Abstract
Arctic benthic ecosystems are facing high-speed environmental changes, such as decreased sea ice coverage, increased temperature and precipitations, as well as the invasion by non-indigenous species. Few sub-arctic fjords have the particularity to have an inner-most part forming a basin in which water remains very cold. Those fjords may offer a refugee for cold-water arctic species as well as a small-scale “laboratory” of the changes that arctic assemblages located at higher latitudes might face soon. The Porsangerfjord in Northern Norway is a sub-arctic fjord with an inner arctic part and face red king crabs Paralithodes camtchasticus invasion since the end of the 1990s. It offers a case study of the dynamics of arctic ecosystems facing multiple stressors, i.e., climate change and invasive species. Based on a time series of megabenthic invertebrates and bentho-demersal fishes over 2007–2019, a complex multivariate analysis (STATICO) was used to identify the trends in the relationship between taxa and the environment. We showed the main environmental changes in the fjord were the freshening of the water, the increase of the seabed current, and the decrease of the maximum sea ice extent. A strong along-fjord gradient was visible for both benthic and fish assemblages. Species richness and Shannon diversity of fishes significantly increased into the fjord, due to the arrival of warm-water species over time that overlapped with cold-water species that have seen their biomass significantly reduced. No significant decrease in the biomass of the cold-water benthic species was visible, which could indicate an efficient refugee effect of the inner fjord. Yet, this refugee effect could be unbalanced by the red king crab invasion as it is a predator of several arctic species. In the Porsangerfjord, fish species thus respond to climate change while megabenthic assemblages are more threatened by invasive species.
Highlights
Climate change has marked effects in the Arctic region: over the last 50 years, it has warmed three times faster than the rest of the world, the extent of sea ice decreased by 43%, and rainfall increased by 24% which increased freshwater runoff and surface stratification (Arctic Monitoring Assessment Program, 2021)
We used a complex multivariate analysis to identify the trends of the co-variation with the environment of megabenthic and fish taxa in a sub-arctic fjord facing both climate change effects and invasion by red king crabs
Our results show that the fish assemblages are increasingly driven by bottom temperature, especially after 2016
Summary
Climate change has marked effects in the Arctic region: over the last 50 years, it has warmed three times faster than the rest of the world, the extent of sea ice decreased by 43%, and rainfall increased by 24% which increased freshwater runoff and surface stratification (Arctic Monitoring Assessment Program, 2021). Fjords ‘Biodiversity Facing Anthropogenic Impacts aquaculture and fisheries, and mitigate global climate change because of high productivity and carbon storage (Johnsen et al, 2009; Smith et al, 2015; Ortega et al, 2019) as well as providing suitable areas for spawning grounds and nurseries for commercially important species (Olsen et al, 2010). A key variable to study climate change impacts from the terrestrial influence is coastal light attenuation. Changes in the open ocean conditions can affect the dynamics of species inside the fjords, notably through coastal wind changes resulting in upwelling or downwelling and the vertical stratification at the coast (Svendsen, 1995; Asplin et al, 1999). Local fjord conditions may attract open ocean megafauna such as humpback whales (Megaptera novaeangliae) and killer whales (Orcinus orca) feeding on wintering Norwegian spring-spawning herring (Clupea harengus) (Jourdain and Vongraven, 2017)
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