Relationships between the deep chlorophyll maximum and hydrographic characteristics across the Atlantic, Indian and Pacific oceans

The Malaspina-2010 circumnavigation expedition on board R/V Hesperides surveyed tropical and subtropical regions of the Atlantic, Indian and Pacific oceans between December 2010 and July 2011. This article examines the relationships between the distribution of chlorophyll a (Chl a), major inorganic nutrients and other hydrographic variables. A deep chlorophyll maximum (DCM) was found at most stations between 60 and 150 m depth; it occurred close to the level of 1% surface photosynthetically active radiation and was associated with the nitracline. There was a negative relationship between total Chl a at surface and the DCM depth, and between Chl a concentration at the DCM and DCM depth. In terms of Chl a concentration, picophytoplankton was the dominant size class at all sampled light intensities (surface, 20% of surface PAR and PAR at DCM), oceans and geoclimatic zones, except at some stations influenced by upwellings or divergences. Within the Chl a concentration ranges found in this study, the proportion of picophytoplankton increased with total Chl a, in contrast with some previous findings. Vertically integrated Chl a was positively correlated with surface Chl a, with similar slopes for the whole data set and for the different oceans and zones. In turn, surface Chl a and sea surface temperature showed a negative correlation for the Indian Ocean and the subtropical zone, a positive correlation for the Atlantic, and non-significant relationships for the remaining oceans and zones.