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Koestner, D, Stramski D, Reynolds RA.  2019.  Assessing the effects of particle size and composition on light scattering through measurements of size-fractionated seawater samples. Limnology and Oceanography. 64   10.1002/lno.11259   Abstract

Measurements of the particulate volume scattering function [β_p(ψ)] at light wavelength of 532 nm, particle size distribution (PSD), and several metrics of particulate concentration and composition were made on eight contrasting seawater samples from near-shore and coastal oceanic environments including river estuary and offshore locations. Both β_p(ψ) and PSDs were measured on original (unfiltered) samples and particle-size fractionated samples obtained through filtration using mesh filters with pore sizes of 5 and 20 micrometers. We present results based on direct size-fractionated measurements and data adjusted for imperfect fractionation, which provide insights into the roles played by particle size and composition in angle-resolved light scattering produced by highly variable natural assemblages of aquatic particles. Despite intricate interplay between the effects of particle size and composition, small particles (< 5 micrometers in size) consistently produced a major or dominant contribution (~50–80%) to particulate backscattering coefficient, b_bp, in organic, either phytoplankton or non-algal, dominated samples regardless of significant variations in PSD between these samples. The notable exception was a sample dominated by large-celled diatoms from microphytoplankton size range, which exemplifies a scenario when large particles (> 20 micrometers) can produce a considerable contribution (~40%) to b_bp. We also observed a trend for inorganic-dominated samples exhibiting consistently weaker contributions (~30−40%) of small particles to b_bp. The particle size-based budget for the particulate scattering coefficient, b_p, indicates a significant decrease in the role of small particles accompanied by an increase in the role of larger particles compared to the b_bp budget.

Reynolds, RA, Stramski D.  2019.  Optical characterization of marine phytoplankton assemblages within surface waters of the western Arctic Ocean. Limnology and Oceanography. 64   10.1002/lno.11199   Abstract

An extensive dataset of measurements within the Chukchi and Beaufort Seas is used to characterize the optical properties of seawater associated with different phytoplankton communities. Hierarchical cluster analysis of diagnostic pigment concentrations partitioned stations into four distinct surface phytoplankton communities based on taxonomic composition and average cell size. Concurrent optical measurements of spectral absorption and backscattering coefficients and remote-sensing reflectance were used to characterize the magnitudes and spectral shapes of seawater optical properties associated with each phytoplankton assemblage. The results demonstrate measurable differences among communities in the average spectral shapes of the phytoplankton absorption coefficient. Similar or greater differences were also observed in the spectral shapes of non-phytoplankton absorption coefficients and the particulate backscattering coefficient. Phytoplankton on average, however, contributed only 25% or less to the total absorption coefficient of seawater. Our analyses indicate that the interplay between the magnitudes and relative contributions of all optically significant constituents generally dampens any influence of varying phytoplankton absorption spectral shapes on the total absorption coefficient, yet there is still a marked discrimination observed in the spectral shape of the ratio of the total backscattering to total absorption coefficient and remote-sensing reflectance among the phytoplankton assemblages. These spectral variations arise mainly from differences in the bio-optical environment in which specific communities were found, as opposed to differences in the spectral shapes of phytoplankton optical properties per se. These results suggest potential approaches for the development of algorithms to assess phytoplankton community composition from measurements of seawater optical properties in western Arctic waters.

Stramski, D, Reynolds RA, Gernez P, Röttgers R, Wurl O.  2019.  Inherent optical properties and particle characteristics of the sea-surface microlayer. Progress in Oceanography. 176:102117.   Abstract

The sea-surface microlayer (SML) is known to have physical, chemical, and biological properties that are distinctly different from the underlying subsurface water (USW). However, only a few studies in the past reported on measurements of the optical properties of the SML and were limited to light absorption. In this study we present results for the main inherent optical properties (IOPs), the spectral absorption coefficients and the volume scattering function, as well as particle size distribution (PSD), from measurements of the SML and USW in contrasting ocean environments with near-surface chlorophyll-a concentration ranging from 0.06 mg/m3 in waters off Hawaiian Islands to 1 mg/m3 in the Santa Barbara Channel. Our observations also included prominent surface slick conditions associated with a dense bloom of Trichodesmium. Significant and highly variable enhancements of the optical properties and particle concentration, including significant changes in the shape of PSD, were observed in the SML compared with USW at all investigated sites. In clear oligotrophic waters the total concentration of particles larger than 0.7 micrometers in size was enriched in the SML more than 8-fold. In all examined waters the enrichment was consistently higher for larger particles (> 10 micrometers) than smaller particles. The highest enhancement of light absorption coefficients, > 100-fold for particulate absorption and > 200-fold for phytoplankton absorption in the near-UV and red spectral regions, was observed during the Trichodesmium bloom. In clear oligotrophic waters the particulate absorption coefficient was enhanced by as much as 45-fold in the green spectral region and the non-algal component exhibited consistently higher enhancement than phytoplankton component across the examined spectrum. In contrast to absorption, the volume scattering function was enhanced more in clear oligotrophic waters (> 15-fold at scattering angles between about 65° and 80°) than in the situation of Trichodesmium bloom. With the exception of Trichodesmium bloom, we consistently observed significantly lower values of the degree of linear polarization of light scattered by suspended particles and whole seawater samples (by as much as 30% in the vicinity of scattering angle of 90°) in the SML compared with USW. This result indicates that the SML can have important effect on the state of polarization of downwelling light entering the ocean and upwelling light leaving the ocean across the air-sea interface. The determinations of IOPs in the SML can extend the capabilities for characterizing constituents of microlayer and provide useful information for radiative transfer and remote-sensing related studies.

