Plastic pollution is considered an important environmental problem by the United Nations Environment Programme, and it is identified, alongside climate change, as an emerging issue that might affect biological diversity and human health. However, despite research efforts investigating plastics in oceans, relatively little studies have focused on freshwater systems. The aim of this study was to estimate the spatial distribution, types, and characteristics of macro-, meso-, and microplastic fragments in shoreline sediments of a freshwater lake. Food wrappers (mainly polypropylene and polystyrene), bags (high- and low-density polyethylene), bottles (polyethylene terephthalate), and disposable Styrofoam food containers (expanded polystyrene) were the dominant macroplastics recorded in this study. Contrary to other studies, herein macroplastic item surveys would not serve as surrogates for microplastic items. This is disadvantageous since macroplastic surveys are relatively easier to conduct. Otherwise, an average of 25 mesoplastics (mainly expanded polystyrene) and 704 microplastic particles (diverse resins) were recorded per square meter in sandy sediments. Comparisons with other studies from freshwater and marine beaches indicated similar relevance of plastic contamination, demonstrating for the first time that plastic pollution is a serious problem in the Paraná floodplain lakes. This study is also valuable from a social/educational point of view, since plastic waste has been ignored in the Paraná catchment as a pollutant problem, and therefore, the outcome of the current study is a relevant contribution for decision makers.

Fish assemblages recorded from August 2010 to July 2013 in two lotic and lentic environments of the Middle Paraná River floodplain were studied in the light of flood and flow pulse variations. Three flood pulses occurred during the period of study, each with a different magnitude, duration and timing. Instead four flow pulses were recorded during this period. The varied hydrological conditions had an influence on the ichthyofauna. The fish species richness in both lentic and lotic habitat was negatively correlated with the maximum water level and fish assemblages were spatially and temporally structured according to their different reproductive strategies. Long- and short-distance migratory species were dominant after the flood pulse of the highest intensity and longest duration, whereas sedentary fish and internal fertilisation prevailed during flow pulses and lower flood pulses. Fish assemblage similarity was generally low between habitats during flood and flow pulses, but increased after those events depending on their magnitude, duration and timing. Apparently, flow pulses would also function like floods, as mechanisms of spatio-temporal structuring of fish assemblages.

Fluctuations of temperature and water levels are the two main drivers of aquatic life in river floodplain ecosystems. The large Middle Paraná River floodplain exhibits marked seasons and important interannual hydrological changes. Using a three-factor-based approach (fish reproductive traits, hydroclimatic conditions, and floodplain recruitment patterns), we analyzed how fish life history evolves within this fluctuating environment. We observed that hydroclimatic conditions can be considered through three main interannual variations that prompt the most abundant Paraná species to adopt four different main reproductive strategies: (i) typical periodic strategists are dependent on large spring–summer floods and juveniles strongly predominate in the floodplain when such a condition occurs, (ii) periodic–opportunistic strategists are associated with floods, regardless of their timing, (iii) periodic–equilibrium strategists take advantage of spring–summer floods whatever the intensity and duration, and (iv) equilibrium strategists have low flood dependence and higher stability in temporal fish recruitment. This work brings forth a first synthesis of fish life history in the Middle Paraná River and evidences how important both hydrological and temperature fluctuations are to interpret its complexity.

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Temporal variations of fish abundance and species richness in two sites of a secondary channel of the Middle Parana ´Floodplain (Argentina) have been analyzed from 2010 to 2012. Relationships of these ecological attributes with alternations of floods and dry periods were appraised. Permutational analyses of variance were applied to determine significance in temporal changes of the fish community, taking into account catch per unit effort and species richness of the whole community, by groups of distinct reproductive strategy and considering fish sizes. The results reveal significant differences in structure and composition of fish assemblages at distinct stages of the hydrological regime. Those temporal differences are associated with changes in timing, duration, and magnitude of floods since the three groups have shown distinct responses to the hydrological variations. Long-distance migrants depend on major inundations for a successful reproduction, short-distance migrants seem to have distinct responses to floods according to the kind of species, and sedentary fish-developing parental care essentially needs high floodplain connectivity for juvenile development.

This study examines the effects of the flow and flood pulses on spatial dispersion of fish assemblages in the floodplain of the Paraná River in Argentina. We tested the hypothesis that high water levels and greater lateral connectivity promote fish dispersal and spatial homogenization of assemblage structure. We sampled four sites during different phases of the annual hydrologic cycle from 2010 to 2016. Water surface in the area was estimated during each phase. We computed multivariate statistics and estimates of ß‐diversity to analyse assemblage variations in relation to hydrological phases. Three hydrological phases were defined: low flow pulses (water levels between 2.3 and 3.2, approximately 10% of the floodplain covered by water), high flow pulses (between 3.2 and 4.5, from 11 to 84%), and floods (> 4.5 m, more than 84%). Although difference between high flow pulses and flood was not significant, ß‐diversity values for these stages were higher than for low flow pulses. This suggests that floods and high flow pulses increase the spatial variability of fish assemblages, whereas homogenization processes occur later during low flow periods. This work provides further knowledge about the flood homogenization effect in a large unregulated floodplain where lateral connectivity still plays a significant role on ecological structuring processes.

Fish are subject to diverse stimulus in floodplain rivers resulting from interactions between environmental factors. In such complex and interconnected ecosystems, fish assemblages are thus highly dynamic across space and time. Here, we analysed fish assemblage structures in lotic/lentic habitats of a geomorphic unit of the Middle Paraná River during summer, autumn, and winter of seven consecutive years. We observed that three main factors trigger significant structural changes: seasonality, habitat characteristics, and floodplain connectivity. The last one appears to have a determinant role on fish structures. Processes of connection/isolation of floodplain environments produced significant variations in fish abundance and species richness.Main results showed that some ecological processes can be largely explained by connectivity variations, such as fish dispersal in the floodplain, fish concentration in remnant floodplain lakes, and migrations back to permanent floodplain water bodies.This study provided a quantitative measurement of connectivity to understand fish dynamics in a giant floodplain and highlights the importance of similar investigations to preserve the fish fauna in these rather unique ecosystems.

The La Plata basin is the second largest basin of South America and has supported important river fisheries for more than a century. In this paper, we evaluate for the first time the historical trends of landings of 21 fish taxa and the recent population trends of 27 species of commercial fishes in the lower La Plata basin (Argentina). We compiled three kinds of data sets: Total fishery landings (between 1934 and 1986) and exports (1994‒2019), fisheries monitoring programs of Chaco and Santa Fe provinces in the Paraná River (2009‒2019), and surveys of fish populations in the Upper (Corrientes, 1993‒2020) and Middle (EBIPES, 2005‒2020) Paraná River. The analysis of the historical landings showed more species declining in the lower portion of the basin than in the upper basin. Regarding recent population trends, Pimelodus spp., Hoplias spp., Salminus brasiliensis, Luciopimelodus pati, and Ageneiosus spp. declined in more than one region, while Megaleporinus spp., Pterodoras granulosus, and Oxydoras kneri showed stable to positive trends, with the other species varying in their trends between regions. These tendencies could be associated to a combination of factors such as overfishing and environmental changes that would require an ecosystem approach for their adequate management.