Marcel Wernand | Royal Netherlands Institute for Sea Research (original) (raw)

Papers by Marcel Wernand

In the thesis introduction issues are discussed on the historical background of marine optics and... more In the thesis introduction issues are discussed on the historical background of marine optics and on marine optical devices that were used over the past centuries to observe and measure; as in all sciences, in marine optics we can see a steady development: that of ‘measuring’, beginning many centuries ago, to 'knowing' and since less than a century to the understanding of the phenomenon. Hereafter, six themes are treated successively. The first theme, ‘Ocean optics from 1600 (Hudson) to 1930 (Raman), shift in interpretation of natural water colouring’, addresses the question of why it took so long a time to explain the phenomenon ‘the colouring of the sea’, especially the blue colour, despite the age-long interest of sailors, for practical purposes of navigation and detection of fish – of which more later. The second theme ‘On the history of the Secchi disc’, describes the search to establish methods for the determination of (sea) water clarity concerning purposes of navigat...

How is the color determined? By means of a Forel-Ule scale (historical method, 1890), from which ... more How is the color determined? By means of a Forel-Ule scale (historical method, 1890), from which the color is compared to the color above a submersed white disk (Secchi-disk) or by means of hyperspectral light meters Science Besides water temperature, salinity and clarity, 'the color of water' belongs to the oldest observations of lakes, seas and oceans. 'The color of water' is an essential climate variable defined by the World Meteorological Organization (WMO) Forel-Ule color composite of al observations done between 1890 en 2000. Color change of the North-Atlantic Ocean since 1890. This ocean is greening (by plankton, so more food) Color comparison between a painted and the color of the sea. How to determine the color of the sea from marine art? i) Using de Modern Forel-Ule color comparator scale (historical method, 1890) and compare with painted sea. ii) Taking a picture of the painted sea including Gray-scale. Then from RGB to XYZ to x,y chromaticity derived Fore...

Ground truth measurements ar necessary for the validation of remotely sensed data. Rapid ship or ... more Ground truth measurements ar necessary for the validation of remotely sensed data. Rapid ship or aircraft spectral measurements of the upwelling and downwelling radiance are needed to determine the reflectance of the water column as well as to intercalibrate with satellite sensors. Inter calibrations are hindered by the application of different instruments with varying spectral bands. It has been found

Participatory science is not, as perhaps is believed, something of the 21st century. In this manu... more Participatory science is not, as perhaps is believed, something of the 21st century. In this manuscript we show that over a century ago it were not only scientists who collected oceanographic data but also merchant sailors. A good example of such globally collected data are Forel-Ule observations, from which the first date back to 1889. This hardly explored (NOAA) dataset, containing around 228,000 of so-called ocean colour observations, was recently analysed on trends. Some of the material here presented refers to a recent publication ‘Trends in Ocean Colour and Chlorophyll Concentration from 1889 to 2000, Worldwide’ (Wernand et al., 2013). Since the launch of satellite-mounted sensors globe-wide monitoring of chlorophyll, a phytoplankton biomass proxy, became feasible. Just as satellites, the Forel-Ule (FU) scale record (a hardly explored database of ocean colour) has covered all seas and oceans - but already since 1889. We provided evidence of the usefulness of the Forel-Ule scal...

ABSTRACT Introduction Framed within the European Project CITCLOPS (Citizens' Observat... more ABSTRACT Introduction Framed within the European Project CITCLOPS (Citizens' Observatory for Coast and Ocean Optical Monitoring), the aim of this study is to present a number of tools that can be employed by citizens to estimate the color of natural waters. Firstly, a scale that accurately matches the original Forel-Ule (FU) colors was developed using accessible and affordable materials. This Modern FU scale is presented as a 'Do-It-Yourself' kit that can be prepared using high-quality illumination filters and a frame made of a white Plexiglas (or other white material). Secondly, a smartphone application (APP) prototype that could be used by anyone willing to participate in environmental monitoring is presented. This application includes a digitalized color-comparator scale, simulating the colors of the original Forel-Ule scale, to be compared to the color of water bodies, and allows the observer to take a picture of the water body to calculate the FU number using a specific algorithm. It also offers an option to include a Secchi disk depth estimate and the Forel-Ule number obtained with the Modern FU scale, if the observer is in possession of these tools. The first inputs provided by selected volunteers and researchers, offer initial comparisons between the two monitoring tools, the Modern FU scale and the digital scale included in the smartphone application. The idea is to provide a water quality index appropriate for participatory science that allows for rapid estimates and interpretation of color changes occurring in the aquatic environment, and that could be used by local or global authorities as an assessing tool. Background

