Do steady state assemblages occur in shallow lentic environments from wetlands?

O' FARRELL, I.; SINISTRO, R.; IZAGUIRRE, I.; UNREIN, F.

HYDROBIOLOGIA

Kluwer Academic Publishers

Año: 2003 vol. 502 p. 197 - 209

0018-8158

This study was conducted in a wetland on the Lower Paraná River (Argentina). Algal assemblages from a permanent shallow lake, three relict oxbow lakes (ROL) and an isolated small pond were sampled fortnightly during the spring-summer time of 1998–99. The algal assemblages strongly differed among the permanently vegetated ROLs and the shallow lake and small pond, both temporarily covered by floating macrophytes. Anoxic conditions and poor light penetration have been pointed out in previous studies to be the leading factors in the selection of algal species in this wetland. Two or three Cyanobacteria species and some large pennate diatoms dominated the algal biomass in the ROLs, whereas only the former taxa contributed significantly to density. Assemblages were characterised by their low diversity and species richness; the highest SIMI values between consecutive samplings were registered in the ROLs. Even though some characteristics of the Z and S2 Reynolds associations describe the ROLs assemblages, they do not strictly match with them. Changes in algal density and biomass were more pronounced in the shallow lake. An alternative dominance of Cryptomonas spp., Peridinium spp., Euglena spp., Monoraphidium spp., Oscillatoriales and Chroococcales occurred, with a consequent replacement of the Y, Lo, W1 X1,S2, K, and F Reynolds associations. Variation in the macrophytic cover of the small pond accounted for the fluctuations in its algal composition. When the cover was complete, the assemblages resembled those of the ROLs, whereas when its surface was free of plants, they were more similar to those of the shallow lake. Steady state assemblages related to the persistence of a dense macrophyte cover may be found in wetlands in environments with constant and extreme ecological conditions such as those recorded in the ROLs. In contrast, ‘unpredictable’ assemblages can be expected in shallow lakes with fluctuating conditions.