Underwater light attenuation

The determination of quantity of light available underwater in the visible spectral range is crucial for a wide community of marine users (i.e. ecosystem modelers, sub-marine operations, divers). The photosynthetically available radiation (PAR) is a key parameter in ecosystem modeling and governs, with the nutrient limitation, the primary production. In the proposed model of PAR attenuation (KPAR) a parameterization of the diffuse attenuation of the downwelling attenuation (K_d) has been established using radiative transfer simulations (Hydrolight). Hydrolight was fed by a wide range of inherent optical properties of water (absorption, a, backscattering, bb, Cf IOCCG dataset), and a selection of illumination conditions (sun zenith angles and cloud cover). The generated profiles of downwelling irradiances were then used to calculate K_d at the euphotic depth Z_eu1%, the depth at which the downwelling irradiance is 1% of its value at the surface of water. In a second step, a link between K_d at 590nm and KPAR was performed via regression analysis applied to Hydrolight derived K_d and KPAR, which yielded a simple logarithmic relationship between K_d590 and KPAR. Validation of this model is ongoing.

More information in this publication.

Relationship between IOPs and TSM

Mass-specific inherent optical properties (IOPs) provide the essential link between optical properties of the water column and the optically significant constituents suspended and dissolved in seawater. This issue is important as conversion models linking optical properties to biogeochemical properties are being used in an accelerated rate both in biogeochemical models with optical modules and in inversion of ocean color to biogeochemical models. Previous studies have demonstrated considerable variability in mass-specific IOPs but the causes of observed variability have been inadequately parameterized. The REMSEM team investigates the causes of variability in mass-specific particulate attenuation and backscattering coefficients with respect to particle size, density and composition for the first time. Based on a large in situ dataset covering a broad range of coastal and offshore waters, it is shown that particle apparent density strongly drives the mass-specific beam attenuation, while particle composition is the main factor driving mass specific backscatter. Overall, recent studies offer new insight into the sources of optical variability in natural waters and contributes to the development of future sampling strategies and algorithm formulation.

Near similarity spectrum

The shape of the water leaving reflectance spectrum in the NIR wavelengths is determined largely by pure water absorption and is thus almost invariant. As such, a universal or "similarity" spectrum in the NIR is defined by normalization at 780nm. This similarity spectrum is calculated from seaborne reflectance measurements and is valid over waters ranging from moderately turbid (water-leaving reflectance at 780nm of order 10^-4 or total suspended matter concentration of order 0.3 gm^-3) to extremely turbid waters (water-leaving reflectance at 780nm of order 10^-1 or total suspended matter concentration of order 200 gm^-3). The similarity spectrum is presented in the figure below and can be downloaded here (Source: Ruddick et al., 2006; see publications). Important applications of the NIR similarity spectrum includeatmospheric correction of ocean colour data over turbid waters, quality control of seaborne, airborne, and space borne reflectance measurements in turbid waters.

Measured water-leaving reflectance spectra normalized by reflectance at 780 nm (using different scales for axes) for the 27 spectra considered. The average of the six brightest measurements, the similarity spectrum ρ_wn780 (λ), is shown as a thick black line, with standard deviation over these six measurements indicated by
the thick dashed lines.

SWIR research

The retrieval of water optical properties and concentrations of in-water constituents from reflectance spectra in coastal waters has been a subject of increased interest in recent research. A critical step in remote sensing of coastal waters is the effective removal of the contribution of the atmosphere to the total signal that reaches the sensor at the top of the atmosphere, i.e. the atmospheric correction process. In open ocean waters the atmospheric correction for ocean colour sensors, such as SeaWiFS and MODIS, has been carried out using the two near-infrared (NIR) bands with assumption of the negligible ocean contributions (black pixel assumption) at the two NIR bands. However, alternative algorithms had to be developed for more turbid waters given that the ocean has significant radiance contributions at the NIR bands. Moreover, it’s been realized that as the sediment concentration increases the use of longer wavelength bands for atmospheric correction is needed. The Short Wave Infrared (SWIR, 1-3 um) range offers new opportunities in this regards due to the high absorption of pure water in this spectral region and the lower contribution of ocean substances compared to Visible and Near-InfraRed (VNIR) wavelengths.

The use of SWIR bands for turbid water atmospheric corrections and Total Suspended Matter (TSM) retrieval and the consequent incorporation of SWIR spectral bands into ocean colour satellites is part of REMSEM current research.

