Distribution and contribution of major phytoplankton groups to carbon cycling across contrasting conditions of the subtropical northeast Atlantic Ocean

The relation between trophic regime and phytoplankton composition and function in oceanic systems is well accepted in oceanography. However, the relative dynamics and carbon cycling contributions of different phytoplankton groups across gradients of ocean richness are not fully understood. In this work we investigated phytoplankton dynamics along two transects from the NW African coastal upwelling to open-ocean waters of the north Atlantic subtropical gyre. We adopted a pigment-based approach to characterize community structure and to quantify group-specific growth and grazing rates and associated carbon fluxes. Changes in pigment cell concentration during the incubation experiments due to photoadaptation were corrected to obtain reliable rates. The oceanic region was dominated by Prochlorococcus (PRO) (45±7% of total chlorophyll a) while diatoms dominated in upwelling waters (40±37%). Phytoplankton grew faster (γ=0.78±0.26d^-1) and free of nutrient limitation (γ/γ_n=0.98±0.42) in the coastal upwelling region, with all groups growing at similar rates. In oceanic waters, the growth rate of bulk phytoplankton was lower (γ=0.52±0.16d^-1) and nutrient limited (γ/γ_n=0.68±0.19d^-1). Diatoms (0.80±0.39d^-1) and Synechococcus (SYN) (0.72±0.25d^-1) grew faster than Prymnesiophyceae (PRYMN) (0.62±0.26d^-1) and PRO (0.46±0.18d^-1). The growth rates of PRO and SYN were moderately nutrient limited (γ/γ_n=0.81 and 0.91, respectively), while the limitation for diatoms (γ/γ_n=0.71) and PRYMN (γ/γ_n=0.37) was more severe. Microzooplankton grazing rate was higher in upwelling (0.68±0.32d^-1) than in oceanic waters (0.37±0.19d^-1), but represented the main loss pathway for phytoplankton in both systems (m/γ=0.90±0.32 and 0.69±0.24, respectively). Carbon flux through phytoplankton, produced and grazed, increased from offshore to coastal (2 to 200γg CL^-1d^-1), with diatoms dominating the flux in the upwelling region (52%) while PRYMN (40%) and PRO (30%) dominated in the open ocean.