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Engineered aeration is one solution for increasing oxygen concentrations in highly eutrophic estuaries that undergo seasonal hypoxia. Although there are various designs for engineered aeration, all approaches involve either destratification of the water column or direct injection of oxygen or air through fine bubble diffusion. To date, the effect of either approach on estuarine methane dynamics remains unknown. Here we tested the hypotheses that 1) bubble aeration will strip the water of methane and enhance the air-water methane flux to the atmosphere and 2) the addition of oxygen to the water column will enhance aerobic methane oxidation in the water column and potentially offset the air-water methane flux. These hypotheses were tested in Rock Creek, Maryland, a shallow-water sub-estuary to the Chesapeake Bay, using controlled, ecosystem-scale deoxygenation experiments where the water column and sediments were sampled in mid-summer, when aerators were ON, and then 1, 3, 7, and 13 days after the aerators were turned OFF. Experiments were performed under two system designs, large bubble and fine bubble approaches, using the same observational approach that combined discrete water sampling, long term water samplers (OsmoSamplers) and sediment porewater profiles. Regardless of aeration status, methane concentrations reached as high as 1,500 nmol L⁻¹in the water column during the experiments and remained near 1,000 nmol L⁻¹through the summer and into the fall. Since these concentrations are above atmospheric equilibrium of 3 nmol L⁻¹, these data establish the sub-estuary as a source of methane to the atmosphere, with a maximum atmospheric flux as high as 1,500 µmol m⁻²d⁻¹, which is comparable to fluxes estimated for other estuaries. Air-water methane fluxes were higher when the aerators were ON, over short time frames, supporting the hypothesis that aeration enhanced the atmospheric methane flux. The fine-bubble approach showed lower air-water methane fluxes compared to the larger bubble, destratification system. We found that the primary source of the methane was the sediments, however,in situmethane production or an upstream methane source could not be ruled out. Overall, our measurements of methane concentrations were consistently high in all times and locations, supporting consistent methane flux to the atmosphere. 

Abstract: The timing of sea ice retreat, light availability, and sea surface stratification largely control the phytoplankton community composition in the Chukchi Sea. This region is experiencing a significant warming trend, an overall decrease in sea ice cover, and a documented decline in annual sea ice persistence and thickness over the past several decades. The consequences of earlier seasonal sea ice retreat and a longer sea-ice-free season on phytoplankton community composition warrant investigation. We applied multivariate statistical techniques to elucidate the mechanisms that relate environmental variables to phytoplankton community composition in the Chukchi Sea using data collected during a single field campaign in the summer of 2011. Three phytoplankton groups emerged that were correlated with sea ice, sea surface temperature, nutrients, salinity, and light. Longer ice-free duration in a future Chukchi Sea will result in warmer sea surface temperatures and nutrient depletion, which we conclude will favor other phytoplankton types over larger diatoms. Plain Language Summary: In the Chukchi Sea, the seasonality of sea ice shapes ecosystem structure of the water column under both sea-ice-covered and sea-ice-free conditions. As such, phytoplankton community composition under both conditions responds to water column structure and nutrient availability. Owing to recent warming in the Arctic, sea ice is thinner and retreats earlier. To date, we do not fully understand the long-term consequences of earlier sea ice retreat on phytoplankton community composition and carbon biomass. To this end, we used environmental and phytoplankton data to relate how differences in ecosystem function under sea-ice-covered and sea-ice-free conditions govern phytoplankton communities. The results from this data set suggest that a future, sea-ice-free Chukchi Sea will exhibit lower phytoplankton biomass, impacting the food web and carbon export.