Rick A. Reynolds

Massive phytoplankton blooms under Arctic Sea ice

In midsummer, diatoms have taken advantage of thinning ice cover to feed in nutrient-rich waters. 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.

A chlorophyll‐dependent semianalytical reflectance model derived from field measurements of absorption and backscattering coefficients within the Southern Ocean

A semianalytical model was developed for the prediction of spectral remote sensing reflectance (Rrs) 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(λ), and backscattering, bb(λ), coefficients of seawater which were derived from field measurements. The relationships between a(λ) 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, Rrs(490)/Rrs(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 Rrs(490)/Rrs(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 Rrs versus Chl relationships between the two regions.

Optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at Imperial Beach, California

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 mm, and the mass concentrations of suspended particulate matter (SPM), particulate organic carbon (POC), and chlorophyll a (Chl). The particulate assemblage spanned a wide range of concentrations and composition, from the dominance of mineral 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. Citation: Woźniak, S. B., D. Stramski, M. Stramska, R. A. Reynolds, V. M. Wright, E. Y. Miksic, M. Cichocka, and A. M. Cieplak (2010), Optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at Imperial Beach, California, J. Geophys. Res., 115, C08027, doi:10.1029/2009JC005554.

Optical characterization of the oceanic unicellular cyanobacterium Synechococcus grown under a day‐night cycle in natural irradiance

The optical properties of the oceanic cyanobacterium Synechococcus (clone WH8103) were examined in a nutrient-replete laboratory culture grown under a day-night cycle in natural irradiance. Measurements of the spectral absorption and beam attenuation coefficients, the size distribution of cells in suspension, and microscopic analysis of samples were made at intervals of 2–4 hours for 2 days. These measurements were used to calculate the optical properties at the level of a single “mean” cell representative of the actual population, specifically, the optical cross sections for spectral absorption , scattering , and attenuation . In addition, concurrent determinations of chlorophyll a and particulate organic carbon allowed calculation of the Chl a- and C-specific optical coefficients. The refractive index of cells was derived from the observed data using a theory of light absorption and scattering by homogeneous spheres. Low irradiance because of cloudy skies resulted in slow division rates of cells in the culture. The percentage of dividing cells was unusually high (>30%) throughout the experiment. The optical cross sections varied greatly over a day-night cycle, with a minimum near dawn or midmorning and maximum near dusk. During daylight hours, and can increase more than twofold and by as much as 45%. The real part of the refractive index n increased during the day; changes in n had equal or greater effect than the varying size distribution on changes in and . The contribution of changes in n to the increase of during daylight hours was 65.7% and 45.1% on day 1 and 2, respectively. During the dark period, when decreased by a factor of 2.9, the effect of decreasing n was dominant (86.3%). With the exception of a few hours during the second light period, the imaginary part of the refractive index n′ showed little variation over a day-night cycle, and was largely controlled by variations in cell size. The real part of the refractive index at λ = 660 nm was correlated with the intracellular C concentration and the imaginary part at λ = 678 nm with the intracellular Chl a concentration. The C-specific attenuation coefficient showed significant diel variability, which has implications for the estimation of oceanic primary production from measurements of diel variability in beam attenuation. This study provides strong evidence that diel variability is an important component of the optical characterization of marine phytoplankton.

Evaluation and improvement of an iterative scattering correction scheme for in situ absorption and attenuation measurements

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.

Effects of inelastic radiative processes on the determination of water-leaving spectral radiance from extrapolation of underwater near-surface measurements.

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.

Determination of the volume scattering function of aqueous particle suspensions with a laboratory multi-angle light scattering instrument

We describe a methodology for determining the volume scattering function β(ψ) 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 β(ψ) from as much as ~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.

Within-day variability of particulate organic carbon and remote-sensing reflectance during a bloom of Phaeocystis antarctica in the Ross Sea, Antarctica

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.