A biological front at the mouth of Saanich Inlet results in higher rates of primary productivity at the inlet mouth relative to the head creating a gradient that could influence zooplankton distribution. A shallow sill (75m) at the inlet mouth restricts circulation below sill depth, isolating the deep basin for much of the year. Anoxia develops in the isolated basin and the depth of the anoxic layer changes during the year. During the day, pelagic zooplankton form a deep scattering layer. Between April 2005 and March 2006. I conducted monthly 200kHz acoustic surveys between the mouth and head of Saanich Inlet to test the hypothesis that zooplankton density was greater near the mouth relative to the head. I was also interested in how changing anoxic layer depth affected the distribution of the deep scattering layer. I found that zooplankton density followed a headward gradient in the spring and summer. with the highest densities near the mouth. Zooplankton density was higher near the mouth or the mid-inlet relative to the head in 75% of transects. I did not observe a zooplankton density gradient during the winter. Zooplankton distribution was affected by dissolved oxygen concentration. Deep scattering layer depth was significantly correlated with the depth of the anoxic layer and vertical compression of the deep scattering layer increased as the anoxic layer moved upwards. When the depth of the anoxic layer was less than 90 meters. zooplankton were nearly absent.
Vertical migration of the deep scattering layer to surface waters at night has been well documented. but zooplankton migration patterns in the shallow waters of Saanich Inlet have not been described. I used 200kHz acoustic data collected by the VENUS observatory (96m) and an autonomous acoustic system deployed at a shallow site (62m) in Patricia Bay to study zooplankton migration patterns. Horizontal movement of the deep scattering layer over shallow depths following vertical migration was infrequent. Over 41 days of observation at the shallow site. I only observed deep scattering layer zooplankton on 12 days. At the shallow site. night-time volume backscatter was dominated by the emergence of benthic zooplankton. The movement of these scatterers into the water column at night resulted in a 14-fold increase in volume backscatter over daytime values. I observed this emergence pattern at both sites. which represents an important component of benthic-pelagic coupling in Saanich Inlet. In contrast to the deer scattering layer. which migrated to the surface each night, emergent zooplankton remained within 30-40 meters of the seafloor and did not ascend into surface waters.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/2287 |
| Date | 01 March 2010 |
| Creators | Beveridge, Ian Alexander |
| Contributors | Tunnicliffe, Verena |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
Page generated in 0.1416 seconds