Arctic Groundwater Discharge Sites Fail to Mimic Methane Seep Ecosystems in Svalbard Fjords

A study of the seabed in two Svalbard fjords found that places where groundwater flows into the sea do not host the dense animal communities seen at methane cold seeps. The groundwater sites held few animals and few species, mostly burrowing worms and small bivalves. Samples were taken in March 2023 from the research vessel Kronprins Haakon. The work appeared in the journal Ecology and Evolution.

The team studied three groundwater discharge sites where freshened porewater had been found in 2022: inner Tempelfjorden at a depth of 78.4 metres, Murdoch Hole in Tempelfjorden at 39.5 metres, and a Hornsund site at 117 metres. For comparison they sampled a cold seep at Hornsundbanken, 126 metres down. Acoustic surveys there had registered gas flares, and the remotely operated vehicle Ægir 6000 confirmed active seepage.

Porewater chloride was below seawater values at all three groundwater sites, down to 251 millimolar at Murdoch Hole. Methane stayed under the detection limit, about 0.09 micromolar, at every one of them. Sulfide was not detected at Tempelfjorden and stayed low at Murdoch Hole and Hornsund, never exceeding 144 micromolar. In one seep core, methane climbed to roughly 158 micromolar, and sulfide reached millimolar levels.

In total 232 individuals from 33 taxa and five phyla were recorded. Mean abundance was 1680.4 individuals per square metre at the groundwater sites and 4742.8 at the seep. Mean biomass was 80.0 grams per square metre against 1139.6. Taxa richness averaged 5.0 against 9.75, and Shannon diversity was 1.27 against 1.85. Annelids made up 75% of individuals overall and 95% at the Hornsund groundwater site, where mollusks were missing. Siboglinid worms and thyasirid bivalves, which dominate high Arctic seep systems, were absent from the groundwater sites. Microbial mats, prominent at Hornsundbanken, were also absent.

The researchers rejected their hypothesis. They said chemosynthesis based on methane is probably not happening at the groundwater sites, and they named freshwater intrusion and glacier-related sedimentation, up to 35 centimetres per year in Hornsund, as further stressors. They added that melt and permafrost thaw may increase groundwater flow, while falling sedimentation as glaciers shift to land-terminating could make its ecological effects detectable.

Correction, 31 October 2026: an earlier version said sulfide never passed 144 micromolar elsewhere than Tempelfjorden. The source says it stayed low at Murdoch Hole and Hornsund, never above 144 micromolar. It also called microbial mats typical of Arctic seeps. The source says they were prominent at Hornsundbanken.

Coverage: Bioengineer