Cultivation grazing response in seagrass may depend on phosphorus availability

Abstract

In terrestrial systems, grazing can have lasting beneficial or damaging effects on plants based on nutrient availability, but it remains unclear how nutrients shape plant responses to grazing in marine systems, especially in resource-poor environments. We chose a phosphorus-limited subtropical turtlegrass Thalassia testudinum meadow in Bermuda to investigate short- and long-term plant responses to repeated herbivory. In a multiyear field experiment, we manipulated nitrogen (N) and phosphorus (P) availability and simulated green turtle Chelonia mydas grazing for 4 months and then tracked recovery through 2 years. In the short term, simulated grazing increased T. testudinum area-specific growth and both foliar N and P content in fertilized plots compared to the control. In unfertilized plots, simulated grazing (trimming) decreased short-term shoot-specific growth and foliar P content. By comparison, long-term data revealed sustained elevated T. testudinum growth after 2 years in fertilized plots in the absence of trimming. While trimming alone did not affect seagrass growth rates 2 years after the treatment application, these plants sustained depleted foliar P concentrations up to 1 year and displayed residual stress symptoms of fewer shoots with thinner leaves. Our data indicate the potential for N and P stoichiometry to influence “cultivation grazing” in marine carbonate environments. Seagrasses apparently responded positively to trimming when the foliar N:P balance did not exceed the “seagrass Redfield ratio” of 30:1. This P-sensitive outcome of herbivory should be evaluated at larger spatial scales and may have important implications for estimating C. mydas carrying capacity of seagrass meadows across resource gradients.