PLoS ONE, 2013

Marine primary productivity is an important agent in the global cycling of carbon dioxide, a majo... more Marine primary productivity is an important agent in the global cycling of carbon dioxide, a major 'greenhouse gas', and variations in the concentration of the ocean's phytoplankton biomass can therefore explain trends in the global carbon budget. Since the launch of satellite-mounted sensors globe-wide monitoring of chlorophyll, a phytoplankton biomass proxy, became feasible. Just as satellites, the Forel-Ule (FU) scale record (a hardly explored database of ocean colour) has covered all seas and oceans -but already since 1889. We provide evidence that changes of ocean surface chlorophyll can be reconstructed with confidence from this record. The EcoLight radiative transfer numerical model indicates that the FU index is closely related to chlorophyll concentrations in open ocean regions. The most complete FU record is that of the North Atlantic in terms of coverage over space and in time; this dataset has been used to test the validity of colour changes that can be translated to chlorophyll. The FU and FU-derived chlorophyll data were analysed for monotonously increasing or decreasing trends with the non-parametric Mann-Kendall test, a method to establish the presence of a consistent trend. Our analysis has not revealed a globe-wide trend of increase or decrease in chlorophyll concentration during the past century; ocean regions have apparently responded differentially to changes in meteorological, hydrological and biological conditions at the surface, including potential long-term trends related to global warming. Since 1889, chlorophyll concentrations have decreased in the Indian Ocean and in the Pacific; increased in the Atlantic Ocean, the Mediterranean, the Chinese Sea, and in the seas west and north-west of Japan. This suggests that explanations of chlorophyll changes over long periods should focus on hydrographical and biological characteristics typical of single ocean regions, not on those of 'the' ocean. Citation: Wernand MR, van der Woerd HJ, Gieskes WWC (2013) Trends in Ocean Colour and Chlorophyll Concentration from 1889 to 2000, Worldwide. PLoS ONE 8(6): e63766.

Deep Sea Research Part A. Oceanographic Research Papers, 1981

... Conseil Permanent International pour l'Exploration de la Mer, 38, 131-146. ... To keep t... more ... Conseil Permanent International pour l'Exploration de la Mer, 38, 131-146. ... To keep the errors minimal, one should decide whether to measure E+v = (Eo+E)/2 or E_v = (Eo-E)/2. Figure 6 displays variation of the function given by equation (2) for different radiance distributions in ...

Ocean Optics XIII, 1997

ABSTRACT An intensive data collection campaign has been conducted in the coastal waters of the so... more ABSTRACT An intensive data collection campaign has been conducted in the coastal waters of the southern North Sea during which a variety of marine optical parameters have ben measured. This has enabled the angular distribution factor of spectral radiance or Q factor to be calculated from in situ measurements. The Q factor relates upwelled spectral radiance to upwelled spectral irradiance and its precise determination is of importance in ocean color remote sensing. Previous modeling studies, often based on the optical properties of case I waters, have proposed a value of 5 for Q. However, this study suggest that more turbid coastal waters may approach the Lambertian case of Q close to (pi) .

