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Title: Climate influence on phytoplankton phenology in the global ocean
Author: Racault, Marie-Fanny
ISNI:       0000 0004 2734 9886
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2009
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Every year, the oceans absorb one quarter of the carbon dioxide (CO2) emitted to the atmosphere by human activities. This CO2 sink is part of a very active, natural carbon cycle, through which phytoplankton fix CO2 into organic matter in the surface layer of the ocean. Phytoplankton cells tend to aggregate and sink from this surface layer, exporting carbon to the deep ocean and regulating the atmospheric CO2 on long time scales. In the light of the recent unequivocal evidence of global warming, it appears essential to assess its impact on phytoplankton community and to evaluate the subsequent feedback through the oceanic carbon cycle. In recent years, phytoplankton phenology has been suggested as a systematic indicator to monitor the state of the pelagic ecosystem and detect changes triggered by perturbation of environmental conditions. For the first time, the phenology of phytoplankton growing season is estimated at the global scale using remote-sensing ocean colour data. The tropics and subtropics present generally long growing season (15-20 weeks) of low amplitude (< 0.5 mg m−3), whereas the high-latitudes show short growing season (< 10 weeks) of high amplitude (up to 7 mg m−3). Correlation analyses suggest a close coupling between the development of the growing season and the seasonal increase in insolation in the North Atlantic and Southern Ocean. In the tropics and subtropics, light is rarely limiting and the growing season is controlled by nutrient supply enhanced by water mixing. Over the decade 1998-2008, the duration of growing season shows large interannual variability of up to ± 10 weeks. Globally, positive anomalies follow the major 1997-98 El Ni˜no-La Ni˜na events and persist until 2001. Positive phases of climate indices such as the North Atlantic Oscillation and the Southern Annular Mode, associated with enhanced water mixing and nutrients supply, generally sustain longer growing season. Using in-situ observations, we show that the export of carbon can be related to the length of the phytoplankton growing season, with largest export in regions where the growing season is shortest and the blooms most intense. Using satellite observations, from the sensors CZCS and SeaWiFS, we estimate that North of 45◦S, the phytoplankton growing season increased by 2.4 weeks on average between the periods 1979-1986 and 1998-2008. Longer growing seasons are associated with regional patterns of surface warming on the same time scale. We infer from in-situ data a decrease in carbon export of 0.6 Pg C yr−1 (excluding the Southern Ocean) over two decades. This represents an unexpected, and important, feedback between physical and biological processes in the ocean: global warming modifies phytoplankton growth, reducing the capacity of the ocean to absorb atmospheric CO2 leading to a probable aggravation of global warming. Finally, phenological characteristics of the phytoplankton growing season are used to resolve Sverdrup’s critical depth model. Mixed layer integrated plankton community respiration Rmld and net community production NCPmld are estimated at the global scale using remote-sensing data of incident irradiance level and primary production, and a global mixed layer depth climatology. NCPmld estimates agree with in-situ observations and model results in the tropical and North Atlantic regions. The model estimates a net autotrophic imbalance of + 0.65 Pg C yr−1 in the North Atlantic and a net heterotrophic imbalance of − 1 Pg C yr−1 for the whole tropics and subtropics.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available