Diatom Succession and Interaction in Littoral Periphyton and Plankton

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, andMelosira italica (plusM. varians) in January.F. virescens, although never dominant, peaked in October. Periphyton communities were dominated byAchnanthes minutissima, Cocconeis placentula, F. crolonensis andF. 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 andA. 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, whereA. minutissima andC. 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.