Anthropogenic nutrient loading is one of the greatest threats to coastal ecosystems. In seagrass habitats, harmful effects of excess nutrients, such as epiphytic growth, have been well-documented. Another potential impact of nutrient loading is the proliferation of native species, like the upside-down jellyfish, Cassiopea spp., which have been shown to increase in abundance and size in human-disturbed environments. Because of the sessile, epibenthic nature of Cassiopea, they may impact seagrass beds by shading seagrasses and altering community composition. In this study, we evaluated whether high abundances of Cassiopea would affect benthic community structure within sub-tropical seagrass (Thalassia testudinum) habitats. To analyze this question, we conducted benthic surveys in seagrass beds with naturally-varying densities of jellyfish to identify whether community structure differed between areas with high or low jellyfish abundance. Additionally, we conducted a press experiment in which jellyfish were repeatedly added to a seagrass meadow in order to replicate jellyfish blooms in a eutrophied seagrass ecosystem, and assess whether any changes occurred to benthic seagrass community structure.
Results/Conclusions
Overall, we found that an increased abundance of Cassiopea had substantial effects on community biotic variables and structure. In the surveys, high Cassiopea densities were correlated with a reduction in benthic animal abundance (N=28, p<0.05) as well as taxa richness (N=28, p<0.05). In the addition experiment, benthic animal abundance was found to decline following jellyfish additions (N=42, Model p=0.0007, p<0.05), as well as taxa richness (N= 42, Model p=0.0025, p<0.05). No effect of jellyfish additions on animal biomass was observed. Few taxa in both the surveys and experiment responded positively to increased jellyfish densities, however most taxa declined in abundance in the presence of jellyfish. Seagrass biomass (aboveground and belowground) did not differ significantly between jellyfish addition treatment plots before and after the study. However, we observed a more severe decline of seagrass biomass following jellyfish additions than the decline observed in controls. This suggests that jellyfish may have affected seagrass biomass, but physical disturbance of the plots may have weakened the effects of jellyfish on seagrass. Ultimately, these results support the hypothesis that jellyfish ‘blooms’ associated with human disturbance may represent an alternative pathway by which anthropogenic nutrients affect benthic seagrass communities.