Shallow lakes are vulnerable ecosystems impacted by human activities and climate change. The Cladocera occupy a central role in food webs and are an excellent paleoecological indicator of food web structure and trophic status. We conducted a paleolimnological study in Lake Blanca Chica (Argentina) to detect changes on the planktivory and herbivory regimes over the last 250 years. Generalized additive models were fitted to the time series of fish predation indicators (ephippial abundance and size, mucrone size, fish scales, and the planktivory index) and pheophorbide a concentration. The cladoceran assemblage changed from littoral-benthic to pelagic species dominance and zooplankton switched from large-bodied (Daphnia) to small-bodied grazers (Bosmina) ca. 1900 due to increased predation. The shift in planktivory regime (ca. 1920–1930), indicated by fish scales and the planktivory index, as well as herbivory (ca. 1920–1950), was triggered by eutrophication. Changes in planktivory affected the size structure of Bosmina, reducing its body size. This study describes the baseline for the lake as well as the profound changes in the composition and size structure of the zooplankton community due to increased predation and the shift in the planktivory regime. These findings will provide a reference status for future management strategies of this ecosystem.

Shallow lakes are particularly sensitive to climate change and significantly affected by human activities. As hotspots of biodiversity, it is essential to understand how aquatic communities respond to hydroclimatic and terrestrial drivers, and how compositional changes translate into lake functioning. This study examines ecological transitions in Lake Kakel Huincul (Argentina, South America) over the past 400 years, from the late Little Ice Age (LIA) to the Current Warm Period (CWP). We analyzed zooplankton and zoobenthos composition and size structure, and reconstructed submerged macrophyte establishment and herbivory. We used Generalized Additive Models to detect ecological transitions and Dynamic Linear Models to identify regime shifts based on cladoceran remains. The first regime shift occurred around 1880-1900 CE, characterized by changes in zooplankton composition, including the collapse of Daphnia, a decrease in zooplankton size structure, and the recolonization of submerged macrophytes. These changes coincided with dry conditions during the end of the LIA to the onset of the wetter CWP, indicating hydroclimatic variability as the main driver. The second regime shift (1970-1980 CE) led to a eutrophic state, with a decline in pelagic cladocerans, an increase in benthic and rotifer species, and a rise in herbivory. Changes since the 1970s align with increased land use and hydrological imbalances, indicating that humid conditions and agricultural intensification drove the regime shift, reconfiguring ecosystem functions. Our results emphasize the importance of past ecosystem conditions and ecosystem characteristics in understanding shallow lake responses to climate and human impacts.