Selective herbivory on necrotic tissue can promote tolerance to abiotic disturbances in the seagrass Cymodocea nodosa

Abstract

Herbivores play an important role in shaping seagrass community structure, but local stressors can change the ecological significance of herbivory by altering seagrass physiology in ways that affect herbivore preferences. However, few studies have assessed the cumulative influence of diverse local stressors on seagrass ecosystem function in relation to herbivory pressure. Here, we performed a four-month in situ experiment and laboratory feeding trials to examine the effects of two abiotic stressors (light reduction and nutrient enrichment) and simulated herbivory on the physiology of Cymodocea nodosa meadows and associated plant-mesograzer interactions, with the goal of understanding seagrass resilience to multiple disturbances. We found that light reduction primarily affected leaf morphology, resulting in lower shoot surface area, plant biomass and leaf growth rate. Simulated herbivory stimulated the production of phenolic compounds with a potential antimicrobial effect. Nutrient enrichment significantly reduced the C:N ratio and increased seagrass necrosis tissues and growth of opportunistic algae. Further, while higher macroalgal biomass was negatively correlated with C. nodosa performance, epiphyte biomass was positively correlated. Furthermore, our findings evidenced that C. nodosa leaves not only had high nutritional quality under nutrient enrichment, but also the presence of necrotic areas could be a significant driver modulating isopod consumption. We discuss the potential ecological impact of the natural mesograzer preference for necrotic tissue, which may promote the recovery of seagrass communities under local stressors.