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Abstract The tidal tributaries of the lower Chesapeake Bay experience seasonally recurring harmful algal blooms and the significance of submarine groundwater discharge (SGD) as a nutrient vector is largely unknown. Here, we determined seasonal SGD nutrient loads in two tributaries with contrasting hydrodynamic conditions, river‐fed (York River) vs. tidally dominated (Lafayette River). Radon surveys were performed in each river to quantify SGD at the embayment‐scale during spring and fall 2021. Total SGD was determined from a²²²Rn mass balance and Monte Carlo simulations. Submarine groundwater discharge rates differed by a factor of two during spring (Lafayette = 11 ± 17 cm d⁻¹; York = 6 ± 10 cm d⁻¹) and a factor of six during fall (Lafayette = 19 ± 27 cm d⁻¹; York = 3 ± 7 cm d⁻¹). Groundwater N concentrations and fluxes varied seasonally in the York (4–7 mmol N m⁻²d⁻¹). In the Lafayette River, seasonal N fluxes (22–37 mmol N m⁻²d⁻¹) were driven by seasonal water exchange rates, likely due to recurrent saltwater intrusion. Submarine groundwater discharge–derived nutrient fluxes were orders of magnitude greater than riverine inputs and runoff in each system. Additionally, sediment N removal by denitrification and anaerobic ammonium oxidation would only remove ~ 1–11% of dissolved inorganic nitrogen supplied through SGD. The continued recurrence of harmful algal blooms in the Bay's tidal tributaries may be indicative of an under‐accounting of submarine groundwater‐borne nutrient sources. This study highlights the importance of including SGD in water quality models used to advise restoration efforts in the Chesapeake Bay region and beyond. 

Research on the impact of seawater intrusion on nitrogen (N) cycling in coastal estuarine ecosystems is crucial; however, there is still a lack of relevant research conducted underin-situfield conditions. The effects of elevated salinity on N cycling processes and microbiomes were examinedin situseawater intrusion experiments conducted from 2019 to 2021 in the Nakdong River Estuary (South Korea), where an estuarine dam regulates tidal hydrodynamics. After the opening of the Nakdong Estuary Dam (seawater intrusion event), the density difference between seawater and freshwater resulted in varying degrees of seawater trapping at topographically deep stations. Bottom-water oxygen conditions had been altered in normoxia, hypoxia, and weak hypoxia due to the different degrees of seawater trapping in 2019, 2020, and 2021, respectively. Denitrification mostly dominated the nitrate (NO₃^-) reduction process, except in 2020 after seawater intrusion. However, denitrification rates decreased because of reduced coupled nitrification after seawater intrusion due to the dissolved oxygen limitation in 2020. Dissimilatory nitrate reduction to ammonium (DNRA) rates immediately increased after seawater intrusion in 2020, replacing denitrification as the dominant pathway in the NO₃^-reduction process. The enhanced DNRA rate was mainly due to the abundant organic matter associated with seawater invasion and more reducing environment (maybe sulfide enhancement effects) under high seawater-trapping conditions. Denitrification increased in 2021 after seawater intrusion during weak hypoxia; however, DNRA did not change. Small seawater intrusion in 2019 caused no seawater trapping and overall normoxic condition, though a slight shift from denitrification to DNRA was observed. Metagenomic analysis revealed a decrease in overall denitrification-associated genes in response to seawater intrusion in 2019 and 2020, while DNRA-associated gene abundance increased. In 2021 after seawater intrusion, microbial gene abundance associated with denitrification increased, while that of DNRA did not change significantly. These changes in gene abundance align mostly with alterations in nitrogen transformation rates. In summary, ecological change effects in N cycling after the dam opening (N retention or release, that is, eutrophication deterioration or mitigation) depend on the degree of seawater intrusion and the underlying freshwater conditions, which constitute the extent of seawater-trapping. 

The nitrogen (N) loss processes have not been well examined in subterranean estuaries (STEs) between land and sea. We utilized a 15N isotope tracer method, q-PCR, and high-throughput sequencing to reveal the activities, abundances, and community compositions of N loss communities in a STE in Gloucester Point, Virginia, US. The highest activities, abundances and diversity of denitrifiers and anammox bacteria were detected at 50–60 cm depth in the aerobic-anaerobic transition zone (AATZ) characterized by sharp redox gradients. nirS-denitrifiers and anammox bacteria were affiliated to 10 different clusters and three genera, respectively. Denitrification and anammox played equal roles with an estimated N loss of 13.15 mmol N m−3 day−1. A positive correlation between ammonia oxidizing prokaryote abundances and DO as well as NOx− suggested that nitrification produces NOx− which supports the hotspot of denitrification and anammox within the AATZ. Overall, these results highlight the roles of N loss communities in STEs 

