No abstract

No abstract

No abstract

1. Changes in species composition of the periphyton on introduced substrates were determined in an oiigotrophic mountain stream subject to long-term heavy metal contamination.2. At the upstream control site, the numerically most abundant taxa were Bacillariophyta {Achnanthes minutissima, Achnanthes microcephala and Achnanthes linearis) as well as, in summer, the Chlorophyta (Mougeotia spp. and Ulothrix subtilissima).3. At the downstream contaminated site the periphyton community was totally dominated by Bacillariophyta throughout the sampling period. A. minutissima and A. microcephala were co-dominants during spring. Seasonal succession patterns did not parallel those at the upstream site. Chlorophyta were virtually absent and A. minutissima comprised 94% of the community during summer.4. Species diversity, species evenness and dissimilarity index were utilized to delect differences in species composition, abundance and number. Slight differences were found in spring samples while summer samples indicated major differences between sampling sites.

No abstract

No abstract

Periphyton and plankton samples were collected at four littoral stations in a relatively shallow, eutrophic lake (Elk Lake, B .C ., Canada) over a six month period from August 1967 to January 1968 . The most abundant planktonic diatoms demonstrated a pattern of seasonal succession typical of temperate lakes, and all were present in the periphyton . This pattern was identical at all stations : Fragilaria crotonensis was dominant from August to October, Asterionella formosa in November and December, and Melosira italica (plus M. varians) in January . F. virescens, although never dominant, peaked in October . Periphyton communities were dominated by Achnanthes minutissima, Cocconeis placentula, F. crotonensis and F. virescens . Considerable station variation in successional patterns occurred over different exposure periods ; station differences were least in those samples immersed for monthly intervals, and greatest in those immersed for a cumulative four-month period . Interaction between the phytoplankton and periphyton was illustrated by the occurrence of species common to both habitats . A decrease in cell numbers and percent abundance of these species in planktonic populations coincided with their increase in the periphyton, a relationship which appeared dependent on the breakdown of thermal stratification in November . For example, following turnover, F. crotonensis and A . formosa settled out of the plankton and correspondingly increased in percent abundance in the periphyton . This interdependence was less evident in the four-month samples, where A . minutissima and C. placentula dominated throughout the entire period and appeared to out-compete the more typical planktonic components for diminishing substrate area . Species interaction or competition was accentuated as exposure duration and periphyton total cell standing crops increased and species diversity decreased, and appeared to account in part, for station differences in successional patterns .

No abstract