA set of phytoplankton pigments measurements collected on eight quarterly transects from France to New Caledonia is analyzed, in order to identify the main assemblages of phytoplankton, and to relate their occurrence with oceanic conditions. Pigments concentrations are first divided by the sum monovinyl chlorophyll a plus divinyl chlorophyll a to remove the effect of biomass, and, second, normalized, to give an equal weight to all pigments. The resulting 17 pigments ≥ 799 observations matrix is then classified into ten clusters using neural methodology. Eight out of these ten clusters have a well marked regional or seasonal character, thus evidencing adapted responses of the phytoplankton communities. The main gradient opposes two clusters with high fucoxanthin and chlorophyll c1+2 in the north Atlantic in January, April and July, to three clusters in the South Pacific Subtropical Gyre with high divinyl chlorophyll a, zeaxanthin and phycoerythrin. One of the clusters in the South Pacific Subtropical Gyre has relatively high zeaxanthin and phycoerythrin contents and dominates in November and February (austral summer) while another one, with relatively high divinyl chlorophylls a and b dominates in May and August (austral winter). The third one in this area is characterized by high carotene concentration and its occurrence peaks in February and May. In the equatorial currents system, one cluster, rich in chlorophylls b and c1+2, is strictly located in a narrow zone centred at the equator, while another one, with relatively high violaxanthin concentration, is restricted to the high nutrient - low chlorophyll waters in only the southern part of the South Equatorial Current. One cluster with relatively high prasinoxanthin content has a spatial distribution that spans the entire South Equatorial Current. Two clusters have a ubiquitous distribution: one in the equatorial Pacific, in the Carribbean Sea and in the north Atlantic in summer has pigments concentrations that are close to the average of the entire dataset, and the other one in the South Pacific Subtropical Gyre, in the Carribbean Sea and in the north Atlantic in autumn clearly has an oligotrophic character. Many of the differences between clusters are caused by diagnostic pigments of nano or pico flagellates. While the space and time characteristics of the clusters are well marked and might correspond to differences in physical and chemical forcing, knowledge of the ecological requirements of these flagellates is generally lacking to understand how the variability of the environment triggers these clusters