Validation of the endmembers and the model
The coverage of the endmember model, relative to the in situ measured reflectances is shown in Fig. 2. The reflectances of the simulated endmembers with the lowest values were lower than any of the measured spectra, while the reflectances of the endmembers with the highest values were higher than almost all in situ measured spectra (Fig. 2), indicating suitable endmembers for this area. However, the endmembers with the highest reflectances have a somewhat different reflectance in the last band (708 nm) than the in situ spectra with the highest reflectances. This could be due to other SIOPs for SPM at these in situ stations than the median value that was used to generate the endmembers. The discrepancies are not too large: such a spectrum will still be unmixed with the most suitable endmembers and the abundance results will be minimally influenced. The high coverage of the endmembers at 412 nm is due to the pure-water endmember. Although spectra with very low concentrations (and therefore high reflectance in the blue wavelengths) were missing in our in situ set, such an endmember was necessary to unmix spectra at locations with very low concentrations in the German Bight.
Abundances derived from unmixing in situ measured spectra were compared with the in situ data of [Chl], [SPM] and a
CDOM in Table 2. Abundances of endmembers were consistent with total and relative concentrations of the three substances. It seems that the low-concentrations endmember functioned as a basis, as it was present with significant abundances in almost all unmixing results. This endmember was usually found in combination with one and sometimes two other endmembers at higher abundances (> 0.25) and two endmembers at low abundances (< 0.25). The results show a correlation between high abundances of the endmembers dominated by [Chl], [SPM] or a
CDOM, and a relatively high concentration for that substance – in comparison to the concentrations of the other two substances. At stations where concentrations are (relatively) comparable, the SPM-dominated endmember has the highest abundance. The last remark is in agreement with general knowledge of the influence of SPM and its spectral signature (e.g. Lubac and Loisel 2007).
Water class monitoring with unmixed MERIS data
The unmixed MERIS images show clear spatial patterns. Spatial variation in the obtained water classes is visible, as shown in Figs. 3, 4, 5, 6, 7 and 8. RMSE values of the figures presented in this section can be found in Appendix 4.
High abundances (0.5 to 0.9) of the SPM-Chl-dominated endmember were found at the innermost locations of the Wadden Sea and in sheltered areas behind the islands (Fig. 3). This agrees with the trend of much higher concentrations of all three substances in the Wadden Sea than in the North Sea (Hommersom et al. 2009). The basic low-concentrations endmember showed its highest abundances in the North Sea, in winter covering most of the German Bight (Fig. 4a), while in spring and summer its area was reduced (Fig. 4b) in favour of the CDOM-dominated, CDOM-Chl-dominated and Chl-dominated endmembers (e.g. Fig. 5). The pure water endmember had low abundances (maximum, 0.5; generally, 0.2 or 0.3) at the most offshore locations (∼>53.5° N, <5.5° E) in almost every image (not shown). Only in winter was this endmember spread over the German Bight with abundances reaching maximum values of 0.2.
As expected, a gradient was found in Ems-Dollard estuary (Fig. 6), with SPM-dominated water in the Dollard (Fig. 6a), via SPM-Chl-dominated water in the inner estuary (Fig. 6b) to high abundances for the low concentrations endmember in the outer estuary (Fig. 6c). In spring, because of phytoplankton blooms (e.g. Cadée 1980), outside the estuary the Chl-dominated endmember was found (Fig. 6d). This spatial trend is similar to the trend found by Magnuson et al. (2004) in the upper, mid and lower Chesapeake Bay and the inshore Middle Atlantic Bight. This trend is characterised as: high b/a values in the upper estuary (very high scattering due to extreme [SPM] and high absorption by Chl due to high [Chl]), via lower b/a values at high-total a and b in the mid-estuary (high [SPM] and high [Chl]), and again lower b/a values at low-total a and b in the lower estuary (low [SPM] and low [Chl]), to finally low b/a values at low a and b (very low [SPM] and low [Chl]) at sea.
