Summer Blooms of Diatom-Diazotroph Assemblages and Surface Chlorophyll in the North Pacific Gyre: a Disconnect

Abstract

The discovery of large summer chlorophyll blooms in oligotrophic regions of the ocean has led to questions about the relationship between these blooms and the frequently cooccurring outburst of nitrogen-fixing phytoplankton. We compared diatom-diazotroph assemblage (DDA) abundance to size-fractionated chlorophyll (chl) and satellite ocean color chlorophyll estimates to evaluate how DDAs affected ocean color estimates in the eastern and central North Pacific gyre at 28–30°N. DDA blooms were dominated by either Hemiaulus hauckii (in the central Pacific in 2003 and the eastern Pacific in 2002) or by Rhizosolenia (eastern Pacific in 2002), both with nitrogen-fixing Richelia symbionts. The 2002 DDA bloom was measured a week prior to the development of a satellite-observed chlorophyll bloom at the same location. In contrast, the 2003 Hemiaulus bloom was not within a clearly defined satellite feature. Although DDA abundance increased 10⁴–10⁵-fold relative to the background and they dominated the net plankton (≥5 μm or >10 μm chl size) fraction, the in situ chl (maximum ≤0.11 mg m⁻³) never reached the 0.15 mg m⁻³ threshold used to define satellite-observed chlorophyll blooms in oligotrophic waters. The DDA blooms were not evident in the in situ fluorometer data; however, the blooms occurred within high beam attenuation features observed in the transmissometer data. Trichodesmium was not a component of either diatom bloom although elevated levels of Trichodesmium were observed at two stations where DDAs were not abundant. While DDA blooms and satellite ocean chlorophyll blooms are sometimes coincident, our data do not support that DDAs are the sole source of the satellite-observed chlorophyll in summertime blooms. DDA blooms are likely underreported in the North Pacific, particularly in the waters west of Hawaii, due to their frequent lack of distinctive ocean color, fluorescence, and chlorophyll signatures. The source of the ocean color signature in the blooms remains elusive, but scattered literature observations suggest that cooccurring members of the near-surface flora such as the small pennate diatom Mastogloia may play an important role.