Geir Johnsen | Norwegian University of Science and Technology (original) (raw)

Papers by Geir Johnsen

Research paper thumbnail of Remote Sensing of the Tautra Ridge: An Overview of the World’s Shallowest Cold-Water Coral Reefs

Frontiers in Marine Science, Mar 28, 2022

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Research paper thumbnail of Underwater Hyperspectral Imaging of Arctic Macroalgal Habitats during the Polar Night Using a Novel Mini-ROV-UHI Portable System

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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Research paper thumbnail of Berge 2015 Current Biol

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Research paper thumbnail of Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night

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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Research paper thumbnail of Spectral light attenuation in the Barents Sea : Impact of pigment signature and relevance for optical depth and primary production

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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Research paper thumbnail of Pelagic organisms avoid white, blue, and red artificial light from scientific instruments

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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Research paper thumbnail of Phytoplankton Pigments: Preface

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Research paper thumbnail of POLAR NIGHT Marine Ecology

Advances in Polar Ecology

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Research paper thumbnail of Marine Micro- and Macroalgae in the Polar Night

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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Research paper thumbnail of A Methodology for Consistent Georegistration in Underwater Hyperspectral Imaging

IEEE Journal of Oceanic Engineering

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Research paper thumbnail of All-sky camera system providing high temporal resolution annual time series of irradiance in the Arctic

Applied Optics

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Research paper thumbnail of Analysis and Model Development of Direct Hyperspectral Chlorophyll-A Estimation for Remote Sensing Satellites

2021 11th Workshop on Hyperspectral Imaging and Signal Processing: Evolution in Remote Sensing (WHISPERS)

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Research paper thumbnail of Phytoplankton community succession and dynamics using optical approaches

Continental Shelf Research

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Research paper thumbnail of Mapping the Historical Shipwreck Figaro in the High Arctic Using Underwater Sensor-Carrying Robots

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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Research paper thumbnail of Mapping the Historical Shipwreck Figaro in the High Arctic Using Underwater Sensor-Carrying Robots

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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Research paper thumbnail of Algal Colonization of Young Arctic Sea Ice in Spring

Frontiers in Marine Science

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Research paper thumbnail of Information-driven robotic sampling in the coastal ocean

Journal of Field Robotics

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Research paper thumbnail of Physical controls on phytoplankton size structure, photophysiology and suspended particles in a Norwegian biological hotspot

Progress in Oceanography

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Research paper thumbnail of Shallow-Water Habitat Mapping using Underwater Hyperspectral Imaging from an Unmanned Surface Vehicle: A Pilot Study

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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Research paper thumbnail of Underwater hyperspectral imaging as an in situ taxonomic tool for deep-sea megafauna

Scientific Reports

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Research paper thumbnail of Remote Sensing of the Tautra Ridge: An Overview of the World’s Shallowest Cold-Water Coral Reefs

Frontiers in Marine Science, Mar 28, 2022

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Research paper thumbnail of Underwater Hyperspectral Imaging of Arctic Macroalgal Habitats during the Polar Night Using a Novel Mini-ROV-UHI Portable System

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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Research paper thumbnail of Berge 2015 Current Biol

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Research paper thumbnail of Photophysiological cycles in Arctic krill are entrained by weak midday twilight during the Polar Night

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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Research paper thumbnail of Spectral light attenuation in the Barents Sea : Impact of pigment signature and relevance for optical depth and primary production

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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Research paper thumbnail of Pelagic organisms avoid white, blue, and red artificial light from scientific instruments

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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Research paper thumbnail of Phytoplankton Pigments: Preface

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Research paper thumbnail of POLAR NIGHT Marine Ecology

Advances in Polar Ecology

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Marine Micro- and Macroalgae in the Polar Night

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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Research paper thumbnail of A Methodology for Consistent Georegistration in Underwater Hyperspectral Imaging

IEEE Journal of Oceanic Engineering

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Research paper thumbnail of All-sky camera system providing high temporal resolution annual time series of irradiance in the Arctic

Applied Optics

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Research paper thumbnail of Analysis and Model Development of Direct Hyperspectral Chlorophyll-A Estimation for Remote Sensing Satellites

2021 11th Workshop on Hyperspectral Imaging and Signal Processing: Evolution in Remote Sensing (WHISPERS)

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Phytoplankton community succession and dynamics using optical approaches

Continental Shelf Research

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Research paper thumbnail of Mapping the Historical Shipwreck Figaro in the High Arctic Using Underwater Sensor-Carrying Robots

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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Research paper thumbnail of Mapping the Historical Shipwreck Figaro in the High Arctic Using Underwater Sensor-Carrying Robots

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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Research paper thumbnail of Algal Colonization of Young Arctic Sea Ice in Spring

Frontiers in Marine Science

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Research paper thumbnail of Information-driven robotic sampling in the coastal ocean

Journal of Field Robotics

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Physical controls on phytoplankton size structure, photophysiology and suspended particles in a Norwegian biological hotspot

Progress in Oceanography

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Shallow-Water Habitat Mapping using Underwater Hyperspectral Imaging from an Unmanned Surface Vehicle: A Pilot Study

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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Research paper thumbnail of Underwater hyperspectral imaging as an in situ taxonomic tool for deep-sea megafauna

Scientific Reports

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