Grazing and assimilation rate estimates of hydromedusae from a temperate tidal creek system

Abstract

Previous research implicates gelatinous zooplankton grazing for reducing microzooplankton populations and influencing pelagic carbon fluxes. The principal objective of this study was to determine in situ ingestion rates and assimilation efficiencies for seasonal hydromedusae populations in North Inlet, SC, USA. Hydromedusae collected during July 2005, 2006 and January 2006 were exposed to natural plankton communities that were previously radiolabeled with 14C. Ingestion rates and assimilation efficiencies were calculated from measurements of hydromedusae tissue radioactivity after ingestion and egestion, respectively. Net tows determined that the numerically dominant gelatinous predators in this system were Bougainvillia muscus (July 2005, 2006) and Nemopsis bachei (January 2006). Summer ingestion rates for B. muscus significantly increased with increased temperature, but there was no effect of temperature on winter ingestion rates for N. bachei, suggesting temperature responses are species specific. Moreover, there was a significant species effect on ingestion rates for July 2006. Hydromedusae assimilation efficiencies are relatively constant throughout the year and are not significantly affected by temperature, species, or season. Seasonal differences in grazing are likely a function of the pelagic community and temperature-dependent metabolism. These results coupled with seasonal hydromedusae abundance data imply summer populations of hydromedusae graze more heavily on prey than winter populations, while also assimilating more carbon into predator tissue. Effects of hydromedusae predation may include modification of trophic structure, shifts in algal community composition, and decline in ecosystem stability, leading to impacts on sustainable commercial fisheries.