Living in Fresh Water

David Beeson, July 2021

Living in fresh water sets up challenges for organisms. It is quite a different environment from dry land or from salty marine places. And it is a rare space on Earth – 2.5% of the earth’s water is fresh. Yet most of the earth’s fresh water is unavailable: locked up in glaciers, polar ice caps, atmosphere and soil. Additionally, some is highly polluted or lies too far under the earth’s surface to be extracted at an affordable cost. As a result only around 0.5% of the earth’s water is available as running or open fresh water, and much of that is in large lakes (think North American Great Lakes) and major rivers such as The Amazon.

Fresh water is a rare habitat, and with ever demanding human populations flowing waters are extracted with increasing regularity. The 1,450-mile-long North American Colorado River, draining 264,000 square miles, is a mere trickle when it nears the sea most years. (This year it is said to be dry.) In Hampshire (UK) our rare, clear, chalk rivers have water extracted in multiple locations, yet flow well most years.

My hometown, Andover, is located near the head of the River Anton, a tributary of the River Test that reaches the sea at Southampton. It is a crystal clear river – a shock to fishers more used to seeing silty waters. It has brown and introduced rainbow trout, some salmon, eels and many other fish species, plus oddities such as brook lampreys. The fishing is both exclusive and expensive*.

* Artificial lakes offer trout fishing at around £200 for a day, and allowing a maximum of 4 fish, and around £500 a day on the main river in May.

The water seeps out of the chalky hills as small streams and bubbles up from river beds. As the water table rises so the waters emerge higher up the valleys, for many streams are ‘winter bournes’ arising only after the winter’s rich rainfall, and slowly falling back as the summer and autumn progresses. Spending possibly years before the slightly acidic rainfall emerges to the surface it has had time to dissolve the chalk (calcium carbonate) and has become slightly alkaline (pH 7.4 – 8.7), and at a temperature of around 10 degrees Celsius all year. Soon the water will absorb oxygen, but at a level far below the 21% in the air, and often between 4 and 10 parts per million (% is parts per 100, of course.) There is much less oxygen* available for respiration in water than air, so one must expect a lower rate of metabolism (body working). In addition carbon dioxide levels are much higher, and that can be a negative influence.

The relatively warm winter water is the reason this part of the UK has many watercress growing businesses, with the natural plant growing in specially prepared gravel-lined beds and fed with spring water.

*Putting it another way, at 5 degrees a litre of air holds 210 cm³ oxygen, water 9 cm³.

The water’s oxygen levels fluctuate with light levels and the associated photosynthetic release of the gas. Sewage or the breakdown of natural organic material can reduce oxygen levels to zero, killing off most life. (Inputs of fertlizers will encourage weed growth, eutrophication, and when they die back later in the year they can ‘kill’ a water course.) Decay uses up oxygen.

Luckily, water allows good light penetration, so plants and algae can grow on the river’s bed. Exploring stones will show up the attached algae.

The final important factor will be current speed. This can be assessed by throwing an orange (fruit) into the river and timing its movement over a set distance. Of course, this gives only the surface speed, and in amongst the gravel or weed it will be near zero speed. Hence, small, swimming invertebrates live amongst weed and gravel to avoid being washed away or using up too much energy in maintaining location. Snails and many insect larvae will adhere to rocks or vegetation and so try to avoid being washed away.

To extract invertebrates we need to sample amongst the weed or the river’s bed. Traditionally I used a scientific instrument* to disturb the bed and washed the beasties into a net located downstream.

(* a Wellington boot at the end of my leg.)

Plants

The negatives, such as water flow and the possible lack of light, are mitigated by the buoyancy, less need for structural support, no wind pressure on fragile stems and diminished threat of desiccation in water. That water plants need to be near the surface is obvious, to that end they can reduce their density by having oxygen-filled spaces in their structure.

Water can be absorbed over their whole structure, minerals, oxygen too. Just as the leaves of aerial plants, if water plants’ leaves do not contribute sugars to the plant they will die or be shed. It is the decay of such structures than can reduce oxygen levels as decay is mostly an areobic process.

Phloem and a minimum of xylem is in the central core, beyond are the air-filled spaces of this hydrophyte. Little need for structural tissues such as collenchyma. (See articles on plant structure.)

Plants in flowing water will often have bisected leaves to reduce resistance. This is river crowfoot, Ranunculus fluitans.

Once plants emerge from water they will need more support, so woody xylem and collenchyma cells, with thickened cell walls, are found in the stems and leaves. Their roots will need to supply water and nutrients to aerial parts, so need to be more extensive than in a free floater. The roots will have an aerated structure.

Water lilies have their leaves spread on the water’s surface in slow moving water. They have their stomata on the upper surface only.