Journal of the European Optical Society: Rapid Publications, 2010

François Alphonse Forel (1890) and Willi Ule (1892) composed a colour comparator scale, with tint... more François Alphonse Forel (1890) and Willi Ule (1892) composed a colour comparator scale, with tints varying from indigo-blue to cola brown, to quantify the colour of natural waters, like seas, lakes and rivers. For each measurement, the observer compares the colour of the water above a submersed white disc (Secchi disc) with a hand-held scale of pre-defined colours. The scale can be well reproduced from a simple recipe for twenty-one coloured chemical solutions and because the ease of its use, the Forel-Ule (FU) scale has been applied globally and intensively by oceanographers and limnologists from the year 1890. Indeed, the archived FU data belong to the oldest oceanographic data sets and do contain information on the changes in geobiophysical properties of natural waters during the last century. In this article, we describe the optical properties of the FU scale and its ability to cover the colours of natural waters, as observed by the human eye. The recipe of the scale and its reproduction is described. The spectral transmission of the tubes and their respective chromaticity coordinates are presented. The FU scale, in all its simplicity, is found to be an adequate ocean colour comparator scale. The scale is well characterized, stable and observations are reproducible. Thus, the large historic data sets of FU measurements are coherent and well calibrated. Moreover, the scale can be coupled to contemporary multi-spectral observations with hand-held and satellite-based spectrometers. A reintroduction of the FU scale is recommended to expand the historical database and to facilitate a tie-in with present satellite ocean colour observations by tranforming MERIS normalized multi-band reflectance image into a FU indexed image.

Journal of the European Optical Society: Rapid Publications, 2010

In this paper we present an analysis of historical ocean colour data from the North Pacific Ocean... more In this paper we present an analysis of historical ocean colour data from the North Pacific Ocean. This colour is described by the Forel-Ule colour index, a sea colour comparator scale that is composed of 21 tube colours that is routinely measured since the year 1890. The main objective of this research is to characterise colour changes of the North Pacific Ocean at a timescale of lustums. Next to the seasonal colour changes, due to the yearly cycle of biological activity, this time series between 1930 and 1999 might contain information on global changes in climate conditions. From seasonal independent analyses of the long-term variations it was found that the greenest values, with mean Forel-Ule scale, FU, of 4.1 were reached during the period of 1950-1954, with a second high (FU = 3) in the period 1980-1984. The bluest ocean was encountered during the years 1990-1994. The data indicate that after 1955 a remarkable long bluing took place till 1980.

Journal of the European Optical Society: Rapid Publications, 2010

The first records on regular, tabulated, measurements of transparency of natural waters are those... more The first records on regular, tabulated, measurements of transparency of natural waters are those performed by the German naturalist Adelbert von Chamisso during the Russian "Rurik" Expedition 1815-1818 under the command of Otto von Kotzebue. A standardized method to determine the water clarity (transparency) was adopted at the end of the nineteenth century. This method (lowering a white painted disc into the water until it disappeared out of sight) was described by Pietro Angelo Secchi in Il Nuovo Cimento and was published in 1865. The Austrian scientist Josef Roman Lorenz von Liburnau, experimenting with submersible objects, like white discs, in the Gulf of Quarnero (Croatia) in the eighteen-fifties, well before Secchi started his investigations, questioned the naming of the white disc. However, the experiments performed by Secchi and Cialdi in 1864 on such an intensive scale were never performed before. At the beginning of the twentieth century, water transparency observations by means of a 30 cm white disc, was named the Secchi-disc method.

ABSTRACT In this chapter the ideas of explorers and scientists on transparency and colouring issu... more ABSTRACT In this chapter the ideas of explorers and scientists on transparency and colouring issues of natural waters are sketched over a period between 1600 and 1930, from the time of Sir Francis Bacon to Chandrasekhara Raman. This history features famous scientists as well as various amateurs who were intrigued by these optical phenomena. We also discuss are a number of pieces of interesting equipment that were developed to quantify the intrinsically coupled transparency and colour of the sea. The Secchi disc (to determine water transparency) and the Forel–Ule scale (to classify sea and colour) are two, from the end of the nineteenth century, which have withstood the time. The hidden mechanisms for the colouring of water were only solved at the beginning of the twentieth century. It is argued that this delay was partially caused by the fact that the subject was mostly treated as marginal; also, if pure water distillation techniques had been fully developed earlier, this understanding could have been shifted backwards by decades. This chapter ends with Chandrasekhara Venkata Raman, who proved in 1922 that molecular scattering of light in water, in combination with the absorption of the longer wavelengths, caused the blue colour of the sea. In his Nobel lecture ‘On the colour of the sea’ on December 11, 1930 he explained his findings to the world.

