Multibeam acoustic image of Patricia Bay. The location of the VENUS network (96m depth) and the shallow tripod deployment site (62m depth) are shown. Vertical relief is exaggerated by 6:1. Image courtesy of the Canadian Hydrographic Service.
Zooplankton are an important component of marine food-webs, forming the energetic link between primary producers and higher trophic levels. Zooplankton can also play an important role in benthopelagic coupling, facilitating the exchange of biogeochemical material between the pelagic environment and the benthos. To avoid visual predators, many zooplankton species undertake diel vertical migrations (DVM) to minimize the risk of encounter with visual predators. During the day, zooplankton in Saanich Inlet form a dense layer at about 100m depth with densities up to 10 000 individuals m-3. As part of my Masters work, I compared DVM patterns at two sites in Saanich: deep (96m) and shallow (62m) in Patricia Bay.
Acoustic backscatter at the deep and shallow sites in Patricia Bay between March 22 and April 5, 2006 from up-looking transducers positioned near the seabed. Height is the distance above the transducer. Time is UTC and sunset and sunrise are shown as black and red lines, respectively. Zooplankton migrants at the shallow site first appear at the level of the transducer each evening near sunset.
We can detect dense zooplankton layers as they migrate, by using high frequency sound (200 kHz) just like an inverted echosounder (we call the instrument a ‘ZAP’). Our instruments were mounted on the bottom near the VENUS node (deep site) and on an autonomous platform (shallow site) deployed by submersible. At the VENUS site, I observed two vertical migrations at sunset. The deep scattering layer ascended first, followed by a second group that I interpreted as zooplankton emerging from the sediment. At the shallow site, I only observed the emergent group. I observed this regular pattern of emergence on 41 consecutive days of observation over two deployments. This nightly migration represents a significant increase in water column biomass relative to daytime. At the shallow site, vertical migration resulted in a 14-fold increase in zooplankton density at night. Another difference between the migrating scattering layer and the emergent zooplankton was ascent depth. Scattering layer migrators ascended into surface waters each night whereas emergent zooplankton usually remained within 30 meters of the seafloor and did not ascend into the upper 20 meters of the water column. Work at UVic continues to examine the identity of these migrators and their importance to bottom feeders.
To see yesterday’s ZAP record, visit the Data Galleries.
by Ian Beveridge, MSc thesis, Department of Biology, University of Victoria
Sample video:
Clip 1: Dense euphausid and copepod swarm filmed from the ROV ROPOS, Saanich Inlet ROPOS.
