Persistent Enhancement of Micronekton Backscatter at the Summits of Seamounts in the Azores

Abstract

Knowledge of the dynamics of micronekton at seamounts is critical to understanding the ecological role of these ecosystems. Active acoustic techniques are an effective tool to monitor the distribution and movements of pelagic organisms. We carried out several day-and nighttime active acoustic surveys over a 3-year period (2009)(2010)(2011) to characterize the spatial and temporal distribution of micronekton backscatter on two seamounts (Condor and Gigante) in the Azores and in the surrounding open-waters. The highest mean volume backscattering strength (MVBS) was consistently found in the water column over the seamount summits, regardless of the season and diel period. MVBS over the summits was 14-26 times higher than over the slopes, and 10 times higher than in open-waters. Diel variations in backscatter intensity were more pronounced in open-waters and in Gigante seamount, with higher values during the day in open-waters, and at night over the summits and slopes of Gigante. Over Condor seamount, diel changes in backscatter intensity were small, but MVBS was generally higher at night than during the day, as in Gigante. Persistence of strong acoustic backscatter over the summits of Condor and Gigante seamounts is a key finding of this study and may be explained by the presence of a seamount-associated micronekton community and by the retention of vertically migrating micronekton. The latter hypothesis is consistent with observed day-night differences in backscatter, suggesting that nocturnal migrants may be passively transported or actively swim above seamount summits and slopes. Possible physical mechanisms leading to the observed patterns in micronekton distribution are discussed. This study contributes to a better understanding of how seamounts may influence the spatial and temporal dynamics of micronekton assemblages.

Temporal patterns in acoustic presence and foraging activity of oceanic dolphins at seamounts in the Azores

“…Dolphins foraging at Condor and Gigante seamounts could benefit from increased foraging opportunities during the day, compared to dolphins foraging over deep waters. Daytime micronekton densities over the plateaus of Condor and Gigante are significantly higher than densities measured in open-waters, at any time of the day and across all seasons 18 . Micronekton layers are also strongly structured during the day, forming small patches less than 16 m thick, nested into layers extending 111 m vertically.…”

“…Condor and Gigante are two relatively small, shallow-intermediate seamounts (summits at ~190 m depth) that rise more than 1000 m from the surrounding seafloor. Active acoustic surveys conducted at these seamounts found very high densities of micronekton (i.e., assemblages of small (<20 cm) pelagic fish, cephalopods and crustaceans 17 ) over the plateau of both seamounts, throughout the day, seasons and years 18 . Such high densities are believed to be caused by the presence of a seamount-associated micronekton community and by the retention of vertically migrating micronekton at the summits 19 .…”

Fine-scale vertical structure of sound-scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

“…HCA was based on Euclidean distance and Ward's aggregation method (Ward, 1963). We used principal component analysis (PCA) (Chessel et al, 2013) on the same dataset to determine similarities between CTD stations relative to environmental parameters. Physicochemical parameters were standardized a priori because they were measured with different metrics.…”

“…We used R software (R Core Team, 2016) for statistical analyses and to map data. We used the R package "Cluster" (Maechler et al, 2014) for HCA of CTD data, the R package "maps" (Brownrigg, 2017) to map stations, the package "ade4" (Chessel et al, 2013) to run a PCA, and the package "oce" (Kelley, 2015) to display vertical section plots of physicochemical parameters.…”

Fine-scale vertical structure of sound scattering layers over an east border upwelling system and its relationship to pelagic habitat characteristics

“…Aggregations of marine pelagic organisms in ocean water columns can be observed acoustically as sound-scattering layers (SSLs) (Evans and Hopkins, 1981;Cascão et al, 2017).…”