įrom what is currently known, internal nutrient cycling is relatively more important in warm shallow lakes than in their cold temperate counterparts, where external loading exerts a relatively strong control on in-lake nutrient concentrations, making it harder to restore clear water conditions once the system has shifted to a turbid state. Research on tropical and subtropical lakes has resulted in the identification of yet one more alternative stable state, characterised by the dominance of free-floating macrophytes, which, in these systems, can have complex effects and an equally or even more important structuring role than that of submerged aquatic vegetation. The same is true for phytoplankton, as it is now known that dominance by either green algae or filamentous cyanobacteria can also represent self-stabilising alternative states. For instance, we can no longer speak simply of macrophyte-dominated shallow temperate lakes, but whether these are rooted angiosperms (flowering plants) or charophytes (e.g., stoneworts) must now be specified. Ecological processes, such as complex predator–prey interactions and interspecific competition, can also give rise to alternative regimes. Additional hydro-morphological factors affecting lake stability are now widely recognised, such as lake size, lake depth, spatial heterogeneity and internal connectivity. For some systems, it is better to speak of alternative regimes rather than alternative stable states, as they can present transient unstable conditions over relatively long periods. ![]() It is now known that regular oscillations between alternative stable states may occur. Ĭontinuous research has resulted in further development of this simple conceptual model, which has since evolved into a more complex picture that considers the effects of seasonality, chaotic population dynamics and changing weather conditions. This hysteretic response is explained by the stabilising feedbacks between the elements of the system present in each of the two alternative stable states. ![]() Once this turbidification threshold is surpassed, the restoration of clear water conditions through reductions of nutrient concentration only (i.e., without the triggering effect of an external perturbation) requires, however, that a different, lower threshold is crossed, below which only the clear water state is possible. ![]() The latter is, additionally, the only possible state after the nutrient concentration increases above a certain lake-specific threshold that varies with a diverse range of factors such as lake surface area, mean depth, retention time, sediment characteristics, relative extension of surrounding marsh areas and fishing intensity. The original simple conceptual model illustrated how, in some shallow macrophyte-dominated, clear water lakes, a gradual increase in the concentration of a nutrient results in a progressive loss of the system’s resilience to external perturbations, which can thus more easily trigger an abrupt shift to an alternative, phytoplankton-dominated, turbid water state. By discussing knowledge gaps, opportunities for research and challenges for management and restoration, we argue that this case is of high scientific value and that its study can advance theoretical understanding of shallow subtropical lakes.īorn out of the theory of alternative stable states, first proposed by Lewontin in 1969, shallow lakes theory has developed considerably since a version of the former for the specific case of shallow lakes was first thoroughly described by Scheffer et al. ![]() Statistical and other analyses suggest high sensitivity of the system to hydraulic, hydro-morphological and hydro-meteorological alterations arising, respectively, from engineering interventions, land use and climate change. Downward trends in water transparency and chlorophyll-a concentration support the hypothesis that primary production in this lake is fundamentally light limited. An upward trend in total phosphorus concentration confirms ongoing eutrophication of an already eutrophic system, evidenced by consistently high values of trophic state indices. Here, by reconstructing the history of scientific and management-oriented research on this system, we provide a comprehensive assessment of current knowledge and practice to which we contribute our recent, novel findings. Rapid growth of human presence and activities within its basin has led to its environmental degradation, a heartfelt matter of high political concern that compels intervention. Beyond its socioeconomic and cultural significance, it has great ecological importance, supporting a rich biodiversity. Ypacaraí Lake is the most renowned lake in landlocked Paraguay and a major source of drinking and irrigation water for neighbouring towns.
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