Limnology and Oceanography: Methods, 2011

The accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult t... more The accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult to test under natural conditions due to rapid changes in physical and biological forcings and the practical inability to manipulate nutrient conditions and phytoplankton composition in the sea. Therefore, an indoor mesocosm was designed to examine the optical properties of phytoplankton under controlled and manipulated conditions of irradiance, temperature, turbulence, and nutrient availability. Equipped with hyperspectral radiometers and bottom irradiance meters, it is shown that under semi-natural environmental conditions biogeochemically relevant species as Emiliania huxleyi and Phaeocystis globosa can be grown with good precision (± 20%) between duplicate mesocosms and between duplicate sensors (<5% deviation). The accuracy of chlorophyll estimates by absorption, using an Integrating Cavity Absorption Meter, and fluorescence using water-leaving radiance was 74% to 80%, respectively, as it was negatively influenced by changes in phytoplankton physiology. Biomass detection was limitedto 1 to 2 µg chlorophyll/L with an apparent linearity to 50 µg chlorophyll/L. Estimates of the quantum efficiency of fluorescence (j ≈ 0.01) were comparable to real-world estimates derived from satellite observations. It is concluded that the mesocosms adequately simulate natural conditions with sufficient accuracy and precision and that they offer an important tool in validating assumptions and hypotheses underlying remote sensing algorithms and models.

Netherlands Journal of Sea Research, 1989

International Journal of Remote Sensing, 1997

. Ground truth measurements are necessary for the validation of remotely sensed data. Rapid ship ... more . Ground truth measurements are necessary for the validation of remotely sensed data. Rapid ship or aircraft spectral measurements of the upwelling and downwelling (ir)radiance are needed to determine the reflectance of the water column as well, as to intercalibrate with satellite ...

Applied Optics, 1986

Using an optical multichannel analyzer, 168 downswelling irradiance spectra were obtained in vari... more Using an optical multichannel analyzer, 168 downswelling irradiance spectra were obtained in variable atmospheric solar, and cloud cover conditions, and with solar elevations varying between 30 and 60 deg, to retrieve the total spectra in the visible 400-720 nm range. The accuracy of the retrieval is found to be better than that of the determination of some ground truth parameters

The Forel-Ule colour comparator scale has been applied globally and intensively by oceanographers... more The Forel-Ule colour comparator scale has been applied globally and intensively by oceanographers and limnologists since the 19th century, providing one of the oldest oceanographic data sets. Present and future Forel-Ule classifications of global oceanic, coastal and continental waters can facilitate the interpretation of these long-term ocean colour data series and provide a connection between the present and the past that will be valuable for climate-related studies. Within the EC-funded project CITLOPS (Citizens' Observatory for Coast and Ocean Optical Monitoring), with its main goal to empower endusers, willing to employ community-based environmental monitoring, our aim is to digitalize the colours of the Forel-Ule scale to establish the colour of natural waters through smartphone imaging. The objective of this study was to reproduce the Forel-Ule scale following the original recipes, measure the transmission of the solutions and calculate the chromaticity coordinates of the scale as Wernand and Van der Woerd did in 2010, for the future development of a smartphone application. Some difficulties were encountered when producing the scale, so a protocol for its consistent reproduction was developed and is described in this study. Recalculated chromaticity coordinates are presented and compared to measurements conducted by former scientists. An error analysis of the spectral and colourimetric information shows negligible experimental errors.

