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2016
Li, L, Stramski D, Reynolds RA.  2016.  Effects of inelastic radiative processes on the determination ofwater-leaving spectral radiance from extrapolation of underwaternear-surface measurements. Appl. Opt.. 55:7050–7067.: OSA   10.1364/AO.55.007050   AbstractWebsite

Extrapolation of near-surface underwater measurements is the most common method to estimate the water-leaving spectral radiance, Lw(λ) (where λ is the light wavelength in vacuum), and remote-sensing reflectance, Rrs(λ), for validation and vicarious calibration of satellite sensors, as well as for ocean color algorithm development. However, uncertainties in Lw(λ) 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, Lu(λ), and its diffuse attenuation coefficient, KLu(λ), within the top 10 m of the ocean surface layer and assess the uncertainties in extrapolated values of Lw(λ). The inelastic processes generally increase Lu and decrease KLu in the red and near-infrared (NIR) portion of the spectrum. Unlike KLu in the blue and green spectral bands, KLu 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 KLu with depth that is typically employed in the extrapolation method can lead to significant errors in the estimate of Lw. These errors approach ~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 Lu 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.

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(5):1869-1890.   10.1002/lno.10341   AbstractWebsite

The magnitude and spectral shape of the optical backscattering coefficient of particles, bbp(λ), is being increasingly used to infer information about the particles present in seawater. Relationships between bbp 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 bbp(λ) 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 bbp(λ) 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(4):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.

2015
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. Appl. Opt.. 54(22):6763–6782.: OSA   10.1364/AO.54.006763   Abstract

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.

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.   http://dx.doi.org/10.1016/j.rse.2015.09.027   AbstractWebsite

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.

IOCCG.  2015.  Ocean Colour Remote Sensing in Polar Seas. No. 16( Babin M, Arrigo , Bélanger S, Forget M-H, Eds.).:130., Dartmouth, Canada: International Ocean Colour Coordinating GroupWebsite
2014
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.2014.59.6.1839   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.

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 I. 87:53-69.   10.1016/j.dsr.2014.02.005   Abstract

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 (40K) indicates that the solar irradiance dominates over the 40K-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.

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.   10.1016/j.dsr2.2014.03.014   AbstractWebsite

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.

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   Abstract

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.

Arrigo, KR, Perovich DK, Pickart RS, Brown ZW, van Dijken GL, Lowry KE, Mills MM, Palmer MA, Balch WM, Bates NR, Benitez-Nelson CR, Brownlee E, Frey KE, Laney SR, Mathis J, Matsuoka A, Mitchell GB, Moore GWK, Reynolds RA, Sosik HM, Swift JH.  2014.  Phytoplankton blooms beneath the sea ice in the Chukchi sea. Deep Sea Research Part II: Topical Studies in Oceanography. 105:1-16.   http://dx.doi.org/10.1016/j.dsr2.2014.03.018   AbstractWebsite

In the Arctic Ocean, phytoplankton blooms on continental shelves are often limited by light availability, and are therefore thought to be restricted to waters free of sea ice. During July 2011 in the Chukchi Sea, a large phytoplankton bloom was observed beneath fully consolidated pack ice and extended from the ice edge to >100 km into the pack. The bloom was composed primarily of diatoms, with biomass reaching 1291 mg chlorophyll a m−2 and rates of carbon fixation as high as 3.7 g C m−2 d−1. Although the sea ice where the bloom was observed was near 100% concentration and 0.8–1.2 m thick, 30–40% of its surface was covered by melt ponds that transmitted 4-fold more light than adjacent areas of bare ice, providing sufficient light for phytoplankton to bloom. Phytoplankton growth rates associated with the under-ice bloom averaged 0.9 d−1 and were as high as 1.6 d−1. We argue that a thinning sea ice cover with more numerous melt ponds over the past decade has enhanced light penetration through the sea ice into the upper water column, favoring the development of these blooms. These observations, coupled with additional biogeochemical evidence, suggest that phytoplankton blooms are currently widespread on nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in waters where under-ice blooms develop are  10-fold too low. These massive phytoplankton blooms represent a marked shift in our understanding of Arctic marine ecosystems.

