The Mini-312 Program seagrass assessment protocol includes the following steps:
- Onsite field assessment
- Application of an injury recovery model
- Application of an ecological service-scaling model
The Field Assessment
The field assessment involves three quantitative techniques: Determining the injury area, estimating the extent of the injury to seagrass within the injury site, and determining the volume of the blowhole. This information is used to plan restoration actions and to help determine interim service losses from the time of the injury until the time of full recovery.
Determining Injury Area
Mapping with a differential global positioning system (DGPS). To determine injury area, the site is mapped using differentially corrected, surveying-grade DGPS equipment. A field biologist holds the DGPS device (or tows it in an inflatable boat) and physically walks around the perimeter of each injury feature (berm, blowhole, or propeller scar). The GPS coordinates are then downloaded to ArcView geographic information system (GIS) software, which is used to compute the area of each injury feature. The result is a two-dimensional representation of the size and shape of the injury that is referenced to a specific geographic location.
Aerial photography. When injuries are large and geometrically complicated, it is often difficult to measure the area onsite. In these cases, aerial photography is used. Prior to photographing the site, a minimum of six DGPS-positioned ground control targets are placed onsite in a manner that ensures distribution throughout the photograph. The targets move vertically according to changes in tidal elevations but are unable to move horizontally. Using GIS, these vertical aerial photographs are georectified and injury features are quantitatively delineated.
Characterizing Injured Habitat
The damage to the seagrass habitat is determined by comparing pre- and postinjury conditions. Preinjury conditions are determined by observing undisturbed areas immediately adjacent to the injury, called reference or control sites.
The Braun-Blanquet method is used to measure the community composition, percentage cover and density of the benthic community, in both the injury and reference sites. This method involves placing a 0.25 square meter quadrat on the substrate and visually inspecting the content inside the quadrat. The presence of each species (Thalassia testudinum, Syringodium filiforme, and Halodole wrightii) are identified and assigned a cover-abundance scale value. The scale values for each species are:
0 = not present
0.1 = solitary specimen
0.5 = few, with small cover
1 = numerous, but less than 5 percent cover
2 = 5-25 percent cover
3 = 25-50 percent cover
4 = 50-75 percent cover
5 = 75-100 percent cover
A minimum of 10 random samples are taken outside the injury (around the entire area within 1 to 3 meters of the injury to account for any variation exhibited by the habitat), and a minimum of 3 samples are taken inside each injury feature (propeller scar, blowhole, and berm). To determine the percentage cover by individual species, the Braun-Blanquet scores, converted to percentages (using range midpoints), are averaged over all of the quadrats assessed within each feature. The loss of percentage cover of seagrass as a result of the grounding can then be assessed by comparing the percentage cover of the injured area to that of the undisturbed area.
Calculating Injury Volume
In addition to their two-dimensional component, grounding injuries often have a three-dimensional aspect resulting from the creation of blowholes. To determine the volume of the seafloor excavated in the blowhole, an inflatable boat equipped with a Lowrance LCX-15 sonar integrated with a Trimble Pro XR DGPS mounted on the stern is passed back and forth over the injury. Depth readings taken by the Lowrance are incorporated with differentially corrected positions taken on the Trimble. This information is then processed using Arcview GIS, resulting in a three-dimensional view of the area.
Seagrass Recovery Model
Using the information from the field assessment, a spatial model is used to estimate the injury's recovery trajectory. The model estimates how long each species of seagrass in the injured area will take to fill in from the perimeter to preinjury conditions. The model is based on rates of habitat recolonization determined from empirical assessments and literature reviews. The model applies a decision rule that governs conversion from an unfilled to a filled state and the resolution of the model determines the time step represented by each model iteration. It is critical to note that the recovery model assumes ideal conditions.
Habitat Equivalency Analysis
Although many seagrass injuries undertaken by the Mini-312 Program are ultimately restored through primary restoration and are expected to eventually produce the same level of services as they did preinjury, the public and the environment must be adequately compensated for the resource services lost between the time of the injury and the time of full recovery.
Habitat equivalency analysis (HEA) is a framework used to quantify these interim lost resource services and determine the scale of compensatory restoration. The principle concept underlying HEA is that the public and the environment can be made whole for injuries to natural resources in the interim period through the implementation of compensatory restoration projects (in these cases at "orphan" grounding sites, or sites for which the responsible party is unknown) that provide resources and services of the same type, quality, and comparable value as those that were lost. Service losses in the injured areas are calculated using information from the field assessment and the recovery trajectories. Restoration project benefits are calculated using restoration project parameters. The HEA ultimately determines the size of a replacement project such that the benefits of the compensatory project equal the interim losses due to the injury.
Trimble Pro XR DGPS Unit.
Aerial photograph of grounding injury. The light green circle in the center is a blowhole surrounded by the white berms. The yellow circle at the top right is a ground control target.
