Figure 1 - uploaded by Marcel R. Wernand
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1-An example of six differently coloured sea areas with from left to right top: Central Atlantic, Central North Sea, Coastal North Sea, left to right bottom; Coastal North Sea during algal bloom, Wadden Sea with lots of re-suspension of sediment and a coastal outlet dominated by coloured dissolved organic matter (photos: Courtesy of Bert Aggenbach and Annelies Hommersom and by the author himself). 

1-An example of six differently coloured sea areas with from left to right top: Central Atlantic, Central North Sea, Coastal North Sea, left to right bottom; Coastal North Sea during algal bloom, Wadden Sea with lots of re-suspension of sediment and a coastal outlet dominated by coloured dissolved organic matter (photos: Courtesy of Bert Aggenbach and Annelies Hommersom and by the author himself). 

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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...

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... Water color is an important and distinct indicator of water quality. The different colors of natural water bodies have been described for centuries, with records tracing back to writings of the 16th century and paintings of the 18th century [4]. Since the 19th century, a color comparator scale called Forel-Ule was adopted by oceanographers to record water color data [5]. ...
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Developments in digital image acquisition technologies and citizen science lead to more water color observations and broader public participation in environmental monitoring. However, the implications of the use of these simple water color indices for water quality assessment have not yet been fully evaluated. In this paper, we build a low-cost digital camera colorimetry setup to investigate quantitative relationships between water color indices and concentrations of optically active constituents (OACs). As proxies for colored dissolved organic matter (CDOM) and phytoplankton, humic acid and algae pigments were used to investigate the relationship between water chromaticity and concentration. We found that the concentration fits an ascending relationship with xy chromaticity values and a descending relationship with hue angle. Our investigations permitted us to increase the information content of simple water color observations, by relating them to chemical constituent concentrations in observed waters.
... Based on the Forel-Ule Scale, the realistic water color is divided into 21 color levels from dark blue to red-brown [20]. It can be derived from ocean color remote sensing products, including remote sensing reflectance (R rs ) or water leaving radiance [21][22][23]. Wernand and Novoa constructed a complete FUI index CIE chromaticity in 2012 [24]. Van der Woerd and Wernand calculated the reflectance values of different bands in the visible light of different spectrometers and then calculated the color parameters by integral. ...
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Eutrophication is considered to be a significant threat to estuaries and coastal waters. Various localized studies on the world’s oceans have recognized and confirmed that the Forel-Ule Color Index (FUI) or optical measurements are proportional to several water quality variables based on the relatively clear Chl-a-based waters. However, the application potential of FUI in the turbid estuary with complex optics has not been explored. In this study, we selected the coastal waters in the northern Liaodong Bay as the study area, using the field hyperspectral reflectances (Rrs) collected in 2018 to correct the hue angle and verify the Sentinel-2 images algorithm of FUI by in situ FUI in 2019–2020. The results show that there is a good agreement (R2 = 0.81, RMSE = 1.32, MAPE = 1.25%). Trophic Level Index (TLI) was used to evaluate the eutrophication status. The relationship between the in situ FUI and TLI collected in 2018 was discussed based on the difference in the dominant components of waters, while a number of non-algae suspended solids in the estuaries and coastal waters led to the overestimation of eutrophication based on FUI. The R(560)–R(704) (when FUI is between 11 and 15) and R(665)/R(704) (when FUI is between 19 and 21) was employed to distinguish total suspended matter (TSM)-dominated systems in the FUI-based eutrophication assessment. Based on the analysis, a new approach to assessing the eutrophication of coastal waters in Liaodong Bay was developed, which proved to have good accuracy by the field data in 2019 and 2020 (accuracy is 79%). Finally, we used Sentinel-2 images from Google Earth from 2019 to 2020 and locally processed data from 2018 to analyze the FUI spatial distribution and spatial and temporal statistics of the trophic status in the northern Liaodong Bay. The results show that the northern Liaodong Bay always presented the distribution characteristics of high inshore and low outside, high in the southeast and low in the northwest. The nutrient status is the worst in spring and summer.
... Owing to water's spectral selective absorption, blue light penetrates much deeper than green and red light in the ocean [24] and this behavior was primarily quantified since the development of the Forel-Ule Index (FUI) in the 1890s. The FUI system gauges water quality based on the observation of water color [25], which effectively divided natural waters into 21 classes of color (recently expanded to 22 classes [26]), with super blue water as Class 1 and brown water as Class 21 [27]. A general relationship exists where blue water has very low concentrations of suspended and dissolved materials, while brown water is richer in suspended sediments [28]. ...
