Magdalena Licursi

The Pampean Diatom Index (IDP) for assessment of rivers and streams in Argentina

The rivers and streams of the Pampean plain are subjected to the impact of agriculture, cattle-raising and industrial activities. The largest urban center of Argentina is located here. The most important stresses on rivers and streams in the Pampean plain are organic enrichments (discharge of insufficiently treated sewage), nutrients, heavy metals, pathogenic agents, pesticides, herbicides and physical changes produced by dredging and canalisation. The epipelic community is suitable for biomonitoring purposes because it allows for comparing similar substrates along the rivers and streams. A total of 164 samples of epipelic diatoms were collected during 1995-1999 from Pampean rivers and streams. The analysis of these samples resulted in the development of a specific biotic index: the Pampean Diatom Index (IDP). The results were correlated with the main chemical water characteristics and with other biotic indices. This study suggests that the IDP is integrating organic pollution and eutrophication and can be applied for monitoring the biological quality of rivers and streams in the Pampean plain.

Effects of dredging on benthic diatom assemblages in a lowland stream.

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 sites 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 using Principal Components Analysis (PCA), non-metric multidimensional scaling (MDS) ordination 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 the post-dredging period sensitive species were replaced by either 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 demand as a consequence of sediment removal. However, these changes were not so conspicuous in sites 2 and 3, which already presented a marked water quality deterioration before the execution of the dredging works.

Stoichiometric homeostasis in the food web of a chronically nutrient-rich stream

Abstract: The theory of ecological stoichiometry holds that heterotrophs are mostly homeostatic and exhibit less variation in body stoichiometry than do autotrophs. Most studies of stream foodweb stoichiometry have been done in low-nutrient environments. Little is known about foodweb stoichiometry in nutrient-rich streams, in which a higher level of stoichiometric homeostasis should be expected, mainly because imbalances between resources and consumers are low and nutrient availability may meet biotic requirements. We analyzed elemental content (C, N, P) and stoichiometric ratios (C:N, C:P, N:P) of basal resources, macroinvertebrates, and fishes in a nutrient-rich Pampean stream and compared these values to those from other studies. We manipulated P and N in a 1-y fertilization experiment to analyze biotic stoichiometric responses to additional nutrient input to this naturally enriched system. Soluble reactive P concentration in the treatment reach was doubled relative to the background concentration. Consumers had lower C:P and N:P than those in other lotic systems, whereas P content and C:P and N:P of basal resources were within the ranges observed for other systems. Most components of the trophic web were not affected by fertilization, and only epiphyton, fine benthic organic matter, and 2 macroinvertebrate species (Palaemonetes argentinus and Pomacea canaliculata) changed their nutrient content or stoichiometric ratios. Imbalances in C:N and C:P occurred between primary consumers and their resources, particularly among macroinvertebrate collectors and detritivorous fishes feeding on FBOM. Most basal resources and consumers were strictly homeostatic for P content and the stoichiometric ratios, but a lower degree of homeostasis occurred in the epiphyton, P. canaliculata, and collectors feeding on epiphyton. A high degree of stoichiometric homeostasis exists across the various components of the food web in this nutrient-rich stream, regardless of their trophic position.

Planktonic and periphytic coastal algae of the Río de la Plata, Argentina

The riverside of the Río de la P lata River displays a heterogeneous topography (CABRERA & ZARDINI 1978). The large variabiliry o f habitats an d the singulariry of some of them have been the reasons for ereating four natural reserves there. The deterioration of the water qualiry and the destruetion o f the habitats are the prineipal faetors that affeet the biota diversiry and its dynamie (G6MEZ & BAUER 1998a,b). There is an inverse relationship between the eeosystem overexploitation and the searee biologieal information.