Herbivory in multi-species, tropical seagrass beds

Very little is known about the characteristics of herbivory and selection by various grazers in tropical, multi-species seagrass beds. We used an in situ shoot tethering method to quantify grazing on 3 dominant seagrass species (Thalassia hemprichii, Cymodocea rotundata, and C. serrulata) that co-inhabit extensive seagrass beds surrounding Dongsha Island in the South China Sea. We measured the amount of seagrass grazing as well as leaf production in different habitats and seasons. The dominant seagrass grazers in Dongsha Island were parrotfish, followed by meso-invertebrates, suggesting that herbivory by small invertebrates may also be critical in tropical seagrass ecosystems. Our results revealed significant spatial and seasonal differences in leaf biomass losses. The level of grazing was 3.6x higher in the subtidal than in the intertidal zones, and 2.6x higher in the warm than in the cool season. Leaf biomass losses were positively associated with parrotfish density, water depth, leaf production, and starch content. However, the nitrogen and total sugar contents did not affect the preferences of the grazers. The 2 Cymodocea species were preferred by most grazers. Collectively, all grazers typically removed <10% of the leaf production in Dongsha Island, which indicates that most seagrass biomass was not directly used by herbivores and flows into detrital pathways.