Relative influence of nitrogen and phosphorous availability on phytoplankton physiology and productivity in the oligotrophic sub‐tropical North Atlantic Ocean

<jats:p>Nutrient addition bioassay experiments were performed in the low‐nutrient, low‐chlorophyll oligotrophic subtropical North Atlantic Ocean to investigate the influence of nitrogen (N), phosphorus (P), and/or iron (Fe) on phytoplankton physiology and the limitation of primary productivity or picophytoplankton biomass. Additions of N alone resulted in 1.5‐2 fold increases in primary productivity and chlorophyll after 48 h, with larger (~threefold) increases observed for the addition of P in combination with N (NP). Measurements of cellular chlorophyll contents permitted evaluation of the physiological response of the photosynthetic apparatus to N and P additions in three picophytoplankton groups. In both <jats:italic>Prochlorococcus</jats:italic> and the picoeukaryotes, cellular chlorophyll increased by similar amounts in N and NP treatments relative to all other treatments, suggesting that pigment synthesis was N limited. In contrast, the increase of cellular chlorophyll was greater in NP than in N treatments in <jats:italic>Synechococcus</jats:italic>, suggestive of NP co‐limitation. Relative increases in cellular nucleic acid were also only observed in <jats:italic>Synechococcus</jats:italic> for NP treatments, indicating co‐limitation of net nucleic acid synthesis. A lack of response to relief of nutrient stress for the efficiency of photosystem II photochemistry, F<jats:sub>v</jats:sub> :F<jats:sub>m</jats:sub>, suggests that the low nutrient supply to this region resulted in a condition of balanced nutrient limited growth, rather than starvation. N thus appears to be the proximal (i.e. direct physiological) limiting nutrient in the oligotrophic sub‐tropical North Atlantic. In addition, some major picophytoplankton groups, as well as overall autotrophic community biomass, appears to be co‐limited by N and P.</jats:p>