Latitudinal variability and adaptation of phytoplankton in the Atlantic Ocean

This study assessed the variability of a range of phytoplankton groups between repeat cruises over the mid-Atlantic Ocean (50°N-50°S), and demonstrated the important contribution of the pico-phytoplankton to the microalgal biomass in the oligotrophic tropical and sub-tropical regions. Pigment data from two meridional transects were analysed by quantitative chemotaxonomic analysis (CHEMTAX) to yield information concerning the composition of phytoplankton communities along the transects. Total chlorophyll a (TChla) in October–November 2012 (AMT22) and 2013 (AMT23) varied from 0.03 mg m−3 in the southern Sub-Tropical Gyre to 1.13 and 1.92 mg m−3 at 40oS and 42oS respectively. Synechococcus accounted for 35–50% and Prochlorococcus 30–35% of the TChla in oligotrophic surface waters on AMT22, while haptophytes dominated the temperate regions. Prochlorococcus was dominant (30–60%) on AMT23, with Synechococcus contributing 20–40% and haptophytes 10–20%, and it was noted that the dominance of Prochlorococcus occurred in water masses where the inorganic nitrate concentrations were extremely low (≤0.02 mmol m−3). Prochlorococcus and haptophytes dominated the deep chlorophyll maximum (DCM) on AMT22, with the Synechococcus proportion being low, while Prochlorococcus was generally dominant on AMT23, although Synechococcus and haptophytes were also prominent. Photo-pigment indices indicated that chlorophyll b was mainly associated with Prochlorococcus but also related to prasinophytes. Chlorophyll c and photosynthetic carotenoids increased with an increase in the proportion of haptophytes and to a lesser extent with the proportion of diatoms and pelagophytes. Prochlorococcus and Synechococcus were the main contributors to the photoprotective carotenoids and relationships indicated that Synechococcus accounted for more of this pool in 2012, but the Prochlorococcus contribution was greater in 2013. Temperature, stratification, nutrients and light appeared to be the main hydrographic variables influencing phytoplankton composition along the transects.