Stramski, D, Li L, Reynolds RA.  2019.  Model for separating the contributions of non-algal particles and colored dissolved organic matter to light absorption by seawater. Applied Optics. 58:3790-3806.   10.1364/AO.58.003790   Abstract

We evaluated the performance of a recently developed absorption partitioning model [J. Geophys. Res. Oceans 120, 2601 (2015)] that derives the spectral absorption coefficients of non-algal particles, aNAP(λ), and colored dissolved organic matter, ag(λ), from the total absorption coefficient of seawater. The model’s performance was found unsatisfactory when the model was tested with a large dataset of absorption measurements from diverse open-ocean and coastal aquatic environments. To address these limitations, we developed a new model based on a different approach for estimating aNAP(λ) and ag(λ) from the sum of these two coefficients, adg(λ), within the visible spectral region. The very good overall performance of the model is demonstrated, with no tendency for bias and relatively small absolute differences (the median ≤20%) between the model-derived and measured values of aNAP(λ) and ag(λ) over a wide range of aquatic environments.

Ehn, JK, Reynolds RA, Stramski D, Doxaran D, Lansard B, Babin M.  2019.  Patterns of suspended particulate matter across the continental margin in the Canadian Beaufort Sea during summer. Biogeosciences. 16:1583-1605.: Copernicus Publications   10.5194/bg-16-1583-2019   Abstract
Zhang, X, Stramski D, Reynolds RA, Blocker RE.  2019.  Light scattering by pure water and seawater: the depolarization ratio and its variation with salinity. Applied Optics. 58:991-1004.: OSA   10.1364/AO.58.000991   Abstract

We measured the linearly polarized light scattering of pure water and seawater at various salinities and estimated the depolarization ratio using five different methods of data analysis after removing the scattering due to contamination by residual nanoparticles. The depolarization ratio values (δ) estimated for pure water using these different methods are largely consistent with each other and result in a mean value of 0.039±0.001. For seawater, our results reveal a trend of a slight linear increase of δ with salinity (S), δ=0.039+a1×S, where a1 varies in the range of 1×10−4 to 2×10−4 between the methods.

Li, L, Stramski D, Darecki M.  2018.  Characterization of the light field and apparent optical properties in the ocean euphotic layer based on hyperspectral measurements of the irradiance quartet. Applied Sciences. 8(12):2677.   10.3390/app8122677   Abstract

Although the light fields and apparent optical properties (AOPs) within the ocean euphotic layer have been studied for many decades through extensive measurements and theoretical modeling, there is virtually a lack of simultaneous high spectral resolution measurements of plane and scalar downwelling and upwelling irradiances (the so-called irradiance quartet). We describe a unique dataset of hyperspectral irradiance quartet, which was acquired under a broad range of environmental conditions within the water column from the near-surface depths to about 80 m in the Gulf of California. This dataset enabled the characterization of a comprehensive suite of AOPs for realistic non-uniform vertical distributions of seawater inherent optical properties (IOPs) and chlorophyll-a concentration (Chl) in the common presence of inelastic radiative processes within the water column, in particular Raman scattering by water molecules and chlorophyll-a fluorescence. In the blue and green spectral regions, the vertical patterns of AOPs are driven primarily by IOPs of seawater with weak or no discernible effects of inelastic processes. In the red, the light field and AOPs are strongly affected or totally dominated by inelastic processes of Raman scattering by water molecules, and additionally by chlorophyll-a fluorescence within the fluorescence emission band. The strongest effects occur in the chlorophyll-a fluorescence band within the chlorophyll-a maximum layer, where the average cosines of the light field approach the values of uniform light field, irradiance reflectance is exceptionally high approaching 1, and the diffuse attenuation coefficients for various irradiances are exceptionally low, including the negative values for the attenuation of upwelling plane and scalar irradiances. We established the empirical relationships describing the vertical patterns of some AOPs in the red spectral region as well as the relationships between some AOPs which can be useful in common experimental situations when only the downwelling plane irradiance measurements are available. We also demonstrated the applicability of irradiance quartet data in conjunction with Gershun’s equation for estimating the absorption coefficient of seawater in the blue-green spectral region, in which the effects of inelastic processes are weak or negligible.

