The role of leaf nitrogen content in determining turtlegrass (Thalassia testudinum) grazing by a generalized herbivore in the northeastern Gulf of Mexico

In shallow marine environments the variability in grazing on seagrasses has been hypothesized to be controlled, in part, by the nutritive quality (i.e ., nitrogen content) of their leaves. The few existing studies of the relat ionship between leaf nitrogen content and seagrass grazing have all found a positive relationship between leaf nitrogen content and preference by sele ctive vertebrate grazers (i,e., the bucktooth parrotfish, green sea turtles , and dugongs). However, most marine herbivores (both vertebrate and invert ebrate) are thought to be extreme generalists with broad diets of variable nutritive quality (e.g., detritus, living plants, and animals), suggesting the currently held view on the role leaf nutrient content in explaining the variability of seagrass grazing is an oversimplification. In this study, we evaluated how leaf nitrogen content influenced grazing on turtlegrass by a generalist invertebrate herbivore (the pink sea urchin Ly techinus variegatus) in the northeastern Gulf of Mexico. Using a short-term laboratory test and a longer-term field experiment, we tested the hypothes is that leaf nitrogen content controls sea-urchin grazing on seagrass leave s. We hypothesized that if poor nutritive value of seagrasses is responsibl e for reduced rates of feeding, then increasing leaf nitrogen concentration s should lead to increased rates of seagrass consumption by sea urchins. In the field experiment, we significantly enriched seagrass leaf nitrogen c oncentrations (some 10-20% depending on month) in experimental plots with a commercial fertilizer and we manipulated grazing intensity by enclosing ad ult sea urchins at densities that bracketed the range of average densities observed in the region (i.e., 0, 10 and 20 individuals/m(2)). Comparisons o f changes in aboveground seagrass production and biomass showed no evidence that sea urchins grazed significantly more in treatments where leaf nitrog en was enriched. Because the statistical power of our test to detect such d ifferences was low and aboveground seagrass production varied significantly among treatments, we also used a mass balance equation to estimate sea urc hin consumption of nitrogen-enriched and unenriched leaves. This showed tha t sea urchins compensated for low nitrogen levels in our unenriched treatme nts by eating more leaves than in treatments where leaf nitrogen was elevat ed. Using a laboratory test, we also found that sea urchins ate less nitrog en-enriched seagrass than unenriched seagrass. In combination, these result s show that, in contrast to findings reported for vertebrate herbivores, se a urchins feed at higher rates when offered seagrass leaves of lower leaf n itrogen content, and that low levels of leaf nitrogen are not always an eff ective defense against herbivores. (C) 2001 Elsevier Science B.V. All right s reserved.