Dariusz Stramski

Light scattering by microorganisms in the open ocean

Abstract Recent enumeration and identification of marine particles that are less than 2μm in diameter, suggests that they may be the major source of light scattering in the open ocean. The living components of these small particles include viruses, heterotrophic and photoautotrophic bacteria and the smallest eucaryotic cells. In order to examine the relative contribution by these (and other) microorganisms to scattering, we have calculated a budget for both the total scattering and backscattering coefficients (at 550nm) of suspended particles. This budget is determined by calculating the product of the numerical concentration of particles of a given category and the scattering cross-section of that category. Values for this product are then compared to values for the particulate scattering coefficients predicted by the models of GORDON and MOREL (1983) and MOREL (1988). In order to make such a comparison, we have estimated both the total scattering and backscattering cross-section of various microbial components that include viruses, heterotrophic bacteria, prochlorophytes, cyanobacteria, ultrananoplankton (2–8μm), larger nanoplankton (8–20μm) and microplankton (>20μm). Such determinations are based upon Mie scattering calculations and measurements of the cell size distribution and the absorption and scattering coefficients of microbial cultures. In addition, we have gathered published information on the numerical concentration of living and detrial marine particles in the size range from 0.03 to 100μm. The results of such a study are summarized as follows. The size distribution of microorganisms in the ocean roughly obeys an inverse 4th power law over three orders of magnitude in cell diameter, from 0.2 to 100μm. Thus, the size distribution of living organisms is similar to that for total particulate matter as determined by electronic particle counters. For representative values of refractive index, it appears that most of the scattering in the sea comes from particles less than 8μm in diameter, and that most of the backscattering comes from particles less than 1μm. Among the microorganisms that are found in this size range, free-living heterotrophic bacteria may be often most important. These microbes account typically for 10 to 50% of the total particulate scattering and 5 to 20% of backscattering in oligotrophic waters that contain less than 0.5mg chlorophyll per m3. The second most important source of microbial scattering is cyanobacteria (especially in tropical and temperate waters) and ultrananoplankton. Viruses, which may be very abundant, make little contribution because they have extremely small cellular scattering cross-sections. Large microorganisms, which include nanoplankton >8μm and microplankton and which efficiently scatter light, contribute little because of their low cellular concentrations. While a significant fraction of the total scattering coefficient may come from the combined contributions of viable procaryotic and eucaryotic cells, only a small fraction of the backscattering appears to come from these microbes. Instead, it appears that the major source of particulate backscattering is small (

Modeling the inherent optical properties of the ocean based on the detailed composition of the planktonic community

We describe an approach to modeling the oceans inherent optical properties (IOPs) that permits extensive analyses of IOPs as the detailed composition of suspended particulate matter is varied in a controlled manner. Example simulations of the IOP model, which includes 18 planktonic components covering a size range from submicrometer viruses and heterotrophic bacteria to microplanktonic species of 30-mum cell diameter, are discussed. Input data to the model include the spectral optical cross sections on a per particle basis and the particle-number concentration for each individual component. This approach represents a significant departure from traditional IOP and bio-optical models in which the composition of seawater is described in terms of a few components only or chlorophyll concentration alone. The simulations illustrate how the separation and understanding of the effects of various types of particle present within a water body can be achieved. In an example simulation representing an oligotrophic water body with a chlorophyll a concentration of 0.18 mg m(-3), the planktonic microorganisms altogether are the dominant particulate component in the process of light absorption, but their relative contribution to light scattering is smaller than that of nonliving particles. A series of simulations of water bodies with the same chlorophyll a concentration but dominated by different phytoplankton species shows that composition of the planktonic community is an important source of optical variability in the ocean.

Diffuse Attenuation Coefficient of Downwelling Irradiance: An Evaluation of Remote Sensing Methods

coastal waters, with Kd(490) ranging from � 0.04 to 4.0 m � 1 . The derived values are compared with the data calculated from in situ measurements of the vertical profiles of downwelling irradiance. The comparisons show that the two standard methods produced satisfactory estimates of Kd(l) in oceanic waters where attenuation is relatively low but resulted in significant errors in coastal waters. The newly developed semianalytical method appears to have no such limitation as it performed well for both oceanic and coastal waters. For all data in this study the average of absolute percentage difference between the in situ measured and the semianalytically derived Kd is � 14% for l = 490 nm and � 11% for l = 443 nm.

Modeling the optical properties of mineral particles suspended in seawater and their influence on ocean reflectance and chlorophyll estimation from remote sensing algorithms

The optical properties of mineral particles suspended in seawater were calculated from the Mie scattering theory for different size distributions and complex refractive indices of the particles. The ratio of the spectral backscattering coefficient to the sum of the spectral absorption and backscattering coefficients of seawater, b(b)(lambda)/[a(lambda) + b(b)(lambda)], was analyzed as a proxy for ocean reflectance for varying properties and concentrations of mineral particles. Given the plausible range of variability in the particle size distribution and the refractive index, the general parameterizations of the absorption and scattering properties of mineral particles and their effects on ocean reflectance in terms of particle mass concentration alone are inadequate. The variations in the particle size distribution and the refractive index must be taken into account. The errors in chlorophyll estimation obtained from the remote sensing algorithms that are due to the presence of mineral particles can be very large. For example, when the mineral concentration is 1 g m(-3) and the chlorophyll a concentration is low (0.05 mg m(-3)), current global algorithms based on a blue-to-green reflectance ratio can produce a chlorophyll overestimation ranging from approximately 50% to as much as 20-fold.

