Herbivory effects on Thalassia testudinum leaf growth and nitrogen content

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Abstract
The pattern of turtle grazing on T. testudinum in St. Croix [USA Virgin Islands] 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 N and a lower lignin content than the leaf tips, in addition to lacking epiphytes. The grazing areas seem to be abandoned when the sediment NH4+ concentration is reduced, leading to reduced growth rates of T. testudinum. Effects of sea urchin grazing on T. testudinum were similar to effects of turtle grazing but were reduced in magnitude as a consequence of lower urchin grazing pressure.