Geir Johnsen | Norwegian University of Science and Technology (original) (raw)
Papers by Geir Johnsen
Frontiers in Marine Science, Mar 28, 2022
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Remote Sensing
We describe an Underwater Hyperspectral Imager (UHI) deployed on an instrument-carrying platform ... more We describe an Underwater Hyperspectral Imager (UHI) deployed on an instrument-carrying platform consisting of two interconnected mini-ROVs (Remotely Operated Vehicle) for the mapping and monitoring of Arctic macroalgal habitats in Kongsfjorden (Svalbard) during the Polar Night. The mini-ROV-UHI system is easy to transport, assemble and deploy from shore, even under the dark, icy and cold conditions of the Arctic Polar Night. The system can be operated by two persons, keeping the operational costs low. In vivo hyperspectral reflectance of collected specimens of brown, red and green macroalgae was measured with a spectrometer in the lab to provide a spectral library for supervised pigment group classification based on UHI photomosaics. The in situ UHI-photomosaics provided detailed information of the areal coverage of the seafloor substrate (16%), as well as brown (51% habitat cover), red (18%), and green (14%) macroalgae, with spatial resolution in the range of cm and spectral resol...
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PLoS Biology, 2021
Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is... more Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is central for processes such as vision and the entrainment of the circadian clock. The poles represent extreme light regimes with an annual light cycle including periods of Midnight Sun and Polar Night. The Arctic Ocean extends to the North Pole, and marine light extremes reach their maximum extent in this habitat. During the Polar Night, traditional definitions of day and night and seasonal photoperiod become irrelevant since there are only “twilight” periods defined by the sun’s elevation below the horizon at midday; we term this “midday twilight.” Here, we characterize light across a latitudinal gradient (76.5° N to 81° N) during Polar Night in January. Our light measurements demonstrate that the classical solar diel light cycle dominant at lower latitudes is modulated during Arctic Polar Night by lunar and auroral components. We therefore question whether this particular ambient light...
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Spectral light attenuation in the Barents Sea : Impact of pigment signature and relevance for opt... more Spectral light attenuation in the Barents Sea : Impact of pigment signature and relevance for optical depth and primary production
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In situ observations of pelagic fish and zooplankton with optical instruments usually rely on ext... more In situ observations of pelagic fish and zooplankton with optical instruments usually rely on external light sources. However, artificial light may attract or repulse marine organisms, which results in biased measurements. It is often assumed that most pelagic organisms do not perceive the red part of the visible spectrum and that red light can be used for underwater optical measurements of biological processes. Using hull-mounted echosounders above an acoustic probe or a baited video camera, each equipped with light sources of different colours (white, blue and red), we demonstrate that pelagic organisms in Arctic and temperate regions strongly avoid artificial light, including visible red light (575–700 nm), from instruments lowered in the water column. The density of organisms decreased by up to 99% when exposed to artificial light and the distance of avoidance varied from 23 to 94 m from the light source, depending on colours, irradiance levels and, possibly, species communities...
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Advances in Polar Ecology
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Advances in Polar Ecology
Microalgae have unique adaptions including low metabolic activity, utilization of lipid storage, ... more Microalgae have unique adaptions including low metabolic activity, utilization of lipid storage, and resting stage formation to survive the Polar Night. Some species are mixotrophic or heterotrophic and do survive periods that are not favorable for photosynthetic (autotrophic) growth, such as the Polar Night. In addition, the autotrophic and mixotrophic species seem to maintain the key components of the photosynthetic apparatus intact during the dark period, which allows them to resume growth rapidly once light comes back in spring. In contrast, some macroalgal species may act as “season anticipators” and utilize the winter darkness or early spring period as their major growth seasons. This chapter elucidates aspects of the ecology of micro- and macroalgae with a focus on the dark season. It is comprised of six parts and starts with an introduction (Sect. “Introduction”) about Arctic marine micro- and macroalgae. Section “The Key Abiotic Environmental Variables Related to Micro- and Macroalgae” reviews the key abiotic environmental variables related to micro- and macroalgal growth and survival. The seasonal development of the different groups of microalgae is described in Sect. “Microalgae”, comprising phytoplankton, microphytobenthos, and sea-ice algae. Section “Macroalgae” introduces the three classes of macroalgae (phaeo-, rhodo-, and chlorophytes) with information about biological variables, seasonal processes, and habitats. Section “Ecophysiology of Algae in the Polar Night” sheds light on the ecophysiology of microalgae and macroalgae in the Polar Night, using selected examples. The last Section “Conclusive Remarks” summarizes our current state of knowledge and provides some conclusions derived from it.
