Increased nutrient availability correlates with increased growth of the benthic jellyfish Cassiopea spp.

Abstract

Jellyfish are an important but underappreciated food source in marine food webs. Human activities influence the abundance of jellyfish, potentially affecting marine trophic dynamics. Human activities such as nutrient loading may also influence the size structure of jellyfish populations, affecting food webs, yet little is known about indirect or direct mechanistic pathways driving shifts in size. In this experiment, we manipulated nutrient concentrations to test if elevated nutrient availability affected growth of the zooxanthellate, benthic jellyfish taxa Cassiopea spp. We found that in the nutrient-enriched treatment, the estimated body mass of jellyfish increased on average 0.24 ± 0.29% d−1, likely as a result of elevated autotrophic carbon supply. Conversely, jellyfish in the ambient nutrient treatment shrank, possibly reflecting reduced food availability as a result of the enclosures that jellyfish were kept in or effects associated with negative density dependence. Our findings suggest that human-derived nutrients may benefit these jellyfish taxa and, more broadly, we provide an example of how environmental conditions can mediate the size structure of jellyfish populations with potential implications for marine food webs.