Groundwater nitrogen concentrations and composition across an arctic subterranean estuary

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2023-12

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Groundwater is considered an important source of dissolved organic matter (DOM) and inorganic nutrients to coastal ecosystems. However, little is known about contemporary groundwater inputs to Arctic coastal waters or how they may change with global warming. Research at Beaufort Lagoon Ecosystems Long Term Ecological Research (BLE LTER) field sites focusing on supra-permafrost groundwater flowing into Simpson Lagoon, Alaska are broadening our understanding of groundwater inputs in this under-studied and rapidly changing region. Field trips to Simpson Lagoon during 2021 and 2022 aimed to quantify patterns in organic matter and inorganic nutrients across the tundra-lagoon interface and get an idea of the diagenetic processing of DOM. Concentrations of inorganic nutrients (nitrogen, phosphorus, and silica) were collected across salinity to consider microbial processing of nutrients. Total hydrolyzable amino acid (THAA) composition was used as an indicator of DOM sources and lability across the salinity gradient. Salinities of groundwater/porewater collected across this interface spanned 0 to 69. Departures from conservative mixing across this salinity gradient were used to assess transformations within the fresh-marine groundwater mixing zone. We found that fresh groundwater along the Beaufort Sea coast has significantly more DOM and THAA than river surface waters. Mixing models suggest that along the subterranean estuary, the influence of fresh groundwater, marine groundwater, and lagoon surface water on inorganic and organic nitrogen varies seasonally. Additionally, principal component analysis (PCA) results relating amino acid composition to environmental variables suggest that groundwater DOM is more degraded than the lagoon DOM. Quantifying DOM and inorganic nutrients and understanding the composition and lability of DOM is imperative to interpreting how groundwater inputs affect lagoon ecosystems.

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