Differential Effects of Nitrate (NO3-), Ammonium (NH4+) and Urea on Phytoplankton Communities in the North Pacific Subtropical Gyre.

Nitrogen (N) has long been known to limit phytoplankton growth and productivity in large regions of the oceans. Likewise, the form and supply of N are important controls on microbial community composition, activity and ultimately ecosystem function. However, the effect of different chemical N species on complex natural phytoplankton communities in the open ocean is not well-known. We used bioassays to examine and compare responses of phytoplankton communities to the addition of either NO₃^-, NH₄⁺ or Urea in the North Pacific Subtropical Gyre during Aug.-Sept. 2014 (NEMO cruise). The effects of iron (Fe) and phosphorus (P) on N assimilation were also examined. Phytoplankton biomass and primary productivity were N-limited in the central and eastern parts of the transect, whereas no response was observed to any of the N substrates (unless added together with P) in the western part of the transect. In response to N substrate additions, the central region of the NPSG had a significant and rapid (within 24 h) shift in heterotrophic and phytoplankton community composition and photosynthetic activity relative to changes in an experiment at the eastern edge of the NPSG. In the experiments where the phytoplankton community was N-limited, urea addition resulted in the highest response in primary productivity and chlorophyll a, largely due to the response by Prochlorococcus. The additions of NO₃^- and urea stimulated Synechococcus populations in both eastern and central regions, but the response in the eastern part of the NPSG was similar to the Fe addition response. Picoeukaryotic population increased most in response to NH₄⁺ in the central NPSG and in response to all N forms and also to Fe in the eastern part. Uptake rates of the different N species were also variable between different phytoplankton groups (Prochlorococcus, Synechococcus, and picoeukaryotic phytoplankton). These variable responses are indicative of diverse ecotypes and physiological states within and between the regions. Our results have implications for understanding how the key N pools impact phytoplankton dynamics and function in the ocean, which is particularly important in light of potential changes in the relative abundance and flux of different N compounds supplied by anthropogenic activities and environmental perturbations.