Aspects of in situ angular scattering measurements in contrasting waters

Rapid changes are observed in oceanic and coastal environments around the world due to global temperatures increases, ocean acidification and changing weather patterns - anthropogenic climate change. These changes have large effects on the ecosystems of the ocean. In order to understand the effects and possibly mitigate their consequences, it is necessary to increase and improve the environmental monitoring of the ocean. Optical properties of natural waters within the visible spectrum is closely linked to properties of phytoplankton, the foundation of oceanic ecosystems, as well as other particles on the micrometer and sub-micrometer scale in the water mass. Optical measurements can thus give us valuable information about the particle content of the water and the state of the ecosystem. The volume scattering function (VSF) is a fundamental optical property describing how much light is scattered by a medium and in what direction the light is scattered. In natural waters, by far most of the light is scattered in the very forward direction, which makes it technically challenging to measure the VSF. The LISST-VSF is the first commercially available instrument for field measurements of the VSF over a large angular domain. To trust the measurements, it is important to validate the performance of instrument and identify any error sources, in particular the valid range of the instrument, given that scattering coefficients of natural waters can span three orders of magnitude. In this thesis, I have characterised LISST-VSF measurements using both in situ measurements of highly contrasting water types, controlled laboratory measurements, and Monte Carlo simulations of instrument geometry. Similar aspects have been investigated for the LISST-200X, which measures the VSF at angles 0.04-13˚ at 670 nm. In Paper I, these two instruments are calibrated and validated using polymer beads and in situ measurements spanning from clear waters on the North Pole to highly turbid glacial meltwater. The measurements demonstrated that due to instrument design, the LISST-200X only gives valid scattering measurements in moderate-to-turbid waters. The LISST-VSF gives valid measurements also in clear waters (however with a loss in precision), but is limited by multiple scattering errors in more turbid waters. Multiple scattering effects on LISST-VSF measurements are investigated in detail in in Paper II and III. For this purpose, a Monte Carlo simulation was developed and validated with experimental data, and subsequently used to simulate LISST-VSF measurements with Fournier-Forand and Henyey-Greenstein phase functions. We demonstrated that the multiple scattering can yield uncertainties of 10% when the scattering coefficient is 1 m-1, which significantly restricts the accurate measurement range of LISST-VSF. LISST-200X is less affected by this error due to its shorter path length. Scattering can be an error source for other optical measurements as well. In Paper IV, we attempt to correct in situ depth profiles of absorption coefficients measured with the ac-s instrument using VSF measurements collected with the LISST-VSF in coastal waters. We show that this method does not show a clear and consistent improvement over existing methods, which are simpler to use but make strong assumptions about absorption and scattering properties. The discrepancies in the VSF correction can be attributed to several different confounding factors, such as spatial variability and multiple scattering, which are exceedingly propagated to the corrected absorption values. Nevertheless, VSF measurements are found useful to analyze the scattering error. We show that the VSF between 5-90˚ can contribute significantly to the scattering error, depending on the phase function and the reflectance efficiency of the reflective tube. Moreover, by simulating the VSF wavelength-dependency using Mie theory, we show that particle sub-populations with diameters close to the wavelength can explain why scaling the scattering error with the scattering coefficient sometimes fails.Doktorgradsavhandlin

From Winter to Late Summer in the Northwestern Barents Sea Shelf: Impacts of Seasonal Progression of Sea Ice and Upper Ocean on Nutrient and Phytoplankton Dynamics

Strong seasonality is a key feature of high-latitude systems like the Barents Sea. While the interannual variability and long-term changes of the Barents Sea are well-documented, the seasonal progression of the physical and biological systems is less known, mainly due to poor accessibility of the seasonally ice-covered area in winter and spring. Here, we use an extensive set of physical and biological in situ observations from four scientific expeditions covering the seasonal progression from late winter to late summer 2021 in the northwestern Barents Sea, from fully ice-covered to ice-free conditions. We found that sea ice meltwater and the timing of ice-free conditions in summer shape the environment, controlling heat accumulation, light and nutrient availability, and biological activity vertically, seasonally, and meridionally. In March and May, the ocean north of the Polar Front was ice-covered and featured a deep mixed layer. Chlorophyll-a concentrations increased from March to May along with greater euphotic depth, indicating the beginning of the spring bloom despite the absence of surface layer stratification. By July and in September, sea ice meltwater created a shallow low-density surface layer that strengthened stratification. In open water, chlorophyll-a maxima were found at the base of this layer as surface nutrients were depleted, while in the presence of ice, maxima were closer to the surface. Solar heating and the thickness of the surface layer increased with the number of ice-free days. The summer data showed a prime example of an Arctic-like space-for-time seasonal variability in the key physical and biological patterns, with the summer situation progressing northwards following sea ice retreat. The amount of sea ice melt (local or imported) has a strong control on the conditions in the northwestern Barents Sea, and the conditions in late 2021 resembled pre-2010 Arctic-like conditions with high freshwater content and lower ocean heat content.acceptedVersio

