Delia Elena Bauer

Global pressures, specific responses: effects of nutrient enrichment in streams from different biomes

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). This study shows that 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.

Phytoplankton from the Southern Coastal Fringe of the Río de la Plata (Buenos Aires, Argentina)

Structural characteristics of spring phytoplankton from the Southern Coastal Fringe of the Río de la Plata between 58°32′ and 57°41′ W, up to 10 km from the coast line are analysed. From 126 recorded taxa, 69 had a relative frequency above 5%. Main frequencies coincided with main quantitative abundances, corresponding to centric diatoms of the genus Aulacoseira – A. granulata var. augustissima especially – Stephanodiscus hantzschii and the coccal green Dictyosphaerium pulchellum and D. subsolitarium; the exception was the Cyanophyceae Microcystis aeruginosa, which showed the greatest quantitative abundance, but was not among the most frequent species. Centric diatoms accounted for most of the biovolume. Average phytoplankton density was 259 cells ml-1 (11–3700 cells ml-1). Diversity index (H′) varied from 0.51 to 2.78 bits ind-1. Phytoplankton size structure was: 43% picoplankton, 56% nanoplankton and 1% microplankton. Quantitative data on selected species were assessed for similarity among stations, 74% are joined in a group dominated by Aulacoseira sp. pl. and codominated by coccal green algae. The groups dominated by Stephanodiscus hantzschii and Microcystis aeruginosa included stations affected by pollution. Algal patchiness was greatest following low tide river currents. The assessed phytoplankton was eutrophic, oligohalophilous and mesosaprobic, a composition in agreement with some characteristics of a true potamoplankton.

An in situ test to explore the responses of Scenedesmus acutus and Lepocinclis acus as indicators of the changes in water quality in lowland streams

This bioassay was designed with the aim of exploring the responses of two wild planktonic microalgae exposed in situ (72 h) as indicators of the changes in water quality. Monocultures of both strains within dialysis membrane bags were placed at two sites in a small lowland stream. Site 1 is located at a suburban area with low horticultural activity and Site 2 is impacted by toxic industrial discharges and urban land use. There was a decrease in population growth of both species at Site 2 compared with Site 1. The comparison of the algae exposed in situ with the normal specimens cultured at the laboratory indicated a significant increase in the cellular volume for both species at both sites. Abnormal shape was recorded at both sites, the percentage being significantly greater for Scenedesmus acutus at Site 2. Significant changes in pyrenoids size were observed in S. acutus and in the percentage of fragmented nuclei in Lepocinclis acus. Also in the latter abnormal paramylon grains were observed. These responses were accentuated at Site 2. This bioassay was sensitive, short term, low cost, and therefore is a suitable tool to contribute with the monitoring and ecological risk assessment of lowland streams.

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.

Microplastics integrating the coastal planktonic community in the inner zone of the Río de la Plata estuary (South America)

This study explores in plankton samples the abundance, distribution, size, types (fibres and fragments), colours of the microplastics (MPs) and its relation with the characteristics of the plankton (size and morphology) of the Río de la Plata estuary. Water samples were collected in triplicate in freshwater-mixohaline tidal zone of the estuary, in ten sampling sites located along 150 km of coast, in two periods (September-November 2016 and April-June 2017). The results revealed the presence of MPs in all the samples analysed, with a dominance of fibres and sizes >500 ≤ 1000 μm, and blue colour being more frequent. The MPs distribution was significantly different among sampling sites, being more abundant in the most urbanized sites, sewage discharges and near the maximum turbidity front. The mean density, in the two samplings analysed, were 164 and 114 MPs m-3. The fibres amount was significantly different among sites. The MPs integrated a planktonic community dominated by pico-microphytoplankton, mainly conformed by filaments/chains and solitary forms and by micro-mesozooplankton. The comparative analysis of plankton and MPs demonstrated that a fraction of the latter showed a frequency range of size that coincides with the most common sizes of plankton (≤500 μm). The mean percentage of MPs items in relation to zooplankton was 0.36% (sampling 1) and 1.20% (sampling 2) and for phytoplankton was 0.0002% (sampling 1) and 0.0005% (sampling 2). The correlations between the MPs concentration and habitat quality (IHRPlata index) were statistically significant, on the contrary correlations between the MPs concentration and measured environmental variables were not found. The findings of this study emphasises the need for a better treatment of urban waste, which would contribute to reducing the entry of this pollutant into the ecosystem. The presence of microplastics in plankton samples on the coast of the Río de la Plata estuary.