Carbon Outwelling and Uptake Along a Tidal Glacier‐Lagoon‐Ocean Continuum

Tidewater glaciers are highly vulnerable to climate change due to warming from both atmospheric and seawater sources. Most tidewater glaciers are rapidly retreating, but little is known about how glacial melting modifies coastal biogeochemical cycles. Here, we investigate carbonate and nutrient dynamics and fluxes in an expanding proglacial tidal lagoon connected to Europe's largest glacier in Iceland (Vatnajökull). The lagoon N:P:Si ratios (2:1:30) imply a system deficient in nitrogen. The large variations in the freshwater endmembers highlighted the complexity of resolving sources and transformations. The lagoon acted as a sink of dissolved inorganic carbon (DIC). Floating chamber incubations revealed a CO2 uptake of 26 ± 15 mmol m−2 d−1. Lagoon waters near the glacier had a 170% higher CO2 uptake than near the lagoon mouth, likely driven by primary production stimulated by nitrogen‐rich bottom water upwelling. The lateral DIC and total alkalinity (TA) flux rates (outwelling) from the lagoon to the ocean were −1.5 ± 0.1 (export to ocean) and 23 ± 5 mmol m−2 d−1 (import into the lagoon) respectively. All samples were undersaturated with respect to aragonite due to glacial meltwater dilution of TA and CO2 uptake. This implies dilution of oceanic alkalinity, lowering the nearshore buffering capacity against ocean acidification. Plain Language Summary Marine terminating glaciers are rapidly retreating and releasing freshwater, sediments, carbon, and nutrients to the ocean. We investigated glacier‐ocean exchange in a climate change hotspot in Iceland. The glacier‐fed lagoon was a sink of atmospheric carbon dioxide with greater uptake close to the glacier where primary production is enhanced. Nutrients, dissolved inorganic and organic carbon were exported from the glacier‐fed lagoon to the ocean. Substantial amounts of dissolved inorganic carbon were consumed within the estuarine lagoon prior to exchange with the ocean. Glacier meltwater diluted alkalinity and acidified the lagoon, contributing to local ocean acidification. We emphasize the need to resolve carbon transformations and transport at the land‐ocean interface in areas impacted by glaciers. Key Points The tidewater glacier enhanced CO2 uptake within the lagoon Estuarine transformations within the lagoon modify carbon and nutrient transport to the coastal ocean Glacial meltwater diluted lagoon alkalinity and enhanced local ocean acidification