Freshwater Reports

Jul 02, 2024

Here I review some recent research papers that caught my eye and got me thinking, or suggested some practical actions we could take for ponds. Results from PONDERFUL are also making valuable additions to our understanding of ponds so keep an eye on the scientific publications page of our website for updates.

Ponds and other small freshwaters

Chironomids are one of the most abundant groups of freshwater invertebrates, but they are a taxonomically challenging group and much less frequently surveyed than other macroinvertebrate groups like dragonflies, water beetles and water snails. This makes the paper by Přidalová and colleagues on chironomids in Central European ponds all the more important for giving us an unusual insight into this groups of animals. They describe chironomid diversity in 246 ponds in Slovakia, from the Pannonian Plain to the Carpathians, recording 225 taxa including 192 species from six subfamilies (Podonominae, Tanypodinae, Diamesinae, Prodiamesinae, Orthocladiinae and Chironominae) representing around 85% of the total diversity of pond-dwelling chironomids. The ponds surveyed were waterbodies with surface area of under two hectares and maximum depth of eight metres, and included temporary, semi-permanent and permanent ponds of both natural and artificial origin. Both alpha and gamma diversity were greater at lower altitudes. Diversity peaked at higher temperature but was lower as the proportion of surrounding forests increased. Consequently, ponds with a mean annual air temperature of approximately 4.8°C and a low proportion of surrounding forests were found to support the most diverse chironomid communities. Interestingly there was no relationship between chironomid diversity and urban land cover or pond size.

In a previous edition of Freshwater Reports I wrote about the work of the Swiss team who described the benefits of a ‘massive’ pond creation programme for amphibian conservation, clearly sharing similar ideas to the PONDERFUL team. Now, the same authors have written more about the details of pond networks considering 12 amphibian species in Switzerland. They found that ponds created where there were between two and four occupied ponds within a radius of approximately 0.5 km had more than 3.5 times higher incidence of target species than isolated ponds. Species had individual preferences regarding pond characteristics, but breeding sites with larger (≥100 m2) total water surface area, that temporarily dried, and that were in surroundings with maximally 50% forest benefitted multiple target species (

Peatlands are a hotspot for ponds and are also major sinks of carbon. A new study brings these two facts together to advise researchers that they must include pond emissions to avoid overestimating carbon removal by peatlands. Specifically, net peatland carbon budgets must include emissions from open waters. Working in Quebec, Canada, Taillardat et al. (2024) found significant carbon and methane emission from pools that made up about 5% of the surface area of the peatlands. The study pools were supersaturated in CO2 and CH4 and were net carbon greenhouse gas sources. As shown by other pond studies, including PONDERFUL, CO2 and CH4 concentrations and fluxes fluctuated over the seasons and were explained by water table level fluctuations and temperature changes. The authors caution that there is a need to take account of these emissions to void overestimating sequestration potential provided by peatlands.

Other interesting papers

Many people reading this newsletter will already be aware of the on-going debate on whether freshwater invertebrate communities are becoming richer or declining in diversity. This research is based on river data where there are large datasets available for analysis and results remain unresolved with some studies reporting recovery slowing or stagnting (biodiversity recovery in European rivers has stagnated since 2010), other reporting a mixed picture and still others reporting unequivocally positive outcomes (significant improvements in freshwater invertebrate biodiversity in all types of English rivers). Of course, none of these studies include data on ponds.

A recent addition to this debate perhaps explains some of the inconsistency. Sinclair et al. (2024) noted that multi-decadal improvements in the ecological quality of European rivers are not consistently reflected in biodiversity metrics. They found that river invertebrate communities improved, becoming more like reference conditions, from 1992 until the 2010s, after which improvements plateaued. Improvements were generally reflected by higher taxon richness, providing evidence that certain community metrics can broadly indicate anthropogenic impacts. However, richness responses were highly variable among sites, and there were no consistent responses in community abundance, evenness or composition. They concluded that that, without sufficient data and careful metric selection, many common community metrics cannot reliably reflect anthropogenic impacts, helping explain the prevalence of mixed biodiversity trends.

