Seagrass Siting Model - Overview

Establish Suitable Sites

ArcView LegendSuitability is determined based on factors that drive eelgrass biology coupled with available geospatial data. Five data layers were used in identifying suitable sites, based on work completed by Dr. Fred Short and his colleagues at the University of New Hampshire:

Depth is a critical element to identifying potential eelgrass transplant sites. Depth often determines whether light can penetrate to reach the eelgrass leaves, thereby allowing photosynthesis to occur. Based on an analysis completed by eelgrass researchers at the University of Rhode Island (URI) and Save The Bay, the Seagrass Site Selection Tool considers locations to be highly suitable from 1 to 6.4 feet, suitable at 0 to 1 and 6.4 to 12 feet, and unsuitable if deeper than 12 feet. The highly suitable category is shallow enough to allow for proper light conditions, yet deep enough to lessen damage to the eelgrass caused by ice shear and/or desiccation. [More details]

The project technical advisory team indicated that there exists substantial evidence that eelgrass will not survive above 25º Celsius (77º Fahrenheit). At temperatures warmer than 25ºC, eelgrass senesces, or loses its leaves. In addition, epiphytes and algal blooms may shade out eelgrass, or photosynthesis may shut down if salinity increases with enhanced evaporation. Eelgrass also appears to be more susceptible to wasting disease at higher temperatures.

Because temperature is so variable and dynamic, this was a challenging data layer to capture. Researchers at the Brown University Department of Geological Sciences Planetary Geosciences Group have been collecting Thematic Mapper Satellite imagery of temperature. They provided a consolidated raster data layer identifying areas that reached or exceeded 25ºC. The data were compiled from 26 seasonally diverse dates of information from the Landsat 4, 5, and 7 sensors, covering cloudless scenes from 1984 through 2002. Areas remaining below 25ºC received a value of 1 (suitable). Areas where temperatures reached or exceeded 25ºC received a value of 0 (unsuitable).

Water clarity is critical to maintaining healthy eelgrass habitats. Light controls the depth at which eelgrass can grow since it is dependent upon photosynthesis. Eelgrass generally requires light intensities of 15 to 25 percent of that available at the surface. Factors that reduce available light, such as shading from docks, turbid water, and nutrient-driven plankton growth, harm eelgrass beds. Light data were developed from three sources in Narragansett Bay collected between 1994 and 1998. Data from the coastal ponds was collected by the U.S. Army Corps of Engineers between 1996 and 1997. For areas that received less than 20 percent light, a value of 0 was assigned. A value of 1 was assigned to areas that received at least 20 percent light or 50 percent light for less than one half the surveys. A value of 2 was given to all areas that received 50 percent light for more than one half the surveys. [More details]

Current Eelgrass Location
Because disturbance of an existing eelgrass bed might cause negative impacts, the Tool exludes these areas from the analysis. The boundaries of existing eelgrass beds are not well defined within the study area, so each bed was given a 100 meter buffer and removed from the analysis. [More details]

Historic Eelgrass Location
Considering where eelgrass was known to exist historically allows one to understand where characteristics of the water were suitable for eelgrass to exist – at least at one point in time. Anecdotal information suggests that Narragansett Bay was once filled with eelgrass, so all areas are considered as suitable.

Data for the coastal ponds were developed from work by URI and was documented by Thorne-Miller and colleagues from work completed in 1983. The authors reported and mapped eelgrass distributions in Ninigret, Green Hill, Potter, Point Judith, and Trustom ponds from aerial photography taken during the summers of 1978, 1979, and 1980. Historical data were digitized for only Ninigret and Green Hill ponds for the Seagrass Site Selection Tool because the other ponds were not included in other model data (e.g., light, current eelgrass).

All areas known to support eelgrass were buffered by 100 meters, and areas known to support eelgrass historically were given a value of 2. All other areas received a value of 1. [More details]

Each of these layers was multiplied against one another to obtain a suitability ranking. Areas within the grid with the highest value are considered the most suitable for establishing eelgrass transplants.

Prioritize Suitable Sites

Prioritize Site Graphic InterfaceEelgrass restoration projects in shallow coastal habitats can potentially conflict with historic and cultural resources, especially historically fished areas. For this reason, the selection tool allows the user to prioritize suitable sites based on factors such as shellfish closure areas, known commercial and recreational fishery areas, and water body classifications.

The results of each of the selected overlays are multiplied together to get a final order of priority, based on the criteria that the user selects. The numbers will not be shown, but the results will be mapped into three groups based on natural breaks. The values selected (1-2) were chosen to allow meaningful multiplication that was consistent for each input feature. One priority factor will not take the total priority to 0, but rather, keep it constant with the original suitability rating.