Abstract
Fixed inorganic nitrogen (N) is persistently scarce in the well-lit regions of the subtropical ocean gyres and its supply plays an important role in controlling phytoplankton productivity. In a series of experiments conducted in the North Pacific Subtropical Gyre (NPSG), we examined changes in primary productivity and eukaryotic phytoplankton community structure in response to additions of nitrate (NO3-), ammonium (NH4+), or urea in conjunction with phosphate and silicic acid. Overall, we observed large increases in rates of 14C-primary productivity (~2- to 27-fold) and concentrations of chlorophyll a (~2- to 7-fold) following N addition. Much of the physiological response was due to larger (>3 µm) phytoplankton, whose contributions to primary productivity increased ~2-fold while picophytoplankton (0.2-3 µm) contributions decreased by a similar proportion. Five experiments, conducted in the spring, summer and winter, revealed apparent seasonally-dependent responses in phytoplankton community structure to N availability. During the summer, pennate diatoms increased significantly following N addition as evidenced by both photosynthetic pigment analyses and high-throughput sequencing of 18S rDNA. For example, following the addition of N substrates, concentrations of fucoxanthin (a diatom pigment biomarker) increased between 23- and 49-fold, and relative abundances of rRNA genes belonging to Pseudo-nitzschia increased from negligible (~0.3%) to 30-60% of the >3 µm phytoplankton assemblage. However, unlike the diatom-driven responses observed in the summer, experiments conducted in the spring and winter demonstrated large increases in concentrations of 19’-butanoyloxyfucoxanthin (a pelagophyte pigment biomarker) together with increases in the relative abundance of rRNA genes clustering among Pelagomonas. Overall, our findings revealed differences in the responses of major taxa during experiments conducted in different times of the year, with concomitant impacts on patterns of phytoplankton diversity. In addition, the overall responses in chlorophyll a, 14C-primary production, and eukaryotic phytoplankton community composition appeared largely independent of the type of N substrate added. Our results highlight seasonal-scale differences on the role of N availability in shaping eukaryotic phytoplankton diversity in the surface waters of the oligotrophic NPSG.
Highlights
In oligotrophic marine ecosystems such as the North Pacific Subtropical Gyre (NPSG), less than 10% of organic matter produced is exported out of the euphotic zone as sinking particles, implying rapid organic matter turnover and nutrient remineralization (Karl et al, 1996)
We sought to determine which eukaryotic phytoplankton assemblages responded to the additions of nitrogenous substrates and over what time-scales, providing insight into how variations in the availability of nitrogenous nutrients contribute to variability in phytoplankton community structure and growth in the euphotic zone of oligotrophic subtropical gyres
During the summer sampling periods the near-surface waters were characterized by relatively warm temperatures (24.8–25.3◦C), the mixed layer depths (MLD) ranged between 58 and 78 m, and the flux of photosynthetic active radiation (PAR) at 25 m was 12.6 mol quanta m−2 d−1 (Table 1, Figure 1)
Summary
In oligotrophic marine ecosystems such as the North Pacific Subtropical Gyre (NPSG), less than 10% of organic matter produced is exported out of the euphotic zone as sinking particles, implying rapid organic matter turnover and nutrient remineralization (Karl et al, 1996). Various field-based studies have concluded that cyanobacteria and eukaryotic phytoplankton in the oligotrophic oceans may rely on different forms of N to support their growth (Fawcett et al, 2011; Shilova et al, 2017) Such results highlight the potential role for different forms of N in controlling timevarying phytoplankton community structure. We sought to determine which eukaryotic phytoplankton assemblages responded to the additions of nitrogenous substrates and over what time-scales, providing insight into how variations in the availability of nitrogenous nutrients contribute to variability in phytoplankton community structure and growth in the euphotic zone of oligotrophic subtropical gyres
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