Population Dynamics of Gelatinous Zooplankton in the Chesapeake Bay and Sargasso Sea, and Effects on Carbon Export

Gelatinous zooplankton (GZ; cnidarians, ctenophores, and pelagic tunicates) periodically are the dominant members of the zooplankton throughout the majority of the world’s oceans. their unique body plans and life cycles allow them to rapidly take advantage of favorable environmental conditions, which has far-ranging consequences for food web dynamics and biogeochemical cycles. GZ populations have been speculated to respond to anthropogenic changes, but few long-term studies exist to test this hypothesis and even fewer have examined the consequent effects on carbon export. I analyzed two long-term time series in the Chesapeake Bay and one in the Sargasso Sea for annual and interannual changes in GZ populations and the environmental drivers of these changes. I also conducted mesocosm experiments in the Chesapeake Bay and developed a carbon flux model for the Sargasso Sea to evaluate the role that GZ play in vertical carbon flux in these two regions. In the Chesapeake Bay, summer populations of the dominant scyphozoan medusae, Chrysaora quinquecirrha, are positively correlated with spring salinity and negatively with dissolved oxygen concentrations. C. quinquecirrha biovolume has been decreasing from 1985-2011, reducing predation pressure on the ctenophore Mnemiopsis leidyi, with cascading effects on copepod abundances. This top-down control of the food web extends to changes in vertical carbon flux, with the presence of M. leidyi reducing copepod fecal pellet flux by 50%. In the Sargasso Sea, large salp blooms can completely dominate the zooplankton community, and both cyclonic mesoscale eddies and seasonal changes in primary production can regulate annual salp population dynamics. Long-term salp population trends are correlated with changes in decadal climate oscillations, and a long-term increase in the most abundant salp species, Thalia democratica, was observed from 1994-2011. During blooms, salps can graze more than 100% of the primary production, and rapidly export carbon to depth through sinking fecal pellets and carcasses, and through active transport via respiration at depth. This carbon export to 200 m (average of 2.3 mg C m-2 d-1) is equivalent to 11% of the measured sediment trap flux at the same depth, but salp fecal pellets and carcasses attenuate slowly and can be equivalent to \u3e 100% of measured sediment trap carbon at 3200 m, representing a large export of carbon to the bathypelagic zone during salp blooms. GZ populations in both the Chesapeake Bay and Sargasso Sea are sensitive to seasonal changes in the environment on annual and interannual time scales. Long-term changes in GZ abundances could continue into the future, causing corresponding changes in carbon export

Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Lapointe, B. E., Brewton, R. A., Herren, L. W., Wang, M., Hu, C., McGillicuddy, D. J., Lindell, S., Hernandez, F. J., & Morton, P. L. Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin. Nature Communications, 12(1), (2021): 3060, https://doi.org/10.1038/s41467-021-23135-7.The pelagic brown macroalgae Sargassum spp. have grown for centuries in oligotrophic waters of the North Atlantic Ocean supported by natural nutrient sources, such as excretions from associated fishes and invertebrates, upwelling, and N2 fixation. Using a unique historical baseline, we show that since the 1980s the tissue %N of Sargassum spp. has increased by 35%, while %P has decreased by 44%, resulting in a 111% increase in the N:P ratio (13:1 to 28:1) and increased P limitation. The highest %N and δ15N values occurred in coastal waters influenced by N-rich terrestrial runoff, while lower C:N and C:P ratios occurred in winter and spring during peak river discharges. These findings suggest that increased N availability is supporting blooms of Sargassum and turning a critical nursery habitat into harmful algal blooms with catastrophic impacts on coastal ecosystems, economies, and human health.This work was funded by the US NASA Ocean Biology and Biogeochemistry Program (80NSSC20M0264, NNX16AR74G) and Ecological Forecast Program (NNX17AF57G), NOAA RESTORE Science Program (NA17NOS4510099), National Science Foundation (NSF-OCE 85–15492 and OCE 88–12055), “Save Our Seas” Specialty License Plate funds, granted through the Harbor Branch Oceanographic Institute Foundation, Ft. Pierce, FL, and a Red Wright Fellowship from the Bermuda Biological Station. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida. D.J.M. gratefully acknowledges the Holger W. Jannasch and Columbus O’Donnell Iselin Shared Chairs for Excellence in Oceanography, as well as support from the Mill Reef Fund