Aquatic plants need to flower. With the Canadian pond weed (Elodea canadensis) the filamentous flower stems (up to 15 cm long) bring the minute flowers to the water’s surface. While flowering in this species is said to be infrequent, no one has told my pond specimens this. They flower yearly in June and July.

Algae are common on rocks, stones and sometimes are free-floating in ponds. I have the common stonewort, Chara globularis. It was useful in the laboratory as the flow of cytoplasm is clearly visible in its long cells under the microscope. Diatoms, also algae, are seen in microscope samples.

Freshwater diatoms, drawings. These are photosynthetic and are at the base of the food chains and pyramids.

Animal life

As on land, there will be herbivores, carnivores, omnivores, detritivores and parasites in the water. Some life will migrate from the water to land, for example newts, dragonflies and many fly species. These movers will have to modify their structures to adjust their physiology.

Of course, not all of these ‘water’ creatures live in the water, some make their homes on its surface with its surface tension providing their solid floor. Locally these will be whirligig beetles and pond skaters. The former can dive underwater if danger threatens, while both are carnivores.

The whirligig (Gyrinus natator) has a streamlined body but with its second and third limbs modified as paddles – being flattened and heavily fringed with hairs. Its eyes are in two sections – so is capable of vision above and below the water line at the same time.

For those living below the surface their oxygen requirement can be acquired in two ways. Either by still employing air’s oxygen or by extracting it from the water. Air breathers will include great diving beetles (Dytiscus marginalis), mosquito and similar fly larvae. They still use spiracles and trachea, with the former in the regions that are protruded from the surface. Another air technique is to carry bubbles of air with you – hence the silver sheen visible on biting water boatmen (Notonecta glauca) that swims upsidedown (or the underwater nests of water spiders). The smaller, but similar looking, lesser water boatman swims normally and is a herbivore.

Certain snails are also semi-aquatic, visiting the surface at intervals to take air into a chamber beneath their mantle, which works as a primitive lung. The great pond snail, Limnaea stagnalis, does this especially when crawling along the underside of the surface film of ponds.

For larger true aquatics they need a gill to allow the absorption, by diffusion, of oxygen into the body and to allow the loss of carbon dioxide. To make this more efficient, especially in mud-living invertebrates, they have a form of haemoglobin – hence the colour of blood worms (normally a fly larva, despite the name! But there are some red annelids, Tubifex sp, too, yet I seldom encounter them.).

Smaller invertebrates that are fascinating include the hydras and protozoans. I find hydras by collecting weed from a still or slow-flowing area and holding it in a tank with one-side illumination. Green hydra are photosynthetic (and carnivorous) and will move to the light and adhere to the tank’s side. They can be kept as pets, being fed on wild water fleas, Daphnia sp or Simocephalus sp. For mobile protozoans merely use mud samples or grow a biofilm on the surface of a water sample kept in the dark.

Daphnia are common in ponds and can be studied by capturing them in well-thinned cotton wool on a microscope slide. You’ll easily spot the beating heart and, if you pre-feed them with food dye-coloured yeast, you will see the gut clearly and its feeding method.

Midge and other fly larva are common in freshwater, as are young mayflies, damselflies, stoneflies etc. These are caught using proper sampling nets.

The list of organisms could go on for ever! In clean rivers I find swan muscles and occasional brook lampreys, and the obvious range of fish from the bottom dwellers to the salmon and trout of the faster flowing sections. Seeing fish is easier in a clean canal – if you live in Hampshire try the Basingstoke Canal near its tunnel at Greywell.

One experiment I once carried out was to look at competition between sticklebacks and leeches. Would the fish eat the small leeches or leeches attach and feed off the fish? (Answer at the very end of the article.)

One speciality I have encountered only once is the fairy shrimp. My students and I had cleaned out a village pond (near Hatherden) that had been lost, even with a tree growing out of it. Shell had donated a huge liner to keep in the water as ‘puddling’ the clay was beyond our abilities. When it naturally refilled the eggs of the shrimps blew in and hatched in their billions. (https://sussexwildlifetrust.org.uk/news/the-fairy-shrimp-just-add-water). It did not end well, however. A drunk driver drove into the pond shortly after we had completed the task and destroyed the liner. By then we were exhausted.

These shrimps sometimes appear in puddles on Salisbury Plain.

The River Test has ploughed its valley through a chalky landscape. The chalky slopes are increasingly being used for wine production with the John Lewis / Waitrose vineyard nearby. Local wines have been winning international prizes. If you want to visit the Test Valley, the hotels at Stockbridge offer a good location.

Answer: the leeches fed off the fish. The fish ate supplied water fleas.

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