Deep Fjords Are Excellent Natural Infrastructure for Climate Impact Studies

Fjords provide valuable research opportunities for marine scientists. They are excellent natural infrastructure for climate impact studies associated with hypoxic episodes and consequences for mesopelagic and deep‐sea ecosystems involving oceanographic circulation processes and basin water renewals. Repeated sampling from the same populations is possible, making fjords excellent systems for developing time series of data for climate impact studies. We provide an overview of the 14 years of data from Norwegian West Coast fjords, focusing on Masfjorden, and report major findings from Oslofjorden in Eastern Norway, exhibiting recurrent hypoxia in the basin waters. We document that the oxygen levels in Masfjorden decreased rapidly by over 60% at 450 m depth in < 8 years, which is much faster than the average rate of deoxygenation in the global ocean. We also discuss the increase in the deep‐sea and low‐light‐adapted coronate jellyfish Periphylla periphylla in view of altered optical conditions of the basin water potentially related to deoxygenation. We argue that fjords like Masfjorden and Oslofjorden are not only macrocosms for ecological processes but also are likely an accelerated version of deep oceans with respect to climate impacts.