Gernez, P, Reynolds RA, Stramski D.  2014.  Within-day variability of particulate organic carbon and remote-sensing reflectance during a bloom of Phaeocystis antarctica within the Ross Sea, Antarctica. International Journal of Remote Sensing. 35(2):454-477.   10.1080/01431161.2013.871598   Abstract

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 (Rrs). 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 Rrs, including the blue-to-green spectral band ratios used in ocean colour algorithms for estimating POC and Chl. The variability in Rrs 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 Rrs was unaffected by within-day variability, implying that short-term variations in surface POC can be potentially monitored by multiple within-day measurements of Rrs, 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.

2013
McKee, D, Piskozub J, Röttgers R, Reynolds RA.  2013.  Evaluation and improvement of an iterative scattering correction scheme for in situ absorption and attenuation measurements. Journal of Atmospheric and Oceanic Technology. 30(7):1527-1541.: American Meteorological Society   10.1175/JTECH-D-12-00150.1   AbstractWebsite

The performance of several scattering correction schemes for reflecting-tube absorption and beam attenuation measurements is evaluated with data collected in European shelf seas. Standard scattering correction procedures for absorption measurements perform poorly because of nonzero absorption in the near infrared and wavelength-dependent scattering phase functions. A previously described iterative correction procedure based on Monte Carlo simulations of the Western Environmental Technologies Laboratories (WET Labs) ac-9 and independent estimates of particle backscattering initially performs poorly, but is greatly improved when realistic losses at flow-tube walls are incorporated into the model. The updated Monte Carlo scattering correction provides excellent agreement with independent absorption and attenuation measurements made with a point-source integrating-cavity absorption meter (PSICAM) and a Laser In Situ Scattering and Transmissometer (LISST, Sequoia Scientific), respectively. Implications for historic datasets and requirements for application to future datasets are discussed.

2012
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.   10.1364/AO.51.003853   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

Arrigo, KR, Perovich DK, Pickart RS, Brown ZW, van Dijken GL, Lowry KE, Mills MM, Palmer MA, Balch WM, Bahr F, Bates NR, Benitez-Nelson C, Bowler B, Brownlee E, Ehn JK, Frey KE, Garley R, Laney SR, Lubelczyk L, Mathis J, Matsuoka A, Mitchell GB, Moore GWK, Ortega-Retuerta E, Pal S, Polashenski CM, Reynolds RA, Schieber B, Sosik HM, Stephens M, Swift JH.  2012.  Massive Phytoplankton Blooms Under Arctic Sea Ice. Science. 336:1408.   10.1126/science.1215065   AbstractWebsite

Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

2011
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.

2010
Uitz, J, Stramski D, Baudoux AC, Reynolds RA, Wright VM, Dubranna J, Azam F.  2010.  Variations in the optical properties of a particle suspension associated with viral infection of marine bacteria. Limnology and Oceanography. 55:2317-2330.   10.4319/lo.2010.55.6.2317   AbstractWebsite

A laboratory experiment was conducted to examine the temporal dynamics of the particle size distribution (PSD) and associated optical variability caused by viral infection of marine heterotrophic bacteria. The PSD covering a broad range of particle size from approximate to 50 nm to 200 mu m was measured in parallel with the spectral particulate absorption and beam attenuation coefficients, from which the particulate scattering coefficient, b(p)(lambda), was determined. Within 12 h following infection, the host bacterial population collapsed, viral abundance increased, and submicron particles were produced as bacteria were disrupted and cell debris released, resulting in a large decrease in b(p)(lambda) seen as an almost complete clearing of the particle suspension. Throughout the remainder of the experiment, significant changes in the PSD occurred primarily within the size range of relatively large particles (> 4 mu m), likely as a result of the aggregation of smaller-sized particles originating from the host lysis. The PSD data were used as input for Mie scattering calculations to evaluate the effects of these particle dynamics in terms of relative contributions of different particle size classes to the scattering and backscattering coefficients. This analysis showed a significant increase in the effect of particle aggregation on light scattering during the second and third days after infection. Viral lysis of bacteria and subsequent particle dynamics produce large variations in the PSD over a broad size range on timescales from hours to a few days, and such processes lead to correspondingly large changes in the suspension optical properties.