In the thesis introduction issues are discussed on the historical background of marine optics and... more In the thesis introduction issues are discussed on the historical background of marine optics and on marine optical devices that were used over the past centuries to observe and measure; as in all sciences, in marine optics we can see a steady development: that of ‘measuring’, beginning many centuries ago, to 'knowing' and since less than a century to the understanding of the phenomenon. Hereafter, six themes are treated successively. The first theme, ‘Ocean optics from 1600 (Hudson) to 1930 (Raman), shift in interpretation of natural water colouring’, addresses the question of why it took so long a time to explain the phenomenon ‘the colouring of the sea’, especially the blue colour, despite the age-long interest of sailors, for practical purposes of navigation and detection of fish – of which more later. The second theme ‘On the history of the Secchi disc’, describes the search to establish methods for the determination of (sea) water clarity concerning purposes of navigat...

How is the color determined? By means of a Forel-Ule scale (historical method, 1890), from which ... more How is the color determined? By means of a Forel-Ule scale (historical method, 1890), from which the color is compared to the color above a submersed white disk (Secchi-disk) or by means of hyperspectral light meters Science Besides water temperature, salinity and clarity, 'the color of water' belongs to the oldest observations of lakes, seas and oceans. 'The color of water' is an essential climate variable defined by the World Meteorological Organization (WMO) Forel-Ule color composite of al observations done between 1890 en 2000. Color change of the North-Atlantic Ocean since 1890. This ocean is greening (by plankton, so more food) Color comparison between a painted and the color of the sea. How to determine the color of the sea from marine art? i) Using de Modern Forel-Ule color comparator scale (historical method, 1890) and compare with painted sea. ii) Taking a picture of the painted sea including Gray-scale. Then from RGB to XYZ to x,y chromaticity derived Fore...

Ground truth measurements ar necessary for the validation of remotely sensed data. Rapid ship or ... more Ground truth measurements ar necessary for the validation of remotely sensed data. Rapid ship or aircraft spectral measurements of the upwelling and downwelling radiance are needed to determine the reflectance of the water column as well as to intercalibrate with satellite sensors. Inter calibrations are hindered by the application of different instruments with varying spectral bands. It has been found

Participatory science is not, as perhaps is believed, something of the 21st century. In this manu... more Participatory science is not, as perhaps is believed, something of the 21st century. In this manuscript we show that over a century ago it were not only scientists who collected oceanographic data but also merchant sailors. A good example of such globally collected data are Forel-Ule observations, from which the first date back to 1889. This hardly explored (NOAA) dataset, containing around 228,000 of so-called ocean colour observations, was recently analysed on trends. Some of the material here presented refers to a recent publication ‘Trends in Ocean Colour and Chlorophyll Concentration from 1889 to 2000, Worldwide’ (Wernand et al., 2013). Since the launch of satellite-mounted sensors globe-wide monitoring of chlorophyll, a phytoplankton biomass proxy, became feasible. Just as satellites, the Forel-Ule (FU) scale record (a hardly explored database of ocean colour) has covered all seas and oceans - but already since 1889. We provided evidence of the usefulness of the Forel-Ule scal...

ABSTRACT Introduction Framed within the European Project CITCLOPS (Citizens&amp;#39; Observat... more ABSTRACT Introduction Framed within the European Project CITCLOPS (Citizens&amp;#39; Observatory for Coast and Ocean Optical Monitoring), the aim of this study is to present a number of tools that can be employed by citizens to estimate the color of natural waters. Firstly, a scale that accurately matches the original Forel-Ule (FU) colors was developed using accessible and affordable materials. This Modern FU scale is presented as a &amp;#39;Do-It-Yourself&amp;#39; kit that can be prepared using high-quality illumination filters and a frame made of a white Plexiglas (or other white material). Secondly, a smartphone application (APP) prototype that could be used by anyone willing to participate in environmental monitoring is presented. This application includes a digitalized color-comparator scale, simulating the colors of the original Forel-Ule scale, to be compared to the color of water bodies, and allows the observer to take a picture of the water body to calculate the FU number using a specific algorithm. It also offers an option to include a Secchi disk depth estimate and the Forel-Ule number obtained with the Modern FU scale, if the observer is in possession of these tools. The first inputs provided by selected volunteers and researchers, offer initial comparisons between the two monitoring tools, the Modern FU scale and the digital scale included in the smartphone application. The idea is to provide a water quality index appropriate for participatory science that allows for rapid estimates and interpretation of color changes occurring in the aquatic environment, and that could be used by local or global authorities as an assessing tool. Background

