Abstract
The northern North Atlantic Ocean and its adjacent shelf seas, are influenced by several large-scale physical processes which can be described by various climate indices. Although the signal of these indices on the upper ocean has been investigated, the potential effects on vulnerable benthic ecosystems remains unknown. In this study, we examine the relationship between pertinent climate indices and bottom conditions across the northern North Atlantic region for the first time. Changes are assessed using a composite approach over a 50 year period. We use an objectively-analysed observational dataset to investigate changes in bottom salinity and potential temperature, and output from a high-resolution ocean model to examine changes in bottom kinetic energy. Statistically significant, and spatially coherent, changes in bottom potential temperature and salinity are seen for the North Atlantic Oscillation (NAO), Atlantic Meridional Overturning Circulation (AMOC), Atlantic Multi-decadal Oscillation (AMO) and Subpolar Gyre (SPG); with statistically significant changes in bottom kinetic energy seen in the subpolar boundary currents for the NAO and AMOC. As the climate indices have multi-annual timescales, changes in bottom conditions may persist for several years exposing sessile benthic ecosystems to sustained changes. Variations in baseline conditions will also alter the likelihood of extreme events such as marine heatwaves, and will modify any longer-term trends. A thorough understanding of natural variability and its effect on benthic conditions is thus essential for the evaluation of future scenarios and management frameworks.
Highlights
The northern North Atlantic region contains a number of deep-sea ecosystems including cold water corals, sponges, and those in hydrothermal fields, with some being classified as Vulnerable Marine Ecosystems (VMEs)
We move on to discussing changes at fourteen case studies (Figure 1 and Table 1) chosen to represent various VMEs across the northern North Atlantic region. As these sites cover a number of management measures, such as ESBAs, Marine Protected Areas (MPAs), and VME closures, and are often considered as a single entity, we show mean conditions averaged across each location
At Case Study 9 on the Reykjanes Ridge (Figure 1), the Subpolar Gyre (SPG) is associated with the largest changes in Sbot in areas shallower than approximately 2000 m, whilst the largest changes in deeper areas are linked to the Atlantic Multi-decadal Oscillation (AMO) (Figure 10)
Summary
The northern North Atlantic region contains a number of deep-sea ecosystems including cold water corals, sponges, and those in hydrothermal fields, with some being classified as Vulnerable Marine Ecosystems (VMEs). These ecosystems are susceptible to changes in climate (Sweetman et al, 2017; Johnson et al, 2018), in order to place future changes in Indice Significance to Bottom Conditions context and evaluate management measures, it is vital to understand natural climate variability. Whilst the signals of various climate indices on upper ocean conditions have been investigated (e.g., Hátún et al, 2005; Frajka-Williams et al, 2017), potential effects on benthic conditions, and deep-sea VMEs, remain unknown. We make some tentative explanations for the physical basis of some of the significant signals, but are careful not to ascribe causality where we investigate only correlation
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