Spatial variability of deep scattering layers shapes the Bahamian mesopelagic ecosystem

Abstract

Deep scattering layers (DSLs) play an important role in pelagic food webs, serving as a vehicle for transferring energy between productive surface waters and the deep sea. We explored the spatial dynamics of DSLs off the Bahamas in shaping oligotrophic ecosystems. We compared 2 areas known to be important foraging habitats for a deep-diving predator, Blainville's beaked whales: the Tongue of the Ocean (TOTO), an oceanographically isolated habitat, and the waters off Abaco Island, an oceanographically connected habitat. Using ship-based multifrequency echosounders and direct net sampling, we identified common layer structures characterized by diffuse, broad layers (>100 m in thickness) observed across the study areas. Within those diffuse layers, we occasionally observed distinctively bounded intense layers (~20 m in thickness) located at the upper edges of the DSLs. We found that spatial variability of layer structures shaped the Bahamian ecosystem. By comparing common layer types across the sampling locations, 2 potential mechanisms were identified. The area off Abaco Island was characterized by diffuse layers comprised of larger animals with greater migration distance and biomass than the habitat in TOTO, suggesting that Abaco may transfer more energy between energy-rich surface waters and the deep sea. Within TOTO, habitat most frequently used by beaked whales was characterized by the highest occurrence of intense layers dominated by thinner layers, suggesting such fine-scale structures may increase foraging efficiency by layer predators. Spatial variability of the DSL structures reveals the dynamics of the Bahamian mesopelagic ecosystem, potentially driving the beaked whales through bottom-up control of their prey.

On the robustness of an eastern boundary upwelling ecosystem exposed to multiple stressors

“…Because of the short life cycles of these species, intermediate communities (zooplankton and micronekton) respond rapidly to mesoscale physical processes associated with currents and frontal structures linked to coastal upwellings 13 – 15 . Zooplanktonic and micronektonic compartments are essential for sustaining organisms higher up the food web 16 – 18 ; consequently, changes to their community could impact the whole marine ecosystem 19 .…”

“…* email: diogoulndague@yahoo.fr upwellings [13][14][15] . Zooplanktonic and micronektonic compartments are essential for sustaining organisms higher up the food web [16][17][18] ; consequently, changes to their community could impact the whole marine ecosystem 19 .…”

Critical threshold identified in the functional relationship between beaked whales and their prey

“…These differences are less pronounced but still statistically significant when comparing only AUTEC West and Abaco. Overall, during the 2 wk timeframe of this study, Abaco appeared to be a more energy efficient ecosystem, with more extensively migrating mesopelagic animals, larger animals from zooplankton to nekton, and a greater degree of vertical overlap amongst trophic steps than TOTO (Sato & Benoit-Bird 2017). This resulted in an approximately 20% difference in foraging habitat quality metrics between similarly sized zones on the parts of the AUTEC range most used by Mesoplodon and Abaco and up to a 20 fold difference from Abaco if the entire TOTO region sampled is considered, though the scale differences in this comparison must be considered.…”

“…During 2−14 July 2015, 8 transects (20 km long) were sampled during the daytime in each of these 5 zones using a surface vessel and an AUV (Sato & Benoit-Bird 2017). Two regions known for relatively high habitat use by beaked whales were sampled at a finer spatial scale, using an expanding box with a maximum dimension of 1.5 km on a side (Moline &…”

“…As shown in previous sampling, the species' foraging effort was higher on the western part of the range than the eastern part Benoit-Bird 2016): an area surrounding Whiskey 1 and the Abaco slope (Claridge 2013). At each of these locations, measurements of the distribution and density of animals in the upper 600 m of the water column were made using multifrequency (38,70,120,200 kHz Simrad EK60s) hydro-acoustics from the R/V 'Sharp', which also conducted net sampling for zooplankton and mesopelagic fishes and vertical CTD plus fluorometer profiles to 600 m. Discussed in detail in Sato & Benoit-Bird (2017), these measurements provide environmental context for comparisons between the study regions. An autonomous echosounder system (38 and 120 kHz Simrad EK60s) integrated into a 600 m depth-capable AUV (Moline et al 2015) was used to measure the distribution and density of squid and other resources from 600− 1200 m, covering much of the depth range over which beaked whales forage (Johnson et al 2008).…”