Abstract Sandy sediment beaches covering 70% of non‐ice‐covered coastlines are important ecosystems for nutrient cycling along the land‐ocean continuum. Subterranean estuaries (STEs), where groundwater and seawater meet, are hotspots for biogeochemical cycling within sandy beaches. The STE microbial community facilitates biogeochemical reactions, determining the fate of nutrients, including nitrogen (N), supplied by groundwater. Nitrification influences the fate of N, oxidising reduced dissolved inorganic nitrogen (DIN), making it available for N removal. We used metabarcoding of 16S rRNA genes and quantitative PCR (qPCR) of ammonia monooxygenase (amoA) genes to characterise spatial and temporal variation in STE microbial community structure and nitrifying organisms. We examined nitrifier diversity, distribution and abundance to determine how geochemical measurements influenced their distribution in STEs. Sediment microbial communities varied with depth (p‐value = 0.001) and followed geochemical gradients in dissolved oxygen (DO), salinity, pH, dissolved inorganic carbon and DIN. Genetic potential for nitrification in the STE was evidenced by qPCR quantification ofamoAgenes. Ammonia oxidiser abundance was best explained by DIN, DO and pH. Our results suggest that geochemical gradients are tightly linked to STE community composition and nitrifier abundance, which are important to determine the fate and transport of groundwater‐derived nutrients to coastal waters. 

Codenitrification is a reactive nitrogen (N) removal pathway producing hybrid dinitrogen (N2) by combining nitrite (NO2–) and a partner-N substrate. Abiotic codenitrification also produces hybrid N2 through nitrosation of organic N by NO2–, but it is poorly constrained in soil N cycles. We determined the importance of abiotic codenitrification in soils and examined factors controlling abiotic codenitrification using live soils, sterile soils, and sterile solutions. Abiotic codenitrification in sterile soils ranged from 0.12 ± 0.001 to 0.60 ± 0.08 nmoles 29N2-N g–1 day–1, which accounts for 2.3 to 8.2% of total N2 production measured in live soils. Increased abiotic N2 production was observed in soils with the addition of an organic N partner (glutamine). Consistent with previous work, higher rates were observed in lower-pH soils, but the highest rate was found in the soil with the highest carbon:nitrogen (C:N) ratio. We further investigated a range of organic N partners and the influence of concentration and pH on abiotic codenitrification in solution. Similar to sterile soil incubations, abiotic 29N2 production was negatively correlated with increasing pH in solution. Greater rates of abiotic 29N2 production were measured as the substrate concentration increased and pH decreased. Solution experiments also showed that addition of organic N partners increased abiotic codenitrification rates, which are positively correlated with the C:N ratios of organic N partners. This is the first study demonstrating the importance of N removal through abiotic codenitrification in acidic soils and the C:N ratio of organic N partners as a controlling factor in abiotic codenitrification. 

Abstract Subterranean estuaries (STEs) form at the land‐sea boundary where groundwater and seawater mix. These biogeochemically reactive zones influence groundwater‐borne nutrient concentrations and speciation prior to export via submarine groundwater discharge (SGD). We examined a STE located along the York River Estuary (YRE) to determine if SGD delivers dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to the overlying water. We assessed variations in STE geochemical profiles with depth across locations, times, and tidal stages, estimated N removal along the STE flow path, measured hydraulic gradients to estimate SGD, and calculated potential nutrient fluxes. Salinity, dissolved oxygen (DO), DIN, and DIP varied significantly with depth and season (p < 0.05), but not location or tidal stage. Ammonium dominated the DIN pool deep in the STE. Moving toward the sediment surface, ammonium concentrations decreased as nitrate and DO concentrations increased, suggesting nitrification. Potential sediment N removal rates mediated by denitrification were <8 mmoles N m⁻² d⁻¹. The total groundwater discharge rate was 38 ± 11 L m⁻² d⁻¹; discharge followed tidal and seasonal patterns. Net SGD nutrient fluxes were 0.065–3.2 and 0.019–0.093 mmoles m⁻² d⁻¹for DIN and DIP, respectively. However, microbial N removal in the STE may attenuate 0.58% to >100% of groundwater DIN. SGD fluxes were on the same order of magnitude as diffusive benthic fluxes but accounted for <10% of the nutrients delivered by fluvial advection in the YRE. Our results indicate the importance of STE biogeochemical transformations to SGD flux estimations and their role in coastal eutrophication and nutrient dynamics. 

Summary Relative sea level rise (SLR) increasingly impacts coastal ecosystems through the formation of ghost forests. To predict the future of coastal ecosystems under SLR and changing climate, it is important to understand the physiological mechanisms underlying coastal tree mortality and to integrate this knowledge into dynamic vegetation models.We incorporate the physiological effect of salinity and hypoxia in a dynamic vegetation model in the Earth system land model, and used the model to investigate the mechanisms of mortality of conifer forests on the west and east coast sites of USA, where trees experience different form of sea water exposure.Simulations suggest similar physiological mechanisms can result in different mortality patterns. At the east coast site that experienced severe increases in seawater exposure, trees loose photosynthetic capacity and roots rapidly, and both storage carbon and hydraulic conductance decrease significantly within a year. Over time, further consumption of storage carbon that leads to carbon starvation dominates mortality. At the west coast site that gradually exposed to seawater through SLR, hydraulic failure dominates mortality because root loss impacts on conductance are greater than the degree of storage carbon depletion.Measurements and modeling focused on understanding the physiological mechanisms of mortality is critical to reducing predictive uncertainty.