Presenter: Kieran Curran (University of New Hampshire)

Description:
Within the oligotrophic subtropical ocean, summertime DIC drawdown in surface waters and subsurface oxygen consumption in the absence of Redfieldian stoichiometric nitrate changes are two phenomena still awaiting a full mechanistic characterization. The release of polysaccharide precursors by phytoplankton resulting in the production of carbon-rich, nitrogen-poor transparent exopolymer particles (TEP), their slow vertical settling, and subsequent respiration below the subsurface chlorophyll maximum (SCM) is one plausible driver to link non-Redfieldian nutrient and carbon cycling in these environments. To investigate the significance of TEP cycling to these phenomena, we undertook a multi-faceted study including: a time-series (Jan 2020 – May 2021) of TEP dynamics at Station ALOHA (22˚45’,158 ˚W), along a transect from 22˚45 to 31˚N, and within incubation assays to assess abiotic and biotic TEP production and consumption rates, particle size distribution, and glucosidase enzyme activity. Five-day incubations were set up using water from 5m and 125m (SCM) from 2 stations at the extremes of our transect: Station ALOHA and at 31˚N. Surface and SCM TEP concentrations were 30-50% higher in the North Pacific gyre than at Station ALOHA. Vertical profiles of TEP concentrations at Station ALOHA through 2020-21 indicate greater seasonal range in surface waters (5-15 µg L-1) and at the SCM (7-12 µg L-1) than below the SCM (5-8 µg L-1), consistent with seasonality of phytoplankton production and possibly community structure. No TEP incubation treatment exhibited monotonic declines in TEP concentrations from heterotrophic consumption, though incubations of ALOHA water exhibited greater variation in TEP concentration than gyre-core water throughout the incubations. We therefore assume that TEP cycling at these sites may occur over daily timescales, as production and consumption (or degradation) seemingly occurs independent of depth and TEP concentration.