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Evaluation of MERIS products from Baltic Sea coastal waters rich in CDOM
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
2014 (English)In: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 10, no 3, 377-396 p.Article in journal (Refereed) Published
Abstract [en]

In this study, retrievals of the medium resolution imaging spectrometer (MERIS) reflectances and water quality products using four different coastal processing algorithms freely available are assessed by comparison against sea-truthing data. The study is based on a pair-wise comparison using processor-dependent quality flags for the retrieval of valid common macro-pixels. This assessment is required in order to ensure the reliability of monitoring systems based on MERIS data, such as the Swedish coastal and lake monitoring system (http://vattenkvalitet.se). The results show that the pre-processing with the Improved Contrast between Ocean and Land (ICOL) processor, correcting for adjacency effects, improves the retrieval of spectral reflectance for all processors. Therefore, it is recommended that the ICOL processor should be applied when Baltic coastal waters are investigated. Chlorophyll was retrieved best using the FUB (Free University of Berlin) processing algorithm, although overestimations in the range 18-26.5 %, dependent on the compared pairs, were obtained. At low chlorophyll concentrations (< 2.5 mg m(-3)), data dispersion dominated in the retrievals with the MEGS (MERIS ground segment processor) processor. The lowest bias and data dispersion were obtained with MEGS for suspended particulate matter, for which overestimations in the range of 8-16% were found. Only the FUB retrieved CDOM (coloured dissolved organic matter) correlate with in situ values. However, a large systematic underestimation appears in the estimates that nevertheless may be corrected for by using a local correction factor. The MEGS has the potential to be used as an operational processing algorithm for the Himmerfjarden bay and adjacent areas, but it requires further improvement of the atmospheric correction for the blue bands and better definition at relatively low chlorophyll concentrations in the presence of high CDOM attenuation.

Place, publisher, year, edition, pages
2014. Vol. 10, no 3, 377-396 p.
National Category
Biological Sciences
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-106536DOI: 10.5194/os-10-377-2014ISI: 000338651700006OAI: oai:DiVA.org:su-106536DiVA: diva2:737072
Note

AuthorCount:3;

Available from: 2014-08-11 Created: 2014-08-11 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Uncertainty measurements and validation of ocean colour data in optically complex waters
Open this publication in new window or tab >>Uncertainty measurements and validation of ocean colour data in optically complex waters
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis addresses validation activities associated with the use of in situ and satellite-based radiometers to assess water quality parameters, such as chlorophyll-a (Chl-a), total suspended matter (TSM) and coloured dissolved organic matter (CDOM- also known as yellow substances, YEL) in CDOM-dominated waters. In paper I, an inter-comparison of in situ radiometers is presented. Here, a new hand-held radiometer, the Water Insight Spectrometer (WISP-3) was tested and evaluated for routine water monitoring against other common radiometers used for validation. The WISP-3 measures the reflectance at the surface, and thus also works in shallow depths. The WISP-3 is designed for validation in places where other radiometers are difficult to deploy. As it is hyperspectral it can be used to develop in-water algorithms for the retrieval of water-quality information. In paper II, satellite data from the 3rd reprocessing archives of the MEdium Resolution Imaging Spectrometer (MERIS) sensor on board of ENVISAT is evaluated. MERIS level 2 reflectance and water products are assessed against in situ data. This assessment is required in order to ensure the reliability of  monitoring systems based on MERIS data such as the Swedish coastal and lake monitoring system www.vattenkvalitet.se. The evaluation of reflectance data is based on a pair-wise comparison of the standard MEGS processor and three coastal processors that are provided as source free plug-ins for the VISAT BEAM software; all pairs are compared to data measured in situ. The derived water products are evaluated both on a pair-wise comparison of processors as well as on an individual comparison of some processors to sea-truthing data. The studied processors improved the retrievals of MERIS reflectance when used the latest MERIS FR 3rd reprocessing, equalized and “smile” corrected and a land adjacency effects were corrected using the improved contrast between ocean and land (ICOL). The blue spectral bands remain problematic for all processors. Chlorophyll was retrieved best using FUB with an overestimation between 18% - 26.5% (MNB) dependent on the compared pairs. At low chlorophyll < 2.5 mg m-3, random errors dominates the retrievals of MEGS. MEGS showed lower bias and random errors when deriving suspended particulate matter (SPM) with an overestimation in the range 8-16% (MNB). All processors failed to retrieve CDOM correctly, but FUB could at least resolve variations in CDOM, however with a systematic underestimation that may be corrected for by using a local correction factor . MEGS has shown already potential to be used as operational processor in the Himmerfjärden bay and adjacent areas, but it requires further improvement of the atmospheric correction for the blue bands and better definition at relatively low chlorophyll concentrations in presence of CDOM.

