Multi-parametric study of behavioural modulation in demersal decapods at the VENUS cabled observatory in Saanich Inlet, British Columbia, Canada

Abstract

Understanding biological rhythms in benthic ecosystems and their modulation by habitat cycles has important implications for resource and ecosystem management. The recent development of permanent, multi-sensor seafloor observatories in deep-water environments provides opportunities for the in situ investigation of the behaviour of benthic organisms in relation to habitat variability. This paper describes a multi-disciplinary investigation at the VENUS observatory platform in Saanich Inlet, an intermittently anoxic fjord (Vancouver Island, Canada). A remotely operated digital camera (103 m depth) was used to document changes in the abundance of shrimp ( Spirontocaris spp.) and the squat lobster ( Munida quadrispina), as well as bacterial mat coverage ( Beggiatoa spp.). These data were used as proxies of diel rhythms related to day–night and internal tidal cycles. Seafloor photos were acquired hourly during consecutive days, before, during and after oxygen intrusion events in the fall of 2009. In order to relate biological fluctuations to habitat cycles, bottom water pressure, temperature, dissolved oxygen, and nitrate data were also acquired from the observatory database. Periodogram analysis showed a weak internal-tide-associated rhythmicity for Spirontocaris spp. that was absent in M. quadrispina and in bacterial mat coverage. Waveform analysis confirmed the absence of any day–night fluctuation in all tested species. However, a rapid intrusion of oxygenated water at the study site influenced visual counts of species, possibly blurring detectable activity rhythms. Temperature and nitrate fluctuations were more accentuated during spring tides but cross-correlation analysis indicated an absence of species responses to these habitat variables. Results are discussed within the context of the complex oceanographic dynamics of Saanich Inlet and with respect to understanding the ecological consequences of expanding hypoxia in the global ocean.