LAUSR

Monitoring of ocean temperatures is crucial for climate studies, ocean circulation modeling and assessing potential ecosystem impacts. However, obtaining observations from the subsurface ocean is difficult and costly. We present a 3‐year time series of distributed fiber optic sensing (sensitive to temperature changes and strain) obtained at 3 to 6‐month intervals, using a network of telecommunication cables in Guadeloupe (Lesser Antilles). We demonstrate that this technique can track seasonal and annual seafloor water temperature changes to within 0.1°C, in a well‐mixed, shelf sea environment. We observe a marine heatwave, with a temperature increase of +1.5°C between 2022 and 2024 at the sea‐floor, causing coral bleaching with 30% mortality. These trends are confirmed by satellite observations of the Sea Surface Temperature in the same location. This successful demonstration, in a shallow‐water environment, opens the path forward for widespread use of submarine cables for long‐term environmental monitoring of the seafloor.