INSTITUTO DE LIMNOLOGIA "DR. RAUL A. RINGUELET"
Unidad Ejecutora - UE
Effects of dredging on benthic diatom assemblages in a lowland stream.
LICURSI M.; GÓMEZ N.
JOURNAL OF ENVIRONMENTAL MANAGEMENT
The objective of this study was to assess the effects of dredging on the structure and composition of diatom assemblages from a lowland stream and to investigate whether the response of diatom assemblages to the dredging is also influenced by different water quality. Three sampling site were established in Rodríguez Stream (Argentina); physico-chemical variables and benthic diatom assemblages were sampled weekly in spring 2001. Species composition, cell density, diversity and evenness were estimated. Diatom tolerance to organic pollution and eutrophication were also analyzed. Differences in physico-chemical variables and changes in benthic diatom assemblages were compared between the pre- and post-dredging periods using a t-test. Data were analyzed by a Principal Components Analysis (PCA), non-metric Multi-Dimensional Scaling ordination (MDS) and cluster analysis. The effects of dredging in the stream involve two types of disturbances: i - in the stream bed, by the removal and destabilization of the substrate, and ii - in the water column, by generating chemical changes and an alteration of the light environment of the stream. Suspended solids, soluble reactive phosphorus and dissolved inorganic nitrogen were significantly higher in post-dredging periods. Physical and chemical modifications in the habitat of benthic diatoms produced changes in the assemblage; diversity and species numbers showed an immediate increase after dredging, decreasing at the end of the study period. Changes in the tolerance of the diatom assemblage to organic pollution and eutrophication were also observed as a consequence of dredging; in post-dredging period sensitive species were replaced either by tolerant or most tolerant species. These changes were particularly noticeable in site 1 (characterized by its lower amount of nutrients and organic matter previous to dredging), which showed an increase in the amount of nutrients and oxygen demands as a consequence of sediment removal. However, these changes were not so conspicuous in sites 2 and 3, that already presented a marked water quality deterioration before the execution of the dredging works.