Abstract
Several factors have been found to structure the spatial and temporal patterns of deep scattering layers (DSLs) including temperature, oxygen, salinity, light, and physical oceanographic conditions. We examine the variance in acoustically detected DSLs in the northern Gulf of Mexico and investigate the importance of multiple biotic and abiotic factors including mesoscale oceanographic conditions (e.g., Loop Current-origin water (LCOW), frontal boundaries (FB), and common water (CW)) in structuring DSLs. Results indicate heterogeneity in the vertical position and acoustic backscatter of DSLs relative to oceanographic conditions and light intensity. LCOW regions displayed consistent decreases (by a factor of two and four) in acoustic backscatter in the upper 200 m relative to FB and CW, respectively. DSLs had considerably greater backscatter at night in comparison to the day (25X for FB, 17X for LCOW, and 12X for CW). The importance of biotic (primary productivity) and abiotic (sea surface temperature, salinity) factors varied across oceanographic conditions and depth intervals, suggesting that the patterns in distribution and behavior of the mesopelagic assemblage in the northern Gulf of Mexico are highly dynamic.
Highlights
The oceanic biome is approximately 71% of the planet’s area and much more of the planet’s living space by volume, yet it remains vastly understudied (Childress, 1983; Webb et al, 2010)
We examined the effect of relative light availability at 5 m depth by computing the instantaneous photosynthetically active radiation (IPAR; W m−2) along each transect: IPAR5m = IPARsurf ∗ 2.72(−Kd(490)∗z) where the surface light intensity IPARsurf was approximated by NOAA’s Geostationary Satellite Server (GOES); Kd(490) represents NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) derived diffuse attenuation coefficient at 490 nm, and z represents water depth (5 m)
Patterns in acoustic backscatter at 38 kHz (NASC, m2 nmi−2) and the center of mass (m) of sound scattering layers were examined by time of day, and across the depth intervals (D’Elia et al, 2016). We investigated these patterns relative to the three oceanographic feature classes (LCOW, frontal boundary, and Common Water), using a linear mixed effects model, implemented in R (R Core Team, 2013) with the library “nlme.” Since the variation in the residuals differed by day and night and across the three intervals of depth for both Nautical Area Scattering Coefficient (NASC) and center of mass, a weighting option was added to the model using the varComb and varIdent structure to allow for different variances by time of day and depth domain
Summary
The oceanic biome is approximately 71% of the planet’s area and much more of the planet’s living space by volume, yet it remains vastly understudied (Childress, 1983; Webb et al, 2010). Perhaps the most conspicuous features of this biome are the persistent and ubiquitous sound scattering layers (Marshall, 1954; Barham, 1966; Gjosaeter and Kawaguchi, 1980; Irigoien et al, 2014; Cade and Benoit-Bird, 2015; Davison et al, 2015) formed by zooplankton and micronekton (Kloser et al, 2002; Irigoien et al, 2014; Béhagle et al, 2017) These organisms are responsible for the Earth’s largest animal migration, a process known as diel vertical migration (DVM). In spite of the fundamental ecological importance for open-ocean functioning, and increasing interest in commercial exploitation, the mesopelagic community remains one of the least-studied components of oceanic systems (Handegard et al, 2013; Irigoien et al, 2014; Davison et al, 2015)
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