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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(λ), 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.

Antoine, D, Babin M, Berthon J-F, Bricaud A, Gentili B, Loisel H, Maritorena S, Stramski D.  2014.  Shedding Light on the Sea: André Morelʼs Legacy to Optical Oceanography. Annual Review of Marine Science. 6:1-21.   10.1146/annurev-marine-010213-135135   Abstract

André Morel (1933–2012) was a prominent pioneer of modern optical oceanography, enabling significant advances in this field. Through his forward thinking and research over more than 40 years, he made key contributions that this field needed to grow and to reach its current status. This article first summarizes his career and then successively covers different aspects of optical oceanography where he made significant contributions, from fundamental work on optical properties of water and particles to global oceanographic applications using satellite ocean color observations. At the end, we share our views on André's legacy to our research field and scientific community.

Neukermans, G, Reynolds RA, Stramski D.  2014.  Contrasting inherent optical properties and particle characteristics between an under-ice phytoplankton bloom and open water in the Chukchi Sea. Deep Sea Research Part II: Topical Studies in Oceanography. 105:59-73.   AbstractWebsite

Abstract Variability in the inherent optical properties (IOPs) of seawater and characteristics of the particle assemblage were examined along a transect in the Chukchi Sea during July 2011. This transect encompassed samples from open waters of the marginal ice zone exhibiting low concentrations of chlorophyll-a (Chla<1 mg m−3), as well as an extensive phytoplankton bloom (Chla>30 mg m−3) beneath consolidated pack ice. Measurements included the spectral coefficients for particulate beam attenuation, backscattering, and absorption, bulk indicators of particle concentration and composition, and the particle size distribution. Within the bloom microphytoplankton contributed >95% to the total Chla, and relatively small amounts of nonalgal particles were present. This assemblage exhibited low Chla-specific phytoplankton absorption coefficients (0.006 m2 mg−1 at 676 nm) indicating a strong influence of pigment packaging, and a weak spectral dependence of the particulate backscattering coefficient. In contrast, the phytoplankton community in nutrient-depleted surface waters outside the sea ice had a strong contribution of picoplankton to Chla (54%) and an increased abundance of nonalgal particles. The Chla-specific phytoplankton absorption coefficient approached maximum values at 676 nm (0.023 m2 mg−1) and particle backscattering had much stronger spectral dependence. Additional samples from near the sea floor exhibited characteristics of either mineral-dominated assemblages or a mixture of mineral and organic particles, and also displayed optical differentiation from the surface samples. The marked contrast in absorption, attenuation, and backscattering properties of these ecological regimes suggest that they can be discriminated from optical measurements available on a variety of in situ and remote-sensing platforms.

Ferreira, A, Stramski D, Garcia CAE, Garcia VMT, Ciotti AM, Mendes CRB.  2013.  Variability in light absorption and scattering of phytoplankton in Patagonian waters: Role of community size structure and pigment composition. Journal of Geophysical Research-Oceans. 118:698-714.   10.1002/jgrc.20082   AbstractWebsite

