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Interrelationships between optical parameters, biological parameters and particle size distributions in Monterey Bay

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  • Spectral beam attenuation coefficients, spectral volume scattering function meaurements at 45°, 90°, and 130° particle size distributions and chlorophyll a and phaeophytin pigment concentrations were measured during May and September, 1977 in Monterey Bay, California. This data is examined and statistical interrelationships between the parameters measured are explored. It was found that beam attenuation and suspended particulate volume concentration are highly correlated with the slope of the particle size distribution, low slopes corresponding to higher particle concentrations. Correspondingly, the slope of the spectrum of the particulate beam attenuation decreases with increasing particle concentration. This decrease is not as much as theoretical values based on hyperbolic size distributions extending to zero particle size would predict, but such theoretical values are based on diameters which we cannot measure nor do we expect the hyperbolic distribution to hold to such small sizes. Changes in the type of particles present with changing particle concentration also affect the spectral properties. Variations in the spectral behavior with location both horizontally and vertically were found. These variations were partially explained by corresponding changes in particle size distributions and biological parameters. The volume scattering functions behaved similarly to the beam attenuation coefficients except that the particle size distributions were not able to explain much additional variation in the scattering beyond that explained by total attenuation and location parameters. Unmeasured parameters such as particle shape may be more important for scattering than attenuation in explaining spectral and angular differences between locations. The particle size distributions and especially the larger particles were more important than the pigment parameters in predicting certain ratios of particulate spectral beam attenuation coefficients such as Cp(450)/Cp(500), Cp(400)/Cp(650) and CP450/Cp(650). The ratio of pigments to suspended volume was also significant in determining these ratios, and is believed to be related to the effective index of refraction for the collection of suspended particles. An exception was the parameter Cp(400)/Cp(450) which was not predicted well by the slope of the size distribution. Instead, the pigment-suspended volume ratio and total suspended volume were foremost in importance followed by the concentration of the smallest measured particles (1.75-2.5 um diameter) and the ratio of the small particles to medium-sized particles (6.2-10 um). This particle ratio may extrapolate to the very, small size particles better than the overall slope and thus be related to Rayleigh (a-4) scattering which might affect Cp(400)/Cp(450) while the large particle scattering dominates the other ratios. Particle size distributions were best predicted by the sum of the attenuation values and Cp(400)/Cp(650). While several other parameters removed a statistically significant amount of additional variance, they did not improve the models for practical purposes, Particles less than 10 um diameter had much smaller original variance (on a logarithmic scale) than the larger particles and not much variance was removed by the attenuation parameters. Better correlations would have resulted if an electronically tripped rosette sampler had been attached to the transmissometer package instead of relying on separate bottle costs. The findings of this report were not even consistent between. May and September and therefore it is highly unlikely that they could be applied to other regions. A better approach would be to confine the area of interest both vertically and geographically, determine typical particle collections on which to do laboratory experiments and then use matrix minimization techniques to decompose measured optical spectra into the predetermined possible components.
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