Aspects of in situ angular scattering measurements in contrasting waters

Rapid changes are observed in oceanic and coastal environments around the world due to global temperatures increases, ocean acidification and changing weather patterns - anthropogenic climate change. These changes have large effects on the ecosystems of the ocean. In order to understand the effects and possibly mitigate their consequences, it is necessary to increase and improve the environmental monitoring of the ocean. Optical properties of natural waters within the visible spectrum is closely linked to properties of phytoplankton, the foundation of oceanic ecosystems, as well as other particles on the micrometer and sub-micrometer scale in the water mass. Optical measurements can thus give us valuable information about the particle content of the water and the state of the ecosystem. The volume scattering function (VSF) is a fundamental optical property describing how much light is scattered by a medium and in what direction the light is scattered. In natural waters, by far most of the light is scattered in the very forward direction, which makes it technically challenging to measure the VSF. The LISST-VSF is the first commercially available instrument for field measurements of the VSF over a large angular domain. To trust the measurements, it is important to validate the performance of instrument and identify any error sources, in particular the valid range of the instrument, given that scattering coefficients of natural waters can span three orders of magnitude. In this thesis, I have characterised LISST-VSF measurements using both in situ measurements of highly contrasting water types, controlled laboratory measurements, and Monte Carlo simulations of instrument geometry. Similar aspects have been investigated for the LISST-200X, which measures the VSF at angles 0.04-13˚ at 670 nm. In Paper I, these two instruments are calibrated and validated using polymer beads and in situ measurements spanning from clear waters on the North Pole to highly turbid glacial meltwater. The measurements demonstrated that due to instrument design, the LISST-200X only gives valid scattering measurements in moderate-to-turbid waters. The LISST-VSF gives valid measurements also in clear waters (however with a loss in precision), but is limited by multiple scattering errors in more turbid waters. Multiple scattering effects on LISST-VSF measurements are investigated in detail in in Paper II and III. For this purpose, a Monte Carlo simulation was developed and validated with experimental data, and subsequently used to simulate LISST-VSF measurements with Fournier-Forand and Henyey-Greenstein phase functions. We demonstrated that the multiple scattering can yield uncertainties of 10% when the scattering coefficient is 1 m-1, which significantly restricts the accurate measurement range of LISST-VSF. LISST-200X is less affected by this error due to its shorter path length. Scattering can be an error source for other optical measurements as well. In Paper IV, we attempt to correct in situ depth profiles of absorption coefficients measured with the ac-s instrument using VSF measurements collected with the LISST-VSF in coastal waters. We show that this method does not show a clear and consistent improvement over existing methods, which are simpler to use but make strong assumptions about absorption and scattering properties. The discrepancies in the VSF correction can be attributed to several different confounding factors, such as spatial variability and multiple scattering, which are exceedingly propagated to the corrected absorption values. Nevertheless, VSF measurements are found useful to analyze the scattering error. We show that the VSF between 5-90˚ can contribute significantly to the scattering error, depending on the phase function and the reflectance efficiency of the reflective tube. Moreover, by simulating the VSF wavelength-dependency using Mie theory, we show that particle sub-populations with diameters close to the wavelength can explain why scaling the scattering error with the scattering coefficient sometimes fails

Inherent optical properties of dissolved and particulate matter in an Arctic fjord (Storfjorden, Svalbard) in early summer

Lysforholdene i havet spiller en nøkkelrolle for marin primærproduksjon og dermed for det hele marine økosystemet. I polområdene er det spesielt stor sesongvariasjon i lysforholdene, og mange ulike prosesser kan påvirke lyset i vannsøylen, både under havisen og i kystnære farvann. Samtidig er havisen i drastisk endring. Ettersom havisen reflekterer mesteparten av sollyset tilbake ut i verdensrommet, blir det enda viktigere å forstå hva som påvirker lyset i vannsøylen uten havis, slik at dette kan bli bedre representert i økosystem- og klimamodeller. Denne studien er den første detaljerte studien om lysforhold i Storfjorden ved Svalbard på sommertid og om hva som påvirker strålingstransport i vannsøylen. Storfjorden har mye havisdannelse på vinteren, noe som bidrar til formasjon av kaldt bunnvann. Is- og bresmelting fører til sterk avrenning om sommeren, og til oppblomstring av planteplankton som kan bli påvirket av ulike havstrømmer. Alle disse faktorene påvirker strålingstransporten, men i ulik grad i forskjellige deler og vannlag av fjorden. I overflatelaget, med smeltevann fra havis og avrenning fra land, er det først og fremst avrenning fra land som påvirker strålingstransporten. Under overflatelaget blir de optiske egenskapene til vannet i stedet bestemt av planteplankton. I bunnen av fjorden, i vannlaget som er formet vinterstid fra havis-produksjon, plukker målingene også opp de optiske egenskapene til partikler som er virvlet opp fra havbunnen. De relativt kompliserte forholdene for strålingstransport i fjorden viser at det er vanskelig å forbedre representasjon i modeller. Samtidig viser studien også at optiske målinger kan gi ny og detaljert informasjon om vannmassene i polare kystområder.publishedVersio