Koestner, D, Stramski D, Reynolds RA.  2018.  Measurements of the volume scattering function and the degree of linear polarization of light scattered by contrasting natural assemblages of marine particles. Applied Sciences. 8(12):2690.   10.3390/app8122690   Abstract

The light scattering properties of seawater play important roles in radiative transfer in the ocean and optically-based methods for characterizing marine suspended particles from in situ and remote sensing measurements. The recently commercialized LISST-VSF instrument is capable of providing in situ or laboratory measurements of the volume scattering function, βp(ψ), and the degree of linear polarization, DoLPp(ψ), associated with particle scattering. These optical quantities of natural particle assemblages have not been measured routinely in past studies. To fully realize the potential of LISST-VSF measurements, we evaluated instrument performance, and developed calibration correction functions from laboratory measurements and Mie scattering calculations for standard polystyrene beads suspended in water. The correction functions were validated with independent measurements. The improved LISST-VSF protocol was applied to measurements of βp(ψ) and DoLPp(ψ) taken on 17 natural seawater samples from coastal and offshore marine environments characterized by contrasting assemblages of suspended particles. Both βp(ψ) and DoLPp(ψ) exhibited significant variations related to a broad range of composition and size distribution of particulate assemblages. For example, negative relational trends were observed between the particulate backscattering ratio derived from βp(ψ) and increasing proportions of organic particles or phytoplankton in the particulate assemblage. Our results also suggest a potential trend between the maximum values of DoLPp(ψ) and particle size metrics, such that a decrease in the maximum DoLPp(ψ) tends to be associated with particulate assemblages exhibiting a higher proportion of large-sized particles. Such results have the potential to advance optically-based applications that rely on an understanding of relationships between light scattering and particle properties of natural particulate assemblages.

Boss, E, D'Sa EJ, Freeman S, Fry E, Mueller JL, Pegau S, Reynolds RA, Roesler C, Rottgers R, Stramski D, Twardowski M, Zaneveld JRV.  2018.  Inherent optical property measurements and protocols: Absorption coefficient. , Dartmouth: International Ocean Colour Coordinating Group   10.25607/OBP-119   Abstract

Inherent Optical Properties Measurements and Protocols: Absorption Coefficient (v1.0) is a document that serves as a comprehensive overview of calibration, measurement and analysis protocols for the state-of-the-art technologies that measure the absorption of particles in the water or on a filter pad.

Le, CF, Zhou XY, Hu CM, Lee ZP, Li L, Stramski D.  2018.  A color-index-based empirical algorithm for determining particulate organic carbon concentration in the ocean from satellite observations. Journal of Geophysical Research-Oceans. 123:7407-7419.   10.1029/2018jc014014   AbstractWebsite

An empirical algorithm for estimating particulate organic carbon (POC) concentration in the surface ocean from satellite observations is formulated and validated using in situ POC data and remote-sensing reflectance (R-rs) data obtained from match-up satellite ocean color measurements. The algorithm builds upon the band-difference algorithm concept, which was originally developed for estimating chlorophyll-a concentration in clear waters. This algorithm utilizes three spectral bands centered approximately at 490, 550, and 670nm to determine a color index (CIPOC), from which POC can be estimated from satellite measurements. For comparison, the blue-green band-ratio algorithm is also formulated using the same data set of in situ POC and satellite-derived R-rs. Results show that the statistical parameters characterizing the differences between the satellite-derived POC and matchup in situ POC are similar when the CIPOC and band ratio algorithms are applied to open ocean waters where the values of CIPOC are relatively low. In coastal waters where the values of CIPOC are generally higher, the statistical parameters of algorithm performance are better for the CIPOC algorithm. In addition, because the CIPOC algorithm is less sensitive to errors and noise in the satellite-derived R-rs, the image quality obtained with this algorithm can be improved for both open-ocean and coastal waters. Particulate organic carbon (POC) in the global ocean is linked to many important ocean biogeochemical processes and is responsible for large carbon fluxes. POC variations occur over a broad range of spatial (from regional to global) and temporal (from seasonal to decadal) scales due to various factors. Ocean color data acquired from satellite sensors, such as Sea-viewing Wide-Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS), and Medium-Resolution Imaging Spectrometer (MERIS), can be used to quantify POC, with the capability for uninterrupted long-term observations and global coverage. This study demonstrates that the color-index (band-difference) approach is applicable to the POC retrieval from remote-sensing reflectance in both open ocean and coastal waters.

