Fall Seminar Series

Thursday, October 17th听 @ 3 pm

Room 200, Oceanography & Physics Building

Or Via Zoom

Matt Church

Flathead Lake Biological Station, University of Montana

The low nutrient subtropical gyres: Emergent patterns and enduring enigmas

ABSTRACT

The subtropical ocean gyres are some of Earth鈥檚 largest ecosystems, comprising ~70% of the surface area of the ocean. These ecosystems are key to planetary climate and globally important in carbon cycling. The upper ocean of these ecosystems is characterized by persistent thermal stratification, low inorganic nutrient concentrations, high water clarity, and low algal biomass. Decades of research in the gyres highlights that the productive upper ocean in these ecosystems functions as a two-layer habitat, with a light-replete, but nutrient-poor, top layer that is superimposed above a dim, nutrient-enriched subsurface layer.听 While phytoplankton dynamics in the top layer are observable by satellite remote sensing, the subsurface layer remains invisible to satellites. This poses a problem: ocean carbon cycle predictions increasingly rely on satellite observations, yet only the thin canopy (< 30 m) of the top layer of the photic zone is visible to ocean color satellites, leaving gaps in understanding of remaining ~100 m of the photic zone. For this presentation, I will use decades of research conducted in the North Pacific Subtropical Gyre to explore differences in the functioning of these photic zone layers. For example, nitrogen supply to each layer differs, with N2 fixation heavily subsidizing the upper layer, while diapycnal supply of nitrate and nitrite are important to the subsurface layer. Enigmatically, N2 fixation supports large, recurrent, summertime phytoplankton blooms and these blooms alter the biogeochemical functioning of both photic zone layers. The availability of iron also appears to regulate plankton biology differently in the two layers, with variability in iron supply to the top layer an important control on N2 fixation, while lower iron concentrations found in the subsurface layer appear to impact organic matter decomposition and remineralization. Taken together, the extant observations on the functioning of the subtropical gyres motivates improving mechanistic understanding of biogeochemistry in these expansive ecosystems.

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