Herbivory Effects on \u3cem\u3eThalassia testudinum\u3c/em\u3e Leaf Growth and Nitrogen Content

The pattern of turtle grazing on Thalassia testudinum in St. Croix seagrass beds begins with the establishment of a grazing plot by initial removal of leaf blades, followed by repeated grazing of several centimeter-long leaf blades within a maintained grazing area. Plants within the grazed area exhibit increased specific growth rate as a consequence of increased light flux to unepiphytized leaf bases. Leaf width is reduced in the grazed area as a consequence of grazing stress. The leaf bases contain a higher proportion of nitrogen and a lower lignin content than the leaf tips, in addition to lacking epiphytes. Our data suggest that the grazing areas are abandoned when the sediment ammonium concentration IS reduced, leading to reduced growth rates of T. testudinum. Effects of sea urchin grazing on T testudinum were similiar to effects of turtle grazing but were reduced in magnitude as a consequence of lower urchin grazing pressure