Development, biological regulation, and fate of ctenophore blooms in the York River estuary, Chesapeake Bay

Abstract

Blooms of the lobate ctenophore Mnemiopsis leidyi proliferate in estuaries and coastal regions worldwide. However, their role in food web structure and carbon flow between trophic lev- els is not fully understood. During 2003-2006, we conducted field surveys along a salinity gradient in the lower York River, a sub-estuary of the Chesapeake Bay, to determine factors controlling the tim- ing and magnitude of M. leidyi blooms, and to evaluate effects of gelatinous zooplankton on carbon cycling. Samples for density, biovolume, and carbon content of ctenophores, scyphomedusae, and mesozooplankton were collected using surface net tows and quantified in the laboratory. Historical published records on ctenophores and physical data from regional databases were used to comple- ment this dataset. The highest biomass of M. leidyi occurred in early summer (up to 50 mgC m -3 ), although blooms also appeared in winter and spring. Peaks in ctenophore biomass in the mesohaline York River occurred in May, 1 mo earlier than further downriver in the polyhaline mainstem Chesa- peake Bay, likely due to higher ctenophore reproductive potential and larval dispersal upriver. Copepod biomass remained low (<10 mg C m -3 ) during ctenophore blooms but was not limiting for M. leidyi. Rather, high predation by Chrysaora quinquecirrha scyphomedusae on M. leidyi (scypho- medusae requirements of up to 240% of M. leidyi C d -1 ) appears to cause the rapid decline in sum- mer ctenophore blooms, and we hypothesize that, subsequently, medusae become carbon-limited. Long-term trends suggest M. leidyi blooms have shifted forward 1 mo, possibly due to localized tem- perature increases over the past 40 yr. These results suggest that M. leidyi blooms have a pivotal, and potentially transformative, role in carbon transfer in estuarine and coastal food webs.