Size and Density of Upside-Down Jellyfish, \u3ci\u3eCassiopea\u3c/i\u3e sp., and Their Impact on Benthic Fluxes in a Caribbean Lagoon

Anthropogenic disturbances may be increasing jellyfish populations globally. Epibenthic jellyfish are ideal organisms for studying this phenomenon due to their sessile lifestyle, broad geographic distribution, and prevalence in near-shore coastal environments. There are few studies, however, that have documented epibenthic jellyfish abundance and measured their impact on ecological processes in tropical ecosystems. In this study, the density and size of the upside-down jellyfish (Cassiopea spp.) were measured in Codrington Lagoon, Barbuda. A sediment core incubation study, with and without Cassiopea, also was performed to determine their impact on benthic oxygen and nutrient fluxes. Densities of Cassiopea were 24–168 m−2, among the highest reported values in the literature. Under illuminated conditions, Cassiopea increased oxygen production \u3e300% compared to sediment alone, and they changed sediments from net heterotrophy to net autotrophy. Cassiopea increased benthic ammonium uptake, but reduced nitrate uptake, suggesting they can significantly alter nitrogen cycling. Future studies should quantify the abundance of Cassiopea and measure their impacts on ecosystem processes, in order to further determine how anthropogenic-related changes may be altering the function of tropical coastal ecosystems

Eastern oyster (Crassostrea virginica) filtration, biodeposition, and sediment nitrogen cycling at two oyster reefs with contrasting water quality in Great Bay Estuary (New Hampshire, USA)

Benthic deposition of carbon (C) and nitrogen (N)-rich oyster biodeposits may increase denitrification, or anaerobic respiration of nitrate (NO3 −) to di-nitrogen gas (N2). However, environmental drivers of C and N dynamics in oyster biodeposits and reef-adjacent sediments require clarification. In July 2012, we collected intact sediment cores adjacent to and 15–20 m away from two oyster reefs (Crassostrea virginica) in Great Bay, New Hampshire, USA: one reference site and one site with cultural eutrophication. We also measured seston, chlorophyll a, and in situ oyster feeding and biodeposition. Cores were incubated in continuous-flow chambers where inflow water received 15N-ammonium (NH4 +), 15NO3 −, or no isotopes (control). We quantified fluxes of dissolved nutrients and gasses (oxygen, 28N2, 29N2, 30N2, and argon) after 24 h. Finally, we measured size-fractionated sediment organic matter. At the eutrophic site, abundant phytoplankton in the 5–28 µm size range was correlated with enhanced oyster feeding rates and biodeposit quality (lower C:N). This site had greater denitrification rates in reef-adjacent cores relative to distal cores. Low production of 29,30N2 in 15NH4 + amended cores suggested water column or biodeposit NH4 + were unlikely to be converted to N2. At both sites, reef-adjacent cores had more shell and higher 29,30N2 production with 15NO3 − addition relative to distal cores, suggesting direct denitrification enhancement near reefs. Oysters likely increased sediment N2 production via high quality biodeposits (eutrophic site), and NO3 − diffusion via structural complexity of reef-adjacent sediment (both sites). Overall, results suggest oyster-mediated ecosystems services may be expected to vary with environmental conditions

Nitrogen regulation by natural systems in “unnatural” landscapes: denitrification in ultra-urban coastal ecosystems

Dense cities represent biogeochemical hot spots along the shoreline, concentrating fixed nitrogen that is subsequently discharged into adjacent coastal receiving waters. Thus, the ecosystem services provided by natural systems in highly urban environments can play a particularly important role in the global nitrogen cycle. In this paper, we review the recent literature on nitrogen regulation by temperate coastal ecosystems, with a focus on how the distinct physical and biogeochemical features of the urban landscape can affect the provision of this ecosystem service. We use Jamaica Bay, an ultra-urbanized coastal lagoon in the United States of America, as a demonstrative case study. Based on simple areal and tidal-based calculations, the natural systems of Jamaica Bay remove ~ 24% of the reactive nitrogen discharged by wastewater treatment plants. However, this estimate does not represent the dynamic nature of urban nitrogen cycling represented in the recent literature and highlights key research needs and opportunities. Our review reveals that ecosystem-facilitated denitrification may be significant in even the most densely urbanized coastal landscapes, but critical uncertainties currently limit incorporation of this ecosystem service in environmental management