30th March 2018
By Jeremy Biggs. A recent Twitter thread here prompted some discussion about what kind of landscapes make for good quality freshwaters. I promised a few more thoughts that needed a bit more than 280 characters…
A recent Twitter thread here prompted some discussion about what kind of landscapes make for good quality freshwaters. So my slightly longer answer to Paul: it is (nearly) all about the catchment, and the landscape, but we need to readjust our idea of what comprises a catchment. For the smallest ponds that catchment needs only to be tiny – a hectare or even a few square metres (but groundwater-fed ponds which can still have much bigger catchments). For headwater streams, it may still only be a few hectares. It’s possible to find a lot of these in areas with low intensity less or non-polluting landuse – which is one reason why these small habitats are so important for freshwater biodiversity.
Once we get to modest sized lakes, say 5 ha (which nearly always have inflows) or streams a couple of metres wide, the catchments are much bigger, and it’s harder to maintain that clean landscape.
A Knepp sized area (c.1000 ha) can pack in a lot of clean ponds and maybe a couple of clean headwaters, and maybe a low intensity more-or-less clean floodplain (floodplains can be clean if you don’t have too much impact from polluted floodwater, and they are supplied by a mainly local clean water source). But at Knepp, a clean larger stream, probably not, because it would need another 10,000 ha or more upstream of clean land.
When we reach the size of the New Forest we can get reasonable-sized rivers into good condition and some lakes. The same is true in the rest of the National Parks where there is space enough for clean water. But in these bigger landscapes, there’s also more chance of point source pollution from large and small sewage works, septic tanks, roads and so on. And quite a lot of the area of National Parks have ordinary farmland in them too. But it’s one of the great strengths of the National Parks that they do have large areas generating clean water, and deintensifying around these already low intensity areas would be one of the best ways of helping freshwater biodiversity in those areas.
So if we could make all the landscape like the New Forest with little pollution from the land mixed with lower densities of animals which can graze/trample hard in some places, and more gently in others (the variation is important), and cut out the point sources which affect all kinds of ponds, streams, rivers and lakes, big and small, that would be near perfect.
The complications come in the nuances – so where we are now in Britain, there are urban areas and roads which produce just as much pollution as farmland, there are great swathes of the countryside with nitrogen in the groundwater which will last for 50-100 years if we stopped polluting tomorrow, and we also have a huge legacy of excess phosphorus in the landscape. Lots of running waters have more fine sediment than is natural as well, and not just obviously running off of maize fields, and we usually have way more than natural animal densities – for obvious reasons. We have air pollution adding nitrogen and other pollutants, and now it looks like plastic in a lot of waterbodies as well – though again we can guess that smaller waters, with cleaner land around them, will have less plastic.
For FHT, our major objective is to create what we have called for now – rather un-imaginatively – The Freshwater Network. This is a network of the clean, and biologically most important, freshwater habitats which is established by ensuring we know where existing clean waters are now, where the important bits for biodiversity are, making sure they are recognised and properly protected, and then linking those up by increasing the extent of clean habitats in between (they don’t have to be physically linked). We can do this either by de-polluting existing waters (often difficult or simply impossible) or by habitat creation and restoration to add new freshwater habitats and wetlands to the landscape, which is usually easier and cheaper (like the Million Ponds Project or the rewetting of bogs – evidence for both looks positive). But just to note: evidence suggests restoration is less effective than protection generally.
Finally – are bigger blocks better? Yes, but to get the best from this we need to look at what ‘bigger’ means, because it’ll vary depending on the size of catchment a waterbody has. And we need to be a bit more precise about what ‘catchment scale’ means – often this is just a portmanteau term for doing some things across a patch of landscape which all happens to drain the same way to the sea. Probably as important, we need to get the existing clean, biologically rich bits, closer together by increasing the area of ‘good’ land around them and adding new clean waters in between. Reducing the distance between clean waters – currently often miles rather than metres – will be essential even to stop the ongoing decline in freshwater biodiversity.