Four images, acquired in June and July 2006, with calm weather (average daily wind conditions over the days of acquisition of the four images ranging 2.9-6.1 m s−1, wind direction north-east to south-east) were selected to analyse tidal changes. With these four images half of the tidal cycle was covered. As the tide varies over the area, the part of the cycle that was covered depends on the location (e.g. for the area near Den Helder the images cover high water (HW) to low water (LW)). The expectation was to find higher influence of SPM about 2 h before slack tide, due to tidal currents (Hommersom et al. 2009; Poremba et al. 1999, Postma 1982). Fig. 7a–d shows the abundances of the SPM-Chl-dominated endmember, which was the most representative endmember in the Wadden Sea in the four selected images. Indeed, over the examined part of the tidal cycle the following trends in the abundances of the SPM-Chl-dominated endmember were visible (Fig. 7a–d):
The lowest abundances occurred just after HW, when relatively clear North Sea water had entered the Wadden Sea and SPM and benthic algae have had the time to settle out (Den Helder, Fig. 7b; the East Frisian Wadden Sea, Fig. 7c).
Abundances increased during ebb tide (Den Helder, Fig. 7c; Eastern Dutch Wadden Sea, Fig. 7d; Ems estuary/area near island Borkum, Fig. 7d; East Frisian Wadden Sea, Fig. 7d).
At LW and the first hour(s) after, large percentages of the surface were covered with surfacing tidal flats. Abundances in the surrounding water were similar to or lower than during ebb (East Frisian Wadden Sea, Fig. 7a; near Den Helder, Fig. 7d). This makes sense, as SPM and benthic algae will be subject to sedimentation in the calm water.
The highest abundances were seen during flood, about 2.5 h before HW (Eastern Dutch Wadden Sea, Figs. 7a, b; Ems estuary/area near island Borkum, Fig. 7b).
Seasonal changes became visible in the abundances as well. In winter, the abundances of the CDOM-dominated, CDOM-Chl-dominated and Chl-dominated endmember were reduced to zero, while the abundances of the SPM-dominated endmember were relatively high compared with summer (Fig. 8). This could be explained by higher SPM-resuspension because of the windier weather (e.g. Lemke et al. 2009; Grossart et al. 2004) and a lack of benthic organisms that stabilise the sediment (Austen et al. 1999). Because of many days with clear weather in (early) spring 2007, results could show the appearance, development and movement of the spring bloom of phytoplankton (e.g. Cadée 1980), in March–May 2007 (Fig. 5). The bloom drifted in the direction of the residual current in the German Bight (Postma 1982). Unfortunately, May 2007 was very cloudy so that this bloom could not be followed further.
Wind induced resuspension, as seen by for example Stanev et al. (2009) and Badewien et al. (2009), became visible through high abundances of the SPM-Chl-dominated endmember (Fig. 9a, b) and the SPM-dominated endmember (Fig. 10a, b) in the Wadden Sea and on the outer side of the islands. Abundances of these endmembers were much lower during calm weather (Fig. 3). The south-westerly wind at December 10th 2006 had apparently blown relatively clear North Sea water into the western Dutch Wadden Sea (low abundances of the SPM-Chl-dominated endmember), while resuspension (due to wind waves and/or tide against wind) caused relatively high abundances of the SPM-Chl-dominated and SPM-dominated endmembers in the rest of the Wadden Sea, and north of the Dutch and western German islands (Figs. 9a and 10a). Waves reach the bottom at shallow locations, including the tidal flats, while at other locations mixing increases. As the Wadden Sea is always well mixed (Postma, 1982), due to tidal changes, water masses of shallow and deep locations easily mix, so resuspended matter can move over relatively large areas. The north–north-easterly wind of late March 2007 caused much higher resuspension than the south-westerly wind early December 2006, visible as very high abundances of the SPM-Chl-dominated endmember and relatively high abundances of the SPM-dominated endmember along the Northern German and Danish islands in the German Bight (Fig. 9b). The wind-wave-induced mixing kept the sediment in the East-Anglian plume (Eleveld et al. 2008; Doerffer and Fischer 1994) in suspension, which is visible as high abundances of the SPM-Chl-dominated and the SPM-dominated endmembers in the image of March 25th, north of 54.5° N and west of 6° E (Figs. 9b and 10b).