PLoS ONE, 2013

Marine primary productivity is an important agent in the global cycling of carbon dioxide, a majo... more Marine primary productivity is an important agent in the global cycling of carbon dioxide, a major 'greenhouse gas', and variations in the concentration of the ocean's phytoplankton biomass can therefore explain trends in the global carbon budget. Since the launch of satellite-mounted sensors globe-wide monitoring of chlorophyll, a phytoplankton biomass proxy, became feasible. Just as satellites, the Forel-Ule (FU) scale record (a hardly explored database of ocean colour) has covered all seas and oceans -but already since 1889. We provide evidence that changes of ocean surface chlorophyll can be reconstructed with confidence from this record. The EcoLight radiative transfer numerical model indicates that the FU index is closely related to chlorophyll concentrations in open ocean regions. The most complete FU record is that of the North Atlantic in terms of coverage over space and in time; this dataset has been used to test the validity of colour changes that can be translated to chlorophyll. The FU and FU-derived chlorophyll data were analysed for monotonously increasing or decreasing trends with the non-parametric Mann-Kendall test, a method to establish the presence of a consistent trend. Our analysis has not revealed a globe-wide trend of increase or decrease in chlorophyll concentration during the past century; ocean regions have apparently responded differentially to changes in meteorological, hydrological and biological conditions at the surface, including potential long-term trends related to global warming. Since 1889, chlorophyll concentrations have decreased in the Indian Ocean and in the Pacific; increased in the Atlantic Ocean, the Mediterranean, the Chinese Sea, and in the seas west and north-west of Japan. This suggests that explanations of chlorophyll changes over long periods should focus on hydrographical and biological characteristics typical of single ocean regions, not on those of 'the' ocean. Citation: Wernand MR, van der Woerd HJ, Gieskes WWC (2013) Trends in Ocean Colour and Chlorophyll Concentration from 1889 to 2000, Worldwide. PLoS ONE 8(6): e63766.

Deep Sea Research Part A. Oceanographic Research Papers, 1981

... Conseil Permanent International pour l'Exploration de la Mer, 38, 131-146. ... To keep t... more ... Conseil Permanent International pour l'Exploration de la Mer, 38, 131-146. ... To keep the errors minimal, one should decide whether to measure E+v = (Eo+E)/2 or E_v = (Eo-E)/2. Figure 6 displays variation of the function given by equation (2) for different radiance distributions in ...

Ocean Optics XIII, 1997

ABSTRACT An intensive data collection campaign has been conducted in the coastal waters of the so... more ABSTRACT An intensive data collection campaign has been conducted in the coastal waters of the southern North Sea during which a variety of marine optical parameters have ben measured. This has enabled the angular distribution factor of spectral radiance or Q factor to be calculated from in situ measurements. The Q factor relates upwelled spectral radiance to upwelled spectral irradiance and its precise determination is of importance in ocean color remote sensing. Previous modeling studies, often based on the optical properties of case I waters, have proposed a value of 5 for Q. However, this study suggest that more turbid coastal waters may approach the Lambertian case of Q close to (pi) .

