Midwater macroplankton of British Columbia studied by submersible PISCES IV

• Published in: Journal of plankton research 1985, Vol.7 (6), p.753-777
• Main Author: MACKIE, G. O
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Brackish; Fundamental and applied biological sciences. Psychology; Marine; Sea water ecosystems; Synecology
• ISSN: 0142-7873; 1464-3774
• DOI: 10.1093/plankt/7.6.753

Data are reported from 30 dives during winter and spring 1980–83 at sites in the Strait of Georgia and inlets running off it, and in inlets on the west coast of Vancouver Island. Observations were made from the surface to the bottom (maximum 733 m) but most attention was given to the midwater plankton community. The vertical distribution and abundance of hydromedusae, siphonophores, ctenophores, euphausiids, pelagic worms and molluscs were recorded systematically, along with data for one copepod species (Neocalanus plumchrus). The midwater environment was found to be stable in terms of species composition and depth ranges, which permitted the data for several years and many locations to be pooled. Four categories of plankton are recognized: (a) epipelagic (concentrated in the top 50 m); (b) mesopelagic (50–175 m); (c) bathypelagic (below 175 m); and (d) meso-bathypelagic (forms living in both meso- and bathypelagic zones). Species in this last category behave like mesopelagic forms at the upper end of their ranges, migrating to the surface at night. Deeper-lying members of the same species do not migrate. For six such species, the cut-off point between migratory and non-migratory components was found to lie at a mean depth of 175 m. This depth is therefore taken as the demarcation point between the meso- and bathypelagic zones. Taking account of published data on light penetration, it is estimated that, for the whole region, daytime light intensity at 175 m, and hence the effective limit for phototaxis of the species in question, lies in the range 10−8–10−9 μW cm−2.