Freshwater algal populations and water quality
Date
1972
Authors
Brown, Sharon-Dale
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Abstract
Periphyton communities on vertically oriented artificial substrata at four sampling stations in the shallow (1.83 to 2.75 m) littoral region of Elk Lake (Vancouver Island, B.C.) were quantitatively sampled over a six month period from August 1967 to January 1968. Statistical methods were employed to determine the degree of variability in community structure between station locations (and exposure periods) and the significance of this variation in relation to fluctuations in environmental data (15 variables) concurrently monitored.
An exposure-frame supporting 50 x 75 mm glass slides was devised to replicate sampling at monthly and overlapping time intervals (TS) of 27 to 135 days at each station. Periphyton species numbers, numbers of individuals, relative abundances and species diversity (Shannon-Weaver index H") were computed from count data obtained using permanent membrane filter preparations. While count estimates (cells or colonies/mm2) were within recommended error limits and most organisms were randomly distributed on the filter surfaces, analysis of the experimental design indicated that the counting stage contributed the largest variance component with percent error of 17 and 30% for total and individual species counts. By contrast, variance due to sampling (5-15%) and filtering (0-5%) was usually lower for both.
Of the 29 periphyton taxa counted, 28 were diatoms and 20 of these had constancy values of 100%. Four species, Achnanthes minutissima, Cocconeis placentula, Fragilaria crotonensis, and F. virescens, composed >50% of total cell popu~ations in all periphyton samples and the latter two species also dominated the net planktonic diatom assemblage.
Elk Lake, a second order warm monomictic lake of enriched eutrophic condition, is typical of the Insular Lowland Limnological Region. In general, normal seasonal fluctuations in physico-chemical variables were found, their variation being effected to some extent by warm dry summer weather followed by heavy rainfall, high flush rates and flood conditions.
Whereas the four stations were similar in measured physico-chemical variates, periphyton species numbers and species composition, as well as plankton species composition and relative abundances, statistically significant station variation occurred in total cell numbers, individual species populations and species diversity of periphyton communities in each of the 11 different TS's. While the temporal variation in cell numbers and species diversity was statistically related to fluctuations in measured physico-chemical variates (total and orthophosphate, calcium and total hardness) and length of slide exposure, the station variation within the lake for any one TS was not.
Since species diversity of the periphyton decreased with increasing exposure duration and increasing total cell populations, station variation in periphyton communities over the same exposure periods and under the same measured physico-chemical conditions may be due in part to species interaction as hypothesized for two species, Achnanthes minutissima and Cocconeis placentula, which appeared to compete for substrate area. Another factor inducing station variability was the occurrence of species more typically found in the plankton. Species such as F. virescens and A. formosa appeared to settle out into the Elk Lake periphyton following the November breakdown of thermal stratification.
While periphyton reflect the sum total of environmental conditions within a lake, these communities are so sensitive to change in ecological variables that more refined experimental and statistical designs, including more frequent and accurate measurements of physico-chemical variates are necessary to elucidate causal relationships. Similarly, the close relationship between net plankton and periphyton requires that both habitats be studied simultaneously in periphyton studies. Analysis of H" indicates that it is less appropriate and useful than commonly believed.