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IEEE Journal of Oceanic Engineering
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Applied Optics
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2021 11th Workshop on Hyperspectral Imaging and Signal Processing: Evolution in Remote Sensing (WHISPERS)
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Continental Shelf Research
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Remote Sensing
In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian... more In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian Hydrographic Service. Using (1) a REMUS 100 autonomous underwater vehicle (AUV) equipped with a sidescan sonar (SSS) and (2) a Seabotix LBV 200 mini-remotely operated vehicle (ROV) with a high-definition (HD) camera, the wreck was in 2015 identified as the Figaro: a floating whalery that sank in 1908. The Figaro is to our knowledge currently the northernmost wreck in the world to be investigated by archaeologists. As the wreck is protected by law as an underwater cultural heritage (UCH) site, only non-intrusive methods could be used during surveys. In this study, we demonstrate how using multiple complementary remote sensing techniques can be advantageous with respect to acquiring a holistic overview of a recently discovered wreck site. In January 2016, the wreck was revisited, and a full photogrammetric survey of the site was conducted with a Sperre Subfighter 7500 medium class ROV. In...
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Remote Sensing, 2020
In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian... more In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian Hydrographic Service. Using (1) a REMUS 100 autonomous underwater vehicle (AUV) equipped with a sidescan sonar (SSS) and (2) a Seabotix LBV 200 mini-remotely operated vehicle (ROV) with a high-definition (HD) camera, the wreck was in 2015 identified as the Figaro: a floating whalery that sank in 1908. The Figaro is to our knowledge currently the northernmost wreck in the world to be investigated by archaeologists. As the wreck is protected by law as an underwater cultural heritage (UCH) site, only non-intrusive methods could be used during surveys. In this study, we demonstrate how using multiple complementary remote sensing techniques can be advantageous with respect to acquiring a holistic overview of a recently discovered wreck site. In January 2016, the wreck was revisited, and a full photogrammetric survey of the site was conducted with a Sperre Subfighter 7500 medium class ROV. In...
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Frontiers in Marine Science
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Journal of Field Robotics
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Progress in Oceanography
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Remote Sensing
The impacts of human activity on coastal ecosystems are becoming increasingly evident across the ... more The impacts of human activity on coastal ecosystems are becoming increasingly evident across the world. Consequently, there is a growing need to map, monitor, and manage these regions in a sustainable manner. In this pilot study, we present what we believe to be a novel mapping technique for shallow-water seafloor habitats: Underwater hyperspectral imaging (UHI) from an unmanned surface vehicle (USV). A USV-based UHI survey was carried out in a sheltered bay close to Trondheim, Norway. In the survey, an area of 176 m2 was covered, and the depth of the surveyed area was approximately 1.5 m. UHI data were initially recorded at a 1-nm spectral resolution within the range of 380–800 nm, but this was reduced to 86 spectral bands between 400-700 nm (3.5-nm spectral resolution) during post-processing. The hyperspectral image acquisition was synchronized with navigation data from the USV, which permitted georeferencing and mosaicking of the imagery at a 0.5-cm spatial resolution. Six spectr...
