Sea surface kinetic energy as a proxy for mesoscale light limitation for phyoplankton in the Southern Ocean
Date
2018
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Publisher
University of Delaware
Abstract
The Southern Ocean is classified as a high nutrient, low chlorophyll region as surface waters are typically macro-nutrient (e.g. nitrogen and phosphorous) rich in the austral summer, yet they are characterized by patchy, intense phytoplankton blooms within a generally unproductive environment. The mechanisms controlling phytoplankton abundance include turbulent mixing, available light, respiration and predation rates, and nutrient availability. While there is much debate about the initiation of phytoplankton blooms in the Southern Ocean, there is also significant variation in phytoplankton bloom dynamics within a growing season, suggesting many different mechanisms are contributing to patchy phytoplankton distributions. ☐ Using data from 2003 to 2017, a surprising relationship between in situ and satellite-derived chlorophyll and modelled kinetic energy was observed. High chlorophyll concentrations were not observed in either remotely sensed or in situ data while kinetic energy was high throughout the entire Southern Ocean. Consequently, low kinetic energy appears to be a necessary, but not sufficient, precondition for phytoplankton blooms throughout the Southern Ocean. In December, surface phytoplankton concentration appears to decrease due to high kinetic energy simply diluting their concentration in the water column. However, in January and February high kinetic energy appears to limit phytoplankton abundance through a mesoscale light limitation by decreasing the time they spend in the euphotic zone. Analysis of this relationship through a time dependent, probabilistic model, combined with in-situ observations of water column structure show that high kinetic energy deepens the mixed layer through turbulent mixing, potentially limiting the availability of light. These probabilistic models suggest that the spatial pattern in kinetic energy in the Southern Ocean explains up to 30% of the variability in the distribution of chlorophyll concentration. These findings indicate that KE is a significant factor in structuring the spatial distribution in Southern Ocean chlorophyll.