The propagation of sunlight and the size distribution of suspended particles in a municipally polluted ocean water

Abstract

Waste discharge by the Los Angeles County Sanitation District (LACSD) affects sunlight in the ocean. Increased sunlight attenuance affects productivity and changes in ocean surface color could be useful for monitoring dispersal. Temperature, sunlight irradiance in five colors, light beam attenuance, and particle size distributions were simultaneously measured as a function of water depth. A limited number of samples were analyzed by a co-worker for particulate percent organic carbon and particulate carbon isotope ratio. Background field stations included Catalina, Dana Point, and Rocky Point. Stations in the vicinity of the LACSD outfall at Whites Point represented a dispersing sewage field. The most significant result was that pollutants decreased the optical albedo of ocean water, increasing the extinction of sunlight irradiance as a function of optical depth. Additional findings were that 1) pollutants decreased euphotic zone depths by as much as 60 percent (depending upon particle concentrations), 2) water surface color spectra indicated that pollutants increased light absorption at short wavelengths (violet, blue and green), 3) the average particle concentration in polluted waters was twice that of background waters, 4) the LACSD discharge caused a bimodal increase in particle numbers as a function of particle size; there was a dramatic increase for particles less than 1.5 microns and a secondary increase at 8 microns, 5) assuming real particle refractive indices (no light absorption by particles), scattering by natural and sewage particles was maximum between particle diameters 3 and 8 microns, 6) calculations assuming complex refractive indices (absorbing particles) indicated that light absorption by sewage particles was maximum for particles less than 1.5 microns in diameter, and 7) heavy (high specific gravity) particles had lower percent organic carbon and a lower carbon isotope ratio.