Green turtle (Chelonia mydas) grazing plot formation creates structural changes in a multi-species Great Barrier Reef seagrass meadow

Abstract

The Great Barrier Reef (GBR) contains extensive seagrass meadows with abundant and diverse herbivore populations. Typically, meadows in the region are multi-species and dominated by fast growing opportunistic seagrass species. However, we know little about how herbivores modify these types of seagrass meadows by grazing. We conducted the first megaherbivore exclusion study in the GBR at Green Island (Queensland) to understand how green turtle grazing structures these multi-species tropical seagrass meadows. After excluding green turtles for three months, we found that grazing only impacted seagrasses at one site, where green turtles created a grazing plot by actively feeding on both above and below ground seagrass structures, a rare observation for the species. Within this grazing plot at the end of the experiment, the un-caged control treatments open to grazing had a 60% reduction in both above and below ground biomass, and shoot height was reduced by 75%, but there was no impact of grazing on the seagrass species mix. Our study shows that grazing plot formation by green turtles occurs in GBR fast growing seagrass communities and reduces both above and below ground seagrass biomass, this behaviour may be targeting elevated leaf nutrients, or nutritional content of rhizomes. This study is the first documented case of grazing plot formation by green turtles in the GBR and suggests that grazing pressure has a major influence on seagrass meadow structure.