Seagrass Injury Types

This grounding resulted in all three injury types. The deep water of the foreground is the blowhole. The light, sandy areas in the middle of the picture are the berms. Inbound propeller scars lead to the berm from the top right of the picture. Photo courtesy of NOAA.

When vessels run aground on seagrass beds, the resulting injuries usually fall into three categories: propeller scars, blowholes, and sediment berms.

Propeller scars. Propeller scars are formed by the dredging effect of the turning propellers as a boat travels over a shallow bank. They often result in a shallow, bare trench. The width of a propeller scar depends on the size of the vessel and the extent to which the propeller is forced into the seagrass bed. Of the three injury types, propeller scars are the fastest to recover to pregrounding conditions.

Blowholes. Once grounded, many vessel operators attempt to use their engines to power off the seagrass bank. The concentrated force of the propeller wash excavates sediment, creating a blowhole. The depth and area of a blowhole depend on the size of the vessel, extent of engine power used, and type of substrate sediment. Newly created blowholes often have steep sides and are likely to expand as the sides collapse inward over time. Blowholes are the most severe category of injury and take the longest to recover.

Berms. When blowholes are formed, the sediment excavated by the propeller wash is thrown onto adjacent areas of the seagrass bed. These sediments form mounds, called berms, which smother seagrass. A berm's area and thickness are related to the size of the blowhole injury. Larger blowholes mean larger and thicker berms.

The figure at right illustrates an actual seagrass injury. The vessel was traveling from the southwest to the northeast and had two propellers. As the seagrass bank got shallower, the propellers started to dig into the sediment and created the inbound propeller scars. The vessel then got stuck, and the operator created a blowhole as he attempted to power off the bank. The sediment from the blowhole created a berm to the east, south, and west of the blowhole. Once the operator got the vessel moving again, the vessel created two more outbound propeller scars. The total area of this injury is 235 square meters, and scientists predict that even with active restoration, it will take 35 years for the area to completely return to pregrounding conditions.