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Scientific Reports
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Frontiers in Marine Science, Mar 28, 2022
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Remote Sensing
We describe an Underwater Hyperspectral Imager (UHI) deployed on an instrument-carrying platform ... more We describe an Underwater Hyperspectral Imager (UHI) deployed on an instrument-carrying platform consisting of two interconnected mini-ROVs (Remotely Operated Vehicle) for the mapping and monitoring of Arctic macroalgal habitats in Kongsfjorden (Svalbard) during the Polar Night. The mini-ROV-UHI system is easy to transport, assemble and deploy from shore, even under the dark, icy and cold conditions of the Arctic Polar Night. The system can be operated by two persons, keeping the operational costs low. In vivo hyperspectral reflectance of collected specimens of brown, red and green macroalgae was measured with a spectrometer in the lab to provide a spectral library for supervised pigment group classification based on UHI photomosaics. The in situ UHI-photomosaics provided detailed information of the areal coverage of the seafloor substrate (16%), as well as brown (51% habitat cover), red (18%), and green (14%) macroalgae, with spatial resolution in the range of cm and spectral resol...
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PLoS Biology, 2021
Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is... more Light plays a fundamental role in the ecology of organisms in nearly all habitats on Earth and is central for processes such as vision and the entrainment of the circadian clock. The poles represent extreme light regimes with an annual light cycle including periods of Midnight Sun and Polar Night. The Arctic Ocean extends to the North Pole, and marine light extremes reach their maximum extent in this habitat. During the Polar Night, traditional definitions of day and night and seasonal photoperiod become irrelevant since there are only “twilight” periods defined by the sun’s elevation below the horizon at midday; we term this “midday twilight.” Here, we characterize light across a latitudinal gradient (76.5° N to 81° N) during Polar Night in January. Our light measurements demonstrate that the classical solar diel light cycle dominant at lower latitudes is modulated during Arctic Polar Night by lunar and auroral components. We therefore question whether this particular ambient light...
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Spectral light attenuation in the Barents Sea : Impact of pigment signature and relevance for opt... more Spectral light attenuation in the Barents Sea : Impact of pigment signature and relevance for optical depth and primary production
Bookmarks Related papers MentionsView impact
In situ observations of pelagic fish and zooplankton with optical instruments usually rely on ext... more In situ observations of pelagic fish and zooplankton with optical instruments usually rely on external light sources. However, artificial light may attract or repulse marine organisms, which results in biased measurements. It is often assumed that most pelagic organisms do not perceive the red part of the visible spectrum and that red light can be used for underwater optical measurements of biological processes. Using hull-mounted echosounders above an acoustic probe or a baited video camera, each equipped with light sources of different colours (white, blue and red), we demonstrate that pelagic organisms in Arctic and temperate regions strongly avoid artificial light, including visible red light (575–700 nm), from instruments lowered in the water column. The density of organisms decreased by up to 99% when exposed to artificial light and the distance of avoidance varied from 23 to 94 m from the light source, depending on colours, irradiance levels and, possibly, species communities...
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Bookmarks Related papers MentionsView impact
Advances in Polar Ecology
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Advances in Polar Ecology
Microalgae have unique adaptions including low metabolic activity, utilization of lipid storage, ... more Microalgae have unique adaptions including low metabolic activity, utilization of lipid storage, and resting stage formation to survive the Polar Night. Some species are mixotrophic or heterotrophic and do survive periods that are not favorable for photosynthetic (autotrophic) growth, such as the Polar Night. In addition, the autotrophic and mixotrophic species seem to maintain the key components of the photosynthetic apparatus intact during the dark period, which allows them to resume growth rapidly once light comes back in spring. In contrast, some macroalgal species may act as “season anticipators” and utilize the winter darkness or early spring period as their major growth seasons. This chapter elucidates aspects of the ecology of micro- and macroalgae with a focus on the dark season. It is comprised of six parts and starts with an introduction (Sect. “Introduction”) about Arctic marine micro- and macroalgae. Section “The Key Abiotic Environmental Variables Related to Micro- and Macroalgae” reviews the key abiotic environmental variables related to micro- and macroalgal growth and survival. The seasonal development of the different groups of microalgae is described in Sect. “Microalgae”, comprising phytoplankton, microphytobenthos, and sea-ice algae. Section “Macroalgae” introduces the three classes of macroalgae (phaeo-, rhodo-, and chlorophytes) with information about biological variables, seasonal processes, and habitats. Section “Ecophysiology of Algae in the Polar Night” sheds light on the ecophysiology of microalgae and macroalgae in the Polar Night, using selected examples. The last Section “Conclusive Remarks” summarizes our current state of knowledge and provides some conclusions derived from it.
