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Abstract Subterranean estuaries (STEs) form in the subsurface where fresh groundwater and seawater meet and mix. Subterranean estuaries support a variety of biogeochemical processes including those transforming nitrogen (N). Groundwater is often enriched with dissolved inorganic nitrogen (DIN), and transformations in the STE determine the fate of that DIN, which may be discharged to coastal waters. Nitrification oxidizes ammonium (NH₄⁺) to nitrate, making DIN available for N removal via denitrification. We measured nitrification at an STE, in Virginia, USA using in situ and ex situ methods including conservative mixing models informed by in situ geochemical profiles, an in situ experiment with¹⁵NH₄⁺tracer injection, and ex situ sediment slurry incubations with¹⁵NH₄⁺tracer addition. All methods indicated nitrification in the STE, but the ex situ sediment slurries revealed higher rates than both the in situ tracr experiment and mixing model estimations. Nitrification rates ranged 55.0–183.16 μmol N m⁻² d⁻¹based on mixing models, 94.2–225 μmol N m⁻² d⁻¹in the in situ tracer experiment, and 36.6–109 μmol N m⁻² d⁻¹slurry incubations. The in situ tracer experiment revealed higher rates and spatial variation not captured by the other methods. The geochemical complexity of the STE makes it difficult to replicate in situ conditions with incubations and calculations based on chemical profiles integrate over longer timescales, therefore, in situ approaches may best quantify transformation rates. Our data suggest that STE nitrification produces NO₃⁻, altering the DIN pool discharged to overlying water via submarine groundwater discharge. 

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. 

Abstract Coastal habitat‐forming species provide protection and essential habitat for fisheries but their ability to maintain these services are under threat from novel stressors including rising temperatures. Coastal habitat restoration is a powerful tool to help mitigate the loss of habitat‐forming species, however, many efforts focus on reintroducing a single, imperilled species instead of incorporating alternatives that are more conducive to current and future conditions. Seagrass restoration has seen mixed success in halting local meadow declines but could begin to specifically utilize generalist seagrasses with climate change‐tolerant and opportunistic life history traits including high reproduction rates and rapid growth.Here, we built on decades of successful eelgrass (Zostera marina) restoration in the Chesapeake Bay by experimentally testing seed‐based restoration potential of widgeongrass (Ruppia maritima)—a globally distributed seagrass that can withstand wide ranges of salinities and temperatures. Using field experiments, we evaluated which seeding methods yielded highest widgeongrass survival and growth, tested if seeding widgeongrass adjacent to eelgrass can increase restoration success, and quantified how either seagrass species changes restored bed structure, invertebrate communities, and nitrogen cycling.We found that widgeongrass can be restored via direct seeding in the fall, and that seeding both species maximized total viable restored area. Our pilot restoration area increased by 98% because we seeded widgeongrass in shallow, high temperature waters that are currently unsuitable for eelgrass survival and thus, would remain unseeded via only eelgrass restoration efforts. Restored widgeongrass had higher faunal diversity and double animal abundance per plant biomass than restored eelgrass, whereas restored eelgrass produced three times greater plant biomass per unit area and higher nitrogen recycling in the sediment.Synthesis and applications.Overall, we provide evidence that supplementing opportunistic, generalist species into habitat restoration is a proactive approach to combat climate change impacts. Specifically, these species can increase trait diversity which, for our study, increased total habitat area restored—a key factor to promote seagrass beds' facilitation cascades, stability, and grass persistence through changing environments. Now, we call for tests to determine if the benefits of restoration with generalist species alone or in conjunction with historically dominant taxa are broadly transferrable to restoration in other marine and terrestrial habitats. 

Abstract Terrestrial groundwater travels through subterranean estuaries before reaching the sea. Groundwater‐derived nutrients drive coastal water quality, primary production, and eutrophication. We determined how dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved organic nitrogen (DON) are transformed within subterranean estuaries and estimated submarine groundwater discharge (SGD) nutrient loads compiling > 10,000 groundwater samples from 216 sites worldwide. Nutrients exhibited complex, nonconservative behavior in subterranean estuaries. Fresh groundwater DIN and DIP are usually produced, and DON is consumed during transport. Median total SGD (saline and fresh) fluxes globally were 5.4, 2.6, and 0.18 Tmol yr⁻¹for DIN, DON, and DIP, respectively. Despite large natural variability, total SGD fluxes likely exceed global riverine nutrient export. Fresh SGD is a small source of new nutrients, but saline SGD is an important source of mostly recycled nutrients. Nutrients exported via SGD via subterranean estuaries are critical to coastal biogeochemistry and a significant nutrient source to the oceans.