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We assessed the effects of nutrient enrichment on three stream ecosystems running through distinct biomes (Mediterranean, Pampean and Andean). We increased the concentrations of N and P in the stream water 1.6-4-fold following a before-after control-impact paired series (BACIPS) design in each stream, and evaluated changes in the biomass of bacteria, primary producers, invertebrates and fish in the enriched (E) versus control (C) reaches after nutrient addition through a predictive-BACIPS approach. The treatment produced variable biomass responses (2-77% of explained variance) among biological communities and streams. The greatest biomass response was observed for algae in the Andean stream (77% of the variance), although fish also showed important biomass responses (about 9-48%). The strongest biomass response to enrichment (77% in all biological compartments) was found in the Andean stream. The magnitude and seasonality of biomass responses to enrichment were highly site specific, often depending on the basal nutrient concentration and on windows of ecological opportunity (periods when environmental constraints other than nutrients do not limit biomass growth). The Pampean stream, with high basal nutrient concentrations, showed a weak response to enrichment (except for invertebrates), whereas the greater responses of Andean stream communities were presumably favored by wider windows of ecological opportunity in comparison to those from the Mediterranean stream. Despite variation among sites, enrichment globally stimulated the algal-based food webs (algae and invertebrate grazers) but not the detritus-based food webs (bacteria and invertebrate shredders).Environ. Res. Lett. 8 (2013) 014002 J Artigas et al nutrient enrichment tends to globally enhance the biomass of stream biological assemblages, but that its magnitude and extent within the food web are complex and are strongly determined by environmental factors and ecosystem structure.

The aim of this study was to propose an index [Index of Biotic Integrity for the Río de la Plata (IBIRP)] formulated from a minimum of information-rich metrics related to biological integrity for evaluation of the coastal freshwater tidal zone of the Río de la Plata estuary. A total of 23 sites were selected along 170 km of shoreline (34°27′10″ S, 58°30′21″ W and 35°23′28″ S, 57°08′50″ W), being influenced by different land uses. We explored 36 biotic descriptors (designated candidate metrics), each of which is considered as an environmental indicator, and selected four metrics: two related to phytoplankton assemblage (planktonic cyanobacterial density and phytoplanktonic total density) and two linked to benthic assemblages, the diatoms (percent most tolerant taxa), and the invertebrates (percent Tanaidacea). The metrics that were chosen to integrate the IBIRP provide valuable information about the biotic integrity both in terms of time and space. The multimetric index can assume values between 0 (very bad status) and 10 (very good status). The IBIRP correlated significantly with the second (p<0.001) and fourth (p < 0.01) factors of the factor analysis-those being related to variables indicative of eutrophication and organic pollution. This index constitutes an approach that is capable of reflecting the anthropogenic impacts produced along the coast of the Río de la Plata, with the inherent flexibility of being subject to modification and ultimate validation for estuaries of similar characteristics.

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