Reynolds, RA, Stramski D, Wright VM, Wozniak SB.  2010.  Measurements and characterization of particle size distributions in coastal waters. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc005930   AbstractWebsite

The particle size distribution (PSD) plays a central role in understanding many facets of the aquatic ecosystem, yet it is rarely measured in field studies and no single method provides a complete description of the PSD. In this study, size distributions of diverse particle suspensions were measured using a laser diffractometer (LISST-100X), an electrical impedance particle sizer (Coulter Counter), and a particle imaging system (FlowCAM). All three instruments provided similar estimates of average particle size for suspensions of known standards. For broad polydisperse assemblages of particles a generally good agreement was found between the LISST and Coulter over a large portion of the size spectrum (from similar to 1-3 mm to 50 mu m), with the exception of suspensions exhibiting narrow features which were not accurately resolved with the LISST measurement. For featureless PSDs, however, the LISST provides an adequate proxy and has the capability for in situ measurements with high spatial and temporal resolution. We examined LISST field measurements from coastal regions within the context of a commonly used parameterization of the PSD. Analysis of nearly 5500 size distributions suggest that the average slope of the power law distribution for particles larger than 3 mu m is -3.5. However, in many coastal waters this model provides a poor description of the PSD owing to the presence of significant peaks in the distribution. The combination of these data with Mie scattering calculations suggest that such departures from the idealized PSD can significantly impact the prediction of seawater optical properties.

Wozniak, SB, Stramski D, Stramska M, Reynolds RA, Wright VM, Miksic EY, Cichocka M, Cieplak AM.  2010.  Optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at Imperial Beach, California. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc005554   AbstractWebsite

[1] We examined optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at Imperial Beach, California, over a period of 1.5 years. Measurements included the hyperspectral inherent optical properties (IOPs) of seawater (particulate beam attenuation, particulate and CDOM absorption coefficients within the spectral range 300-850 nm), particle size distribution (PSD) within the diameter range 2-60 mu m, and the mass concentrations of suspended particulate matter (SPM), particulate organic carbon (POC), and chlorophyll a (Ch1). The particulate assemblage spanned a wide range of concentrations and composition, from the dominance of mineral particles (POC/SPM < 0.06) with relatively steep PSDs to the high significance or dominance of organic particles (POC/SPM > 0.25) with considerably greater contribution of larger-sized particles. Large variability in the particulate characteristics produced correspondingly large variability in the IOPs; up to 100-fold variation in particulate absorption and scattering coefficients and several-fold variation in the SPM-specific and POC-specific coefficients. Analysis of these data demonstrates that knowledge of general characteristics about the particulate composition and size distribution leads to improved interpretations of the observed optical variability. We illustrate a multistep empirical approach for estimating proxies of particle concentration (SPM and POC), composition (POC/SPM), and size distribution (median diameter) from the measured IOPs in a complex coastal environment. The initial step provides information about a proxy for particle composition; other particulate characteristics are subsequently derived from relationships specific to different categories of particulate composition.

2008
Huot, Y, Morel A, Twardowski MS, Stramski D, Reynolds RA.  2008.  Particle optical backscattering along a chlorophyll gradient in the upper layer of the eastern South Pacific Ocean. Biogeosciences. 5:495-507.   10.5194/bg-5-495-2008   AbstractWebsite