Loisel, H, Stramski D, Dessailly D, Jamet C, Li L, Reynolds RA.  2018.  An Inverse Model for Estimating the Optical Absorption and Backscattering Coefficients of Seawater From Remote-Sensing Reflectance Over a Broad Range of Oceanic and Coastal Marine Environments. Journal of Geophysical Research: Oceans. 123:2141-2171.   10.1002/2017JC013632   Abstract

We present an inverse model (referred to as LS2) for estimating the inherent optical properties (IOPs) of seawater, specifically the spectral absorption, a(λ), and backscattering, bb(λ), coefficients within the ocean surface layer, from measurements of ocean remote-sensing reflectance, Rrs(λ). The non-water absorption, anw(λ), and particulate backscattering, bbp(λ), coefficients can be derived after subtracting pure seawater contributions. The LS2 requires no spectral assumptions about IOPs and provides solutions at arbitrary light wavelengths in the visible spectrum independently of one another. As the LS2 can operate with the inputs of Rrs(λ) and solar zenith angle it is applicable to satellite ocean color remote sensing. The model can also operate with additional input of the diffuse attenuation coefficient of downward irradiance, which provides somewhat improved model performance for applications using in situ radiometric measurements as inputs. The evaluation of LS2 with a synthetic dataset that is free of measurement errors indicates good performance for IOPs in the visible spectrum, except for anw(λ) in the long-wavelength portion of the spectrum where anw(λ) contributes only a few percent to a(λ) under typical open ocean conditions. The good performance is characterized by a median absolute percentage difference between the model-derived and true values of IOPs, which is generally <20%, and the median ratio of model-derived to true values <10%. The satisfactory model performance is also demonstrated through validation analysis based on extensive datasets comprising coincident in situ measurements of Rrs(λ) and IOPs as well as a match-up dataset comprising satellite-derived Rrs(λ) and in situ IOP measurements.

Werdell, PJ, McKinna LIW, Boss E, Ackleson SG, Craig SE, Gregg WW, Lee Z, Maritorena S, Roesler CS, Rousseaux CS, Stramski D, Sullivan JM, Twardowski MS, Tzortziou M, Zhang XD.  2018.  An overview of approaches and challenges for retrieving marine inherent optical properties from ocean color remote sensing. Progress in Oceanography. 160:186-212.   10.1016/j.pocean.2018.01.001   AbstractWebsite

Ocean color measured from satellites provides daily global, synoptic views of spectral water-leaving reflectances that can be used to generate estimates of marine inherent optical properties (IOPs). These reflectances, namely the ratio of spectral upwelled radiances to spectral downwelled irradiances, describe the light exiting a water mass that defines its color. IOPs are the spectral absorption and scattering characteristics of ocean water and its dissolved and particulate constituents. Because of their dependence on the concentration and composition of marine constituents, IOPs can be used to describe the contents of the upper ocean mixed layer. This information is critical to further our scientific understanding of biogeochemical oceanic processes, such as organic carbon production and export, phytoplankton dynamics, and responses to climatic disturbances. Given their importance, the international ocean color community has invested significant effort in improving the quality of satellite-derived TOP products, both regionally and globally. Recognizing the current influx of data products into the community and the need to improve current algorithms in anticipation of new satellite instruments (e.g., the global, hyperspectral spectroradiometer of the NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission), we present a synopsis of the current state of the art in the retrieval of these core optical properties. Contemporary approaches for obtaining IOPs from satellite ocean color are reviewed and, for clarity, separated based their inversion methodology or the type of IOPs sought. Summaries of known uncertainties associated with each approach are provided, as well as common performance metrics used to evaluate them. We discuss current knowledge gaps and make recommendations for future investment for upcoming missions whose instrument characteristics diverge sufficiently from heritage and existing sensors to warrant reassessing current approaches.

