Periphyton community dynamics in a temperate oligotrophic lake

Microalgal communities growing on Plexiglas slides were studied over a two year period. Changes in the total numbers and relative abundances of taxa of manipulated and unmanipulated substrata were analysed using ordination and modelling. The sequence of taxa replacements on the slides was orderly and repeatable during the first months. The "direction" of succession involved early appearance of bacteria and dominance by coccoid greens during the first 3-4 weeks, followed by increases in Gomphonema, Eunotia, Dinobryon, Microcystis, and Merismopedia. Coccoid greens were co-dominant even on the older slides. Mature communities underwent a yearly quasi-cyclical sequence of taxa replacements, without reaching stationarity in community structure. Winter and summer were periods of relative stability, whereas spring and fall were transition periods; the spring and fall communities resembled each other. In general, high diatom densities occurred in winter, whereas coccoid greens, filamentous greens, and diatoms typified summer assemblages. The sequence of changes varied with depth. The relative abundance of diatoms increased with depth. Spatio-temporal variations in light and temperature seem to have strongly influenced periphyton periodicity and spatial heterogeneity. Community age was more important than seasonality in determining community structure. Biotic differences between spatially separated stations increased with community age. Phytoplankton and periphyton differed greatly in community structure; only two abundant taxa were established in both communities. A summer Dinobryon/Epipyxi s pulse began simultaneously in the two communities. Phytoplankton recovered faster from the pulse, suggesting that generation times were shorter, and turnover rates higher, in phytoplankton than in per iphyton. Communities exposed to experimentally-induced environmental fluctuations of one- and two-week periods could not "track" the fluctuations. Instead, succession was slowed, and the communities remained in a state intermediate between the two extreme states of the fluctuation cycles. The response of communities differing in initial composition to a Dinobryon/Epipyxi s pulse suggested that development towards maturity proceeded along a rather rigidly determined trajectory. Convergence experiments involving communities whose structure differed initially showed that community structure at the family/genus level was regulated. The rate of regulation was commensurate with average cell division rates estimated independently, but regulation was slow in comparison to other waterbodies. Community structure was seemingly regulated towards a single, highly dynamic equilibrium. The estimated characteristic response time for the communities was 12 weeks, indicating that the communities should track yearly cyclical environmental variation with a lag. Environmental changes occurring over periods shorter than 12 weeks, such as those seen in spring and fall, might have left the community far from equilibrium for extended periods of time. The rates of change in community structure in Gwendoline Lake were low in comparison with other waterbodies, probably because cell division rates were also low in comparison with other systems. Phosphorus scarcity might have been the main factor underlying the sluggishness of periphyton responses.Science, Faculty ofZoology, Department ofGraduat