Evaluation of Total Suspended matter and Turbidity algorithms in different regions of the world

Regional algorithms have been designed to map TSM (Nechad et al. 2010) and turbidity (T) (Nechad et al. 2009) from in situ or satellite-derived marine reflectance in turbid North Sea waters. In order to test its general applicability, these algorithms have been applied to different coastal regions of the world. They have been tested in the Gulf of Gabés (Tunisia) and in the La Plata River (Argentina) where very high TSM concentration have been reported  (100-300 gm^-3) providing an interesting extreme case for testing these algorithms. The algorithms are applied to satellite-derived marine reflectance to yield TSM and turbidity maps for each region and, if in situ measurements are available, they are use to validate TSM and T maps and, where necessary, recalibrate the primary algorithm calibration parameter (TSM- or turbidity-specific particulate backscatter).

Since 2001, REMSEM collects hyperspectral measurements of marine reflectance (above-water), in combination with pigment concentrations and suspended matter concentration. These measurements are used to establish algorithms for the retrieval of e.g. chl a and TSM from remotely sensed imagery. Since 2007, TU is also recorded. The figure below shows the Belgian Continental Shelf and the location of the 10 stations which are monitored regularly by the Belgian science Vessel ‘Belgica’. With the installation of the so-called “FerryBox”, the Belgica becomes one of the leading platforms in the field of autonomous measurements at sea. The system provides the continuous measurement of salinity, temperature, dissolved oxygen, pH, plantpigments, turbidity, nutrients (nitrate, nitrite, phosphate and silica), pCO2 and radiation. The output is a major contribution to marine research with, in particular, the study of eutrophication and it’s effects, the development and testing of models, the research of climate change and ocean acidification. It can also be seen as the next step in the continuous monitoring of the marine environment.

Geographical area showing limit of Belgian marine waters (black line), 12 nautical mile limit (red line) and existing locations used for in situ monitoring of chlorophyll a in the framework of the OSPAR eutrophication strategy and the EU Water Framework.

In situ datasets can be freely downloaded from the Belgian Marine Data centre (BMDC).

Chlorophyll a maps and Algae Bloom detection/timing

The purpose of REMSEM’s services is to provide satellite-based support for the water quality assessment of chlorophyll a concentration. Satellite ocean colour observation provides daily maps of surface chlorophyll a concentration for the global oceans. MUMM provides products based on multi-temporal analysis of chlorophyll a (CHL) products for different sub-regions of the North Sea and the Mediterranean Sea for use in eutrophication-related water quality reporting (OSPAR eutrophication strategy, EU Water Framework Directive). Products include monthly/3-monthly and annual mean and 90 percentile (P90) CHL products, algae bloom detection and timing products and time series of CHL at in situ monitoring stations. In addition, where required an expert analysis (descriptive report) is provided as tailored to the specific user/application. These products are generated within the MARCOAST 2 project using specific temporal binning and classification procedures which are described in more detail on the project website which is mentioned in the section "Data availability / users support"

Currently, the REMSEM team is developing specialized procedures to upgrade the Algae Bloom (AB) detection service to a Harmful Algae Bloom (HAB) detection service. Such a service is wanted in many regions where toxic or otherwise harmful algae may impact on economic resources such as fisheries, aquaculture or tourism. Satellite chlorophyll information can be very useful for (H)AB detection, where the harmfulness is related to the high biomass, as is the case for Phaeocystis globosa in Belgian waters, or where the HAB species dominates the phytoplankton community. In general the satellite data will be combined with some in situ data.

Total Suspensed matter (TSM)

TSM is used to describe the dry weight concentration of suspended particulate matter and is measured by filtering a known volume of water on a glass fiber filter. Filters are then dried and weighed in the laboratory to obtain the dry mass concentration of particles, TSM. The term “Total” refers to inclusion of organic as well as inorganic matter, although it can be misleading since particles smaller than the pore size of the filter (usually 0.45micrometer) are not retained.

In remote sensing studies TSM is estimated via its optical properties assuming, either directly or indirectly, a constant relationship between particle mass and particulate (back)scattering or absorption. It is well-known that the relationship between particle mass and optical properties in reality varies as function of particle size and composition (refractive index) (e.g. Neukermans et al. 2011), giving an important limit on the generality and accuracy of remote sensing algorithms for estimating TSM. TSM is the parameter generally required for sediment transport applications.

Hover over the image for alternate view.

