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Oligotrophic Oceanic Regions

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Oligotrophic Oceanic Regions

Certain regions of the world’s oceans are oligotrophic, i.e. they contain very low levels of nutrients and are thus almost free of biogenic activity. As a consequence, solar irradiation partly penetrates into the uppermost water layers. This has two effects on the spectra of the backscattered light: first, the ocean water causes rather broadband absorptions according to its cross section. In addition, vibrational Raman scattering occurs causing high-frequent spectral structures (Vasilkov et al. 2002; Vountas et al. 2003) similar to the atmospheric Ring effect. Global maps of fit results for vibrational Raman scattering show consistent global patterns with high values for the oligotrophic oceanic regions and have been obtained from SCIAMACHY data by applying differential optical absorption spectroscopy in the UV-A region (Vountas et al. 2007).

Oceanic Phytoplankton Characteristics

In open water, marine phytoplankton is the basis of the marine food web. They contribute 50% to the global primary production via photosynthesis. Microscopic algae also play an important role in the global carbon cycle. For photosynthesis, sunlight is absorbed by certain pigments, such as chlorophyll. The absorption spectrum is typical for particular groups of algae due to their characteristic pigment composition. The absorption signatures can be identified in SCIAMACHY data, allowing the quantitative evaluation of the global distribution of phytoplankton. By including a phytoplankton absorption spectrum in the DOAS retrieval, Vountas et al. (2007) was able to retrieve global maps of marine chlorophyll concentrations from SCIAMACHY data successfully. Furthermore, Bracher et al. (2009) could even distinguish different types of phytoplankton, namely diatoms from cyanobacteria (fig. 3-28). The resulting global maps are in good agreement with biochemical models and with independent in situ measurements obtained during cruises with the German research vessels Polarstern, Maria S. Merian and Sonne in the Atlantic and Pacific Oceans. These comparisons again proved the validity of the phytoplankton concentrations retrieved from SCIAMACHY measurements.

 

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fig. 3-28

image
Global biomass distributions of diatoms (left hemisphere) and cyanobacteria (right hemisphere) in November 2007 as derived from SCIAMACHY data using the PhytoDOAS method. The insets show members of the two algae groups. (Graphics: A. Bracher IUP-IFE, University of Bremen and Alfred Wegener Institutefor Polar and Marine Research, adapted from Bracher et al. 2009; photo diatoms: E. Allhusen, cyanobacteria: S. Kranz; both Alfred Wegener Institutefor Polar and Marine Research)
 

 

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