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Analyzing Benthic Data: Examples

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Summary Puerto Rico Coral Mapping

Narragansett Bay Eelgrass

Narragansett Bay Eelgrass

Click image for a full view of the final eelgrass map.

In 2001, a small eelgrass bed in Narragansett Bay, Rhode Island was mapped to provide baseline data for assessment of impacts from a planned dredging project. Data were collected using a combination of side-scan sonar, plan view photography, and diver observation. A GIS was used to analyze and visually display the data.

Initial surveys of the study area were conducted using side-scan sonar and single-beam bathymetry. Survey lanes were closely spaced (25 feet apart) to provide dense data for more accurate interpolation. Kriging, a data interpolation method, was used to transform bathymetric data to a raster surface. This phase of the project provided a baseline for plan-view photographic surveys. For details on data interpolation, see the Spatial Analysis section.

Several factors were considered in the planning phase of the plan-view photographic survey:

1. The scale of the eelgrass habitat – What sizes are the eelgrass patches?
2. Scale of analysis – At what resolution could the eelgrass cover and density best be represented?
3. Navigational precision and accuracy – How close could differential Global Positioning Systems (dGPS) get researchers to the target sample locations?
4. Vessel maneuverability – How well could the survey boat stay on station while in shallow water?
5. Budget – What could reasonably be accomplished within the time and funding allotted for the study?
6. Repeatability – Could the survey techniques be easily replicated in sequential monitoring surveys?

Based on these considerations, 200 stations were chosen at 20 feet intervals within a sample area that extended beyond the eelgrass bed. At each target station, three plan view photographs were collected. Percent cover and density (plants per square meter) of eelgrass were calculated using these photographs.

Click image for a full view of the final eelgrass map.

To validate the accuracy of percent cover and density calculations, diver transect surveys were conducted. Along each transect, quadrats (one square meter) were sampled every three meters. Density and percent cover were measured through visual observation by the diver.

For final analysis, all eelgrass coverage and density measurements within 20 feet of each of the 200 target stations were averaged and assigned to that station. These point data were then converted to a raster with 20-foot cell resolution. Because the sample points were evenly spaced and matched the cell resolution, interpolation was not necessary for this conversion. Results of this study indicate that diver-validated plan-view photography is an effective, thorough, and cost-efficient method for mapping shallow eelgrass beds of this size.

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