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IEEE Journal of Oceanic Engineering
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Applied Optics
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2021 11th Workshop on Hyperspectral Imaging and Signal Processing: Evolution in Remote Sensing (WHISPERS)
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Continental Shelf Research
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Remote Sensing
In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian... more In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian Hydrographic Service. Using (1) a REMUS 100 autonomous underwater vehicle (AUV) equipped with a sidescan sonar (SSS) and (2) a Seabotix LBV 200 mini-remotely operated vehicle (ROV) with a high-definition (HD) camera, the wreck was in 2015 identified as the Figaro: a floating whalery that sank in 1908. The Figaro is to our knowledge currently the northernmost wreck in the world to be investigated by archaeologists. As the wreck is protected by law as an underwater cultural heritage (UCH) site, only non-intrusive methods could be used during surveys. In this study, we demonstrate how using multiple complementary remote sensing techniques can be advantageous with respect to acquiring a holistic overview of a recently discovered wreck site. In January 2016, the wreck was revisited, and a full photogrammetric survey of the site was conducted with a Sperre Subfighter 7500 medium class ROV. In...
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Remote Sensing, 2020
In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian... more In 2007, a possible wreck site was discovered in Trygghamna, Isfjorden, Svalbard by the Norwegian Hydrographic Service. Using (1) a REMUS 100 autonomous underwater vehicle (AUV) equipped with a sidescan sonar (SSS) and (2) a Seabotix LBV 200 mini-remotely operated vehicle (ROV) with a high-definition (HD) camera, the wreck was in 2015 identified as the Figaro: a floating whalery that sank in 1908. The Figaro is to our knowledge currently the northernmost wreck in the world to be investigated by archaeologists. As the wreck is protected by law as an underwater cultural heritage (UCH) site, only non-intrusive methods could be used during surveys. In this study, we demonstrate how using multiple complementary remote sensing techniques can be advantageous with respect to acquiring a holistic overview of a recently discovered wreck site. In January 2016, the wreck was revisited, and a full photogrammetric survey of the site was conducted with a Sperre Subfighter 7500 medium class ROV. In...
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Frontiers in Marine Science
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Journal of Field Robotics
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Progress in Oceanography
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Remote Sensing
The impacts of human activity on coastal ecosystems are becoming increasingly evident across the ... more The impacts of human activity on coastal ecosystems are becoming increasingly evident across the world. Consequently, there is a growing need to map, monitor, and manage these regions in a sustainable manner. In this pilot study, we present what we believe to be a novel mapping technique for shallow-water seafloor habitats: Underwater hyperspectral imaging (UHI) from an unmanned surface vehicle (USV). A USV-based UHI survey was carried out in a sheltered bay close to Trondheim, Norway. In the survey, an area of 176 m2 was covered, and the depth of the surveyed area was approximately 1.5 m. UHI data were initially recorded at a 1-nm spectral resolution within the range of 380–800 nm, but this was reduced to 86 spectral bands between 400-700 nm (3.5-nm spectral resolution) during post-processing. The hyperspectral image acquisition was synchronized with navigation data from the USV, which permitted georeferencing and mosaicking of the imagery at a 0.5-cm spatial resolution. Six spectr...
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Scientific Reports
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