Herbivory as a limiting factor for seagrass proximity to fringing reefs in Moorea, French Polynesia

Seagrasses are important foundational species that frequently display distinct depth distributions, although the drivers of these patterns can be spatially and temporally variable. While the pantropical seagrass Halophila decipiens is known from waters as shallow as 1 m deep, in Moorea, French Polynesia we only found it > 6.4 m deep. To explore factors affecting depth distribution, we transplanted H. decipiens into 3 habitats: the existing seagrass bed (control), just outside the seagrass bed, and shallower habitat adjacent to a fringing coral reef. Results showed that growth was not significantly different between the control and just outside of the seagrass bed; however, number of shoots and rhizome length were significantly reduced adjacent to the reef. Transplanting seagrass into the shallow reef site with and without herbivore exclusion cages showed that H. decipiens grew in herbivore exclusion treatments, but lost both shoots and rhizomes in the control. These results indicate that H. decipiens can grow in shallow habitats adjacent to reefs on Moorea, but that herbivory pressure, presumably from the reef, limits its depth distribution.