UBC Theses and Dissertations
The analysis of zooplankton population fluctuations in the strait of Georgia, with emphasis on the relationships between Calanus Plumchrus Marukawa and Calanus Marshallae frost Gardner, Grant Allan
In 1971, changes were observed in the overwintering population sizes of Calanus plumchrus Marukawa, Galanus marshallae Frost and Calanus pacificus californicus Brodsky in the Strait of Georgia, British Columbia. Calanus plumchrus and C. pacificus were less common than in previous years, while C. marshallae was more common. Based on scattered data taken since the turn of the century, these changes appeared to be abnormal. Because Calanus plumchrus constituted a significant proportion of the biomass of the zooplankton community, it was possible that the observed fluctuations were indicative of changes in the structure of the zooplankton community within the Strait. Thus a unique opportunity was presented to study a zooplankton community and Its relationship to environmental parameters. Multiple correlation analysis, cluster analysis, multiple regression analysis, factor analysis and principal components analysis were used to analyse zooplankton concentrations and hydrographic data taken in overwintering periods from I969 to 1974. Additional hydrographic data were used to examine relationships between physical and biological data three and six months out of phase. The multivariate techniques allowed an efficient analysis of the relationships within and between the biological and physical data banks. More than one multivariate method was used as each method gives a slightly different viewpoint on the data. A combination of methods thus produces a more complete picture of the system being analysed, while points of overlap between the techniques act as internal checks on the consistency of the analysis. The analysis indicates a recent shift in the hydrographic regime of the Strait of Georgia. The shift is most obvious in the salinity, but can also be seen in the temperature, and in both cases is strongest in Strait of Georgia deep water. It involves subtle changes in salinity and temperature structure. These changes axe of uncertain biological significance but indicate fluctuations in the process of deep water formation. Deep water is formed in the Southern Passages by the mixing of incoming Strait of Juan de Fuca intermediate and deep water with outflowing near surface fresher water. Changes in either of these components, or in the degree of mixing, may produce some changes in the quality of the deep water, which intrudes into the Strait of Georgia in late summer. These changes in quality appear to affect the zooplankton community. Individual zooplankton species are strongly influenced by temperature and stability characteristics or related factors. Temperature and stability during the fall intrusion are particularly important to the overwintering zooplankton community three months later. The same two factors in spring also affect zooplankton in the following winter. The concentrations of Calanus plumchrus and C. marshallae have significant (p<O.05) linear regressions with concurrent temperature at 350 m. The regression lines have opposite slopes and intersect in the region of normal ambient temperature at 350 m. This result suggests that deep water temperature, or a temperature associated, factor, strongly affects the relative fluctuations in the numbers of both species. Principal components and factor analysis of the hydrographic data both suggest that the most important factor in the structure of the water column is its subdivision into near surface, intermediate and deep water. However, in both temperature and salinity components a portion of the variance is associated with a temporal trend within the deep water. Principal components of the zooplankton similarly associate 15% of the zooplankton variance with a temporal trend. No species is strongly weighted on these components, and the association appears to be a function of the whole community, rather than of individual species. As an adjunct to this investigation, ecological separation between Calanus plumchrus and C. marshallae was investigated. Both species have similar distributions and life histories. Feeding competition between them is minimized by a separation in their ability to filter small particles from the water. Calanus plumchrus can feed readily on particles above 3>5 ym in diameter, while C. marshallae can not efficiently filter particles below about 10.5 ym in diameter. Thus, Calanus plumchrus can exploit a potentially rich food source with no competition from Calanus marshallae. This advantage may maintain Calanus plumchrus within the Strait of Georgia despite the detrimental effect of a shift in deep water temperature or related factors. It also suggests that, given a more "normal" physical climate, Calanus plumchrus could revert to its traditional dominance. If Calanus plumchrus continues to drop, or remains at suppressed levels, the economically important fish species that utilize it as food will have to shift prey species, probably to Calanus marshallae. Feeding on C. marshallae will involve a greater energy expenditure to obtain the same ration, and may be detrimental to some predators.
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