Daniela Voß

Physical, Bio-Optical State and Correlations in North–Western European Shelf Seas

Color of seawater has become an integral tool in understanding surface marine ecosystems and processes. In this paper we seek to assess the correlations and consequently the potential of using shipborne remote sensing products to infer marine environmental parameters. Typical seawater parameters are chlorophyll–a (chl–a), colored dissolved organic material (CDOM), suspended particulate material (SPM), Secchi–disk depth (SDD), temperature, and salinity. These parameters and radiometric quantities were observed from a total of 60 stations covering German Bight, North Sea, Inner Seas, Irish Sea, and Celtic Sea. Bio-optical models developed in this study were used to predict the in situ measured parameters, with low mean unbiased percent differences and absolute percent difference less than 35%. Our investigations show that the use of ocean color products namely the Forel–Ule Index to infer seawater parameters is encouraging. The constrained spatial and temporal span of measured in situ parameters does limit the accuracy of our models. Absorption coefficients of the main color producing agents CDOM, chl–a, and inorganic fraction of SPM (iSPM) were determined to estimate absorption budgets. During the field campaign, iSPM was the primary light absorber over the spectral range (400–700 nm) although variabilities were observed in the regional seas.

Modern approaches to shipborne ocean color remote sensing

In this paper, modernized shipborne procedures are presented to collect and process above-water radiometry for remote sensing applications. A setup of five radiometers and a bidirectional camera system, which provides panoramic sea surface and sky images, is proposed for the collection of high-resolution radiometric quantities. Images from the camera system can be used to determine sky state and potential glint, whitecaps, or foam contamination. A peak in the observed remote sensing reflectance RRS spectra between 750–780xa0nm was typically found in spectra with relatively high surface reflected glint (SRG), which suggests this waveband could be a useful SRG indicator. Simplified steps for computing uncertainties in SRG corrected RRS are proposed and discussed. The potential of utilizing “unweighted multimodel averaging,” which is the average of four or more common SRG correction models, is examined to determine the best approximation RRS. This best approximation RRS provides an estimate of RRS based on various SRG correction models established using radiative transfer simulations and field investigations. Applying the average RRS provides a measure of the inherent uncertainties or biases that result from a user subjectively choosing any one SRG correction model. Comparisons between inherent and apparent optical property derived observations were used to assess the robustness of the SRG multimodel averaging approach. Correlations among the standard SRG models were completed to determine the degree of association or similarities between the SRG models. Results suggest that the choice of glint models strongly affects derived RRS values and can also influence the blue to green band ratios used for modeling biogeochemical parameters such as for chlorophyll a. The objective here is to present a uniform and traceable methodology for determining shipborne RRS measurements and its associated errors due to glint correction and to ensure the direct comparability of these measurements in future investigations. We encourage the ocean color community to publish radiometric field measurements with matching and complete metadata in open access repositories.

Classifying Natural Waters with the Forel-Ule Colour Index System: Results, Applications, Correlations and Crowdsourcing

Societal awareness of changes in the environment and climate has grown rapidly, and there is a need to engage citizens in gathering relevant scientific information to monitor environmental changes due to recognition that citizens are a potential source of critical information. The apparent colour of natural waters is one aspect of our aquatic environment that is easy to detect and an essential complementary optical water quality indicator. Here we present the results and explore the utility of the Forel-Ule colour index (FUI) scale as a proxy for different properties of natural waters. A FUI scale is used to distinguish the apparent colours of different natural surface water masses. Correlation analysis was completed in an effort to determine the constituents of natural waters related to FUI. Strong correlations with turbidity, Secchi-disk depth, and coloured dissolved organic material suggest the FUI is a good indicator of changes related to other constituents of water. The increase in the number of tools capable of determining the FUI colours, (i) ocean colour remote sensing products; (ii) a handheld scale; and (iii) a mobile device app, make it a versatile relative measure of water quality. It has the potential to provide higher spatial and temporal resolution of data for a modernized classification of optical water quality. This FUI colour system has been favoured by several scientists in the last century because it is affordable and easy to use and provides indicative information about the colour of water and the water constituents producing that colour. It is therefore within the scope of a growing interest in the application and usefulness of basic measurement methodologies with the potential to provide timely benchmark information about the environment to the public, scientists and policymakers.

Fjord light regime: Bio‐optical variability, absorption budget, and hyperspectral light availability in Sognefjord and Trondheimsfjord, Norway

Optically active constituents (OACs) in addition to water molecules attenuate light via processes of absorption and scattering and thereby determine underwater light availability. An analysis of their optical properties helps in determining the contribution of each of these to light attenuation. With an aim to study the bio-optical variability, absorption budget and 1% spectral light availability, hydrographical (temperature and salinity), and hyperspectral optical (downwelling irradiance and upwelling radiance) profiles were measured along fjord transects in Sognefjord and Trondheimsfjord, Norway. Optical water quality observations were also performed using Secchi disc and Forel-Ule scale. In concurrence, water samples were collected and analyzed via visible spectrophotometry, fluorometry, and gravimetry to quantify and derive inherent optical properties of the water constituents. An absorption model (R2u2009=u20090.91, nu2009=u200936, pu2009<u20090.05) as a function of OACs is developed for Sognefjord using multiple regression analysis. Influenced by glacial meltwater, Sognefjord had higher concentration of inorganic suspended matter, while Trondheimsfjord had higher concentrations of CDOM. Increase in turbidity caused increased attenuation of light upstream, as a result of which the euphotic depth decreased from outer to inner fjord sections. Triangular representation of absorption budget revealed dominant absorption by CDOM at 443–555 nm, while that by phytoplankton at 665 nm. Sognefjord however exhibited much greater optical complexity. A significantly strong correlation between salinity and acdom440 is used to develop an algorithm to estimate acdom440 using salinity in Trondheimsfjord.

Next generation fluorescence sensor with multiple excitation and emission wavelengths — NeXOS MatrixFlu-UV

Ocean health observations have been performed for hundreds of years. Optimizing the process of gathering information on status as well as changes of oceanographic parameters through research cruises is however still challenging and recently developed robotic opportunities need to be enhanced. Using mobile and autonomous platforms like gliders, buoys and surface platforms (here sailbuoys) is an innovative and cost-effective way to obtain large datasets even under unfavorable conditions. Developing suitable sensors, like the optical fluorescence sensor MatrixFlu-UV, which can be deployed on those platforms, is one of the major goals of “Next generation Low-Cost Multifunctional Web Enabled Ocean Sensor Systems Empowering Marine, Maritime and Fisheries Management” (NeXOS, a European Union funded project). The sensor concept of multiple excitation and emission configuration, its proof-of-principle and verification results for colored dissolved organic matter related chemical compound humic acid are represented. An extended concentration series, using sensor and reference detector in parallel, showed clearly a linear correlation (coefficient of determination of R2 = 0.99) between data measured with the sensor and the reference measurement. In combination with the other wavelengths excitation-emission combinations, the MatrixFlu-UV offers a true ultra-compact multi-parameter option for upcoming ocean observation activities.