Ponds in good condition can contribute to mitigating the effects of climate change

Ponds are the most extensive freshwater habitat on Earth, as they account for approximately 30% of the planet's standing water, and are the home of 70% of freshwater species. However, studies of them have been limited, despite their ecological importance. Against this backdrop, the European "PONDERFUL" research project has examined how ponds and pond landscapes can be used as nature-based solutions (NBS) to mitigate the effects of climate change, in view of the fact that these bodies of water can both store and release carbon dioxide. This research has shown that the ponds which have suffered the most extensive damage or which are in poor ecological condition (such as those that are poorly oxygenated, with high concentrations of nutrients from agriculture, or with few or no aquatic plants) tend to emit greenhouse gases, including carbon dioxide (CO₂) and methane (CH4). However, ponds in good ecological condition (those that are well oxygenated and with low levels of nitrogen) tend to be carbon sinks, which act as reservoirs. 

The "PONDERFUL" project, led by the Aquatic Ecology Research Group at the University of Vic - Central University of Catalonia (UVic-UCC), has highlighted the importance of restoring and managing ponds, as the production of the two main greenhouse gases (carbon dioxide and methane) in ponds depends on several factors, but above all on levels of oxygen and nutrients, and the amount of organic matter accumulated in the ponds. "The pond's metabolism and its oxygen are the key factors: low levels of oxygen are linked to an increase in emissions," says Sandra Brucet, ICREA researcher at UVic-UCC and the coordinator of the "PONDERFUL" project. 

Emission or retention of gases 

If there are plenty of submerged and rooted plants in ponds, this helps them store greenhouse gases cleanly. Algae and vascular plants can retain carbon dioxide, which is then deposited in the pond's sediments. The processes that take place in ponds which generate or capture greenhouse gases are photosynthesis, respiration, the decomposition of organic matter, microbial activity, sedimentation and chemical reactions. During daylight, phytoplankton, algae and aquatic plants consume carbon dioxide in order to photosynthesise, while they release it at night, when respiration is the predominant process. 

The "PONDERFUL" project has also shown that ponds are more likely to emit carbon dioxide when a rise in the water temperature is combined with a high level of nutrients. At higher water temperatures and with scarcer water, the effects of eutrophication (the pollution process in stagnant waters as a result of nutrient enrichment) become more acute. Brucet says that "more greenhouse gases are released during droughts, when ponds dry up and particularly when the sediment is still wet," and she adds that "it is very important that nutrient levels remain as low as possible when temperatures rise."  

Ponds must be restored and managed, as ponds with high concentrations of dissolved oxygen and low levels of nutrients are more likely to act as sinks rather than sources of greenhouse gases. "By removing the sediments and vegetation that build up in them, for example, restoring ponds reduces greenhouse gas emissions and also increases the amount and diversity of pollinating organisms," says Brucet. The "PONDERFUL" project examined nearly 180 SBN initiatives carried out in 93 ponds and pond landscapes in 24 countries. 

Apart from the global effects of climate change (with the consequent episodes of extreme weather), ponds and pond landscapes are currently also subject to other threats. One of these is the direct destruction of these freshwater habitats (with devastating effects on flora and fauna). Another is pollution affecting the quality of the water, caused by human activities including agriculture, livestock farming, tourism and infrastructure construction. Around 80% of the locations sampled by PONDERFUL had high levels of nutrients, which suggests that pollution due to nitrogen and phosphorus is widespread. Finally, another threat is invasive species, which endanger freshwater ecosystems. 

The relationship between biodiversity and how ponds function 

One of the most important aspects of the "PONDERFUL" project is that it has provided scientific evidence on the relationship between ponds' biodiversity (macrophytes, zooplankton, macroinvertebrates and amphibians) and how they function. This issue had previously been studied primarily on land. This research has shown that greater biodiversity in ponds is related to improvements in the environmental roles they play: increased carbon capture, decomposition and biomass production, and more limited gas emissions. "Although it is still a correlational study, it shows that maintaining biodiversity is very important for ecosystems to continue functioning properly, and to retain the benefits they give us in a context of global change," explains Brucet. 

The factors that affect the numbers of amphibians in ponds was another subject related to protecting the biodiversity of ponds that the project studied. The study found that factors related to climate (temperature and water availability) play an important role, but that local factors also play a role. The project showed that the ideal pond for amphibians would be a pond that is well-preserved, medium-sized and shallow, and with no fish. Some of the first scientific articles from "PONDERFUL" led by UVic-UCC on this subject have recently been published in the journals Ecography and Biodiversity and Conservation.  

Publication of guides and a pond prototype 

"PONDERFUL" has shown that ponds not only have "immense potential to act as an SBN", says Brucet, but they also provide many benefits for society. These include regulating the dangers of extreme events (floods or fires, for example), regulating the quantity and quality of freshwater (by acting as water reservoirs and because of their purification capacity), regulating the climate (with carbon storage, among other factors), protecting and improving biodiversity, encouraging pollination, improving connectivity between habitats and providing climate refuges for a wide variety of species. 

The results of "PONDERFUL", which are based on studies of more than 1,900 ponds and 16 demonstration sites in different countries have been compiled in various documents and resources for policymakers and managers with a view to making ponds SBNs. These documents include a technical guide with best practices for pond management and restoration, an interactive decision-making tool for managing and restoring ponds, a guide to sustainable financing and investment and a guide for policymakers on using ponds as SBNs, as well as priority intervention maps showing the regions where lakes and their landscapes are facing the greatest threats. The project has also created the CLIMA pond concept - a prototype of an ideal pond specifically designed as an SBN to address the challenges of climate change in natural, rural and urban landscapes.

The "PONDERFUL" project (POND Ecosystems for Resilient Future Landscapes in a changing climate), funded by the European Union's Horizon 2020 programme, was carried out by 18 universities and institutions in Germany, Belgium, Switzerland, the United Kingdom, Turkey, Portugal, Denmark, Sweden, France, Spain and Uruguay between 2020 and 2024, and had a budget of almost 7 million Euros.