Abstract. Oligotrophic regions play a central role in global biogeochemical cycles, with microbial communities in
these areas representing an important term in global carbon budgets. While
the general structure of microbial communities has been well documented in
the global ocean, some remote regions such as the western tropical South
Pacific (WTSP) remain fundamentally unexplored. Moreover, the biotic and
abiotic factors constraining microbial abundances and distribution remain not
well resolved. In this study, we quantified the spatial (vertical and
horizontal) distribution of major microbial plankton groups along a transect
through the WTSP during the austral summer of 2015, capturing important
autotrophic and heterotrophic assemblages including cytometrically determined
abundances of non-pigmented protists (also called flagellates). Using
environmental parameters (e.g., nutrients and light availability) as well as
statistical analyses, we estimated the role of bottom–up and top–down
controls in constraining the structure of the WTSP microbial communities in
biogeochemically distinct regions. At the most general level, we found a
“typical tropical structure”, characterized by a shallow mixed layer, a
clear deep chlorophyll maximum at all sampling sites, and a deep nitracline.
Prochlorococcus was especially abundant along the transect,
accounting for 68 ± 10.6 % of depth-integrated phytoplankton
biomass. Despite their relatively low abundances, picophytoeukaryotes (PPE)
accounted for up to 26 ± 11.6 % of depth-integrated phytoplankton
biomass, while Synechococcus accounted for only 6 ± 6.9 %.
Our results show that the microbial community structure of the WTSP is
typical of highly stratified regions, and underline the significant
contribution to total biomass by PPE populations. Strong relationships
between N2 fixation rates and plankton abundances demonstrate the
central role of N2 fixation in regulating ecosystem processes in
the WTSP, while comparative analyses of abundance data suggest microbial
community structure to be increasingly regulated by bottom–up processes
under nutrient limitation, possibly in response to shifts in abundances of
high nucleic acid bacteria (HNA).
Response of ocean phytoplankton community structure to climate change over the 21st century: partitioning the effects of nutrients, temperature and light