Intense phytoplankton blooms were observed along the Patagonian shelf-break with satellite ocean color data, but few in situ optical observations were made in that region. We examine the variability of phytoplankton absorption and particulate scattering coefficients during such blooms on the basis of field data. The chlorophyll-a concentration, [Chla], ranged from 0.1 to 22.3 mg m(-3) in surface waters. The size fractionation of [Chla] showed that 80% of samples were dominated by nanophytoplankton (N-group) and 20% by microphytoplankton (M-group). Chlorophyll-specific phytoplankton absorption coefficients at 440 and 676 nm, a(ph)*(440) and a(ph)*(676), and particulate scattering coefficient at 660 nm, b(p)*(660), ranged from 0.018 to 0.173, 0.009 to 0.046, and 0.031 to 2.37m(2) (mg Chla) (1), respectively. Both a(ph)*(440) and a(ph)*(676) were statistically higher for the N-group than M-group and also considerably higher than expected from global trends as a function of [Chla]. This result suggests that size of phytoplankton cells in Patagonian waters tends to be smaller than in other regions at similar [Chla]. The phytoplankton cell size parameter, S-f, derived from phytoplankton absorption spectra, proved to be useful for interpreting the variability in the data around the general inverse dependence of a(ph)*(440), a(ph)*(676), and b(p)*(660) on [Chla]. S-f also showed a pattern along the increasing trend of a(ph)*(440) and a(ph)*(676) as a function of the ratios of some accessory pigments to [Chla]. Our results suggest that the variability in phytoplankton absorption and scattering coefficients in Patagonian waters is caused primarily by changes in the dominant phytoplankton cell size accompanied by covariation in the concentrations of accessory pigments. Citation: Ferreira, A., D. Stramski, C. A. E. Garcia, V. M. T. Garcia, A. M. Ciotti, and C. R. B. Mendes (2013), Variability in light absorption and scattering of phytoplankton in Patagonian waters: Role of community size structure and pigment composition, J. Geophys. Res. Oceans, 118, 698-714, doi:10.1002/jgrc.20082.

Zheng, GM, Stramski D.  2013.  A model for partitioning the light absorption coefficient of suspended marine particles into phytoplankton and nonalgal components. Journal of Geophysical Research-Oceans. 118:2977-2991. AbstractWebsite

We developed a model for partitioning the spectral absorption coefficient of suspended marine particles, a(p)(), into phytoplankton, a(ph)(), and nonalgal, a(d)(), components based on the stacked-constraints approach. The key aspect of our model is the use of a set of inequality constraints that account for large variability in the a(ph)() and a(d)() coefficients within the world's oceans. The bounds of inequality constraints were determined from the analysis of a comprehensive set of 505 field determinations of absorption coefficients in various oceanic environments. The feasible solutions of the model are found by simultaneously satisfying all inequality constraints. The optimal solutions represented by the median values of feasible solutions for a(ph)() and a(d)() generally agree well with field measurements and are superior in terms of error statistics compared with previous partitioning models. For example, on the basis of comparisons of optimal model solutions with field determinations of absorption coefficients, the systematic error calculated as the median ratio of model-derived to measured values for both a(ph)(443) and a(d)(443) is within 1% for our model. The random error represented by the mean absolute percent difference for a(ph)(443) and a(d)(443) is <5% and <20%, respectively. This study suggests that our model has the potential for successful applications with input data of a(p)() which can be collected from various oceanographic platforms.

Zheng, GM, Stramski D.  2013.  A model based on stacked-constraints approach for partitioning the light absorption coefficient of seawater into phytoplankton and non-phytoplankton components. Journal of Geophysical Research-Oceans. 118:2155-2174.   10.1002/jgrc.20115   AbstractWebsite

Partitioning of the total non-water absorption coefficient of seawater, anw() (i.e., the light absorption coefficient after subtraction of pure water contribution), into phytoplankton, aph(), and non-phytoplankton, adg(), components is important in the areas of ocean optics, biology, and biogeochemistry. We propose a partitioning model based on stacked-constraints approach, which requires input of anw() at a minimum of six specific light wavelengths. Compared with existing models, our approach requires much less restrictive assumptions about the spectral slope of adg() and the spectral shape of aph(). Our model is based on several inequality constraints determined from an extensive, quality-verified set of field data covering oceanic and coastal waters from low to high-latitudes. With these constraints, the model first derives a wide range of speculative solutions for adg() and aph() and then identifies feasible solutions. Final model outputs include the optimal solution and a range of feasible solutions for adg() and aph(). The optimal solutions agree well with measurements. For example, the median ratio of the model-derived optimal solutions to measured adg() and aph() at 443nm is very close to 1, i.e., 1.004 and 0.988, respectively. The median absolute percent difference between the optimal solutions and measured values of adg(443) and aph(443) is 6.5% and 12%, respectively. The range of feasible solutions encompasses the measured adg() and aph() with a probability >90% at most wavelengths. Our results support the prospect for the applications of the partitioning model using the input data of anw() collected from various oceanographic and remote-sensing platforms.