Estimation of the inherent optical properties of natural waters from the irradiance attenuation coefficient and reflectance in the presence of Raman scattering

By means of radiative transfer simulations we developed a model for estimating the absorption a, the scattering b, and the backscattering b(b) coefficients in the upper ocean from irradiance reflectance just beneath the sea surface, R(0-), and the average attenuation coefficient for downwelling irradiance, 1, between the surface and the first attenuation depth. The model accounts for Raman scattering by water, and it does not require any assumption about the spectral shapes of a, b, and b(b). The best estimations are obtained for a and b(b) in the blue and green spectral regions, where errors of a few percent to <10% are expected over a broad range of chlorophyll concentration in water. The model is useful for satellite ocean color applications because the model input, R(0-) and 1, can be retrieved from remote sensing and the model output, a and b(b), is the major determinant of remote-sensing reflectance.

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.

Bio-optical relationships and ocean color algorithms for the north polar region of the Atlantic

Received 30 October 2001; revised 30 October 2001; accepted 11 February 2003; published 13 May 2003. [1] Up to now, relatively few bio-optical measurements have been made in the high northern latitude waters, which allow sound relationships for ocean color remote sensing to be determined. We collected optical and chlorophyll a concentration, Chl, data in the north polar region of the Atlantic in summer season. The investigated region includes subarctic and arctic waters between 70� N and 80� N within the meridional zone between 1� E and 20� E. Our measurements show that the current NASA global algorithms, OC2, OC4, and chlor-MODIS, generally overpredict Chl in the investigated waters by a factor of about 2 at low pigment concentrations (<0.2 mg m � 3 ) and underpredict Chl at higher concentrations (20–50%at2–3mgm � 3 ).Forourdataset,thebesttwo-bandalgorithmforChlinvolvesthe ratio of remote-sensing reflectance, Rrs(442)/Rrs(555), at 442-nm and 555-nm light wavebands. We found that the general trend of variation in the blue-to-green reflectance ratio, Rrs(442)/Rrs(555) or Rrs(490)/Rrs(555), with Chl was driven primarily by Chldependent change in the green-to-blue ratio of absorption by pure seawater and particles. The effect of the blue-to-green backscattering ratio was of secondary importance. We observed acharacteristic opticaldifferentiation ofwaterswithin theinvestigated region. The majority of waters, which are here hypothesized to be dominated by diatoms, exhibited a relatively high blue-to-green reflectance ratio. The waters at several other stations, presumably dominated by dinoflagellates and/or prymnesiophytes, showed much lower reflectance ratio. Our data also show that the seemingly random variations in particulate absorption and backscattering coefficients at any given Chl are significant (more than a factor of 2) in the investigated waters. INDEXTERMS: 4552 Oceanography: Physical: Ocean optics; 4275 Oceanography: General: Remote sensingand electromagnetic processes (0689); 9325 Information Related to Geographic Region: Atlantic Ocean; 9315 Information Related to Geographic Region: Arctic region;

Optical properties of photosynthetic picoplankton in different physiological states as affected by growth irradiance

Abstract A phycocyanin-rich cyanobacterium belonging to the genus Synechocystis has been adapted and grown under differing irradiances (PAR), ranging from 16 to 1450 μE m −2 s −1 , and differing spectral compositions (“white”, “blue” and “green”). Chlorophyll-specific as well as carbon-specific spectral absorption and scattering coefficients were determined for all conditions. Due to drastic changes in chlorophyll and phycocyanin content per cell in response to the radiative level imposed to the culture, these coefficients undergo extreme variations, in a range wider than the inter-specific range already reported for eucaryotic algae. The optical dimensionless efficiency factors have been computed and used to calculate the bulk refractive index (in the range 1.05–1.06 with respect to the index of water). The optical properties of this picoplanktonic species is typical of “small” optical particles, with a scattering efficiency increasing towards the blue, and a backscattering efficiency increasing towards the red end of the spectrum. Superimposed on this pattern are features associated with the presence of pigments, including the phycocyanin signature. Although the cellular pigment concentrations are high (particularly at low irradiances), the package effect remains negligible. Thus Synechocystis is well suited for harvesting light, even if the presence of biliprotein appears to be useless in regards to the spectral quality of the light available in the deep layers of the euphotic zone.

Bubble entrainment by breaking waves and their influence on optical scattering in the upper ocean

Breaking waves at the oceans surface inject bubbles and turbulence into the water column. During periods of rough weather the scales of wave breaking will increase with increasing sea states and result in mixing of the surface waters and the turbulent transport of bubbles to depth. Depending on their concentrations and size distribution, the entrained bubbles can significantly change the optical properties of water, introducing potentially significant errors in retrieval of remotely sensed hyperspectral data products. In this paper, the effects of bubbles on optical scattering in the upper ocean are investigated through optical scattering calculations based on field measurements of bubble populations. The field measurements were obtained offshore Point Conception, California, in June 1997, using an acoustical technique which measured the bubble size distribution at 2 Hz from a surface buoy designed to follow the longer waves. The effects of the bubbles on the bulk optical scattering and backscattering coefficients, b and bb, respectively, are determined by using the acoustically measured size distributions, and size-dependent scattering efficiencies based on Mie scattering calculations. Time series of the bubble distributions measured in rough conditions (wind speed, U10 = 15 m/s, significant wave height, H1/3 = 3.2 m) suggest that the bubble contribution to light scattering is highly variable near the ocean surface, with values spanning roughly 5 decades over time periods of O(10) minutes. Bubble size distributions measured at a 0.7-m depth indicate that the optical effects of the bubbles on bb, and hence the remote sensing reflectance, will be significant at bubble void fractions above 10−6 and that the bubble contribution to total bb will exceed values of 10−2 m−1 inside bubble clouds.

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.