Effects of propeller scarring on macrofaunal use of the seagrass Thalassia testudinum

Abstract

Propeller scarring within seagrass beds is common in shallow coastal waters. Scarring has the potential to fragment seagrass beds, resulting in habitat loss, decreased productivity, and the pos- sibility for further erosion and degradation. We conducted a study in Thalassia testudinum beds in Puerto Rico to determine whether seagrass macrofauna are affected by this disturbance. Four sampling zones (propeller scar, seagrass-scar interface, homogeneous seagrass located 5 m from the scar, and homo- geneous seagrass located 10 m from the scar) were compared among 10 replicate seagrass beds. Scar- ring modified faunal assemblages at the scale of the propeller scar; there was significantly lower total macrofaunal abundance and fewer species in scars. When individual taxa were considered, shrimp and mollusc abundances were lower in scars compared to the other sampling zones. Resident fish abundance was not significantly different among zones. Dominant shrimp species in scars differed from seagrass zones. Crabs and molluscs responded negatively to scarring as indicated by significantly lower densi- ties of these 2 taxa up to 5 m from scars. The extent to which these results 'scale up' remains unknown and future studies should focus on larger, more intensely scarred areas.