Parrotfish grazing on turtlegrass Thalassia testudinum:: evidence for the importance of seagrass consumption in food web dynamics of the Florida Keys National Marine Sanctuary

The widespread occurrence and persistence of modern day seagrass habitats has led many to hypothesize that grazing on seagrasses is minimal. On a global scale this may well be true as the numbers of large vertebrate herbivores (e.g. sea turtles, manatees and dugongs) and waterfowl, grazers that can greatly alter seagrass density, have been dramatically reduced in coastal ecosystems. Nonetheless, numerous observations indicate that smaller herbivores (e.g. the bucktooth parrotfish Sparisoma radians), grazers that may not be able to reduce seagrass density substantially, still commonly feed on seagrasses in the subtropical and tropical western Atlantic Ocean. These observations led us to quantify the role that seagrass herbivory plays in modern-day seagrass food webs. In the first phase of this study, digitally scanned seagrass leaves were clipped to ropes and placed at 3 sites in Hawk Channel, in the northern Florida Keys (USA). Areal loss from the tethered leaves provided a daily estimate of seagrass grazing rates. These losses, coupled with local estimates of net aboveground primary production, allowed us to determine the proportion of seagrass production consumed at our study sites during 4 separate seasons. We found that seagrass grazing varied greatly both spatially and seasonally at our sites but, on average, grazers consumed virtually all of the aboveground production at 2 of the 3 sites. When experiments were repeated in the summer of a second year at 6 sites, seagrass grazing again varied greatly among sites, but at 3 of the sites most of the daily production of seagrass shoots was consumed by small herbivorous fishes. These results suggest that while it is undoubtedly true that modern day grazing by mammals, turtles and waterfowl on seagrass is reduced, small vertebrate grazers consume substantial amounts of seagrass production in the northern Florida Keys.