Yang, Q, Stramski D, He M-X.  2013.  Modeling the effects of near-surface plumes of suspended particulate matter on remote-sensing reflectance of coastal waters. Applied Optics. 52:359-374.   10.1364/AO.52.000359   Abstract

A radiative transfer model was applied to examine the effects of vertically stratified inherent optical properties of the water column associated with near-surface plumes of suspended particulate matter on spectral remote-sensing reflectance, Rrs(λ), of coastal marine environments. The simulations for nonuniform ocean consisting of two layers with different concentrations of suspended particulate matter (SPM) are compared with simulations for a reference homogeneous ocean whose SPM is identical to the surface SPM of the two-layer cases. The near-surface plumes of particles are shown to exert significant influence on Rrs(λ). The sensitivity of Rrs(λ) to vertical profile of SPM is dependent on the optical beam attenuation coefficient within the top layer, c1(λ), thickness of the top layer, z1, and the ratio of SPM in the underlying layer to that in the top layer, SPM2/SPM1, as well as the wavelength of light, λ. We defined a dimensionless spectral parameter, P(λ)=c1(λ)×z1×(SPM2/SPM1), to quantify and examine the effects of these characteristics of the two-layer profile of SPM on the magnitude and spectral shape of Rrs(λ). In general, the difference of Rrs(λ) between the two-layer and uniform ocean decreases to zero with an increase in P(λ). For the interpretation of ocean color measurements of water column influenced by near-surface plumes of particles, another dimensionless parameter P′(λ) was introduced, which is a product of terms representing homogenous ocean and a change caused by the two-layer structure of SPM. Based on the analysis of this parameter, we found that for the two-layer ocean there is a good relationship between Rrs(λ) in the red and near-infrared spectral regions and the parameters describing the SPM(z) profile, i.e., SPM1, SPM2, and z1.

Uitz, J, Stramski D, Gentili B, D'Ortenzio F, Claustre H.  2012.  Estimates of phytoplankton class-specific and total primary production in the Mediterranean Sea from satellite ocean color observations. Global Biogeochemical Cycles. 26   10.1029/2011gb004055   AbstractWebsite

An approach that combines a recently developed procedure for improved estimation of surface chlorophyll a concentration (Chl(surf)) from ocean color and a phytoplankton class-specific bio-optical model was used to examine primary production in the Mediterranean Sea. Specifically, this approach was applied to the 10 year time series of satellite Chl(surf) data from the Sea-viewing Wide Field-of-view Sensor. We estimated the primary production associated with three major phytoplankton classes (micro, nano, and picophytoplankton), which also yielded new estimates of the total primary production (P-tot). These estimates of P-tot (e.g., 68 g C m(-2) yr(-1) for the entire Mediterranean basin) are lower by a factor of similar to 2 and show a different seasonal cycle when compared with results from conventional approaches based on standard ocean color chlorophyll algorithm and a non-class-specific primary production model. Nanophytoplankton are found to be dominant contributors to P-tot (43-50%) throughout the year and entire basin. Micro and picophytoplankton exhibit variable contributions to P-tot depending on the season and ecological regime. In the most oligotrophic regime, these contributions are relatively stable all year long with picophytoplankton (similar to 32%) playing a larger role than microphytoplankton (similar to 22%). In the blooming regime, picophytoplankton dominate over microphytoplankton most of the year, except during the spring bloom when microphytoplankton (27-38%) are considerably more important than picophytoplankton (20-27%).

