Ms. Brenda E. Hargreaves Profile

Brenda E. Hargreaves

Event Marketing Manager at Xmark Media Ltd

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Proceedings Article | 6 February 1997 Paper

UVR attenuation in lakes: relative contibutions of dissolved and particulate material

L. M. Ayoub, Brenda Hargreaves, D. P. Morris

Proceedings Volume 2963, (1997) https://doi.org/10.1117/12.266426

KEYWORDS: Signal attenuation, Absorption, Optical filters, Phase modulation, Absorption filters, Ultraviolet radiation, Spectrophotometry, Absorbance, Scattering, Particles

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Solar radiation in lakes is attenuated by dissolved material, especially dissolved organic material (DOC), particulate material (PM), and water. DOC is a strong predictor of the attenuation of Ultraviolet Radiation (UVR) in lakes. Phytoplankton and detritus are known to contribute substantially to PAR attenuation but relatively little is known about their role in attenuation of UVR. This study investigated the relative contributions of dissolved and particulate material to the attenuation of UVR in lakes by combining an adapted Quantitative Filter Technique (QFT) with laboratory measurements of absorption by DOC and field measurements of UVR diffuse attenuation. The absorption of filtrate and PM filtered onto glass fiber filters were scanned by a UV-visible dual beam spectrophotometer. Total absorption coefficient (a_t) was computed as the sum of the absorption coefficients for water (a_w), dissolved material (a_d), and PM (a_p). The value of a_t was compared with the diffuse attenuation coefficient (K_d) measured directly in the lakes by a Profiling UV radiometer. The ratio a_t/K_d ranged between 0.9 and 1.3 for UVR. Ratios less than unity may be attributed to scattering and to sun angle effects, especially at the longer UVR wavelengths. Ratios occasionally were measured above unity, suggesting errors in estimating a_t. Particles played a significant and seasonally-varying role in lake UVR attenuation. Within oligotrophic, low DOC Lake Giles, the relative contribution of a_p varied from 30 percent to 53 percent for 320 nm UVR. In mesotrophic, higher DOC Lake Lacawac the seasonal range was 7.6 percent to 57 percent. In each case, the highest contribution of PM was found during early spring and late fall.

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