Ponds are often identified by their small size and shallow depths, but the lack of a universal evidence-based definition hampers science and weakens legal protection. Here, we compile existing pond definitions, compare ecosystem metrics (e.g., metabolism, nutrient concentrations, and gas fluxes) among ponds, wetlands, and lakes, and propose an evidence-based pond definition. Compiled definitions often mentioned surface area and depth, but were largely qualitative and variable. Government legislation rarely defined ponds, despite commonly using the term. Ponds, as defined in published studies, varied in origin and hydroperiod and were often distinct from lakes and wetlands in water chemistry. We also compared how ecosystem metrics related to three variables often seen in waterbody definitions: waterbody size, maximum depth, and emergent vegetation cover. Most ecosystem metrics (e.g., water chemistry, gas fluxes, and metabolism) exhibited nonlinear relationships with these variables, with average threshold changes at 3.7 ± 1.8 ha (median: 1.5 ha) in surface area, 5.8 ± 2.5 m (median: 5.2 m) in depth, and 13.4 ± 6.3% (median: 8.2%) emergent vegetation cover. We use this evidence and prior definitions to define ponds as waterbodies that are small (< 5 ha), shallow (< 5 m), with < 30% emergent vegetation and we highlight areas for further study near these boundaries. This definition will inform the science, policy, and management of globally abundant and ecologically significant pond ecosystems.

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An experimental study using mesocosms was conducted in the main shallow lake of a temperate wetland (Otamendi Natural Reserve, Argentina) to analyse the impact of zooplankton on phytoplankton and the microbial assemblages. The lake is characterised by the presence of a fluctuating cover of floating macrophytes, whose shading effects shape the phytoplakton community and the ecosystem functioning, which was absent during the study period. The experiment was run in situ using polyethylene bags, comparing treatments with and without zooplankton. The cascade effect of zooplankton on phytoplankton and the lower levels of the microbial food web (ciliates, heterotrophic nanoflagellates (HNF) and picoplankton) were analysed.A significant zooplankton grazing on the nano-phytoplankton fraction (3-30 mm) was observed. Conversely, large algae (filamentous cyanobacteria, colonial chlorophytes and large diatoms) increased in all mesocosms until day 10, suggesting that they were not actively grazed by zooplankton during this period. However, from day 10 until day 17 this fraction decreased in the enclosures with mesozooplankton, probably due to an increase in the abundance of large herbivores.The results of the experiment would also indicate a trophic cascade effect on the lower levels of the microbial community. In the treatment where zooplankton was removed, the abundance of ciliates followed the same increasing pattern as the abundance of HNF, but with a time lag in its response. In the enclosures without zooplankton, HNF remained relatively constant throughout the experiment, whereas ciliates strongly decrease during the last week. Total picoplankton abundance increased in the enclosures with mesozooplankton, thus supporting the existence of a fourlink trophic cascade (copepods-microzooplankton-HNF-picoplankton). Zooplankton composition changed significantly from the beginning until the end of the experiment; cyclopoid nauplii and rotifers were notoriously dominant at t 0 , whereas 10 days later the community showed a more equitable proportion of cyclopoids, calanoids, nauplii, cladocerans and rotifers.

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Shallow lakes from the Pampa plain (Argentina)

We analyzed experimentally the relative contribution of phytoplankton and periphyton in two shallow lakes from the Pampa Plain (Argentina) that represent opposite scenarios according to the alternative states hypothesis for shallow lakes: a clear lake with submerged macrophytes, and a turbid lake with high phytoplankton biomass. To study the temporal changes of both microalgal communities under such contrasting conditions, we placed enclosures in the littoral zone of each lake, including natural phytoplankton and artificial substrata, half previously colonized by periphyton until a mature stage and half clean to analyze periphyton colonization. In the clear vegetated shallow lake, periphyton chlorophyll a concentrations were 3-6 times higher than those of the phytoplankton community. In contrast, phytoplankton chlorophyll a concentrations were 76-1,325 times higher than those of periphyton in the turbid lake. Here, under light limitation conditions, the colonization of the periphyton was significantly lower than in the clear lake. Our results indicate that in turbid shallow lakes, the light limitation caused by phytoplankton determines a low periphyton biomass dominated by heterotrophic components. In clear vegetated shallow lakes, where nitrogen limitation probably occurs, periphyton may develop higher biomass, most likely due to their higher efficiency in nutrient recycling.

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