Variability in specific-absorption properties and their use in a semi-analytical ocean colour algorithm for MERIS in North Sea and Western English Channel Coastal Waters

Abstract

Coastal areas of the North Sea are commercially important for fishing and tourism, and are subject to the increasingly adverse effects of harmful algal blooms, eutrophication and climate change. Monitoring phytoplankton in these areas using Ocean Colour Remote Sensing is hampered by the high spatial and temporal variation in absorption and scattering properties. In this paper we demonstrate a clustering method based on specificabsorption properties that give accurate water quality products from the Medium Resolution Imaging Spectrometer (MERIS). A total of 468 measurements of Chlorophyll a (Chla), Total Suspended Material (TSM), specific-(sIOP) and inherent optical properties (IOP) were measured in the North Sea between April 1999 and September 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 organic material at 442 nm (a CDOM (442)) was 0.02 to 0.26 m-1. The variation in absorption properties of phytoplankton (a ph) and non-algal particles (a NAP) were an order of magnitude greater than that for a ph normalized to Chla (a ph *) and a NAP normalized to TSM (a NAP *). Hierarchical cluster analysis on a ph *, a NAP * and a CDOM reduced this large data set to three groups of high a NAP *-a CDOM , low a ph * situated close to the coast, medium values further offshore and low a NAP *-a CDOM , high a ph * in open ocean and Dutch coastal waters. The median sIOP of each cluster were used to parameterize a semi-analytical algorithm to retrieve concentrations of Chla, TSM and a CDOM (442) from MERIS data. A further 60 measurements of normalized water leaving radiance (nL w), Chla, TSM, a CDOM (442) and a NAP (442) collected between 2003 and 2006 were used to assess the accuracy of the satellite products. The regionalized MERIS algorithm showed improved performance in Chla and a CDOM (442) estimates with relative percentage differences of 29 and 8% compared to 34 and 134% for standard MERIS Chla and a dg (442) products, and similar retrieval for TSM at concentrations >1 gm-3 .

Physical, Bio-Optical State and Correlations in North–Western European Shelf Seas

“…This diversity was likely a result of rapid changes in seawater especially during bloom/post-bloom collapse phase, in these waters spring bloom have been reported from late February to late May [47,48]. Furthermore, the measured environmental parameters in Table 1 fall within the range of previous investigations in these north-western shelf seas [9,33,40,42,49]. These regions are typically fed by some of the major rivers introducing both organic and inorganic material enhancing locally observed concentrations of CDOM and water transparency.…”

“…The mass specific scattering coefficient of iSPM was the mean of the calcareous sand, red clay, yellow clay, and brown earth in Hydrolight (Sequoia Scientific Inc., Seattle, WA, USA). These estimates are relatively within the range of other studies [42,57]. We make the assumption here that the water bodies investigated have similar iSPM so as to get a general picture.…”

“…It is presumed that perceived color of seawater in the NWES is strongly driven by CDOM and SPM although chl-a plays a seasonal and local role for example in the Central North Sea [32][33][34][35]. In situ station (blue dots) sampling was done in the German Bight (1)(2)(3)(4)(5)(6)(7)(8), North Sea (9-26), Inner Seas (27)(28)(29)(30)(31)(32)(33)(34), Irish Sea (35)(36)(37)(38)(39)(40)(41)(42)(43), and Celtic Sea (44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)(58)(59)(60). Region naming is based on maps from The Germany Federal Maritime and Hydrographic Agency [34] and International Hydrographic Organization [35].…”

“…In this paper the focus is on assessing the quality of fit (R), statistical significance F-statistic (p-value), measure of average uncertainty in model prediction MAPD and UPD as widely presented in literature [8,21,[42][43][44].…”

Photochemical oxidation of dimethylsulphide to dimethylsulphoxide in estuarine and coastal waters

“…The mean a 350 of these sample groupings broadly agrees with previous work in the study area (Stubbins et al, 2011). Furthermore, the mean CDOM absorption coefficient for our coastal samples (a 350 ¼ 1.4 m -1 ) is similar to the mean a 350 value of 1.3 m -1 for the North Sea, obtained by extrapolation from a 442 ¼ 0.348 m -1 (Tilstone et al, 2012) using a CDOM spectral slope of 0.014 nm À1 (Table 1). Our mean coastal DMS photo-oxidation rate constant, k DMS , was 1.0 d À1 , 2-fold and 5-fold lower than our means for the lower and upper estuary, respectively (Table 4).…”