SURFACE AND SUBSURFACE PHYTOPLANKTON BIOMASS AND STOICHIOMETRY AT STATION ALOHA OVER THREE DECADES

Marine phytoplankton contribute approximately half of Earth’s primary production, fuelling marine ecosystems and playing a key role in global climate regulation. However, uncertainties remain regarding their response to climate variability, particularly with respect to vertical distribution patterns. While previous research has advanced our understanding of surface or column-integrated phytoplankton biomass, vertical structuring has received less attention. Recent research in a seasonally stratified region of the subtropics has shown contrasting seasonal and temporal trends in surface and subsurface phytoplankton. In this study, we apply a similar conceptual modelling tool that divides the vertical profile of phytoplankton biomass into two distinct communities—one in the surface mixed layer and another in the subsurface layer—to over 30 years of chlorophyll-a, particulate organic carbon (POC) and particulate organic nitrogen (PON) data from Station ALOHA. Preliminary analyses reveal contrasting seasonal trends between these communities, with surface phytoplankton appearing to adjust their carbon-to-chlorophyll ratio, potentially reflecting photoacclimation processes, while subsurface ratios remain relatively stable. Longer-term trends in these patterns, along with the roles of photoacclimation and changing stoichiometry are examined in the context of ocean warming. Given that satellite remote sensing only captures surface signals, sustained in situ measurements of subsurface phytoplankton are essential for understanding their response to climate change.

Presentation Preference: Oral

Primary Presenter: Johannes Viljoen, University of Exeter (j.j.viljoen@exeter.ac.uk)

Authors:

Johannes Viljoen, University of Exeter  (j.j.viljoen@exeter.ac.uk)

Xuerong Sun, University of Exeter  (x.sun8@exeter.ac.uk)

Robert Brewin, University of Exeter  (r.brewin@exeter.ac.uk)