Babin, M, Stramski D, Reynolds RA, Wright VM, Leymarie E.  2012.  Determination of the volume scattering function of aqueous particle suspensions with a laboratory multi-angle light scattering instrument. Applied Optics. 51:3853-3873. AbstractWebsite

We describe a methodology for determining the volume scattering function beta(psi) of aqueous particle suspensions from measurements with a laboratory multi-angle light scattering instrument called DAWN (Wyatt Technology Corporation). In addition to absolute and angular calibration, the key component of the method is the algorithm correcting for reflection errors that reduce the percent error in beta(psi) from as much as similar to 300% to <13% at backward scattering angles. The method is optimized and tested with simulations of three-dimensional radiative transfer of exact measurement geometry including the key components of the instrument and also validated experimentally using aqueous suspensions of polystyrene beads. Example applications of the method to samples of oceanic waters and comparisons of these measurements with results obtained with other light scattering instruments are presented. (C) 2012 Optical Society of America

Dickey, T, Banner ML, Bhandari P, Boyd T, Carvalho L, Chang G, Chao Y, Czerski H, Darecki M, Dong C, Farmer D, Freeman S, Gemmrich J, Gernez P, Hall-Patch N, Holt B, Jiang S, Jones C, Kattawar G, LeBel D, Lenain L, Lewis M, Liu Y, Logan L, Manov D, Melville WK, Moline MA, Morison R, Nencioli F, Pegau WS, Reineman B, Robbins I, Rottgers R, Schultz H, Shen L, Shinki M, Slivkoff M, Sokolski M, Spada F, Statom N, Stramski D, Sutherland P, Twardowski M, Vagle S, Van Dommelen R, Voss K, Washburn L, Wei J, Wijesekera H, Wurl O, Yang D, Yildiz S, You Y, Yue DKP, Zaneveld R, Zappa CJ.  2012.  Introduction to special section on Recent Advances in the Study of Optical Variability in the Near-Surface and Upper Ocean. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc007964   AbstractWebsite

Optical variability occurs in the near-surface and upper ocean on very short time and space scales (e.g., milliseconds and millimeters and less) as well as greater scales. This variability is caused by solar, meteorological, and other physical forcing as well as biological and chemical processes that affect optical properties and their distributions, which in turn control the propagation of light across the air-sea interface and within the upper ocean. Recent developments in several technologies and modeling capabilities have enabled the investigation of a variety of fundamental and applied problems related to upper ocean physics, chemistry, and light propagation and utilization in the dynamic near-surface ocean. The purpose here is to provide background for and an introduction to a collection of papers devoted to new technologies and observational results as well as model simulations, which are facilitating new insights into optical variability and light propagation in the ocean as they are affected by changing atmospheric and oceanic conditions.

Torrecilla, E, Stramski D, Reynolds RA, Millan-Nunez E, Piera J.  2011.  Cluster analysis of hyperspectral optical data for discriminating phytoplankton pigment assemblages in the open ocean. Remote Sensing of Environment. 115:2578-2593.   10.1016/j.rse.2011.05.014   AbstractWebsite

Optical measurements including remote sensing provide a potential tool for the identification of dominant phytoplankton groups and for monitoring spatial and temporal changes in biodiversity in the upper ocean. We examine the application of an unsupervised hierarchical cluster analysis to phytoplankton pigment data and spectra of the absorption coefficient and remote-sensing reflectance with the aim of discriminating different phytoplankton assemblages in open ocean environments under non-bloom conditions. This technique is applied to an optical and phytoplankton pigment data set collected at several stations within the eastern Atlantic Ocean, where the surface total chlorophyll-a concentration (TChla) ranged from 0.11 to 0.62 mg m(-3). Stations were selected on the basis of significant differences in the ratios of the two most dominant accessory pigments relative to TChla, as derived from High Performance Liquid Chromatography (HPLC) analysis. The performance of cluster analysis applied to absorption and remote-sensing spectra is evaluated by comparisons with the cluster partitioning of the corresponding HPLC pigment data, in which the pigment-based clusters serve as a reference for identifying different phytoplankton assemblages. Two indices, cophenetic and Rand, are utilized in these comparisons to quantify the degree of similarity between pigmentbased and optical-based clusters. The use of spectral derivative analysis for the optical data was also evaluated, and sensitivity tests were conducted to determine the influence of parameters used in these calculations (spectral range, smoothing filter size, and band separation). The results of our analyses indicate that the second derivative calculated from hyperspectral (1 nm resolution) data of the phytoplankton absorption coefficient, a(ph)(lambda), and remote-sensing reflectance, R(rs)(lambda), provide better discrimination of phytoplanIcton pigment assemblages than traditional multispectral band-ratios or ordinary (non-differentiated) hyperspectral data of absorption and remote-sensing reflectance. The most useful spectral region for this discrimination extends generally from wavelengths of about 425-435 nm to wavelengths within the 495-540 nm range, although in the case of phytoplankton absorption data a broader spectral region can also provide satisfactory results. (C) 2011 Elsevier Inc. All rights reserved.

