Coastalareas of the North Sea are commercially important for fishing and tourism, andare subject to the increasingly adverse effects of harmful algal blooms,eutrophication and climate change. Monitoring phytoplankton in these areasusing Ocean Colour Remote Sensing is hampered by the high spatial and temporalvariations in absorption and scattering properties. In this paper wedemonstrate a clustering method based on specific-absorption properties thatgives accurate water quality products from the Medium Resolution ImagingSpectrometer (MERIS). A total of 468 measurements of Chlorophyll a (Chla),Total Suspended Material (TSM), specific- (sIOP) and inherent opticalproperties (IOP) were measured in the North Sea between April 1999 andSeptember 2004. Chla varied from 0.2 to 35 mg m− 3, TSM from 0.2 to 75 g m− 3 and absorption properties of coloured dissolved organicmaterial at 442 nm (aCDOM(442)) was 0.02 to 0.26 m− 1. The variation in absorption properties of phytoplankton (aph) and non-algal particles (aNAP) were an order ofmagnitude greater than that for aph normalized toChla (aph*) and aNAP normalized to TSM (aNAP*). Hierarchicalcluster analysis on aph*, aNAP* and aCDOM reduced this large data set to three groups of high aNAP*–aCDOM, low aph* situated close to the coast, medium values further offshoreand low aNAP*–aCDOM, high aph* in open ocean and Dutch coastal waters. The median sIOP ofeach cluster were used to parameterize a semi-analytical algorithm to retrieveconcentrations of Chla, TSM and aCDOM(442) from MERISdata. A further 60 measurements of normalized water leaving radiance (nLw), Chla, TSM, aCDOM(442) and aNAP(442) collected between 2003 and 2006 were used to assess theaccuracy of the satellite products. The regionalized MERIS algorithm showedimproved performance in Chla and aCDOM(442) estimateswith relative percentage differences of 29 and 8% compared to 34 and 134% forstandard MERIS Chla and adg(442) products, and similar retrieval for TSM at concentrations> 1 g− 3.
|Number of pages||19|
|Journal||Remote Sensing of Environment|
|Publication status||Published - 15 Mar 2012|