Quantifying Contributions to Light Attenuation in Estuaries and Coastal Embayments: Application to Narragansett Bay, Rhode Island

Abstract

In Narragansett Bay, light attenuation by total suspended sediments (TSS), colored dissolved organic matter (CDOM), and phytoplankton chlorophyll-a (chl-a) pigment is 129, 97, and 70%, respectively, of that by pure seawater. Spatial distribution of light attenuation indicates higher values in the upper Bay, where rivers with sediment and nutrient-rich waters enter and elevate TSS, CDOM, and chl-a concentrations. The temporal trends of light attenuation during the summer months (July–August) differed at various locations in the Bay, having the highest values in July. For the same period, spectral methods overestimated attenuation throughout the Bay. These findings quantify the behavior of light attenuation in space and time, providing information that can guide decisions related to improving water clarity and help understanding the effects of various environmental and management scenarios on it.

Appendix A. Spectrum Distribution Function (Table 2)

The spectrum distribution function is calculated from the extraterrestrial solar irradiance at the top of the atmosphere. Gregg and Carder (1990) present mean extraterrestrial irradiance values at all wave lengths λ within the range 350 to 700 nm. Values of the spectrum distribution function are identified from this information. First, irradiance values at 5-nm increments are extracted, f(λ _j ) (W m⁻² nm⁻¹), λ _j is the wave length (nm), and j is a counter for values within the PAR (400–700 nm) (j = 1 to 61). Second, the normalized values, which represent the distribution function f(λ _j ), are obtained by dividing each f(λ _j ) by the total sum of all the extracted values, i.e.,

$$ f\left({\lambda}_j\right)=\frac{F\left({\lambda}_j\right)}{\sum_{j=1}^{61}F\left({\lambda}_j\right)} $$

(20)

where f(λj) (dimensionless) is the distribution function which has a unit area, i.e.,

$$ \sum_{j=1}^{61}f\left({\lambda}_j\right)=1.0 $$

(21)

It is assumed that the spectrum distribution function, f(λ _j ), maintains its shape and unit area throughout the atmosphere down to the water surface (Table 2). The 5-nm increment cancels out from the numerator and denominator in Eq. (20).