[1] 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 ∼1-3 mm to 50 mm), 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 mm 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.

From laboratory measurements, we determined the spectral mass-specific absorption, a ranged from about 0.5 to 1.5 m 2 g 21 , and the spectral behavior varied from nearly flat spectra to the spectral dependency ,l 2g with a slope g as high as ,1.3 for the sample with the largest contribution of small particles.

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.

[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 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 < 0.06) 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. Cieplak (2010), Optical variability of seawater in relation to particle concentration, composition, and size distribution in the nearshore marine environment at