Response of tropical seagrass palatability based on nutritional quality, chemical deterrents and physical defence to ammonium stress and its subsequent effect on herbivory.

Seagrass-herbivore interactions play a principal role in regulating the structure and function of coastal food webs, which were affected by nutrient enrichment. Seawater nutrient enrichment might change seagrass palatability by altering seagrass physical and chemical traits, consequently modulating herbivory patterns, but this remains elusive. In this study, the dominant tropical seagrass Thalassia hemprichii was cultured in different ammonium concentrations to examine the response of seagrass nutritional quality, deterrent secondary metabolites, and leaf toughness, as well as the subsequent effect of the changed physical (e.g., leaf toughness) and chemical traits (e.g., nitrogen content; total phenol) on the grazing activity of the herbivorous snail Cerithidea rhizophorarum. Ammonium enrichment enhanced seagrass nutritional quality and decreased physical defence. Low ammonium enrichment increased total phenol content, while high ammonium enrichment reduced it. Both low and high ammonium enrichment enhanced the grazing intensity of C. rhizophorarum on seagrass. Interestingly, nutritional quality mostly determined the herbivory preference of C. rhizophorarum on the intact seagrass having physical structure, with a chemical deterrent (total phenol) playing a secondary role. In contrast, chemical deterrent mainly determined the grazing intensity on agar seagrass food which was made artificially to exclude physical structure. This indicated that seagrass leaf physical structure might hinder phenol compounds from deterring herbivores. Overall, the results presented here demonstrate that ammonium enrichment remarkably increased seagrass palatability and subsequently induced higher susceptibility to herbivory, which might induce seagrass loss. Schematic pictures of the effects of ammonium enrichment on seagrass palatability and subsequent effects on herbivory. Symbols of Thalassia hemprichii are courtesy of the Integration and Application Network, University of Maryland Center for Environmental Science (http://ian.umces.edu/symbols/). The different sizes of symbols represent the relative sizes of seagrass leaf traits. [Display omitted] • Ammonium enrichment significantly increased seagrass palatability. • Nutritional quality mostly determined the feeding preference on intact seagrass. • Seagrass physical structure had an effect on reducing efficiency of chemical deterrents. [ABSTRACT FROM AUTHOR]