High Latitude Epipelagic and Mesopelagic Scattering Layers—A Reference for Future Arctic Ecosystem Change

Abstract

Scattering structures, including deep (>200 m) scattering layers are common in most oceans, but have not previously been properly documented in the Arctic Ocean. In this work, we combine acoustic data for distribution and abundance estimation of zooplankton and fish with biological sampling from the region west and north of Svalbard, to examine high latitude meso-and epipelagic scattering layers and their biological constituents. Our results show that typically, there was strong patchy scattering in the upper part of the epipelagic zone (<50 m) throughout the area. It was mainly dominated by copepods, krill, and amphipods in addition to 0-group fish that were particularly abundant west of the Spitsbergen Archipelago. Off-shelf there was a distinct deep scattering layer (DSL) between 250 and 600 m containing a range of larger longer lived organisms (mesopelagic fish and macrozooplankton). In eastern Fram Strait, the DSL also included and was in fact dominated by larger fish close to the shelf/slope break that were associated with Warm Atlantic Water moving north toward the Arctic Ocean, but switched to dominance by species having weaker scattering signatures further offshore. The Weighted Mean Depths of the DSL were deeper (WMD > 440 m) in the Arctic habitat north of Svalbard compared to those south in the Fram Strait west of Svalbard (WMD ∼400 m). The surface integrated backscatter [Nautical Area-Scattering Coefficient, NASC, s A (m 2 nmi −2 )] was considerably lower in the waters around Svalbard compared to the more southern regions (62-69 • N). Also, the integrated DSL nautical area scattering coefficient was a factor of ∼6-10 lower around Svalbard compared to the areas in the south-eastern part of the Norwegian Sea ∼62 • 30 ′ N. The documented patterns and structures, particularly the DSL and its constituents, will be key reference points for understanding and quantifying future changes in the pelagic ecosystem at the entrance to the Arctic Ocean.

Evidence of Diel Vertical Migration of Mesopelagic Sound-Scattering Organisms in the Arctic

“…In the eastern Norwegian Sea, the DSL also consisted of far fewer species of which the most important were krill (M. norvegica), mesopelagic fish (Maurolicus muelleri, B. glaciale), shrimps (S. arcticus, Pasiphaea multidenta, Hymenodora glaciale), and the Scyphozoan (Periphylla periphylla; Melle et al, 1993;Torgersen et al, 1997;Knutsen and Serigstad, 2001), but also included blue whiting (Micromesistius poutassou). An important difference in species composition between the earlier studies in the southeastern Norwegian Sea is the nearly complete absence of blue whiting and Mueller's pearlside (M. muelleri) found during this study in the Svalbard region and confirmed by Knutsen et al (2017).…”

“…Our results fit well with the results from these more southerly regions of the Norwegian Sea that show acoustic energy from the mesopelagic layer decreases with increasing latitude. Knutsen et al (2017) also observed this pattern of DSL backscattering intensity decreasing from the south-eastern Norwegian Sea region to Arctic waters north of Svalbard. This pattern agrees with ≪photoperiod constraint hypothesis≫ (Kaartvedt, 2008).…”

“…In the high Arctic as during the present study with a near 24-h light regime, the abundance of larger pelagic fish predators in the surface waters were limited, although frequent sightings of baleen whales, possibly feeding on schooling krill and amphipods, and dense registrations of copepods (see Knutsen et al, 2017), were recorded. It is however, difficult to assess the predation risk for migrating mesopelagics here, compared to regions further south.…”

“…The results from the SI_ARCTIC 2014 and SI_ARCTIC 2015 surveys revealed that although less substantial in terms of integrated acoustic backscatter than further south, a DSL was found in the Northern Fram Strait and further to the north of Svalbard at latitudes between 79 and 82 • N (Knutsen et al, 2017;this manuscript). This finding corroborates previous observations that scattering layers are present in high latitudes, at least in summer (Hunkins, 1965;Kutschale, 1969;Hansen and Dunbar, 1971).…”

“…The results also showed clear differences in vertical migration when comparing the localities across a longitudinal gradient in the northern Fram Strait; localities in the eastern part spanned a substantial larger WMD range than the localities in the western part (Figures 3, 4). The DSL in the eastern part, which is closer to the shelf break, was dominated by larger fishes (including cod) as compared to further west (Knutsen et al, 2017, Table S1). Larger fishes also migrate on daily cycles, and e.g., cod migrations can span ranges up to 250 m (e.g., Godø and Michalsen, 2000;Michalsen et al, 2013).…”