Two studies in the Netherlands have recently taken a different approach to analysing the recovery of freshwater invertebrates by focussing on two invertebrate groups widely known to be sensitive to river pollution. Assessing 40 years of change in mayfly and caddis fly populations van Kouwen et al. (2024) and Becker et al. (2024) found interesting patterns. For the area’s 35 Ephemeroptera species, there was a general initial recovery of biodiversity, up to about the year 2000, which subsequently appeared to halt. The general lack of further recovery after 2000 suggested that, while extensive hydromorphological restoration measures were taken (dos Reis Oliveira et al., 2020), these were ineffective in restoring Ephemeroptera biodiversity. Recovery after the year 2000 was limited to only a few Ephemeroptera species, likely as a result of slight further improvements in water quality (Van Puijenbroek et al., 2014). Van Kouwen et al. concluded that before additional investment in large scale restoration projects are made, those stressors that hamper biodiversity recovery should first be identified and alleviated. For caddis flies there was a different outcome following initial recovery. Richness and abundance of all Trichoptera significantly increased from 1980 to around 2010. After this point, caddis generally declined except for the 5 most abundant species. Trichoptera increasing in abundance were less sensitive to climate change and poor water quality, or were sensitive species which benefited from local restoration measures. Species with stable or declining abundances showed higher sensitivity to climate change.

Practical actions

An interesting example of the need to maintain networks of ponds, even in the most highly protected sites, comes from Doñana National Park, Iberia’s largest wetland – and an iconic pondscape. The decline of the Doñana ponds is well-known and a conservation cause celebre. A recent paper highlights the critical threat to the amphibians of this highly protected area. In 2003-2004, 6 species of amphibian were present across more than 50% of the area studied by Díaz-Paniagua and colleagues. In contrast, 18 years later, this was the case for only two species, Pelophylax perezi and Hyla meridionalis. Declines were greatest for Epidalea calamita, followed by Pelobates cultripesTriturus pygmaeus, and Lissotriton boscai. The mean number of species per sampling unit dropped from 4.3 to 3.1. To preserve Doñana’s amphibian community, the park’s pond network must be restored, which implies reducing regional groundwater overexploitation. The availability of temporary and permanent ponds in Doñana is determined by rainfall, the sandy substrate, and the ground waters that sustain these wetlands. Climate change and aquifer overexploitation are jointly drying up ponds in Doñana, reducing the extent and hydroperiod even of permanent or semipermanent ponds to the point that terrestrial vegetation (e.g., pines, heath, rockroses) is colonising what used to be pond basins just a few years ago.

There are many examples of the importance of ponds for endangered species and new evidence suggests that endangered Little Brown Bats in North America may also benefit from ponds. Working in the Yukon in Canada Kukka et al. (2024) assessed the use of ponds along an urban-rural gradient by Little Brown Bats. Using ultrasonic detectors to survey 99 ponds they found that virtually all (98%) of the ponds were used by the bats. Bats selected more isolated ponds and there was less bat activity at ponds surrounded by open water and wetland habitat which the research team did not expect. They concluded that ponds surrounded by additional wetland habitat may have been too exposed for bats at high latitudes, where nights are short and not completely dark. Isolated ponds that are darker, such as those surrounded by mature forest, may be particularly valuable for little brown bats by helping them to avoid predators while foraging.

Finally, a very practical approach to protecting pond biota: make ponds on the top of low hills which ensure they are not directly connected to adjacent freshwaters. In New South Wales, Australia, work to protect the threatened Green and Golden Bell Frog (Litoria aurea) from the invasive Eastern Mosquitofish (Gambusia holbrooki) has taken a novel approach. Because Gambusia moves between aquatic systems during floods, Gould et al. (2024) set out to determine whether constructing ponds on elevated plots were less likely to be colonized by fish. Gambusia presence was monitored at 50 ponds which comprised 24 constructed ponds, 19 of which were on elevated plots, along with 26 historic ponds already present prior to the study. None of the ponds constructed on elevated plots were ever colonized by Gambusia during the study period, while most (three out of five) constructed at lower elevation and historical ponds (25 out of 26) were. The results showed clearly that plateau ponds provided a safe haven for the endangered amphibian, in an otherwise highly disturbed and damaged industrial landscape. Elevation difference, rather than proximity to Eastern Mosquitofish sources, was the key driver preventing colonisation. There are a variety of situations where ponds benefit from isolation and this adds another example.