This multi-year pilot study evaluated a proposed field method for its effectiveness in the collection of a benthic macroinvertebrate sample adequate for use in the condition assessment of streams and rivers in the Neuquén Province, Argentina. A total of 13 sites, distributed across three rivers, were sampled. At each site, benthic macroinvertebrates were collected at 11 transects. Each sample was processed independently in the field and laboratory. Based on a literature review and resource considerations, the collection of 300 organisms (minimum) at each site was determined to be necessary to support a robust condition assessment, and therefore, selected as the criterion for judging the adequacy of the method. This targeted number of organisms was collected at all sites, at a minimum, when collections from all 11 transects were combined. Subsequent bootstrapping analysis of data was used to estimate whether collecting at fewer transects would reach the minimum target number of organisms for all sites. In a subset of sites, the total number of organisms frequently fell below the target when fewer than 11 transects collections were combined.Site conditions where < 300 organisms might be collected are discussed. These preliminary results suggest that the proposed field method results in a sample that is adequate for robust condition assessment of the rivers and streams of interest. When data become available from a broader range of sites, the adequacy of the field method should be reassessed.

The diatom Didymosphenia geminata (D. geminata) is an invasive periphytic species from the Northern Hemisphere. D. geminata occurrence and mat formation in Patagonia Argentina dates from 2010, spreading throughout Patagonia (provinces of Chubut, Neuquén, Río Negro, and Santa Cruz) by 2017. In 2012, the Surveillance and Monitoring Program was implemented by Neuquén government entities. Within this programme, nine sampling campaigns have been conducted at 91 sites including streams, rivers, lakes, reservoirs, and a canal. Presence or absence of D. geminata cells, abundance of D. geminata and other algal species cells as well as environmental variables were determined in samples. Multivariate analysis and generalized linear models were used to determine environmental constraints on habitat suitability and requirements for D. geminata. During 2012-2017, D. geminata cells were detected at 33 out of 91 sites sampled. D. geminata was present in lower streams order, with low values of total and dissolved nutrients, conductivity, alkalinity, ions and solutes, andhigh values of dissolved oxygen and habitat quality, for the ranges of environmental variables measured at sampling sites. D. geminata abundance was negatively related to stream order. Our results prove that D. geminata is expanding its ecological spectrum, showing a wider tolerance to environmental conditions particularly for phosphorus and nitrogen, some ions such as magnesium, and even temperature, and they confirm some authors' hypothesis about the ecological behaviour of this invasive species. Range expansion and environmental preferences of D. geminata could modify the state of susceptibility to invasion in numerous waterbodies.

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