Abstract
Knowledge of the processes shaping deep-sea benthic communities at seasonal scales in cold-seep environments is incomplete. Cold seeps within highly dynamic regions, such as submarine canyons, where variable current regimes may occur, are particularly understudied. Novel Internet Operated Vehicles (IOVs), such as tracked crawlers, provide new techniques for investigating these ecosystems over prolonged periods. In this study a benthic crawler connected to the NEPTUNE cabled infrastructure operated by Ocean Networks Canada was used to monitor community changes across 60 m2 of a cold-seep area of the Barkley Canyon, North East Pacific, at ~890 m depth within an Oxygen Minimum Zone (OMZ). Short video-transects were run at 4-h intervals during the first week of successive calendar months, over a 14 month period (February 14th 2013 to April 14th 2014). Within each recorded transect video megafauna abundances were computed and changes in environmental conditions concurrently measured. The responses of fauna to environmental conditions as a proxy of seasonality were assessed through analysis of abundances in a total of 438 video-transects (over 92 h of total footage). 7698 fauna individuals from 6 phyla (Cnidaria, Ctenophora, Arthropoda, Echinodermata, Mollusca, and Chordata) were logged and patterns in abundances of the 7 most abundant taxa (i.e. rockfish Sebastidae, sablefish Anoplopoma fimbria, hagfish Eptatretus stoutii, buccinids (Buccinoidea), undefined small crabs, ctenophores Bolinopsis infundibulum, and Scyphomedusa Poralia rufescens) were identified. Patterns in the reproductive behaviour of the grooved tanner crab (Chionnecetes tanneri) were also indicated. Temporal variations in biodiversity and abundance in megabenthic fauna was significantly influenced by variabilities in flow velocity flow direction (up or down canyon), dissolved oxygen concentration and month of study. Also reported here for the first time are transient mass aggregations of grooved tanner crabs through these depths of the canyon system, in early spring and likely linked to the crab’s reproductive cycle.
Highlights
The continental margins are characterized by high temporal variability in key benthic habitat variables, with variability often related to depth [1]
In the aphotic deep sea, light intensity is often assumed to be replaced by current regimes [2] as the zeitgeber but recent data suggest that indirect day-night synchronization may occur as a result of the presence or absence of fauna making diel vertical migrations, potentially between different depth strata within the water column [3, 4]
Canyons may have an increased diversity and abundance in fauna when compared to adjacent continental slopes, oxygen minimum zones (OMZs), can prevent colonization by fauna less tolerant to low oxygen concentrations, reducing the benefits of the locally elevated food availability [16]
Summary
The continental margins are characterized by high temporal variability in key benthic habitat variables, (such as current regimes, sedimentation rates, and light intensity and spectral quality), with variability often related to depth [1]. Continuous monitoring of oceanographic variables and hourly to seasonal turnover of species compositions can permit the identification of environment drivers that shape benthic community composition [3, 5, 6]. This lack of knowledge is pronounced in regions with complex topography, such as marine canyons, where detrital funneling and changing flow regimes may occur [7, 8]. Our study site in Barkley Canyon corresponded to the core of the North Pacific OMZ between 600–700 m and 900–1100 [21]
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