Function of grazers in maintaining vertical distribution of algae on tropical intertidal seawalls

The influence of intertidal gastropod grazers on algal communities on hard intertidal shores is well established in temperate systems, but remains poorly understood in the tropics. Nerita undata, a common gastropod grazer on artificial and natural shores in tropical Southeast Asia, is especially abundant on Singapore's seawalls, yet its consumptive effects on algal communities remain unclear. To test whether the presence of grazers such as N. undata influences algal communities in terms of their cover, biomass, and composition, we conducted a six-month exclusion-inclusion grazing experiment at three seawall sites and two shore heights. A total of 108 cleared experimental plots were assigned to three grazing treatments ('No Grazer', 'Single Grazer', 'All Grazers'; six replicates per site × height × treatment). We found that macroalgae extended above their natural vertical limits in the absence of grazers, suggesting a strong top-down influence by grazers on the distribution of algae. However, algal composition did not differ significantly between Single Grazer and No Grazer treatments, likely because single individuals of N. undata resulted in insufficient grazing pressure; though small differences in certain algal groups were documented. Our study also suggests high primary productivity on seawalls in Singapore and highlights the combined roles of abiotic and biotic stressors in shaping algal cover and distribution. These findings contribute to a growing understanding of ecological processes on artificial coastal structures in the tropics, and underlines the critical role of grazers.