Hidden under the surface of our rivers is a microscopic world of beautiful and alien creatures. Most of these animals are the larval form of mayflies, stoneflies and caddisflies who spend most of their lives feeding and growing under the water before hatching as adult flies to reproduce and die. Under the rushing oxygen-rich water flattened mayfly nymphs with bodies streamlined to deal with the fast-flowing currents feed on algae and detritus. Beside them caseless caddis nymphs have hooks on the end of their bodies that allow them to hold on in the torrent whilst they construct shelters and spin nets out of silk (like butterflies and moths which they are closely related to) to catch morsels of food floating by. Also found in the fast flows are blackfly larvae who anchor themselves in the flow and grab particles of food out of the water with hair like fans on their heads. Freshwater shrimp and hoglouse (which are basically a freshwater version of woodlouse) swim and trundle through the slower flowing areas, shredding up the leaves dropped at the end of the previous year. Large predatory beetle larvae with fearsome jaws prey on these smaller invertebrates and of course these creatures all form the base of a complex food web which support freshwater fish and birds and mammals.
As well providing food for fish invertebrates can tell us a lot about the environment simply by whether we find them or not. In this microscopic world not all animals are equal, some are more sensitive to pollution than others. Some mayflies, stoneflies and caddis are particularly sensitive (like the canary in the coalmine) and very quickly disappear from polluted rivers, while worms, snails and hoglouse can tolerate much more before they are affected. These differences in sensitivity are translated into a score by biologists (with the most sensitive animals scoring the highest) and then these scores can be added together. The bigger the number, the healthier the environment.
By looking at the invertebrates in this way we can get a quick check-up of the health of our underwater environment. In order to do this a kick sample is taken. This involves disturbing the bottom of the river by kicking for a set amount of time (usually 3 minutes) and using a net to collect all the animals that are dislodged. These animals can then be identified and counted under a microscope to see who’s present and who’s not. Aquatic ecologist Iain Reid from The Forth Rivers Trust has been doing just this at a number of sites on the river Almond and the initial results may surprise you!
Kick samples taken above and below Limefield falls were found to contain several families of mayfly, stonefly and caddisfly which had the highest sensitivities to pollution. In addition to this the samples contained a wide range of other invertebrates that are less sensitive to pollution including beetles, snails, shrimps, cranefly larvae and worms. In all there were 24 families of different invertebrate found. When all the scores from these different animals were added up and compared with the average values for rivers in the UK, the sites at Limefield were found to range from very good to excellent quality indicating a thriving biological community! Iain still has a lot more beasties to catch and count from other sites around the Almond which will hopefully be as positive as the samples at Limefield. Whatever the results, this monitoring work will allow the Trust to compare the health of the river at different sites and with samples taken in previous years to see if things are getting better or worse and where we need to work in the future.