The particulate scattering, b(p), and backscattering, b(bp), coefficients are determined by the concentration and physical properties of suspended particles in the ocean. They provide a simple description of the influence of these particles on the scattering of light within the water column. For the remote observation of ocean color, bbp along with the total absorption coefficient govern the amount and spectral qualities of light leaving the sea surface. However, for the construction and validation of ocean color models measurements of b(bp) are still lacking, especially at low chlorophyll a concentrations ([Ch1]). Here, we examine the relationships between spectral b(bp) and b(p) vs. [Ch1] along an 8000 km transect crossing the Case 1 waters of the eastern South Pacific Gyre. In these waters, over the entire range of [ Ch1] encountered ( similar to 0.02-2 mg m(-3)), both b(bp) and b(p) can be related to [Ch1] by power functions (i.e. bp or bbp = alpha[ Chl](beta)). Regression analyses are carried out to provide the parameters alpha and beta for several wavelengths throughout the visible for both b(bp) and b(p). When applied to the data, these functions retrieve the same fraction of variability in b(bp) and b(p) ( coefficients of determination between 0.82 and 0.88). The b(bp) coefficient fall within the bounds of previous measurements at intermediate and high [ Ch1] recently published. Its dependence on [ Chl] below similar to 0.1 mg m(-3) is described for the first time with in situ data. The backscattering ratio (i.e. b(bp)/b(p)) with values near 0.01 for all stations appears to be spectrally neutral and not significantly dependent on [ Ch1]. These results should foster the development of improved forward models of the mean optical properties for oceanic Case 1 waters as well as inverse models based upon them.

Stramski, D, Reynolds RA, Babin M, Kaczmarek S, Lewis MR, Rottgers R, Sciandra A, Stramska M, Twardowski MS, Franz BA, Claustre H.  2008.  Relationships between the surface concentration of particulate organic carbon and optical properties in the eastern South Pacific and eastern Atlantic Oceans. Biogeosciences. 5:171-201.   10.5194/bg-5-171-2008   AbstractWebsite

We have examined several approaches for estimating the surface concentration of particulate organic carbon, POC, from optical measurements of spectral remote-sensing reflectance, R-rs(lambda), using field data collected in tropical and subtropical waters of the eastern South Pacific and eastern Atlantic Oceans. These approaches include a direct empirical relationship between POC and the blue-to-green band ratio of reflectance, R-rs(lambda(B)) / R-rs(555), and two-step algorithms that consist of relationships linking reflectance to an inherent optical property IOP (beam attenuation or backscattering coefficient) and POC to the IOP. We considered two-step empirical algorithms that exclusively include pairs of empirical relationships and two-step hybrid algorithms that consist of semianalytical models and empirical relationships. The surface POC in our data set ranges from about 10 mg m(-3) within the South Pacific Subtropical Gyre to 270 mgm(-3) in the Chilean upwelling area, and ancillary data suggest a considerable variation in the characteristics of particulate assemblages in the investigated waters. The POC algorithm based on the direct relationship between POC and R-rs(lambda(B)) / R-rs(555) promises reasonably good performance in the vast areas of the open ocean covering different provinces from hyperoligotrophic and oligotrophic waters within subtropical gyres to eutrophic coastal upwelling regimes characteristic of eastern ocean boundaries. The best error statistics were found for power function fits to the data of POC vs. Rrs(443) / R-rs(555) and POC vs. R-rs(490) / R-rs(555). For our data set that includes over 50 data pairs, these relationships are characterized by the mean normalized bias of about 2% and the normalized root mean square error of about 20%. We recommend that these algorithms be implemented for routine processing of ocean color satellite data to produce maps of surface POC with the status of an evaluation data product for continued work on algorithm development and refinements. The two-step algorithms also deserve further attention because they can utilize various models for estimating IOPs from reflectance, offer advantages for developing an understanding of bio-optical variability underlying the algorithms, and provide flexibility for regional or seasonal parameterizations of the algorithms.

2007
Hopkinson, BM, Mitchell G, Reynolds RA, Wang H, Selph KE, Measures CI, Hewes CD, Holm-Hansen O, Barbeau KA.  2007.  Iron limitation across chlorophyll gradients in the southern Drake Passage: Phytoplankton responses to iron addition and photosynthetic indicators of iron stress. Limnology and Oceanography. 52:2540-2554.   10.4319/lo.2007.52.6.2540   AbstractWebsite

Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron-enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1-2 nmol L-1) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream high-chlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (F-v: F-m, sigma(PSII), and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.

