Due to the great climatic variety and the peculiar north–south orographic distribution, Argentinean lake systems include a wide diversity of aquatic environments. The deepest lakes are situated in the Andean Patagonia and Tierra del Fuego, and range from ultraoligotrophic to oligotrophic. Patagonian Plateau lakes are shallower than Andean lakes and usually range from mesotrophic to eutrophic. All lakes in the Chaco‐Pampa Plain are very shallow and range from eutrophic to hypertrophic or salt lakes. Most of the lakes situated in the central–western and northwestern arid regions are reservoirs or salt lakes, and range from mesotrophic to eutrophic. More than half of the northwestern reservoirs have very low dissolved oxygen concentrations in the hypolimnion during mid‐summer. Argentina has more than 400 lakes with surface area >5 km2, but for some of them not even major ion data are available. In order to synthesize and to extract general characteristics and patterns, issues have been selected which adequately reflect the character of the lake environment in Argentina. We divided Argentina according to geographical regions into six major divisions: Puna, Chaco‐Pampa Plain, Peri‐Pampean Sierras, Andean Patagonia, Patagonian Plateau, and Misiones Plateu and Brazilian Shield‐related systems. Most of the lakes situated in Patagonia are undisturbed lakes. However, the Chaco‐Pampa Plain lakes are usually lightly impacted by agricultural operations. Moreover, most of the reservoirs located in the Argentinean arid ‘corridor’ are highly impacted by agriculture. Other lakes in both north and south Argentina range from salt lakes, through dark humic‐stained lakes and large river floodplain lakes, to dilute high altitude glacial lakes. Argentina still has a larger proportion of its lake waters in natural conditions. However, expected developments for natural resources indicates that Argentinean goals for lake water management should include the preservation of some proportion of pre‐European natural lake environment, and the evolution of a stable, managed lake environment in the more developed regions.

Climatic and hydrological variability is usually high in the Pampa Plain (Argentina). However it has not studied yet how this variability may affect the phytoplankton and zooplankton biomass and community structure in aquatic systems of this region. The main purpose of this study was to assess flushing effects on nutrient and plankton dynamics in two interconnected very shallow lakes of the Pampa Plain. In order to study the impact of hydrology on the plankton biomass and community structure, we compared the summer plankton community among three consecutive years with contrasting hydrological characteristics. Water residence time varied an order of magnitude among years and this variability was correlated to strong changes in physicochemical and biological lake characteristics. Depending on the water discharge level, the hydrological regime within the lakes ranged from lentic to more lotic conditions. Nutrient and phytoplankton biomass were positively related to water discharges. During high flushing periods, nutrients import from intensive agriculture lands leads to a dramatic increase in trophic conditions. On the other hand, macrozooplankton biomass was positively related to water residence time and showed a dramatic decrease during high flushing years. Rotifers biomass was not affected by interannual water discharge variability during the study period. Our results support that in case of lakes with high flushing rates, zooplankton development is dependent on water residence time and that hydrology may have stronger effects on macrozooplankton biomass than top-down control by planktivores.

Ecological processes in lowland rivers are mostly dominated by hydrology and its interactions with other environmental factors. Fish-habitat relationships in rivers are also influenced by human impacts.In this study, we describe patterns of abundance and distribution of fish species in a group of natural lowland river lakes along spatial anthropogenic and abiotic gradients when four hydrologically different summers are compared. We also describe the proportion of the total variances in fish species abundances that can be accounted for by selected abiotic (water conductivity), hydrological (water residence time) and human activity-derived (total phosphorus (TP) concentration and NO 3 :NH 4 ) variables. Consequently, our main purpose is to explore how abiotic and anthropogenic factors interact to affect fish abundance and distribution together with consistent results across different hydrological conditions. We conclude with a briefly discussion of some management implications.The anthropogenic impacts on water quality, the extreme hydrological variability and the fluctuating abiotic environment affected fish abundance and distribution. Pampa inland silverside Odontesthes bonariensis was benefited from a less human disturbed environment with higher water residence time and total salinity, whereas species as Cyphocharax voga, Parapimelodus valenciennis and Cyprinus carpio found these conditions largely disadvantageous. On the other hand, while most species showed stronger-either negative or positive-response to anthropogenic, hydrological or abiotic factors Oligosarcus jenynsii was only slightly affected. This paper identified the ecological function of a lowland river under its natural flow regime. There are not many opportunities to study unmodified rivers worldwide. Therefore, our findings may help in assessment programmes of fish communities in flow altered and human disturbed aquatic ecosystems.

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Relationships between total fish biomass and chlorophyll, nutrient levels, and morphometric and climatic factors are shown for a set of 106 Argentinian lakes and reservoirs. The total data base is highly heterogeneous. Relative fish biomass (CPUE) was estimated from gill net catches. A data screening process was applied to the environmental data base to homogenize it. Nutrient, total organic matter content, and mean depth were most important in explaining relative fish biomass variability between lakes and reservoirs. In most heterogeneous sets total organic nitrogen (TON) explained most of CPUE variability. After screening for limnological anomalies (i.e. turbidity derived from inorganic sources, unsuitable conditions for fish, and TN/TP (total nitrogen/total phosphorus) < 37 (molar basis)), TP and TON were highly correlated with CPUE. Mean annual air temperature was significantly related to fish biomass, but the relationship was not significant after nutrients, morphometry, and environmental oxygen conditions had been accounted for. Several possible causes are discussed. The results support the hypothesis that fish/phytoplankton, and fish/zooplankton biomass ratios are inversely related to lake trophy.