Nitrification is a central process of the aquatic nitrogen cycle that controls the supply of nitrate used in other key processes, such as phytoplankton growth and denitrification. Through time series observation and modeling of a seasonally stratified, eutrophic coastal basin, we demonstrate that physical dilution of nitrifying microorganisms by water column mixing can delay and decouple nitrification. The findings are based on a 4-y, weekly time series in the subsurface water of Bedford Basin, Nova Scotia, Canada, that included measurement of functional (amoA) and phylogenetic (16S rRNA) marker genes. In years with colder winters, more intense winter mixing resulted in strong dilution of resident nitrifiers in subsurface water, delaying nitrification for weeks to months despite availability of ammonium and oxygen. Delayed regrowth of nitrifiers also led to transient accumulation of nitrite (3 to 8 μmol · kgsw−1) due to decoupling of ammonia and nitrite oxidation. Nitrite accumulation was enhanced by ammonia-oxidizing bacteria (Nitrosomonadaceae) with fast enzyme kinetics, which temporarily outcompeted the ammonia-oxidizing archaea (Nitrosopumilus) that dominated under more stable conditions. The study reveals how physical mixing can drive seasonal and interannual variations in nitrification through control of microbial biomass and diversity. Variable, mixing-induced effects on functionally specialized microbial communities are likely relevant to biogeochemical transformation rates in other seasonally stratified water columns. The detailed study reveals a complex mechanism through which weather and climate variability impacts nitrogen speciation, with implications for coastal ecosystem productivity. It also emphasizes the value of high-frequency, multiparameter time series for identifying complex controls of biogeochemical processes in aquatic systems.

Isotopic enrichment factors are key to using stable isotope signatures in biogeochemical studies. However, these are typically determined in laboratory experiments and their applicability to environmental conditions is difficult to test. Here, we analyzed nitrogen stable isotope changes associated with nitrification in a coastal basin using weekly time-series measurements of δ 15 N in particulate nitrogen, ammonium, nitrite, and nitrate. Two year-long time series were selected as contrasting natural experiments in the ammonium-rich, aphotic bottom water of Bedford Basin, Nova Scotia, Canada. In 2014, ammonia oxidation (AO) was associated with Thaumarchaeota and nitrite concentrations remained low (< 0.5 μmol kg À1 ). In contrast, transient nitrite accumulation (~8 μmol kg À1 ) and a more rapid δ 15 N NH4 increase in the fall of 2017 were likely caused by ammoniaoxidizing bacteria, associated with higher AO rates and, possibly, stronger nitrogen-isotope enrichment ( 15 ε AO ). Estimates of 15 ε AO (21.8 AE 2.2‰, 24.1 AE 1.1‰) were derived empirically using Rayleigh models applied to field data from restricted periods during which the bottom waters approximated a closed system and influence on 15 ε AO from other processes was demonstrably insignificant. Using a numerical reactive-transport model, we

Deoxygenation of the ocean has been occurring over the last half century, particularly in poorly ventilated coastal waters. In coastal and estuarine environments, both the water column and sediments play key roles in controlling oxygen variability. In this study, we focus on controls of oxygen concentration in Bedford Basin (BB), a 70 m deep, seasonally hypoxic semi-enclosed fjord on the West Atlantic coast in Nova Scotia. The basin is connected to the Scotian Shelf via a narrow 20 m deep sill that restricts the resupply of bottom water. Hypoxia was recorded seasonally in 2018, 2019 and 2021 with minimum oxygen concentration of 5, 6.7 and 2.7 μM, respectively. Using a 1-D benthic-pelagic coupled model we investigate oxygen consumption and resupply processes during these years. The model was constrained with weekly water column measurements of temperature, salinity, chlorophyll-a fluorescence and dissolved oxygen from a monitoring station in the central basin together with seasonal measurements of benthic diffusive oxygen uptake. Our model suggested that 29-81%, and up to 36% of bottom water re-oxygenation occurred during the winter mixing period and through summer/fall intrusions of Scotian Shelf water, respectively. Occasional shelf water intrusions occurred rapidly, on a timescale of a few hours, and delivered equivalent amounts of oxygen as winter mixing and were sufficient to end bottom water hypoxia. Collectively, these mechanisms supplied the majority of the oxygen delivered to the bottom water. Oxygen supply to bottom waters during periods of water column stratification accounted for 19-36% of the annual flux. The mean benthic uptake was 12 ± 8 mmol m-2 d-1 and contributed ~20% of the total oxygen consumption below the sill depth. In 2021, sea surface temperature (SST) was unusually high and likely resulted in 50% less bottom water oxygenation compared to 2018 and 2019 due to increased stratification; SST in BB was found to be increasing at a rate of 0.11 ± 0.02 °C/year. Climate control on water column stratification are discussed and numerical experiments are used to compare the effects of different water column mixing scenarios on bottom water oxygenation.

We have computed superconformal partial wave for the mixed correlators involving J, φ and φ † , where J is the superconformal primary of 4D N = 2 multiplet, φ and φ † are chiral and anti-chiral scalars respectively. We have used the superembedding formalism and compute conformal partial wave corresponding to the exchange of long multiplet using supershadow approach. This study provides necessary ingredients for study of conformal bootstrap for mixed correlator in 4D N = 2 superconformal field theories.

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