2001
Reynolds, RA, Stramski D, Mitchell BG.  2001.  A chlorophyll-dependent semianalytical reflectance model derived from field measurements of absorption and backscattering coefficients within the Southern Ocean. Journal of Geophysical Research-Oceans. 106:7125-7138.   10.1029/1999jc000311   AbstractWebsite

A semianalytical model was developed for the prediction of spectral remote sensing reflectance (R-rs) as a function of fluorometric chlorophyll a concentration (Chl) for two regions within the Southern Ocean: the Ross Sea and the Antarctic Polar Front Zone (APFZ). The model is based upon Chl-dependent parameterizations of the spectral absorption, a(lambda), and backscattering, b(b)(lambda), coefficients of seawater which were derived from field measurements. The relationships between a(lambda) and Chl were similar in both regions, but for comparable Chl the particulate backscattering was on average 4 times greater in the APFZ. Measurements of particle size distributions suggest that particle assemblages in the APFZ were characterized by a greater predominance of smaller particles, consistent with the observed regional differences in backscattering properties. The model is used to examine the separate influences of absorption and backscattering on the blue to green ratio of reflectance, R-rs(490)/R-rs(555). Variability in the spectral absorption ratio, resulting principally from changes in the relative contribution of water to total absorption in each band, contributes >75% to changes in the R-rs(490)/R-rs(555) ratio as a function of Chl. However, variability in the spectral backscattering ratio appears to be the primary cause for the observed differentiation in the R-rs versus Chl relationships between the two regions.

1999
Stramski, D, Reynolds RA, Kahru M, Mitchell BG.  1999.  Estimation of particulate organic carbon in the ocean from satellite remote sensing. Science. 285:239-242.   10.1126/science.285.5425.239   AbstractWebsite

Measurements from the Southern Ocean show that particulate organic carbon (POC) concentration is welt correlated with the optical backscattering by particles suspended in seawater. This relation, in conjunction with retrieval of the backscattering coefficient from remote-sensing reflectance, provides an algorithm for estimating surface POC from Satellite data of ocean color. Satellite imagery from SeaWiFS reveals the seasonal progression of POC, with a zonal band of elevated POC concentrations in December coinciding with the Antarctic Polar Front Zone. At that time, the POC pool within the top 100 meters of the entire Southern Ocean south of 40 degrees S exceeded 0.8 gigatons.

1997
Reynolds, RA, Stramski D, Kiefer DA.  1997.  The effect of nitrogen limitation on the absorption and scattering properties of the marine diatom Thalassiosira pseudonana. Limnology and Oceanography. 42:881-892. Abstract[link to article]

Optical properties of the marine diatom Thalassiosira pseudonana were examined in nitrate-limited semicontinuous cultures for growth rates (mu(N)) varying from 0.22 to 1.32 d(-1). Two experiments were conducted at each growth rate in which sampling of the culture was done at either the dark-to-light (D-L) or light-to-dark (GD) transition of a 12-h photoperiod. Optical cross sections and efficiency factors for absorption, scattering, and attenuation were calculated from measurements of the spectral absorption and beam attenuation coefficients, cell concentration, and the size distribution of cells in suspension. The refractive index of cells, relative to seawater, was calculated from these data using an inverse method. Nitrogen limitation strongly influences the optical properties of this species. Absorption cross sections increased more than twofold with increasing mu(N), predominantly caused by increases in the imaginary part of the refractive index, n'. At the red absorption peak, n'(673) increased 140% and was strongly correlated with intracellular Chi a concentration. For most visible light wavelengths, scattering cross sections were not a strong function of growth rate but were consistently largest in the GD transition experiments. The difference between D-L and GD experiments at any given mu(N) increased with increasing growth rate; for example, scattering cross sections at 660 nm in the GD experiments were 25% (mu(N) = 0.22 d(-1)) and 90% (mu(N) = 1.32 d(-1)) larger than in the respective D-L experiments. The real part of the refractive index at lambda = 660 nn varied from 1.024 to 1.039 among all experiments and was positively correlated with intracellular carbon concentration. The Chl a-specific absorption coefficient decreased with increasing mu(N) (e.g. 43% decrease in the blue Soret band) in response to decreases in the carotenoid-to-Chl a ratio and increased pigment packaging. The Chi a-specific scattering coefficient at 660 nm decreased from 0.55 to 0.09 m(2) (mg Chi a)(-1) with increases in mu(N). Our results suggest that the optical properties of phytoplankton populations in nitrogen-limited surface waters may be significantly altered from those in deeper portions of the water column.