Place, publisher, year, edition, pages
Stockholm, Sweden: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2013. 22 p.
Keyword
Ocean colour validation, MERIS, in situ radiometry, water quality, CDOM
National Category
Oceanography, Hydrology, Water Resources
Research subject
Marine and Brackish Water Ecology
Identifiers
urn:nbn:se:su:diva-90128 (URN)
Presentation
2013-06-10, 105, Frescati Backe, Svante Arrhenius väg 21-A, Stockholm, 17:28
Opponent
Supervisors
Funder
EU, FP7, Seventh Framework Programme, 251527
Note

Main research funders:

Swedish National Space Board (Dnr. 99/09)

European Space Agency (contract no. 21524/08/I-OL)

Available from: 2013-06-04 Created: 2013-05-23 Last updated: 2015-12-07Bibliographically approved
2. Remote sensing in optically complex waters: water quality assessment using MERIS data
Open this publication in new window or tab >>Remote sensing in optically complex waters: water quality assessment using MERIS data
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This PhD study focusses on the use of MEdium Resolution Imaging Spectrometer (MERIS) data for reliable and quantitative water-quality assessment of optically-complex waters (lake, brackish and coastal waters). The thesis is divided into two parts: A. intercalibration of reflectance measurements in different optically-complex water bodies (Paper I), and validation of various satellite processing algorithms for the coastal zone (Paper II). B. Applications: the use of MERIS data in integrated coastal zone management mostly using Himmerfjärden bay as an example.

Himmerfjärden bay is one of the most frequently monitored coastal areas in the world and it is also the recipient of a large urban sewage treatment plant, where a number of full-scale nutrient management experiments have been conducted to evaluate the ecological changes due to changes in nutrient schemes in the sewage plant.

Paper I describes the development and assessment of a new hyperspectral handheld radiometer for in situ sampling and validation of remote sensing reflectance.  The instrument is assessed in comparison with readily available radiometers that are commonly used in validation.

Paper II has a focus on the validation of level 2 reflectance and water products derived from MERIS data. It highlights the importance of calibration and validation activities, and the current accuracy and limitations of satellite products in the coastal zone.  Bio-optical in situ data is highlighted as one of the key components for assessing the reliability of current and future satellite missions. Besides suspended particulate matter (SPM), the standard MERIS products have shown to be insufficient to assure data quality retrieval for Baltic Sea waters. Alternative processors and methods such as those assessed and developed in this thesis therefore will have to be put in place in order to secure the success of future operational missions, such as Sentinel-3.

The two presented manuscripts in the applied part B of the thesis (paper III and IV), showed examples on the combined use of in situ measurements with optical remote sensing to support water quality monitoring programs by using turbidity and suspended particulate matter as coastal indicators (manuscript III). The article also provides  a new turbidity algorithm for the Baltic Sea and a robust and cost-efficient method for research and management.  A novel approach to improve the quality of the satellite-derived products in the coastal zone was demonstrated in manuscript IV. The analysis included, the correction for adjacency effects from land and an improved pixel quality screening.  The thesis provides the first detailed spatio-temporal description of the evolution of phytoplankton blooms in Himmerfjärden bay  using quality-assured MERIS data, thus forwarding our understanding of ecological processes in in Swedish coastal waters.

It must be noted that monitoring from space is not a trivial matter in these optically-complex waters dominated by the absorption of coloured dissolved organic matter (CDOM). These types of coastal waters are especially challenging for quantitative assessment from space due to their low reflectance.  Papers III and IV thus also provide tools for a more versatile use in other coastal waters that are not as optically-complex as the highly absorbing Baltic Sea waters. The benefits of the increased spatial-temporal data coverage by optical remote sensing were presented, and also compared to in situ sampling methods (using chlorophyll-a as indicator).

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2015. 58 p.
Keyword
Ocean Colour, MERIS, Baltic Sea, Bio-optics, water quality, Chlorophyll, Turbidity, SPM, Himmerfjärden, Remote Sensing
National Category
Ecology Oceanography, Hydrology, Water Resources
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-123724 (URN)978-91-7649-310-6 (ISBN)
Public defence
2016-01-29, Q211, Department of Ecology, Environment and Plant Sciences, Svante Arrhenius väg 20 A, Stockholm, 14:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 165/11Swedish National Space Board, 147/12EU, FP7, Seventh Framework Programme, 251527EU, FP7, Seventh Framework Programme, 295068Ecosystem dynamics in the Baltic Sea in a changing climate perspective - ECOCHANGE, 4315403Baltic Ecosystem Adaptive Management (BEAM), 4315403
Note

Research funders:

European Space Agency (ESA, contract no.21524/08/I-OL)

NordForsk funding: Nord AquaRemS Ref. no. 80106

NordForsk funding:  NordBaltRemS Ref.no. 42041

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2016-01-05 Created: 2015-12-03 Last updated: 2017-04-11Bibliographically approved

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