Remote sensing of phytoplankton in the Southern Ocean
thesisposted on 2023-05-27, 12:07 authored by Johnson, R
One of the greatest challenges for Southern Ocean and Antarctic research is the development of robust methods for assessing the current and future impacts of climate change, and for evaluating regional differences in the rate and direction of that change. The Southern Ocean has been changing rapidly for at least the last 30 years, including measurable changes to phytoplankton communities. Climate projections suggest that they will continue to change. It is predicted that there will be continued southward movement of oceanographic fronts, in- creased warming and freshening (increased precipitation) of the surface ocean, shallowing of the mixed layer (increased stratification) and increased carbon dioxide enrichment and ab- sorption of the upper ocean. In order to capture and monitor the response of phytoplankton across the Southern Ocean, an economical observing system with high resolution in time and space is needed. This thesis examines the ability of ocean colour remote sensing to meet this challenge by accurately assessing and monitoring climate change impacts on phytoplankton. The ever-increasing number of in situ samples from the Southern Ocean, which can be used to calibrate and validate remote sensing algorithms, have the potential to make ocean colour radiometry a robust method for assessing climate change impacts on the Southern Ocean ecosystem. We investigated both calcite and chlorophyll products, two measurements that are key for assessing the impact of climate change on phytoplankton. Chlorophyll is used as a proxy for biomass and calcite is used to identify calcifying plankton, and to detect changes in calcification rates and carbon sequestration impacted by ocean acidification. We found that current satellite algorithms underestimate chlorophyll by as much as 50% and overestimate calcite by up to 400% in the Southern Ocean. Much of the in situ data used in this thesis were collected by ships transiting to and from the Antarctic on station re-supply missions that collected surface samples while the ship was sailing. This methodology naturally raised the question of how well does surface sampling capture the variability with depth in the euphotic zone. We determined that surface sampling of chlorophyll, either from ships or satellites, is an adequate representation of the ecologically important euphotic zone in the well-mixed regions of the Southern Ocean. This thesis concludes that customised ocean colour algorithms can be a robust method for assessing Southern Ocean phytoplankton and presents several methods and improved satellite products for doing so.
Rights statementCopyright 2016 the Author Chapter 2 appears to be the equivalent of a post print version of an article published as: Johnson, R., Strutton, P. G., Wright, S. W., McMinn, A., Meiners, K. M., 2013. Three improved 124 satellite chlorophyll algorithms for the Southern Ocean, Journal of geophysical research, Oceans, 118(7), 3694-3703. An edited version of this paper was published by AGU. Copyright 2013 American Geophysical Union