Freshwater Reports

Nov 21, 2023

Here I review some recent research papers that caught my eye and got me thinking, or suggested some practical actions we could take. In addition, the PONDERFUL consortium is now publishing a growing number of papers with results from the project. Keep an eye on the scientific publications page of our website for updates.

Ponds and other small freshwaters

I’m always on the lookout for papers which illustrate the details of how ponds are made by animals. Such observations help to support the idea that ponds are an essentially natural type of habitat, widely recreated by people. In Europe and North America a lot is known about the ponds made by beavers, but other animals also make ponds. In the Florida Everglades ‘alligator ponds’ are a distinctive feature of the landscape and in a paper published earlier this year, Bradley Strickland and colleagues describe the distinctive features of these habitats, and the effect of alligators as ‘ecosystem engineers’. To make their ponds, alligators repeatedly remove vegetation and push sediment into the banks of the ponds with their claws, snout and tail, maintaining areas of open water and well-vegetated margins. This provides habitat for a range of aquatic plants and animals and maintains unique open-water areas in the Everglades that would fill with vegetation without the activities of alligators. Alligator ponds are modest in size: an earlier study of their basic limnology found that the largest were only 70 m2 and just over a metre deep. These natural reptile-created ponds remind us that large animals have been making ponds for many millions of years, and long before the rise of the mammals.

Meredith Holgerson and her colleagues made another interesting contribution to our understanding to the sequestration of carbon in ponds. As everyone in PONDERFUL will be well aware, ponds influence the global carbon cycle by releasing greenhouse gases and storing carbon in their sediments. Yet, there are very few estimates of carbon burial rates in ponds, making it challenging to determine their global importance. Holgerson and her colleagues surveyed 22 artificial ponds that were all constructed in an identical way and lacked watersheds, allowing them to examine how management activities influenced internal accumulation and storage of organic carbon. They found that ponds sequestered large amounts of carbon in their sediments, with rates significantly influenced by macrophytes, fish and nitrate/phosphate levels. Ultimately, the high carbon sequestration rates observed in ponds suggest that the global carbon sink for lentic waters is substantially underestimated. Earlier this year, Meredith also gave one of the PONDERFUL seminars on the role of ponds in carbon sequestration. If you haven’t already seen the talk, you can view it here.

PONDERFUL results are suggesting that ponds can be a sink for N2O. However, the mechanism of N2O-fixation in freshwater is currently unknown, unlike the long-recognised conversion of nitrogen compounds in anoxic environments to nitrogen (N2) gas. The recent paper Direct biological fixation provides a freshwater sink for N2O, by Yueyue Si and colleagues, suggests a possible mechanism for the N2O sink. They found in a mesocosm study, conducted in the UK, that both N2O and N2 can be fixed by freshwater communities. N2O fixation appeared less sensitive to temperature than N2 fixation, driving a strong sink for N2O in colder months. They also found that, rather than N2O being first reduced to N2 through denitrification, N2O fixation was direct and could explain the widely reported N2O sinks in natural waters.

Turning to larger scale phenomena, everyone in the PONDERFUL team will probably be aware of the long-running debate on how large grazing mammals affected European vegetation: was the land largely cloaked in forests where the climate was suitable or were there open patches maintained by large herbivores? The answer has big implications for freshwaters, and not least for ponds. In a new paper on the history of tree cover, which also has some very good artist reconstruction of how the landscapes looked in the post-glacial period, Elena Pearse and colleagues provide further evidence to support the idea that European forests were open and patchy, not a thick blanket over the land. For ponds this is important because of the long-running practical debates about whether trees are a good or bad thing around ponds. It suggests two important observations: first there were open spaces, suggesting that it would be normal to have open and shaded ponds, and all stages in between. And second it indicates that large animals would have been abundant enough to change the vegetation, further supporting the idea that ponds, and other freshwaters, have long been influenced by large animals.

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. Two recent additions to this debate are of interest: covering the whole of Europe, a large-scale analysis by a multi-author team showed that biodiversity recovery in European rivers has stagnated since 2010. A different take is provided by a paper from the UK by Yueming Qu and colleagues shows significant improvements in freshwater invertebrate biodiversity in all types of English rivers over the past 30 years. Yet another view is provided from a large analysis of United States river monitoring in an analysis spanning 27 years and 6131 stream sites across forested, grassland, urban, and agricultural land uses throughout the United States. Samantha Rumschlag and colleagues found that macroinvertebrate density declined by 11% and richness increased by 12.2%, and insect density and richness declined by 23.3 and 6.8%, respectively, over 27 years. Differences in richness and composition between urban and agricultural versus forested and grassland streams increased over time. Urban and agricultural streams lost the few disturbance-sensitive taxa they once had and gained disturbance-tolerant taxa. The authors suggest that current efforts to protect and restore streams are not sufficient to mitigate anthropogenic effects.

All of these papers make use of the excellent large, long-term, datasets available from river monitoring programmes over the last 30 years. But it’s always worth remembering that these are only part of the story: in the small number of catchments where comparative data from different waterbody types are available, including ponds, invertebrate diversity at landscape scale is greater in ponds than in rivers. PONDERFUL data itself is providing a different perspective on this same issue: the latest update of Williams et al. (2020), which showed whole landscape decline of freshwater biodiversity over the period 2010-2018 in central England, the first such study, shows that this trend continued until to 2023. The study area is now the PONDERFUL Water Friendly Farming demonstration site.

Practical actions

For the practically-minded, a recently published guide to the restoration, creation and management of ponds should be useful. Published by two of the PONDERFUL partners, UCL and Freshwater Habitats Trust and written by Carl Sayer, Jeremy Biggs, Helen Greaves and Penny Williams, the guide is intended for practical conservation work and, although focused on the UK, should be helpful in other areas.