Evers-King, H, Martinez-Vicente V, Brewin RJ, Dall'Olmo G, Hickman AE, Jackson T, Kostadinov TS, Krasemann H, Loisel H, Röttgers R, Roy S, Stramski D, Thomalla S, Platt T, Sathyendranath S.  2017.  Validation and intercomparison of ocean color algorithms for estimating particulate organic carbon in the oceans. Frontiers in Marine Sciences. 4:251.   10.3389/fmars.2017.00251   Abstract

Particulate Organic Carbon (POC) plays a vital role in the ocean carbon cycle. Though relatively small compared with other carbon pools, the POC pool is responsible for large fluxes and is linked to many important ocean biogeochemical processes. The satellite ocean-color signal is influenced by particle composition, size, and concentration and provides a way to observe variability in the POC pool at a range of temporal and spatial scales. To provide accurate estimates of POC concentration from satellite ocean color data requires algorithms that are well validated, with uncertainties characterized. Here, a number of algorithms to derive POC using different optical variables are applied to merged satellite ocean color data provided by the Ocean Color Climate Change Initiative (OC-CCI) and validated against the largest database of in situ POC measurements currently available. The results of this validation exercise indicate satisfactory levels of performance from several algorithms (highest performance was observed from the algorithms of Loisel et al., 2002; Stramski et al., 2008) and uncertainties that are within the requirements of the user community. Estimates of the standing stock of the POC can be made by applying these algorithms, and yield an estimated mixed-layer integrated global stock of POC between 0.77 and 1.3 Pg C of carbon. Performance of the algorithms vary regionally, suggesting that blending of region-specific algorithms may provide the best way forward for generating global POC products.

Stramski, D, Tatarkiewicz JJ, Reynolds RA, Karr M.  2017.  Nanoparticle Analyzer. US Patent. : The Regents of the University of California Abstract

Methods for detecting and analyzing individual nanoparticles of the same, similar, or different sizes co-existing in a fluid sample using multi-spectral analysis are described. A plurality of light sources may be configured to produce a plurality of light beams at different spectral wavebands. An optical assembly may be configured to combine the plurality of light beams into one or more incident light sheets. Each incident light sheet may illuminate one or more nanoparticles in a liquid sample. One or more image detectors may be configured to detect, using a plurality of wavelengths, light scattered or emitted by one or more nanoparticles. The plurality of wavelengths may correspond to the different spectral wavebands of the plurality of light beams. Related apparatus, techniques, and articles are also described.

Li, LH, Stramski D, Reynolds RA.  2016.  Effects of inelastic radiative processes on the determination of water-leaving spectral radiance from extrapolation of underwater near-surface measurements. Applied Optics. 55:7050-7067.   10.1364/ao.55.007050   AbstractWebsite

Extrapolation of near-surface underwater measurements is the most common method to estimate the water-leaving spectral radiance, L-w(lambda) (where lambda is the light wavelength in vacuum), and remote-sensing reflectance, R-rs (lambda),for validation and vicarious calibration of satellite sensors, as well as for ocean color algorithm development. However, uncertainties in L-w(lambda) arising from the extrapolation process have not been investigated in detail with regards to the potential influence of inelastic radiative processes, such as Raman scattering by water molecules and fluorescence by colored dissolved organic matter and chlorophyll-a. Using radiative transfer simulations, we examine high-depth resolution vertical profiles of the upwelling radiance, L-u(lambda) and its diffuse attenuation coefficient, K-Lu (lambda) within the top 10 m of the ocean surface layer and assess the uncertainties in extrapolated values of L-w(lambda) The inelastic processes generally increase L-u and decrease K-Lu in the red and nearinfrared (NIR) portion of the spectrum. Unlike K-Lu in the blue and green spectral bands, K-Lu in the red and NIR is strongly variable within the near-surface layer even in a perfectly homogeneous water column. The assumption of a constant K-Lu with depth that is typically employed in the extrapolation method can lead to significant errors in the estimate of L-w. These errors approach similar to 100% at 900 nm, and the desired threshold of 5% accuracy or less cannot be achieved at wavelengths greater than 650 nm for underwater radiometric systems that typically take measurements at depths below 1 m. These errors can be reduced by measuring L-u within a much shallower surface layer of tens of centimeters thick or even less at near-infrared wavelengths longer than 800 nm, which suggests a requirement for developing appropriate radiometric instrumentation and deployment strategies. (C) 2016 Optical Society of America

Reynolds, RA, Stramski D, Neukermans G.  2016.  Optical backscattering by particles in Arctic seawater and relationships to particle mass concentration, size distribution, and bulk composition. Limnology and Oceanography. 61:1869-1890.   10.1002/lno.10341   AbstractWebsite

