Abstract Factors that affect abundances of organisms in water bodies are influenced by extrinsic and intrinsic drivers that operate from outside and within a system. A high temporal coherence in the dynamics of abiotic parameters and biological communities among neighboring lakes evidences a strong extrinsic control operating similarly across lakes, and allows for prediction of ecosystems evolution in the context of global change and intensive land use. The Pampa region (Argentina) encompasses many shallow lakes submitted to different degrees of anthropic influence and showing contrasting alternative states. We studied an eutrophic clear and a hypertrophic turbid shallow lake during an annual cycle in order to evaluate whether they responded similarly to extrinsic factors or these were overridden by the effects of the steady state of each lake. Physical and chemical variables were highly coherent between both lakes, but accounted little for the large disparities among abundances and dynamics of microorganisms. While communities from the clear lake responded to a combination of extrinsic and intrinsic factors, the turbid lake showed a state less prone to be affected by climatic effects. We hypothesize that clear lakes would perform better as sentinels of climate change in the Pampa wetland.

Laguna Chascomús is an eutrophic, turbid, shallow lake typical of the Pampa region of Argentina. This shallow lake is permanently mixed and displays a high degree of spatial homogeneity. The cyclical periods of excess and shortage of rain, characteristic of this area, result in periodic drying and flooding events. According to newspaper articles published during the first half of the XX century, the lake was originally turbid. The first patches of rooted vegetation appeared after the 1913 y 1914 floods. During most part of the XX century the lake remained in a "clear", vegetated state. During this period the dominant fish species was the silverside (pejerrey), Odontesthes bonariensis, which accounted for over 60% of the total fish catches (in biomass). Instead, by the end of the latest century the lake shifted to a turbid state that persisted until present. Currently, the phytoplankton primary production is limited by light and its values are among the highest ones reported for natural aquatic systems. In light limited systems, the transparency may be expected to be controlled by the amount of incident light, through a negative feedback loop with primary production. These predictions have been confirmed, both in mesocosm experiments and "in situ". The phytoplankton biomass, dominated by nanoplanktonic cyanobacteria accounts for 75% of the total amount of carbon in the water column. The equilibrium predicted by the light limitation theory could not hold if the model assumptions were violated. This situation could happen if the phytoplankton biomass were controlled by herbivore zooplankton. However, such a control of primary producer by herbivores seems to be precluded due to the absence of large-sized zooplankton grazers, presumably due to the composition of the fish assemblage. In contrast to earlier reports of the 1960, the proportion of pejerrey biomass is presently quite low (0.04%) and the community is dominated by omnivore microphagous, such as the sabalito (Cyphocharax voga). The dominance of sabalito could not only explain the composition of the zooplankton community, but it also could contribute to prevent or delay the consolidation of sediments.

La laguna Chascomús es un típico lago somero, eutrófico y turbio de la Pampa Deprimida. Se encuentra permanentemente mezclada y presenta un grado elevado de homogeneidad espacial. La alternancia entre períodos de déficit y de exceso de agua, característica de la región, determina que la laguna sufra ciclos periódicos de sequía e inundación. Las primeras crónicas indican que a principios del siglo XX la laguna era turbia y que las primeras matas de vegetación habrían aparecido después de las inundaciones de 1913 y 1914. Hacia mediados del siglo XX, Chascomús se encontraba en un estado de aguas claras y colonizada de manera profusa por macrófitas. En ese momento, aproximadamente 60% de la biomasa de peces correspondía al pejerrey (Odontesthes bonariensis). En las últimas décadas, esta condición se modificó y en la actualidad la laguna está estabilizada en un estado turbio en el que la producción primaria fitoplanctónica es muy alta y se encuentra limitada por luz. La biomasa de fitoplancton representa el 75% del carbono de la columna de agua y predominan las cianobacterias nanoplanctónicas. Estudios de campo y experimentos en mesocosmos indican que la transparencia del agua está controlada por la cantidad de radiación incidente a través de una retroalimentación negativa con la producción primaria. La biomasa fitoplanctónica elevada se mantiene debido a la ausencia de un control efectivo por parte del zooplancton herbívoro. Como resultado de la presión de depredación que ejerce la comunidad actual de peces, la composición del zooplancton no presenta filtradores eficientes (e.g., Daphnia y otros cladóceros de gran tamaño). En la actualidad el pejerrey representa un porcentaje muy bajo de la biomasa total de peces (0.04%), mientras que los micrófagos omnívoros, como el sabalito (Cyphocharax voga), son dominantes. La dominancia del sabalito no sólo explicaría la baja abundancia del zooplancton, sino que al mismo tiempo contribuiría a evitar o retardar la consolidación del material sedimentado.