PHYTOOPTICS - Marine Phytoplankton studied by global biooptical methods
In order to understand the marine phytoplankton’s role in the global marine ecosystem and biogeochemical cycles it is necessary to derive global information on the distribution of its biomass and primary production, in particular the distribution of major functional phytoplankton types (PFT) in the world oceans. Because phytoplankton pigments absorb light for photosynthesis, satellite sensors detecting the ocean color can monitor phytoplankton on the global scale with reasonable spatial and temporal resolution. Already this global satellite information is available over a decade with resolving the temporal dynamics on a weekly to monthly scale. These satellite data sets are of great importance for global modelling of marine ecosystem and climate change studies but also for coastal zone monitoring concerning pollution and harmful algal blooms.
It has been estimated that marine phytoplankton contributes 30 - 60% to global primary production. The large uncertainty range is a result of the lack of global information on phytoplankton absorption and light penetration depth, which cannot be supplied by the current ocean color satellite sensors, such as SeaWiFS, MODIS and MERIS. The spectral resolution of these sensors is not sufficient to extract the relevant information. The variation of phytoplankton absorption in ocean waters also affects the retrieval of chlorophyll a concentrations (a measure of phytoplankton biomass) derived from satellite data, which are also important input data used in primary production models. It has been shown that specific phytoplankton absorption spectra as well as information on the light penetration depth can be derived (Vountas et al. 2007) and major phytoplankton groups can be determined (Bracher et al. 2009) from measurements of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) with high spectral resolution (operating on board of the third European environmental satellite ENVISAT).
The aim of the Helmholtz-University Young Investigators Group PHYTOOPTICS, a cooperation between AWI Climate Sciences and Institute of Environmental Physics at the University of Bremen (IUP) is to improve estimates of marine primary production by retrieving new biooptical information from the European satellite SCIAMACHY in addition to using the data of the common ocean color sensors. Besides remote sensing retrievals, biooptical and biochemical in-situ parameters of phytoplankton and light conditions are measured during various cruises with the research vessels Polarstern, Maria S. Merian and Sonne and models are used to calculate radiative transfer and primary production. Through a better knowledge of the sinks and sources of CO2 in the ocean a contribution will be made to a better understanding of changes in the world's climate as well as to the understanding of the marine food web.
Scientific objectives
- to retrieve new biooptical information from spectrally highly resolved satellite data and identify globally different phytoplankton groups (phytoplankton functional types) from satellite observations of upwelling radiation
- highly precise field and laboratory measurements of biooptical parameters for characterising different phytoplankton groups. This in-situ information is also used for validation and for the provision of reference spectra for satellite retrieval
- to improve polar primary production estimates and study the effect of changing environmental factors on phytoplankton distribution and growth
- to model the underwater lightfield by means of radiometric and biogeochemical measurements
- to assess the spatial-temporal variability of phytoplankton types in the Southern Ocean
- to improve satellite retrievals of phytoplankton biomass and primary production estimates by incorporating the additional biooptical information
- to improve UV-VIS satellite spectra retrievals by accounting for ocean optical signals in radiative transfer calculations
- to establish a data base on biooptical parameters, phytoplankton functional types and biomass and make it available to the wider community for the purposes of optical and ecosystem modelling
