Light Data Processing

Light data were interpolated by the NOAA Coastal Services Center and University of Rhode Island (URI) staff from the following sources.

Candace Oviatt and colleagues collected the primary source of light data between 1997 and 1998. Data on light extinction was collected biweekly at 16 stations located in Narragansett Bay and the lower Providence River.
A second set of light extinction data was collected in 1994 by Blaine Kopp and colleagues. This data set includes one comprehensive survey of 50 stations throughout Narragansett Bay, the Providence River, Greenwich Bay, and the Sakonnet River (collected in 1995).
The third set of light data was a time series taken within Greenwich Bay and its associated coves between 1996 and 1997.

Only data from May to September, the peak growth months for eelgrass in Narragansett Bay, were used from each time series. Pond data were provided by URI scientists from data collected by the U.S. Army Corps of Engineers during the same time period. Bathymetry data was from NOAA as described in the bathymetry details.

Spatial Analysis
The light extinction coefficient (k) at each station and date was calculated and then a grid for each date was interpolated using the Inverse Distance Weighting (IDW) algorithm. The maximum extent of each interpolation was dictated by the station locations. Using Formula 1, areas were identified where the solution for depth allowed 20 percent and 50 percent light on each particular date. Areas that received less than 20 percent light were given a value of 0, areas that received at least 20 percent light were given a value of 1, and areas that received at least 50 percent light were given a value of 2.

Next, each of the grids representing individual dates were multiplied together, using the map calculator function in ArcView, which yielded a grid depicting the number of surveys at which each area received a certain percentage of light. A high value indicated that the area received 50 percent light for a majority of the survey dates while a value of 0 indicated that the site received less than 20 percent light during at least one survey.

A final grid for each light dataset was produced where a value of 0 was given to all areas that received less than 20 percent light, a value of 1 was given to areas that received at least 20 percent light or 50 percent light for less than one half the surveys, and a value of 2 was given to all areas that received 50 percent light for more than one half the surveys.

Formula 1: I_z/I_o=e^-kz

I_z = Incident Light at Depth
I_o = Incident Light at Surface
e = log
k = extinction coefficient
z = depth