Responses of photosynthetic assemblage structure and physiology to variations in nitrogen substrates

Picophytoplankton (<3 μm) are major contributors to productivity and biomass in the North Pacific Subtropical Gyre (NPSG). Due to the persistent dearth of fixed inorganic nitrogen in the well-lit region, nitrogen availability is central to structuring planktonic communities through competition for this limiting resource. We examined changes in phytoplankton assemblage structure and physiology in response to nitrogen enrichments in five experiments conducted between 2011-2013 (two summer, two spring, and one winter). Natural seawater planktonic communities from the surface water at Station ALOHA were enriched with nitrate (NO₃^-) or ammonium (NH₄⁺) and incubated over 120 to 144 hours. During summer experiments, picophytoplankton exhibited a rapid increase in ¹⁴C-based primary production, with phytoplankton >3 µm becoming increasingly more productive later in the experiment (120 h). Chlorophyll a concentrations increased gradually over the experiment period. During spring experiments, phytoplankton displayed the opposite pattern; chlorophyll a concentrations increased rapidly (48 h, ~3-fold) but rates of photosynthetic carbon fixation did not increase until after 96 h. For the winter experiment, both primary productivity and chlorophyll a increased gradually over the experimental period. HPLC pigment analyses and high-throughput sequencing of 18S rRNA genes revealed that pennate diatoms, haptophytes, and dictyochophytes thrived in both N enrichments, but chrysophytes flourished in the NH₄⁺ additions. Overall, our results indicate that responses in biomass and productivity differed in both time scale and magnitude between experiments conducted during different seasons, shedding insight into the potential controls of phytoplankton nutrient acquisition and growth in the euphotic zone.