Nitrate threshold of tropical seagrass susceptibility to herbivory

Plant-herbivore interaction, susceptible to nutrient enrichment, is fundamental for maintaining a healthy and stable ecosystem. Hence, identifying nutrient thresholds may enable better management and monitoring of ecosystems. However, little information is available for the nutrient threshold for the susceptibility of seagrass, the only marine angiosperm, to herbivory. In present study, we investigated how nitrate enrichment altered the traits of dominant tropical seagrass Thalassia hemprichii at the physiological level (nutritional quality, chemical deterrents, and structural characteristics), changing its susceptibility to the herbivory of Cerithidea rhizophorarum. A set of multiple-choice feeding assays were conducted using nitrate-enriched seagrass pre-cultivated in the lab for 60 days. The present study found that the effect of nitrate enrichment on seagrass susceptibility to herbivory was non-linear, with a threshold effect. Low nitrate enrichment chemically and structurally defended seagrass, subsequently reducing herbivory, while continuously increasing nutrient enrichment crossing the threshold enhanced seagrass susceptibility to herbivory. Meanwhile, seagrass leaf total phenol and free fatty acid contents were the two dominant factors determining the herbivory preference of C. rhizophorarum. Overall, since anthropogenic nitrogen enrichment is increasing in intensity and frequency, our findings are significant for conservation and emphasize the importance of coastal eutrophication control.