Gernez, P, Stramski D, Darecki M.  2011.  Vertical changes in the probability distribution of downward irradiance within the near-surface ocean under sunny conditions. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007156   AbstractWebsite

Time series measurements of fluctuations in underwater downward irradiance, E(d), within the green spectral band (532 nm) show that the probability distribution of instantaneous irradiance varies greatly as a function of depth within the near-surface ocean under sunny conditions. Because of intense light flashes caused by surface wave focusing, the near-surface probability distributions are highly skewed to the right and are heavy tailed. The coefficients of skewness and excess kurtosis at depths smaller than 1 m can exceed 3 and 20, respectively. We tested several probability models, such as lognormal, Gumbel, Frechet, log-logistic, and Pareto, which are potentially suited to describe the highly skewed heavy-tailed distributions. We found that the models cannot approximate with consistently good accuracy the high irradiance values within the right tail of the experimental distribution where the probability of these values is less than 10%. This portion of the distribution corresponds approximately to light flashes with E(d) > 1.5 (E(d)) over bar, where (E(d)) over bar is the time-averaged downward irradiance. However, the remaining part of the probability distribution covering all irradiance values smaller than the 90th percentile can be described with a reasonable accuracy (i.e., within 20%) with a lognormal model for all 86 measurements from the top 10 m of the ocean included in this analysis. As the intensity of irradiance fluctuations decreases with depth, the probability distribution tends toward a function symmetrical around the mean like the normal distribution. For the examined data set, the skewness and excess kurtosis assumed values very close to zero at a depth of about 10 m.

Darecki, M, Stramski D, Sokolski M.  2011.  Measurements of high-frequency light fluctuations induced by sea surface waves with an Underwater Porcupine Radiometer System. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc007338   AbstractWebsite

Under clear skies the underwater light field within the near-surface ocean shows large fluctuations caused by focusing of sunlight by surface waves. The downwelling light at near-surface depths can fluctuate greatly on times scales as short as milliseconds and distances less than 1 cm. Specially designed radiometers and measurement strategies are required to adequately characterize these fluctuations. We developed an Underwater Porcupine Radiometer System which has a capability to measure light fluctuations with a sampling frequency of 1 kHz. This instrument is equipped with 23 radiometric sensors for measuring time series of the downward plane irradiance, E(d)(t), at several light wavelengths lambda and the downwelling radiance L(t) at lambda = 532 nm for different zenith angles within two orthogonal azimuthal planes. We describe the critical components of the Porcupine instrument and measurement protocols, and present example results from measurements made at near-surface depths in the ocean. We show that the irradiance collector of a few millimeters in diameter or smaller is required to provide adequate measurement of light flashes produced by wave focusing. The measurements with larger irradiance collectors can result in deceptive reduction of the measured intensity of fluctuations. The brightest flashes of irradiance or radiance can exceed the time-averaged irradiance or radiance by an order of magnitude and the duration of flashes is typically on the order of milliseconds to tens of milliseconds. The intensity of light fluctuations decreases rapidly with depth and is higher at longer light wavelengths compared with shorter wavelengths within the visible spectral range.