Article
For the first time the vertical variation in light quality in the global ocean is quantified with a single parameter--the hue angle (αE, in degree) in chromaticity of downwelling irradiance. For oceanic waters, αE is ~140° at surface, but it becomes ~230° at the bottom of the euphotic zone; αE changes rapidly near the surface, and we term this layer of rapid change in light quality as chromocline, analogous to the thermocline or pycnocline in oceanography. The 3-D variations in light quality are further highlighted with data from satellite ocean color measurements, where global distributions of αE for depths of 99%, 37%, and 1% of surface photosynthetically available radiation (PAR) are presented. As an example to demonstrate the importance to consider the change in light quality, the relationship between the light quality and the ratio of phytoplankton absorbed light to PAR is presented where this ratio may vary by a factor of 3 or more under different chlorophyll-a concentrations; otherwise, the ratio would be constant vertically. We advocate quantitative measurement and report the light quality in the upper ocean with such a single and objective parameter to accompany the routine measurement and report the light intensity, which will greatly improve our understanding of light-related processes and further bridge ocean optics and oceanography.
... The origins of the discipline are challenging to pinpoint, as it arose from a merging of technology and research coming from limnology and oceanography, as well as building from advances made in terrestrial, atmospheric, and cryospheric remote sensing. Wernand (2011) provides a comprehensive overview of the scientists, their hypothesis and experiments in the historical development of hydrologic optics since the seventeenth century that culminated in the theoretical foundations provided by Raman (Raman, 1922) and Shoulejkin (Shoulejkin, 1923). Field experiments were also conducted at that time, such as those done by the limnologist Edison Pettit who conducted spectroscopic investigations into the volume scattering and absorption processes underlying the color of Crater Lake, OR, USA (Pettit, 1936). ...
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Intensifying pressure on global aquatic resources and services due to population growth and climate change is inspiring new surveying technologies to provide science-based information in support of management and policy strategies. One area of rapid development is hyperspectral remote sensing: imaging across the full spectrum of visible and infrared light. Hyperspectral imagery contains more environmentally meaningful information than panchromatic or multispectral imagery and is poised to provide new applications relevant to society, including assessments of aquatic biodiversity, habitats, water quality, and natural and anthropogenic hazards. To aid in these advances, we provide resources relevant to hyperspectral remote sensing in terms of providing the latest reviews, databases, and software available for practitioners in the field. We highlight recent advances in sensor design, modes of deployment, and image analysis techniques that are becoming more widely available to environmental researchers and resource managers alike. Systems recently deployed on space- and airborne platforms are presented, as well as future missions and advances in unoccupied aerial systems (UAS) and autonomous in-water survey methods. These systems will greatly enhance the ability to collect interdisciplinary observations on-demand and in previously inaccessible environments. Looking forward, advances in sensor miniaturization are discussed alongside the incorporation of citizen science, moving toward open and FAIR (findable, accessible, interoperable, and reusable) data. Advances in machine learning and cloud computing allow for exploitation of the full electromagnetic spectrum, and better bridging across the larger scientific community that also includes biogeochemical modelers and climate scientists. These advances will place sophisticated remote sensing capabilities into the hands of individual users and provide on-demand imagery tailored to research and management requirements, as well as provide critical input to marine and climate forecasting systems. The next decade of hyperspectral aquatic remote sensing is on the cusp of revolutionizing the way we assess and monitor aquatic environments and detect changes relevant to global communities.
... The attenuation depends on the composition of water that may vary over time even at the same geo-location [74]. The optical properties can be divided into inherent and apparent. ...
... The complete McGlamery-Jaffe model involves camera parameters and water properties, such as the volume scattering functions and the point spread function that governs scattering in F. The water property information is particularly difficult to obtain as they are constantly changing even at the same geo-location [74] and can only be measured with specialised devices [5], hence this model is mainly used in underwater navigation or monitoring where the devices are readily available [5]. The Schechner-Karpel model considers how the un-attenuated reflected light (effective scene radiance, J) is degraded along the objects' distance from the camera, encoded in the range map z [8]. ...
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Underwater images are degraded by the selective attenuation of light that distorts colours and reduces contrast. The degradation extent depends on the water type, the distance between an object and the camera, and the depth under the water surface the object is at. Underwater image filtering aims to restore or to enhance the appearance of objects captured in an underwater image. Restoration methods compensate for the actual degradation, whereas enhancement methods improve either the perceived image quality or the performance of computer vision algorithms. The growing interest in underwater image filtering methods--including learning-based approaches used for both restoration and enhancement--and the associated challenges call for a comprehensive review of the state of the art. In this paper, we review the design principles of filtering methods and revisit the oceanology background that is fundamental to identify the degradation causes. We discuss image formation models and the results of restoration methods in various water types. Furthermore, we present task-dependent enhancement methods and categorise datasets for training neural networks and for method evaluation. Finally, we discuss evaluation strategies, including subjective tests and quality assessment measures. We complement this survey with a platform ( https://puiqe.eecs.qmul.ac.uk/ ), which hosts state-of-the-art underwater filtering methods and facilitates comparisons.
... The reason why we use the Secchi disc together is to enhance the color of seawater, and make it easier to compare it to the scale (Wernand 2011;Pitarch 2017). To remove the effect of sun glare, the use of a black umbrella is also recommended. ...
