Life in Freshwater

Lochs, lochans, peaty pools and ephemeral winter ponds are all familiar and characteristic features of our landscape. The 6,000 lochs and 1,375 stream systems together with the water held in our spongy peatbogs and its pools, combine to create a very high ratio of freshwater to land mass in the islands. We might stop to admire the water lilies on a roadside loch or be momentarily entranced by the dancing flight of a neon blue damselfly, but otherwise we barely give them a second thought. However, our watery landscape is an important part of our natural environment, even though most of its biodiversity is hidden from view.

Example of freshwater habitats in the Outer Hebrides

For the curious naturalist it is not the most accessible of habitats and we tend to limit our studies to recording the fascinating array of aquatic plants and the larger insects. The clear yellow flowers of the irises, marsh marigolds and lesser spearworts, the delicate white flowers of the water lobelia and bogbean, the semi-submerged leaves of the pondweeds and spikes of the sedges and rushes are just a modest introduction to the diversity of plant life that is hidden from view. 

Similarly, the eye is drawn to the hawking dragonflies, and we hardly notice the gyrating whirligig beetles and the impossibly elegant pond skaters or think about the dragons which might lurk below the surface. However carefully we look or patiently we wait, we will not discover the mysteries of the aquatic fauna and flora by standing on the bank. It is time to go pond dipping.

Today you don’t have to put your wellies on or get nibbled by the midges, this is a virtual glimpse of life below the surface.  The first trawl with the net will probably produce some of the larger invertebrates. Small dark pond snails and diving beetles are easy to recognise, but many of the other creatures are unfamiliar and give us few clues as to their identity. Some are wriggly and worm-like, others are more insect-like with six legs, a variety of tail spines or large jaws, and don’t be deceived by the bits of twig, they might be the larvae of caddis flies. You don’t have to know what they are to admire the diversity of life and wonder how they evolved and adapted to an aquatic lifestyle.

Some of these animals spend their entire life submerged in freshwater using gills to obtain oxygen from the water. However, gills are not the only way to adapt to life under water. Some diving beetles and water boatmen carry bubbles so that they can spend longer under water, whilst water scorpions use a snorkel-like breathing tube.

In some insects it is only the larvae which are completely aquatic. This is often the most important part of their life history, a time to feed, grow and mature. Some species live quietly hidden in the vegetation or under stones or in the mud feeding on algae or pondweed, whilst others are active predators and, in some dragonflies, not adverse to a bit of cannibalism. It may take several months or more for the adult insect to emerge and in mayflies for example, they survive just long enough to mate and lay the eggs to produce the next generation.

It is easy to be distracted by these slightly alien looking invertebrates with interesting life histories, but the core of the biodiversity of these freshwater habitats consists of micro-organism. A complex network of microscopic algae, diatoms, bacteria, protozoa, and zooplankton (small animals such as copepods, water fleas and rotifers). They may be very small but these basic life forms are abundant with amazing functional and genetic diversity, and essential to the survival of the other larger animals and plants in the freshwater ecosystems. Their elemental role in creating and driving the food webs of aquatic habitats is fundamental to the health of the biosphere. If the equilibrium of the microbial communities is disturbed, it can result in ecosystem collapse.  So perhaps the next time you stop to admire the clear waters of a machair loch or dark waters of a peaty pool, remember that they are rich and dynamic ecosystems teaming with life, and that protecting their biodiversity is as important as saving a rainforest.

Phytoplankton (freshwater algae and cynobacterium) and Zooplankton found in the Outer Hebrides.

Top row: Freshwater Algae (Chaetophora pisiformis); Freshwater Alga (Desmid – Staurastrum spongiosum); Cyanobacterium (Gloeotrichia pisum)
Middle row: Foxtail Stonewort (Lamprothamnium papulosum); Freshwater Alga (Euglenoid – Lepocinclis tripteris); Freshwater Alga (Desmid – Teilingia granulata)
Bottom row: Harpacticoid species (Cocepod); Water Flea (Chydorus sphaericus).
Photographs © Chris Johnson

Discover more in the Outer Hebrides Algae website

This post is based on an article published in Am Pàipear in October 2023

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