Early in July 2006, a patch with a high abundance of the SPM-dominated endmember was seen north-west of the Danish islands (Fig. 11a), indicating high SPM, or at least high reflectances. Also the pure-water endmember was elevated. The shape and location of the patch, the low-wind conditions of July 3rd and the combination with the pure-water endmember reveal that the patch could not be caused by resuspension and the high reflectances would possibly be caused by something else than SPM. Elevated RMSE values in the patch (Fig. 11b) suggest the MERIS spectra had a shape that was not well covered by the spectral shapes of the endmembers: the patch was a novelty (Schiller et al. 2007). It could be a coccolith bloom, as these algae cause a high reflection and have a distinct specific absorption spectrum. This explanation was supported by the presence of an elevated biomass of prymnesiophytes (coccoliths) found by the Danish National Environmental Monitoring Institute (NERI: www.dmu.dk/International/Water/MarineMonitoring/MADS) at the nearest in situ station in the first 2 weeks of July 2006. The spectral shape of the reflectances in the patch showed high similarities with the bloom found by Schiller et al. (2007); the high blue-green ratios explains why the pure-water endmember had high abundances.
Tidal flat detection would in principle be possible as well with the presented unmixing method. To do this, a ‘tidal flat’-endmember should be included. But unfortunately, the neural network of the C2R processor was not trained for land or tidal flat data, and all pixels at land or tidal flat are flagged with the RAD_ERR flag, which means that the reflection at the top of the atmosphere was invalid and reflectances should not be trusted. The C2R level-2 product used in this study indeed showed unrealistically low values at tidal flats and land, so that unmixing did not give high abundances for a tidal flat endmember for these pixels. As an alternative, standard MERIS level-2 data were tested, but it was found that they often have negative values in the blue wavelengths or showed unrealistic spectral shapes in near-coastal waters. Therefore, this data have not been used as modelling input.
Changes in concentration settings during endmember generation were found to lead to small changes in endmember abundances. As shown in Fig. 12, changes in concentrations of 10% during endmember generations resulted in almost 1:1 correlations of abundances, shown for the low-concentrations endmember and the SPM-Chl-dominated endmember. The expectation is that the model is also relatively invulnerable for small changes in shape of the SIOPs (a*Chl, a*SPM, a*CDOM, and b*SPM) used in the Gordon model.
The RMSE data presented in Appendix 4 show that RMSE values are generally <0.005 and always <0.01. As the modelled abundances are presented on a scale from 0 to 1 with steps of 0.1, the error is always <10% relative to the results. Appendix 4 also shows that the shape of the RMSE does not follow patterns in abundances of certain endmembers.
It appeared that ICOL (first version) processing failed for some images. Out of 32 selected images, two images acquired in winter 2006 (Jan 16th and Dec 10th) could not be processed due to an unknown reason. As images without too much cloud cover were scarce, these two were still used with only C2R processing. Since ICOL compensates for the adjacency effect, the influence of ICOL is significant in water pixels nearby land covered with vegetation. Our experience is that ICOL has a large influence on MERIS data processed with the standard MERIS processor, but the effect is negligible for most of the Wadden Sea when images are processed with the C2R processor. This could be explained by the fact that MERIS standard level 2 atmospheric correction relies for a large part on observed reflectances in the near infrared, the part of the spectrum which is most heavily influence by the adjacency effect. C2R on the other hand uses a neural network that uses all spectral bands. Influence of ICOL on C2R data is expected in the inner parts of the Ems estuary (Dollard) and in the pixels bordering the coast.