Chapter
Ecologists study the interactions between organisms and their physicochemical and biotic environment. This spans biochemical, cellular and individual organism processes, as well as biological community and ecosystem levels of organization. Rocky shores have long been a testing ground for ecological theory as they are easily accessible and have strong gradients in abiotic and biotic conditions. A major branch of this field is the population ecology of commercially important species. Marine ecologists also study how species interact within seascapes and how energy and matter flow through ecosystems. This introduction explains the terms used to describe the zonation of marine life and provides an overview of the approaches available to investigate the ecology of Japanese waters where there are wide seasonal changes in temperature and an exceptionally high diversity of algae, plants, and animals.
... The reason why we use the Secchi disc together is to enhance the color of seawater, and make it easier to compare it to the scale (Wernand 2011;Pitarch 2017). To remove the effect of sun glare, the use of a black umbrella is also recommended. ...
Chapter
The marine environmental sciences lay at the interface between physics, chemistry and biology. Training in environmental science techniques provides skills that are highly transferable to the workplace and beyond. This subject typically involves collecting environmental data, collating it and then creating graphs and text to explain the key results. Employers within and outside the marine sciences actively seek people with these skills. Environmental scientists build an ability to conduct risk assessments, prepare reagents, calibrate instruments and design sampling protocols. You learn to plan and assess how to most effectively use your own time and to work in teams to decide upon what to measure to meet a particular set of objectives. By evaluating an environmental problem and honing a set of observations you will use the time-management and communication skills that are required for advising on policy or having input to evidence-based decision-making be it in environmental assessments or in running large organizations.
... The reason why we use the Secchi disc together is to enhance the color of seawater, and make it easier to compare it to the scale (Wernand 2011;Pitarch 2017). To remove the effect of sun glare, the use of a black umbrella is also recommended. ...
Chapter
Life on Earth began in the sea which covers 71% of our planet’s surface and its algae have produced half of the atmospheric oxygen that we breathe. Under the influence of worldwide water currents, marine organisms are distributed as neuston, plankton, nekton, or benthos. This chapter summarizes the major horizontal and vertical gradients in chemical and physical conditions that determine ocean productivity of the sea which is driven by sunlight and algae. Seawater is a dense, viscous medium and so marine life has an array of adaptations to take advantage of this environment. We introduce the different marine phyla present, which have a far greater biodiversity than terrestrial fauna. This introduction explains the highly interdisciplinary nature of marine biology and demonstrates that pioneering research in the life sciences continues to use marine organisms, including evolutionary biology, molecular biology, developmental biology and physiology. The islands of Japan have attracted marine scientists worldwide due to an exceptional variety of conditions (from tropical corals to high latitude kelp forests, abyssal deep sea to shallow lagoons). This gives the region an unusually rich flora and fauna coupled by highly productive fisheries. This overview sets the scene for our book on Japanese marine life.
... The reason why we use the Secchi disc together is to enhance the color of seawater, and make it easier to compare it to the scale (Wernand 2011;Pitarch 2017). To remove the effect of sun glare, the use of a black umbrella is also recommended. ...
Chapter
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In the seventeenth century, Hugo Grotius developed the doctrine of the ‘freedom of the seas’, arguing that the ocean bounty was vast enough to share without ownership. However, the human population has trebled since 1950 with much of the recent growth located in coastal regions where we are witnessing a profound transformation of our relationship with the natural world. Over that time fertilizer consumption has increased from 40 to 280 million tonnes a year, quadrupling inputs of nitrogen to the coastal zone. Motor vehicle use is up from 30 million in the 1950s to 750 million vehicles on the road, and international tourism has risen from <1 million international arrivals of people per year to 600 million today. Our use of natural resources is accelerating and this, coupled with poor management, means the planet has entered a phase of mass extinction with widespread biodiversity loss. Within a generation, fishing using fossil fuels has removed large fish from ecosystems and homogenized continental shelf habitats, with extensive damage even on remote seamounts. This chapter sets out the scale of some of the challenges we face in managing the ways in which humans impact the oceans.
... The measurement lends itself particularly well to citizen science programs (Busch et al., 2016;Garcia-Soto et al., 2017) which create extensive data sets at low cost. There is also a large archive of Secchi disk measurements, including some time series stretching back several decades, unmatched by other types of water quality data (Gallegos et al., 2011;Kratzer et al., 2003;Sanden & Hakansson, 1996;Wernand, 2011). ...
Article
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In the classical theory of the Secchi disk depth, diffuse sunlight falling on the disk is reflected back to the observer's eye along the most direct route, as a beam. The disappearance depth, ZSD, of the disk is then expected to vary inversely with the sum of the beam and diffuse attenuation coefficients: c + KD. Observations presented here show that, in the most turbid waters sampled, the Secchi disk is visible at greater depths (by a factor of up to 4) than predicted by this theory. In these conditions, the disk appears blurry, and it seems likely that some of the light reflected by the disk returns to the eye as diffuse light, photons being scattered one or more times on their journey from the disk surface to the observer. We have modified the theory of the Secchi disk in turbid water to allow for a mixture of beamed and diffuse light contributing to disk visibility. The modified theory corrects the under‐estimate of Secchi depths in turbid waters and gives good agreement with observations over a wide range of turbidity. The insight gained allows a more informed interpretation of Secchi disk measurements in turbid water.