Microbial Ecology of the Island Mass Effect Off the Wai'anae Coast of O'ahu

Abstract

While the Island Mass Effect (IME) has been widely observed and documented for decades, little research has been done regarding how it impacts heterotrophic bacterioplankton communities, and none in Hawaiʻi. My project studied how bacterioplankton communities vary with distance from shore in the context of an IME by synoptically characterizing the taxonomic composition of bacteria over depth along duplicate occupations of a transect from nearshore to offshore over a 24h period. I collected water samples from 6 depth profiles (5 to 250 meters) at three stations (roughly 2, 7, and 23 km from the Waiʻanae Coast of Oʻahu) conducted in September 2023 aboard the research vessel R/V Kilo Moana. I characterized the relative abundances of all bacterial taxa using 16S amplicon sequencing and measured absolute concentrations of the Cyanobacteria Prochlorococcus and Synechococcus as well as total heterotrophic bacterioplankton and picoeukaryotic phytoplankton using flow cytometry. Chlorophyll a measurements showed little variation in the mixed layer but a strengthening and shoaling of the deep chlorophyll maximum (DCM) in the nearshore. I characterized dissolved organic matter, observing vertical shifts in multivariate dissolved organic matter fluorescence signatures but no horizontal shifts along the transect. Bulk dissolved organic carbon also showed little significant horizontal variation. When comparing the community at each station, Prochlorococcus was more abundant offshore within the DCM and Synechococcus was more abundant nearshore in the upper mixed layer in both relative and absolute abundances, suggesting resource partitioning between these two major clades of picocyanobacteria. I observed the greatest horizontal differences in bacterioplankton community makeup between stations in the range of 100 to 125 meters depth; dozens of Bacteria ecotypes including the abundant taxa SAR86, Flavobacteriaceae, and Cryomorphaceae showed clear horizontal gradients. Taken together my work documents a clear IME in heterotrophic microbial communities that manifests throughout the euphotic zone, emphasizing a need for a deeper understanding of the vertical context in understanding how islands affect marine biogeochemistry and microbial ecology.