Distribution patterns of phytoplankton groups along isoirradiance layers in oligotrophic tropical and subtropical oceans

A pigment chemotaxonomic approach was used to find the distribution of phytoplankton groups over a wide extension of the oligotrophic tropical and subtropical Atlantic, Indian and Pacific oceans. The six sampling depths corresponded to percentages of surface irradiance, i.e. “isoirradiances”. Most of the 139 stations sampled presented a subsurface deep chlorophyll maximum (DCM). Only in 19 of the 818 samples the total chlorophyll a (TChl a) concentration was >0.5 mg m−3 and only in one >0.75 mg m−3. The composition of the phytoplankton populations showed low variability. Haptophytes and Prochlorococcus contributed most to TChl a (36% and 35%, respectively), followed by green algae (11%), Synechococcus (6.5%), pelagophytes (6.7%), dinoflagellates (3.2%) and diatoms (1.6%). Specific non-polar Chl c2 pigments were analyzed to estimate the pigment biomass of three types of haptophytes (6, 7 and 8) using CHEMTAX. Haptotophytes-8 was the most abundant (18%), while haptophytes-7 and -6 contribution was 13% and 5.2%, respectively. There was a vertical partition of the groups along the water column. Prochlorococcus, haptophytes-6 and -8, and pelagophytes presented low values in shallow and intermediate layers and a strong increase in pigment biomass at the DCM. Dinoflagellates, haptophytes-7 and green algae also with low values in the upper layers showed a sharp increase at the layer above the DCM, with similar values as in the DCM or even higher for green algae. The pigment biomass of diatoms and Synechococcus were fairly homogeneous throughout the water column. These three patterns were best reflected by estimating the contribution of each group to the TChl a concentration. The Malaspina expedition crossed some well recognized ecosystem/areas of the open ocean such as the Atlantic and Pacific equatorial upwellings, the Costa Rica and Guinea domes, the low oxygen area of the northeastern Pacific and the Great Australian Bight, for which the composition of the phytoplankton populations is provided.