OS11B-03
Chlorophyll-a Variability in the Southern Ocean Mixed Layer and Euphotic Zone From Elephant Seals and Profiling Floats
Monday, 14 December 2015: 08:30
3009 (Moscone West)
Magdalena M Carranza, Scripps Institution of Oceanography, La Jolla, CA, United States, Sarah T Gille, UCSD, La Jolla, CA, United States, Peter J. S. Franks, University of California San Diego, La Jolla, CA, United States, Kenneth S Johnson, Monterey Bay Aquarium Research Institute, Watsonville, CA, United States and James B Girton, Applied Physics Laboratory University of Washington, Seattle, WA, United States
Abstract:
The Southern Ocean contains some of the ocean’s deepest mixed layers. Because deep mixed layers can transport phytoplankton below the euphotic zone, light levels depend on mixed-layer depth (MLD), and phytoplankton growth is hypothesized to be co-limited by iron and light. Estimates of Chl-a fluorescence, particle backscattering and hydrographic profiles collected by southern elephant seals, EM-APEX, and biogeochemical Argo floats are used to evaluate the extent to which MLD influences phytoplankton bloom development and the vertical structure of chlorophyll-a (Chl-a) in the Southern Ocean. We find that surface Chl-a (i.e., mean Chl-a for the upper light penetration depth) is a relatively good proxy of phytoplankton biomass (i.e., depth-integrated Chl-a) within the euphotic zone but gives an inadequate representation of biomass within the mixed layer, particularly in the summer. Although nearly vertically homogeneous Chl-a within the mixed layer prevails in seasonal mean profiles, subsurface Chl-a maxima are not uncommon from spring through fall. Deep Chl-a maxima that correlate with particle backscattering in summer and fall are found near the base of the mixed layer, closer to the nutrient maximum than the light maximum, suggesting that nutrient limitation (i.e., essentially iron) can play a greater role than light limitation in governing productivity.
