REGIONAL CHLOROPHYLL RETRIEVAL ALGORITHMS FOR NORTH SEA WATERS: INTERCOMPARISON AND VALIDATION

摘要

The coastal waters of the North Sea are eutrophic and feature elevated levels of Chlorophyll-a. The complexity of North Sea Water is expressed by the fact that not only Chlorophyll-a but also suspended matter and yellow substance determine the optical characteristics of the water, and by the variation of the specific inherent optical properties (SIOP) with location and time. In the framework of the EU funded research project REVAMP in-situ data were collected in the North Sea in order to characterise the SIOPs of different areas, such as the channel water, the German Bight, the eastern English coast, the central North Sea and the Skagerrak/Kattegat. Five different types of algorithms have been developed which are either tuned to a specific area or implemented using a specific mathematical method. These algorithms include blue-green ratio algorithms, model inversion and neural network techniques. Each algorithm is calibrated with SIOPs. An intercomparison of the different algorithms was performed. The finally developed EVAMP algorithm is a new methodology, which derives the Chlorophyll-a concentration from MERIS spectral surface reflectance spectra based on the standard forward model Hydrolight (using a lookup table approach). With this algorithm a series of monthly, seasonal and yearly maps were generated and compared with in-situ observed values. To obtain these maps, 450 MERIS images were processed. The comparison was done for some transects and for stations of monitoring networks. It was found that REVAMP Chlorophyll-a compares well for most situations, except for situations with probably a very high background concentration of suspended matter. MERIS atmospheric correction was found to be very adequate, except maybe for situations very close to the coast.