Bucktooth parrotfish Sparisoma radians grazing on Thalassia in Bermuda varies seasonally and with background nitrogen content

Herbivory has an important influence on seagrass abundance and distribution, yet there is limited understanding of grazing behavior in communities at geographic margins of species ranges. We used 24-h seagrass-tethering experiments to measure the effect of foliar nitrogen content and time of year on bucktooth parrotfish Sparisoma radians herbivory in high-latitude Thalassia testudinum meadows of Bermuda. Parrotfish preferentially grazed nitrogen-enriched tethered shoots especially at offshore sites that had relatively low background nutrient levels. Selective feeding was pronounced during periods of high grazing intensity. We found strong seasonal patterns, likely because Bermuda is at the northerly limit of these two species. Herbivory rates peaked in late summer when the mean leaf removal rate by S. radians was 3.38 ± 1.11 cm shoot− 1 day− 1, which exceeded the daily seagrass growth rate by 10-fold. Seagrass meadows are in decline in this region and we suggest that seasonal episodes of clustered, intensive grazing could make these habitats more vulnerable to other environmental or human-induced stresses, such as storms and runoff of sediment and nutrients.

► Grazing pressure varies with season in high-latitude Thalassia meadows.
► In summer parrotfish consumption surpasses daily seagrass growth by 10-fold in Bermuda.
► Selective feeding occurs when grazing intensity is highest.
► Nitrogen-stimulated grazing is pronounced when ambient nutrient levels are lowest.