The magnitude and spectral shape of the optical backscattering coefficient of particles, b(bp)(lambda), is being increasingly used to infer information about the particles present in seawater. Relationships between b(bp) and particle properties in the Arctic are poorly documented, and may differ from other oceanic regions which contribute the majority of data used to develop and parameterize optical models. We utilize recent field measurements from the Chukchi and Beaufort Seas to examine relationships between the spectral backscattering coefficient of particles in seawater and the mass concentration, bulk composition, and size distribution of the suspended particle assemblage. The particle backscattering coefficient spanned six orders of magnitude from the relatively clear waters of the Beaufort Sea to extremely turbid waters on the Mackenzie shelf. This coefficient was highly correlated with the mass concentration of particles, and to a lesser extent with other measures of concentration such as particulate organic carbon or chlorophyll a. Increased backscattering and high mass-specific b(bp)(lambda) was associated with mineral-rich assemblages that tended to exhibit steeper size distributions, while reduced backscattering was associated with organic-dominated assemblages having a greater contribution of large particles. Our results suggest that algorithms which employ composition-specific relationships can lead to improved estimates of particle mass concentration from backscattering measurements. In contrast to theoretical models, however, we observe no clear relationship between the spectral slope of b(bp)(lambda) and the slope of the particle size distribution in this environment.

Neukermans, G, Reynolds RA, Stramski D.  2016.  Optical classification and characterization of marine particle assemblages within the western Arctic Ocean. Limnology and Oceanography. 61:1472-1494.   10.1002/lno.10316   AbstractWebsite

We develop an optical classification of marine particle assemblages from an extensive dataset of particle optical properties collected in the Chukchi and Beaufort Seas. Hierarchical cluster analysis of the spectral particulate backscattering-to-absorption ratio partitioned the dataset into seven optically-distinct clusters of particle assemblages, each associated with different characteristics of particle concentration, composition, and phytoplankton taxonomic composition and size. Three phytoplankton-dominated clusters were identified. One was characterized by small-sized phytoplankton that are typically associated with regenerated production, and comprised samples from the subsurface chlorophyll-a maximum in oligotrophic waters of the Beaufort Sea. The other two clusters represented diatom-dominated particle assemblages in turbid shelf waters with differing contributions of photoprotective pigments. Such situations are generally associated with significant new production. Two clusters were dominated by organic nonalgal material, one representing clear waters off the shelf, the other representative of post-diatom bloom conditions in the Chukchi Sea. Another distinct cluster represented mineral-dominated particle assemblages that were observed in the Colville and Mackenzie River plumes and near the seafloor. Finally, samples in a cluster of mixed particle composition were scattered throughout all locations. Optical classification improved performance of predictive bio-optical relationships. These results demonstrate a capability to discriminate distinct assemblages of suspended particles associated with specific ecological conditions from hyperspectral measurements of optical properties, and the potential for identification of ecological provinces at synoptic time and space scales from optical sensors. Analogous analysis of multispectral optical data strongly reduced this capability.

Uitz, J, Stramski D, Reynolds RA, Dubranna J.  2015.  Assessing phytoplankton community composition from hyperspectral measurements of phytoplankton absorption coefficient and remote-sensing reflectance in open-ocean environments. Remote Sensing of Environment. 171:58-74.   10.1016/j.rse.2015.09.027   Abstract

This study assesses the ability of hyperspectral optical measurements to discriminate changes in the composition of phytoplankton communities in open-ocean non-bloom environments. A large set of in situ near-surface measurements, comprising phytoplankton pigment determinations and hyperspectral optical data of phytoplankton absorption coefficient, aph(λ), and remote-sensing reflectance, Rrs(λ), are used in the analysis. Measurements were collected in different ecological provinces in the Pacific and Atlantic Oceans with chlorophyll-a concentrations ranging from about 0.02 to 1.5 mg m− 3. Hierarchical cluster analysis was applied to measured spectra of aph(λ) and Rrs(λ) and the second-derivative spectra of these optical variables. The resulting optical-based classifications of the examined stations compared favorably (similarity index ≥ 0.73) with a classification of phytoplankton community composition calculated from pigment measurements. Similarities between pigment-based and optically-based classifications were better for the optical data of aph(λ) than Rrs(λ), with only slight improvements resulting from the use of the second derivative spectra as opposed to the non-differentiated spectra. An Empirical Orthogonal Function (EOF) analysis was applied to the optical spectra to examine the correlation of dominant modes of variability with several bio-optical and biogeochemical properties. This analysis supports the notion that the performance of the optical approach is strongly associated with the effects of differences in pigment composition, cell size, and intracellular pigment concentration among different phytoplankton communities on the optical properties of the ocean.

