Insights into marine biological responses to Icelandic glacial outburst floods from remote sensing data

Abstract

Cryospheric change has altered discharge patterns of the subglacial Skaftá cauldrons and Grímsvötn volcano beneath the Vatnajökull ice cap, SE Iceland (64.4220° N, 16.7902° W) at least since 1940. Jökulhlaup (glacial outburst floods) frequency has increased but their magnitude has decreased. This study examines jökulhlaup effects on the marine environment at the Gígjukvísl, Skaftá, and Kúðafljót estuaries. Remote sensing data integrated in situ, observational, and modelled data to analyse sea surface salinity (1/8° resolution), temperature (0.05° resolution), and observational chlorophyll-a data (1 km² resolution). Covering 11 jökulhlaups in Gígjukvísl/Skaftá and seven in Kúðafljót, a systematic response emerged in residual data. Salinity reduced by 0.428 ± 0.444 (Gígjukvísl/Skaftá) and 0.280 ± 0.423 (Kúðafljót) on jökulhlaup start-days, with smaller declines one to two weeks later linked to seawater dilution. Temperatures fell by around -0.5 °C pre-flood at both sites (± 0.710 °C at Gígjukvísl/Skaftá and ± 0.658 at Kúðafljót) and up to -0.6 ± 0.209 °C (Kúðafljót) post-flood from ice dam melting and meltwater discharge. This aligns with partially significant chlorophyll-a pre-flood reductions of ~1.2 ± 0.496 mg/m3 and three post-flood peaks with mean increases by up to 1.446 ± 7.968 mg/m3 (Gígjukvísl/Skaftá) and 3.269 ± 11.810 mg/m3 (Kúðafljót), suggesting short algal blooms through increased nutrients and sediments. These results advance the understanding of anomalous glacial freshwater discharges, potentially affecting
regional to global ocean circulation and productivity.

Date of Award	21 Jul 2025
Original language	English
Awarding Institution	University of Plymouth
Supervisor	Tim Daley (Director of Studies (First Supervisor)) & Matt Westoby (Other Supervisor)