Picoplankton community structure within and outside a <i>Trichodesmium</i> bloom in the southwestern Pacific Ocean

Phytoplankton composition and community structure in the south-western Pacific Ocean were examined at sea using flow cytometry and epifluorescence microscopy to explore the relationships among distributions of picophytoplankton populations and a variety of nitrogen fixing cyanobacteria. The cruise track began in New Zealand, extended north via New Caledonia to 13S, and continued east to 13.9S 173.2W. The track crossed a large bloom of the filamentous nitrogen-fixing cyanobacteria Trichodesmium centered around New Caledonia. Within Trichodesmium blooms, abundances of Synechococcus were elevated 10-fold; however, there was no significant enrichment of Prochlorococcus, picoeukaryotic algae, or heterotrophic bacteria. Unicellular coccoid cyanobacteria (> 2 m), which resemble the nitrogen-fixing Crocosphaera spp., were observed in waters > 27 C along the eastward track and were most abundant at the deep oceanic stations where Trichodesmium was absent or present at very low abundances. These Crocosphaera-rich, Trichodesmium-poor stations were characterized by lower dissolved iron concentrations compared to coastal stations where Trichodesmium tended to be more abundant. Given the apparent mutually exclusive distributions of these two groups of cyanobacteria, further examination of N2 fixation within the pico- and nanoplankton components of the phytoplankton community is needed.

Picoplankton community structure within and outside a Trichodesmium bloom in the southwestern Pacific Ocean Academic Article