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Overview
Ponds and small lakes support an extremely rich biodiversity of fascinating organisms. Many people have encountered a few unfamiliar creatures, such as dragonfly nymphs and caddisfly larvae. However, there is a far richer world of microscopic organisms, such as diatoms, desmids and rotifers, which is revealed in this book.
Product Details
| ISBN-13: | 9781784271350 |
|---|---|
| Publisher: | Pelagic Publishing |
| Publication date: | 09/15/2017 |
| Series: | Naturalists' Handbooks , #32 |
| Pages: | 224 |
| Sales rank: | 552,088 |
| Product dimensions: | 5.80(w) x 8.20(h) x 0.70(d) |
Read an Excerpt
CHAPTER 1
Ponds
The great castle of Pembroke is built at the western end of a limestone ridge in south-west Wales. To either side of the ridge, along which the town grew, were two branches of the estuary of the Pembroke River. They ran broadly east-west and discharged ultimately to Milford Haven. In 1946 the southern prong was finally filled in and now forms a piece of land used as roads, car parks and recreational land, with a much engineered stream running through it. The northern prong was dammed, just north of the castle, in the early thirteenth century, to hold a freshwater millpond, the water from which powered a mill set on the dam. The mill burned down and was demolished in 1955 but the dam and its pond (Fig. 1.1) remain and over a couple of decades until 2015 a flock of mute swans on it grew to around 70 birds (Fig. 1.2). The pond has gardens down to the edge on its north side and a masonry embankment, with a walking path paralleling the town wall along the south, so that most of its shoreline is steep or vertical and cannot easily support plants like reeds that would give particularly good habitat for nesting birds and invertebrates. Only at the eastern end, where water enters from another pond, the Upper Millpond, through a narrow tunnel under a railway embankment was there the possibility of some reed growth though the strength of the westerly winds prevented much developing.
In recent years there has been a move to diversify the habitat through provision of wire mesh mats to encourage growth of reeds along the southern shore and at the western end, but the swans interfered with this. They are voracious feeders on young shoots and their number was boosted by the bread daily fed to them by citizens and tourists. In summer, their grazing diminished growth also of underwater plants so that only a fast growing alga, Enteromorpha, looking like stringy lettuce, could keep pace. It was not a very attractive system. In 2015, for unknown reasons, most of the flock decamped elsewhere. The result has been a flourishing growth of the reeds and of a much more diverse plant community in the pond. The algal, invertebrate and fish communities have not been monitored, nor has the water chemistry, so we do not know how they might have changed or why the swans have moved.
There are several lessons from this. First, a heavily modified pond can be made much more attractive, despite concrete banks; secondly, although it is familiar that grazing birds must depend on plant food, the birds, by their grazing, can change the nature of the whole system, just as farm stock can maintain a grassland where otherwise a forest would develop; and thirdly, much information of interest was lost because no-one had time to record properly the plant and invertebrate communities in the pond (although the bird community has been served by several amateur birdwatchers). All I know comes from casual walks around the pond during frequent but short visits. There was potentially an important role for local amateur naturalists, had the opportunity been recognised.
I hope that this book will give insights into the natural history of ponds that might help such opportunities to be taken, and beyond that to bring the enormous fascination and importance of freshwaters, the science of limnology, to a wider audience. Science advances partly on the collection and sifting of information, but mostly on fortuitous or deliberate experiments that can give understanding of mechanisms, so as well as providing advice on sampling and identification of the animals and plants, I also suggest experiments that can be carried out largely with domestic materials and equipment, in a garden and on its pond. If you do not have a pond, or access to one locally, that is no problem. Plastic bowls or buckets offer many possibilities. Any activity carries risks and we now live in a society that has become increasingly unadventurous because of the threat of legal action. Schoolchildren now are rarely allowed close to water without lifejackets, goggles, gloves and close supervision. The risks, however, of drowning, eye and other infections are exceptionally small, especially if common sense is applied. The same is true of the laboratory, or in this context the facilities of a domestic kitchen. My assumptions and those of the publisher are that normal common sense and practical precautions, like covering up skin wounds and abrasions, will be used in carrying out any of the suggested investigations.
Professional limnologists are often asked how a pond differs from a lake. There are many possible definitions (an area of less than 2 ha, with water in it for at least four months a year, is one), all of them arbitrary and to which undermining exceptions can always be found. In the end there are no absolute differences. Ponds are small lakes; lakes are large ponds; they are all bulges of various sizes in drainage systems. They exist in an unbroken range of sizes and they are all linked in systems of runnels, streams and rivers, either on the surface of the land, or under it through the groundwater, in a system that connects the rain and snow ultimately with the ocean. This movement is then made into the water cycle by evaporation, wherever water is exposed, and condensation into rain or snow.
There is thus a continuum of standing bodies of water, from puddles to ocean basins (Fig. 1.3) and along this series there are steadily changing characteristics that operate in many directions. The mistake is to think in terms of separate categories rather than continuous change. Boundaried thinking has meant that work on large bodies of water, where expensive boats and equipment are needed, has tended to gain more prestige than that on ponds and small lakes, at least until recently. But since the 1970s there has been something of a change in balance, as the importance of shallow wetlands has been realised, and ponds have entered the mainstream in scientific research. Thinking in continua gives a much better idea of how the world works. Moreover, there is no absolute distinction between standing waters (puddles, ponds, lakes and oceans) and flowing waters (runnels, streams, rivers and estuaries), though scientists have tended to think of themselves as lake ecologists or stream ecologists depending on their special interests. There is just a continuum concerning how long, on average, a water molecule might remain around the same spot. It is very short in fast-moving waters, perhaps only a second or two, very long in ocean basins, probably some tens of thousands of years. In puddles it may be moved on by outflow or evaporation in days, in ponds in weeks, in small lakes, months and in large lakes a few years. Slow-flowing rivers on the plains and riverine lakes, where, for example, the river is held back by a gorge through a mountain range have much the same characteristics.
