Abstract
The downwelling light in seawater is shaped by natural seawater constituents as well as by some external substances which can occur locally and temporally. In this study we focused on dispersed oil droplets which can be found in seawater after an oil spill or in the consequence of intensive shipping, oil extraction and transportation. We applied our modified radiative transfer model based on Monte Carlo code to evaluate the magnitude of potential influence of dispersed oil droplets on the downwelling irradiance and the depth of the euphotic zone. Our model was validated on the basis of in situ measurements for natural (unpolluted) seawater in the Southern Baltic Sea, resulting in less than 5% uncertainty. The optical properties of dispersed Petrobaltic crude oil were calculated on the basis of Mie theory and involved into radiative transfer model. We found that the changes in downwelling light caused by dispersed oil depend on several factors such as oil droplet concentration, size distribution, and the penetration depth (i.e. vertical range of oil droplets occurrence below sea surface). Petrobaltic oil droplets of submicron sizes and penetration depth of 5 m showed a potentially detectable reduction in the depth of the euphotic zone of 5.5% at the concentration of only 10 ppb. Micrometer-sized droplets needed 10 times higher concentration to give a similar effect. Our radiative transfer model provided data to analyse and discuss the influence of each factor separately. This study contributes to the understanding of the change in visible light penetration in seawater affected by dispersed oil.
Highlights
The processes of attenuation of visible light in seawater have been studied since the development of modern oceanography in the 20th century in order to provide understanding of primary production of the oceans and marine ecosystem dynamics [1, 2]
In order to determine the scattering phase function for natural seawater we run a set of radiative transfer (RT) simulations using our model described in [25] with different Fournier-Forand (F-F) phase functions parameterized by different backscattering ratios, bb/b
As a result of numerous runs of RT simulations we obtained the depth-dependence of downwelling irradiance for natural seawater and seawater virtually polluted by previously described dispersions of Petrobaltic crude oil
Summary
We applied our modified radiative transfer model based on Monte Carlo code to evaluate the magnitude of potential influence of dispersed oil droplets on the downwelling irradiance and the depth of the euphotic zone. We found that the changes in downwelling light caused by dispersed oil depend on several factors such as oil droplet concentration, size distribution, and the penetration depth (i.e. vertical range of oil droplets occurrence below sea surface). Petrobaltic oil droplets of submicron sizes and penetration depth of 5 m showed a potentially detectable reduction in the depth of the euphotic zone of 5.5% at the concentration of only 10 ppb. This study contributes to the understanding of the change in visible light penetration in seawater affected by dispersed oil.
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