Journal of the European Optical Society: Rapid Publications, 2010

François Alphonse Forel (1890) and Willi Ule (1892) composed a colour comparator scale, with tint... more François Alphonse Forel (1890) and Willi Ule (1892) composed a colour comparator scale, with tints varying from indigo-blue to cola brown, to quantify the colour of natural waters, like seas, lakes and rivers. For each measurement, the observer compares the colour of the water above a submersed white disc (Secchi disc) with a hand-held scale of pre-defined colours. The scale can be well reproduced from a simple recipe for twenty-one coloured chemical solutions and because the ease of its use, the Forel-Ule (FU) scale has been applied globally and intensively by oceanographers and limnologists from the year 1890. Indeed, the archived FU data belong to the oldest oceanographic data sets and do contain information on the changes in geobiophysical properties of natural waters during the last century. In this article, we describe the optical properties of the FU scale and its ability to cover the colours of natural waters, as observed by the human eye. The recipe of the scale and its reproduction is described. The spectral transmission of the tubes and their respective chromaticity coordinates are presented. The FU scale, in all its simplicity, is found to be an adequate ocean colour comparator scale. The scale is well characterized, stable and observations are reproducible. Thus, the large historic data sets of FU measurements are coherent and well calibrated. Moreover, the scale can be coupled to contemporary multi-spectral observations with hand-held and satellite-based spectrometers. A reintroduction of the FU scale is recommended to expand the historical database and to facilitate a tie-in with present satellite ocean colour observations by tranforming MERIS normalized multi-band reflectance image into a FU indexed image.

Journal of the European Optical Society: Rapid Publications, 2010

In this paper we present an analysis of historical ocean colour data from the North Pacific Ocean... more In this paper we present an analysis of historical ocean colour data from the North Pacific Ocean. This colour is described by the Forel-Ule colour index, a sea colour comparator scale that is composed of 21 tube colours that is routinely measured since the year 1890. The main objective of this research is to characterise colour changes of the North Pacific Ocean at a timescale of lustums. Next to the seasonal colour changes, due to the yearly cycle of biological activity, this time series between 1930 and 1999 might contain information on global changes in climate conditions. From seasonal independent analyses of the long-term variations it was found that the greenest values, with mean Forel-Ule scale, FU, of 4.1 were reached during the period of 1950-1954, with a second high (FU = 3) in the period 1980-1984. The bluest ocean was encountered during the years 1990-1994. The data indicate that after 1955 a remarkable long bluing took place till 1980.

Journal of the European Optical Society: Rapid Publications, 2010

The first records on regular, tabulated, measurements of transparency of natural waters are those... more The first records on regular, tabulated, measurements of transparency of natural waters are those performed by the German naturalist Adelbert von Chamisso during the Russian "Rurik" Expedition 1815-1818 under the command of Otto von Kotzebue. A standardized method to determine the water clarity (transparency) was adopted at the end of the nineteenth century. This method (lowering a white painted disc into the water until it disappeared out of sight) was described by Pietro Angelo Secchi in Il Nuovo Cimento and was published in 1865. The Austrian scientist Josef Roman Lorenz von Liburnau, experimenting with submersible objects, like white discs, in the Gulf of Quarnero (Croatia) in the eighteen-fifties, well before Secchi started his investigations, questioned the naming of the white disc. However, the experiments performed by Secchi and Cialdi in 1864 on such an intensive scale were never performed before. At the beginning of the twentieth century, water transparency observations by means of a 30 cm white disc, was named the Secchi-disc method.

ABSTRACT In this chapter the ideas of explorers and scientists on transparency and colouring issu... more ABSTRACT In this chapter the ideas of explorers and scientists on transparency and colouring issues of natural waters are sketched over a period between 1600 and 1930, from the time of Sir Francis Bacon to Chandrasekhara Raman. This history features famous scientists as well as various amateurs who were intrigued by these optical phenomena. We also discuss are a number of pieces of interesting equipment that were developed to quantify the intrinsically coupled transparency and colour of the sea. The Secchi disc (to determine water transparency) and the Forel–Ule scale (to classify sea and colour) are two, from the end of the nineteenth century, which have withstood the time. The hidden mechanisms for the colouring of water were only solved at the beginning of the twentieth century. It is argued that this delay was partially caused by the fact that the subject was mostly treated as marginal; also, if pure water distillation techniques had been fully developed earlier, this understanding could have been shifted backwards by decades. This chapter ends with Chandrasekhara Venkata Raman, who proved in 1922 that molecular scattering of light in water, in combination with the absorption of the longer wavelengths, caused the blue colour of the sea. In his Nobel lecture ‘On the colour of the sea’ on December 11, 1930 he explained his findings to the world.