Turbidity

Turbidity, as defined in the standard ISO7027, is a parameter that is routinely measured in many national and regional water quality monitoring programs. While satellite data products are beginning to become available for the closely related parameter, Total Suspended Matter (TSM), the direct estimation of turbidity as a satellite data product has not yet been addressed. In situ measurements of TSM and of turbidity, obtained in the Southern North Sea (SNS), show high correlation (correlation coefficient of 98.6%). A generic multi-sensor algorithm for TSM as function of reflectance has been developed and is extended to the estimation of turbidity from water-leaving reflectance. The algorithm was calibrated using a set of 49 seaborne measurements of reflectance in the spectral range 600-850nm and turbidity in the SNS. Validation of these models was carried out using an independent set of seaborne measurements of turbidity and reflectance and shows low relative errors in turbidity retrieval at 681nm (less than 35%). This wavelength is recommended, provided no significant fluorescence affects this range.

Turbidimeter from HACH LANGE GmbH Düsseldorf, Germany used to collect in situ measurements.

Nechad B., Ruddick K. & Neukermans G. (2009). Calibration and validation of a generic multisensor algorithm for mapping of turbidity in coastal waters. In: SPIE "Remote Sensing of the Ocean, Sea Ice, and Large Water Regions" Conference held in Berlin (Germany), 31 August 2009. Proc. SPIE Vol. 7473, 74730H. Download

Sediment transport (TSM)

At MUMM different numerical models are being developed for the calculation of the transport of the dumped dredging material at the Belgian coast. To better understand the dispersion of the material after dumping, MUMM develops sediment transport models to simulate this transport. The remotely sensed spatial distribution of suspended matter can be used to validate these models.

GRIMAS: a tool for GRIdding and archiving of MArine Satellite data

GRIMAS is a set of generalized routines written in IDL, which can automatically process large numbers of MODIS and MERIS L2 files for any region on earth (described by 2 sets of bounding coordinates). Datasets are imported from the L2 files, projected on a custom generated grid (equirectangular projection), centered on the average latitude of the region. The gridded data can be easily exported to NetCDF files or mapped to PNG files. Extraction of time-series, match-ups and multi-temporal means from the archive of gridded data is also possible.

Standard products that can be generated are total suspended matter, chlorophyll concentration and sea surface temperature (MODIS only). However, with minor configuration, the processing of other parameters and of custom input files is possible.

GRIMAS is a work in progressImprovements are continuously made and new features are added. A web-based interface to GRIMAS is avilable.

Regional OceanColour archive

An archive of MODIS Aqua and MERIS RR level 2 images from the sensor’s acquisition start in mid 2002 to the present was generated using GRIMAS, for three regions: the Southern North Sea and the Channel (SNC, 48.5°N – 52.5° N, 4°W - °E, including SNS and BCZ), the waters around Ireland (IRL, 50°N – 57°N, 2.5°W – 13°W) and the Catalan coast (CTL, 40.5°N – 42.5°N, 0.5°E – 3.5°E). Gridded datasets extracted from these archives are available on request, as are time-series (for given locations) and multi-temporal (daily/monthy/yearly) composites and climatologies (averaged over several years). Time-series extraction from a web-based interface is available. The standard products include total suspended matter concentration, chlorophyll a concentration and sea surface temperature (MODIS only). Contact Quinten Vanhellemont for information on custom products and regions.

Multi-temporal chlorophyll a products / Algal Bloom detection and Timing products

MUMM generates multi-temporal chlorophyll a products in the framework of the “MarCoast-2” project funded by the European Space Agency (ESA) under the GMES Services Element. We aim to make these Multi-temporal chlorophyll a products more accessible to a large community of fisheries and scientific institutions using MUMM’s MarCoast webportal. This archive holds the following products for the period 2003 to present:

• Weekly, 2-weekly, monthly, 3-monthly, and yearly chlorophyll a concentration maps (mean, P90, pixel count)
• Daily algal bloom detection maps
• Annual algal bloom timing maps
• Time Series of satellite-derived chlorophyll a concentrations can be delivered for specific locations on request

The area covered is lon = -8° to 13° and lat = 48° to 62° for the period 2003-2005 and lon = -40° to 55° and lat = 20° to 80° for the period 2006-current.

These OceanColour products can be freely obtained after sending a request to the provided contact.

MUMM’s MarCoast webportal: http://www2.mumm.ac.be/marcoast/
Contact for access request: marcoast@mumm.ac.be