At present, there are around 23,000 ponds in Norfolk. Most of these ponds are found in farmland, often along hedgerows or in the middle of fields (fig.1).
To get a snap-shot view of the current state of Norfolk’s ponds, in terms of the species which they support, their water chemistry, and their physical surroundings, we surveyed 121 randomly selected farmland ponds in the spring of 2013. This survey taught us some important lessons about the current state of Norfolk’s ponds, and what needs to be done to help protect and manage these farmland oasis.
The most striking message from this survey was that many of Norfolk’s ponds are very overgrown and shaded, which has a strong negative effect on their biodiversity.
We already know from smaller scale studies that shading from overhanging trees has a negative effect on pond biodiversity; lack of light reaching the pond surface means that few aquatic plants can grow, so there is little habitat for invertebrates, and little oxygen being released into the water. Falling leaves drop directly into the pond, and their decomposition by bacteria and microbes further uses up what little oxygen is available. These decayed leaves form a very soft and unstable mud in the bottom of the pond, which plants cannot easily root in. Over time, a shaded pond looses many of the species which it once supported, until only a few particularly tolerant species remain (see British Wildlife article for more details).
Fig.2 – A water hoglouse (Asellus aquaticus). These are a particularly hardy species, which can survive low oxygen levels and poor water quality. They are often the only invertebrate species found in very over-shaded ponds.
While we already know quite a lot about the effects of shading on ponds, most of this information has come from studies monitoring a few sites over several years. This survey revealed the extent of the problem across a much wider landscape, with ponds in different areas and different surroundings all responding to shading in similar ways.
During the survey, we measured a wide range of parameters which might influence pond biodiversity. These included:
Water chemistry: pH, dissolved oxygen, conductivity, alkalinity, chlorophyll a concentration (a rough indicator of nutrient enrichment).
Physical properties: pond area, % shading of pond, % cover of aquatic plants, width of buffer strip surrounding the pond.
Spatial properties: density of ponds within the surrounding landscape, distance to the nearest neighbouring pond.
Overall, only pond area, dissolved oxygen in the water column, and shading, had a significant effect on the biodiversity of a pond. Of these factors, shading was by far the most important, having significant impacts on aquatic plant diversity, water beetle diversity, and the presence / absence of amphibians.
Of the 121 ponds we surveyed, 47% were heavily shaded, with over half of their water surface overhung by trees. Of these shaded ponds, almost a third were extremely shaded, with over 90% of their water surface overhung by trees. Shading had a significant effect on aquatic plant diversity, with open ponds (less than 20% of their surface overhung by trees), tending to contain more plant species than more shaded ponds (fig.3).
Shading not only influenced the plant diversity of a pond, but also had an effect on water beetle diversity. Although water beetle diversity was more variable for all levels of pond shading (both heavily shaded ponds and open ponds would sometimes contain no species of water beetle), there were patterns in the range of water beetle species recorded (fig.4).
The maximum species diversity recorded for any one pond was 17 species of water beetle, which were collected from an open pond. In only 21% of open ponds were no water beetles found during the 3 minute sample period. For highly shaded ponds (50 – 90% overhung by trees), the maximum beetle diversity was 7 species from one site, with 26% ponds surveyed returning no water beetles. In the very overgrown ponds (>90% shaded), there was a maximum of 4 species from one pond, and 69% pond surveys returned no water beetles.
Fig.4 – Range & median number of water beetle species found in the different pond-shading categories. The most beetle-diverse ponds were those with the lowest levels of shading, and the least diverse were the extremely shaded ponds.
Pond shading also had an impact on amphibians, although this looks to only be really important in the case of extreme shading (fig.5). Less than a third of extremely shaded ponds had any amphibians present, compared to 90% of open ponds. Considering that a third of the ponds surveyed were extremely shaded, this could have a significant effect on regional amphibian populations.
Fig.5 – Effect of shading on the presence of amphibians in a pond.
Assuming that our 121 survey ponds are reasonably representative of the rest of Norfolk, we can assume that pond shading is a major issue across the county. Unless we actively manage our ponds to create a mosaic of different stages of succession, over time more and more ponds will become extremely shaded, supporting fewer plants, invertebrates and amphibians. Restoring some (but not all), of the more overgrown ponds, combined with resurrecting ghost ponds, could help restore balance to the pond landscape, creating a range of different habitats for all sorts of wildlife.