Stramski, D, Reynolds RA, Kaczmarek S, Uitz J, Zheng G.  2015.  Correction of pathlength amplification in the filter-pad technique for measurements of particulate absorption coefficient in the visible spectral region. Applied Optics. 54:6763-6782.: OSA   10.1364/AO.54.006763   AbstractWebsite

Spectrophotometric measurement of particulate matter retained on filters is the most common and practical method for routine determination of the spectral light absorption coefficient of aquatic particles, ap(&#x3BB;), at high spectral resolution over a broad spectral range. The use of differing geometrical measurement configurations and large variations in the reported correction for pathlength amplification induced by the particle/filter matrix have hindered adoption of an established measurement protocol. We describe results of dedicated laboratory experiments with a diversity of particulate sample types to examine variation in the pathlength amplification factor for three filter measurement geometries; the filter in the transmittance configuration (T), the filter in the transmittance-reflectance configuration (T-R), and the filter placed inside an integrating sphere (IS). Relationships between optical density measured on suspensions (ODs) and filters (ODf) within the visible portion of the spectrum were evaluated for the formulation of pathlength amplification correction, with power functions providing the best functional representation of the relationship for all three geometries. Whereas the largest uncertainties occur in the T method, the IS method provided the least sample-to-sample variability and the smallest uncertainties in the relationship between ODs and ODf. For six different samples measured with 1 nm resolution within the light wavelength range from 400 to 700 nm, a median error of 7.1% is observed for predicted values of ODs using the IS method. The relationships established for the three filter-pad methods are applicable to historical and ongoing measurements; for future work, the use of the IS method is recommended whenever feasible.

Zheng, GM, Stramski D, DiGiacomo PM.  2015.  A model for partitioning the light absorption coefficient of natural waters into phytoplankton, nonalgal particulate, and colored dissolved organic components: A case study for the Chesapeake Bay. Journal of Geophysical Research-Oceans. 120:2601-2621.   10.1002/2014jc010604   AbstractWebsite

We present a model, referred to as Generalized Stacked-Constraints Model (GSCM), for partitioning the total light absorption coefficient of natural water (with pure-water contribution subtracted), a(nw)(), into phytoplankton, a(ph)(), nonalgal particulate, a(d)(), and CDOM, a(g)(), components. The formulation of the model is based on the so-called stacked-constraints approach, which utilizes a number of inequality constraints that must be satisfied simultaneously by the model outputs of component absorption coefficients. A major advancement is that GSCM provides a capability to separate the a(d)() and a(g)() coefficients from each other using only weakly restrictive assumptions about the component absorption coefficients. In contrast to the common assumption of exponential spectral shape of a(d)() and a(g)() in previous models, in our model these two coefficients are parameterized in terms of several distinct spectral shapes. These shapes are determined from field data collected in the Chesapeake Bay with an ultimate goal to adequately account for the actual variability in spectral shapes of a(d)() and a(g)() in the study area. Another advancement of this model lies in its capability to account for potentially nonnegligible magnitude of a(d)() in the near-infrared spectral region. Evaluation of model performance demonstrates good agreement with measurements in the Chesapeake Bay. For example, the median ratio of the model-derived to measured a(d)(), a(g)(), and a(ph)() at 443 nm is 0.913, 1.064, and 1.056, respectively. Whereas our model in its present form can be a powerful tool for regional studies in the Chesapeake Bay, the overall approach is readily adaptable to other regions or bio-optical water types.

Haag, JM, Roberts PLD, Papen GC, Jaffe JS, Li L, Stramski D.  2014.  Deep-sea low-light radiometer system. Optics Express. 22:30074-30091.: OSA   10.1364/OE.22.030074   AbstractWebsite

Two single-waveband low-light radiometers were developed to characterize properties of the underwater light field relevant to biological camouflage at mesopelagic ocean depths. Phenomena of interest were vertical changes in downward irradiance of ambient light at wavelengths near 470 nm and 560 nm, and flashes from bioluminescent organisms. Depth profiles were acquired at multiple deep stations in different geographic regions. Results indicate significant irradiance magnitudes at 560 nm, providing direct evidence of energy transfer as described by Raman scattering. Analysis of a night profile yielded multiple examples of bioluminescent flashes. The selection of high-sensitivity, high-speed silicon photomultipliers as detectors enabled measurement of spectrally-resolved irradiance to greater than 400 m depth.