Limnology and Oceanography: Methods, 2011

The accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult t... more The accuracy of remote sensing algorithms for phytoplankton biomass and physiology is difficult to test under natural conditions due to rapid changes in physical and biological forcings and the practical inability to manipulate nutrient conditions and phytoplankton composition in the sea. Therefore, an indoor mesocosm was designed to examine the optical properties of phytoplankton under controlled and manipulated conditions of irradiance, temperature, turbulence, and nutrient availability. Equipped with hyperspectral radiometers and bottom irradiance meters, it is shown that under semi-natural environmental conditions biogeochemically relevant species as Emiliania huxleyi and Phaeocystis globosa can be grown with good precision (± 20%) between duplicate mesocosms and between duplicate sensors (<5% deviation). The accuracy of chlorophyll estimates by absorption, using an Integrating Cavity Absorption Meter, and fluorescence using water-leaving radiance was 74% to 80%, respectively, as it was negatively influenced by changes in phytoplankton physiology. Biomass detection was limitedto 1 to 2 µg chlorophyll/L with an apparent linearity to 50 µg chlorophyll/L. Estimates of the quantum efficiency of fluorescence (j ≈ 0.01) were comparable to real-world estimates derived from satellite observations. It is concluded that the mesocosms adequately simulate natural conditions with sufficient accuracy and precision and that they offer an important tool in validating assumptions and hypotheses underlying remote sensing algorithms and models.

Netherlands Journal of Sea Research, 1989

International Journal of Remote Sensing, 1997

. Ground truth measurements are necessary for the validation of remotely sensed data. Rapid ship ... more . Ground truth measurements are necessary for the validation of remotely sensed data. Rapid ship or aircraft spectral measurements of the upwelling and downwelling (ir)radiance are needed to determine the reflectance of the water column as well, as to intercalibrate with satellite ...

Applied Optics, 1986

Using an optical multichannel analyzer, 168 downswelling irradiance spectra were obtained in vari... more Using an optical multichannel analyzer, 168 downswelling irradiance spectra were obtained in variable atmospheric solar, and cloud cover conditions, and with solar elevations varying between 30 and 60 deg, to retrieve the total spectra in the visible 400-720 nm range. The accuracy of the retrieval is found to be better than that of the determination of some ground truth parameters

The Forel-Ule colour comparator scale has been applied globally and intensively by oceanographers... more The Forel-Ule colour comparator scale has been applied globally and intensively by oceanographers and limnologists since the 19th century, providing one of the oldest oceanographic data sets. Present and future Forel-Ule classifications of global oceanic, coastal and continental waters can facilitate the interpretation of these long-term ocean colour data series and provide a connection between the present and the past that will be valuable for climate-related studies. Within the EC-funded project CITLOPS (Citizens' Observatory for Coast and Ocean Optical Monitoring), with its main goal to empower endusers, willing to employ community-based environmental monitoring, our aim is to digitalize the colours of the Forel-Ule scale to establish the colour of natural waters through smartphone imaging. The objective of this study was to reproduce the Forel-Ule scale following the original recipes, measure the transmission of the solutions and calculate the chromaticity coordinates of the scale as Wernand and Van der Woerd did in 2010, for the future development of a smartphone application. Some difficulties were encountered when producing the scale, so a protocol for its consistent reproduction was developed and is described in this study. Recalculated chromaticity coordinates are presented and compared to measurements conducted by former scientists. An error analysis of the spectral and colourimetric information shows negligible experimental errors.