ABSTRACT: Environmental characteristics and sediment diatoms of 51 lakes on Southern Vancouver Island and Saltspring Island

CGRG Bibliography of Canadian Geomorphology
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Abstract

A comparison of 51 lakes in the southeastern lowlands of Vancouver Islands and Saltspring Island was based on recent profundal sediment diatoms, morphometric and land drainage characteristics, and physico-chemical attributes of mid-lake surface waters. The study was initiated in Elk Lake to: (1) develop a method of quantitative slide preparation, establish a procedure for counting and expressing absolute diatom numbers, and determine optimum subsampling designs for within lake and between lake comparative studies, (2) investigate the spatial variation in sediment diatom assemblages along a traverse of decreasing depth, and (3) ascertain the extent of seasonal variation in deepwater surface sediment diatoms by monthly sampling in conjunction with measurement of contemporary fluctuation in physico-chemical attributes of the water column. Results were used to develop the sampling program for the comparative survey, and assess the ecological significance of between lake differences in environmental variables. Environmental heterogeneity of the study area, including the glacial history as it relates to lake basin formation, was described based on review of the literature. Variates of physiographic location, morphometry and land drainage, and water chemistry for the 51 lakes were analyzed by methods of classification and ordination. Relationships between the three classes of variables were explored and lakes were grouped on the basis of environmental similarities. Analysis included indirect assessment of trophic status. A total of 427 diatom taxa were identified in the counts from 51 lakes or 56 basins (five double basin lakes were represented). Taxonomic information and cell dimensions of representative specimens were presented. Analysis of the sampling design indicated a high degree of reliability is associated with total cell counts/microscope field for all lakes. Diatom data were expressed as presence/absence, absolute cell numbers/mg dry weight of sediment and relative abundances, and descriptive summary statistics were computed for all taxonomic levels. Relationships between individual taxonomic units and environmental variables were examined using correlation and stepwise multiple regression. As with environmental data, classification and ordination were used to examine relationships between diatoms on the basis of their habitat preferences and those between lakes on the basis of the expression of diatoms. Differences between planktonic and nonplanktonic diatoms and the association with the physical environment were reflected in correlation between taxa and with morphometric variables. Within the planktonic components there was a distinct dichotomy between those which proliferate in environments of high conductivity (eutrophy) and those which reach maximum development in waters of low conductivity (oliogotrophy). These basic patterns recurred throughout the analysis with slight modification in interpretation depending on the taxonomic level involved. Lake ordinations described the variation in the diatom count matrix in terms of gradients of conductivity and maximum basin depth. Differences in sediment diatoms of double basin lakes were insignificant in the context of this study. Within the study area, lakes occur in physiographic regions defined by latitude, longitude, altitude and climate, and these regions reflect underlying geological differences. Lakes in these regions have more physico-chemical properties in common than morphometric similarities. Lake groups formed on the basis of taxonomic similarity at the specific and varietal levels conform most closely to physiographic locations and are similar in water chemistry. The most abundant taxon in 33 basins belongs to the Araphidineae, while centric taxa dominate in the other 23 basins. The most abundant taxon was Fragilaria pinnata Ehrenberg var. pinnata dominant in 25 basins, and the second most abundant taxon, Cyclotella stelligera Cleve et Grunow was dominant in 18 basins. A basic differentiation occurs between upland and lowland basins with regards to conductivity and diatom composition at all taxonomic levels of organization.

Bibliography of Canadian Geomorphology