Zheng, G, Stramski D, Reynolds RA.  2014.  Evaluation of the Quasi-Analytical Algorithm for estimating the inherent optical properties of seawater from ocean color: Comparison of Arctic and lower-latitude waters. Remote Sensing of Environment. 155:194-209.   10.1016/j.rse.2014.08.020   AbstractWebsite

We evaluated the performance of the Quasi-Analytical Algorithm (QAA, v5 with modifications) for deriving the spectral total absorption, a(λ), and backscattering, bb(λ), coefficients of seawater and partitioning of a(λ) into phytoplankton and non-phytoplankton components from input spectrum of remote-sensing reflectance, Rrs(λ), with field data collected in the Arctic and lower-latitude open waters from the Atlantic and Pacific Oceans. The systematic error based on median ratio between QAA-derived and measured a(λ) varied from about 1% to ± 10% depending on light wavelength and the oceanic region. The QAA typically overestimated bb(λ) from 3% to 14% compared with field measurements. These results were obtained with a correction for Raman-scattering contribution to Rrs and separate parameterization of molecular and particulate backscattering in the Rrs vs. bb/a relationship. Without these features the earlier versions of the QAA can overestimate bb(λ) by as much as 35% in clear waters. The use of pure seawater backscattering coefficients accounting for water temperature and salinity improved the accuracy of QAA-derived a(λ) in Arctic waters. The absorption-partitioning component of the QAA significantly underestimated phytoplankton absorption and overestimated non-phytoplankton absorption in both Arctic and lower-latitude waters.

Li, L, Stramski D, Reynolds RA.  2014.  Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations. Deep-Sea Research Part I-Oceanographic Research Papers. 87:53-69.   10.1016/j.dsr.2014.02.005   AbstractWebsite

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200-1000 m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by 9-10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475 nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565 nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (K-40) indicates that the solar irradiance dominates over the K-40-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400-500 m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly-asymptotic regime at depths exceeding 400-500 m. (c) 2014 Elsevier Ltd. All rights reserved.

Gernez, P, Reynolds RA, Stramski D.  2014.  Within-day variability of particulate organic carbon and remote-sensing reflectance during a bloom of Phaeocystis antarctica in the Ross Sea, Antarctica. International Journal of Remote Sensing. 35:454-477.   10.1080/01431161.2013.871598   AbstractWebsite

We examined the within-day variability in seawater optical properties and biogeochemical constituents for a high-latitude location in the Ross Sea, Antarctica, during development of the annual spring phytoplankton bloom. Measurements of particulate organic carbon concentration (POC), chlorophyll-a concentration (Chl), and particle size distribution were conducted at 4-6 hour intervals in parallel with determinations of the spectral absorption and attenuation coefficients of particles, and the spectral remote-sensing reflectance of the surface ocean (R-rs). Surface POC and Chl exhibited more than a twofold variation throughout the day in the continuous presence of natural light. A minimum occurred near local noon coinciding with peak solar irradiance, a maximum in the evening, and a subsequent decrease throughout the night-time hours. These patterns were accompanied by large changes in the magnitude and spectral shape of R-rs, including the blue-to-green spectral band ratios used in ocean colour algorithms for estimating POC and Chl. The variability in R-rs could not be explained by changes in solar zenith angle, but was consistent with observations of within-day variations in spectral absorption and scattering by particles which were influenced by changes in the particle concentration and size distribution. The accuracy of an empirical ocean colour algorithm for estimating POC from R-rs was unaffected by within-day variability, implying that short-term variations in surface POC can be potentially monitored by multiple within-day measurements of R-rs, through means of in situ and remote sensing observations if available. Our findings also suggest that within-day changes in POC can be significant compared with the variability observed on meso-scale spatial scales, potentially confounding the interpretation of remote-sensing data obtained from temporal and spatial compositing of images measured at different times within a single day.

Johnsen, S, Gassman E, Reynolds RA, Stramski D, Mobley C.  2014.  The asymmetry of the underwater light field and its implications for mirror-based camouflage in silvery pelagic fish. Limnology and Oceanography. 59(6):1839-1852.   10.4319/lo.2014.59.6.183   Abstract

Many pelagic species, particularly teleost fish, have silvered lateral surfaces that are thought to primarily serve as a form of camouflage. The underlying argument is that the underwater light field is cylindrically symmetrical around the vertical axis; thus a vertical mirror reflects a region of the water column that matches the region directly behind the mirror. However, the degree of symmetry of the underwater light field itself has not been assessed. Modeled underwater radiances from the surface to a depth of 100 m using measured profiles of inherent optical properties and HydroLight radiative transfer software showed that the horizontal light field under sunny conditions was asymmetrical over a wide range of solar elevations. In addition, the maximum asymmetry at 100 m occurred not when the sun was near the horizon, but when it was 45° above it. We validated these modeled results in Hawaiian waters using a modification of a commercial radiometer. Both modeled and measured radiances showed that the inherent contrast of silvery fish was typically higher at longer wavelengths. However, models of the sighting distances of these surfaces showed that sighting distance was greatest at the peak wavelength of the downwelling irradiance (∼ 480 nm). The modeled and measured asymmetry of the horizontal light field implies that mirror camouflage is not always as successful as originally thought and suggests that there may be further refinements for this form of crypsis that have not been previously considered.