This continuum of retention time, the reciprocal of which is called the turnover rate (the number of times that the water mass is replaced per year), has many consequences. A runnel, where water collects on the land in a temporary channel during a rainstorm, before joining a stream with a permanently recognisable bed, will have water with the chemistry of rain. Streams in areas of hard, poorly weatherable rocks, like granites, and thin soils, will also have the chemistry largely of rain, but in regions where the rain also percolates through porous rocks, deeper soils or peats, the water chemistry will be greatly changed and will reflect the nature of the local geology. The chemistry will change also as a result of the activity of organisms living in the water. Substances are taken up for growth, others are excreted. More changes occur when bodies die and are decomposed. And the longer the retention time, the more these changes will have effect. The volume of the water body will also be important. Small water bodies tend to have a very short retention time. Their water chemistry will be less influenced by their organisms. But retention times beyond a few days will be reflected in much greater changes, many of them caused by the activity of the organisms themselves.
For example, a small rocky tarn, high on the granite of the mountains, will have water that is barely different from rain, with relatively high sodium and chloride concentrations and low calcium and carbonate concentrations (Fig. 1.4). Indeed it is much diluted seawater because rain picks up droplets of seawater suspended in the atmosphere by storms, as well as wind-blown dust. The closer the tarn to the sea, the greater will be the proportions of sodium and chloride. In contrast, a pond in the lowlands, amid glacial drift soils derived from a myriad of rocks, will have relatively more calcium and carbonate. Moreover the absolute concentrations of calcium and carbonate will be high compared with the mountain tarn. The sodium and chloride concentrations will also be higher, but not much higher. The source of these ions is still largely the rain, though the longer retention times, and higher evaporation in the warmer lowlands, allow some concentration in the local groundwater.
The next characteristic that is important along the continuum from puddles to ocean basins is morphometry. This describes the shape and depth of the basin. The smaller the basin, the less permanent is the morphometry. The morphometry of ocean basins is determined by the drifting of continents and long-term sedimentation at the edges from erosion of the land. Such basins change very slowly and their main features are very ancient. Less permanent are the deep lake basins formed by large geological movements that contain lakes like Baikal in Russia, and Tanganyika, Malawi, Edward and Turkana in Africa, which are perhaps a few million years old but have nonetheless undergone huge changes in water level, volume and shoreline during their history. Such old basins are very few; most lakes, even the very large St Lawrence Great Lakes on the USA/Canadian border, the largest lakes of Europe, and every smaller lake and pond including all those of the UK, are very young. Most were formed, one way or another, by the action of ice, and their basins were exposed and filled at most ten to fifteen thousand years ago when the glaciers retreated. Sometimes their floors were bulldozed by the moving ice, sometimes their basins are former river valleys dammed by moraines. Sometimes they formed in great holes left when icebergs calved from the retreating ice front and were buried in glacial drift. The lochs of Scotland, the loughs of Ireland, the llyns of Wales, the lakes of Cumbria and the northwest midland meres were variously formed in these ways. Then there are other ways that basins can naturally form, from the collapse of limestone caverns, the damming of streams by blown sand, shingle or landslides, or the work of beavers, and increasingly of man.
There are many times more small ponds than there are moderate sized or large lakes (Fig. 1.5). It is estimated that on a world scale there are close to three hundred million ponds between 0.2 and 1 ha in area, a further twenty-four million between 1 and 10 ha and two million between 10 and 100 ha, which covers the size range most people would think of as a pond or small lake. In contrast the rest of the worlds' lakes number only 20,000 and their combined surface area (including the Caspian Sea with 378,000 km2) of 2.42 million km2 is only a little greater than the 1.82 million km2 of the smaller bodies that average less than 25 ha each. Many of the smaller bodies dot the tundra regions of Canada, Scandinavia and Russia and were formed by soil movements owing to the freezing and thawing of the surface, but a great many are man-made and in the tropics the numbers are probably still increasing. The small irrigation dam, the pool for stock watering, the water supply reservoir and the village fishpond are there the mainstays of modern pond creation.
Ponds probably numbered millions, compared with the perhaps four hundred thousand up to 2 ha in area, present now in Britain and Ireland, before many were filled in to serve the needs of modern agriculture (Fig. 1.6). They once embraced all the functions they currently fulfil in the tropics, and a myriad of other purposes too (Fig. 1.7). There are decoy ponds (for concentrating wild ducks for food in netting pipes that led off them), dew ponds and droving ponds (for watering cattle in the fields or on the move), dye ponds, flax retting ponds (flax is the fibre left after softer tissues have been rotted away under water), blacksmith's forge ponds, hammer ponds (for supplying industrial steam hammers in the nineteenth century), ice ponds (for cutting ice, which was stored in deep vaults, before refrigerators), marl pits (where chalky soil had been dug out for sweetening acid land), mill ponds (to store the water to drive the machinery for grinding grain), moats (for defence of fortified houses), stew ponds (in which fish were kept for the winter when other meat was scarce), swimming ponds, traction engine ponds (steam engines needed lots of water and it had to be replenished frequently), peat cuttings (sometimes quite large in the Netherlands and constituting the Norfolk and Suffolk Broads), and watercress beds (for when watercress and salt constituted a meal in itself). Since the medieval period ponds have increasingly been made for amenity.
Often the functions of working ponds have become redundant but ponds still have a role in conservation and in the appearance of the landscape. They need not necessarily be permanent. There are plenty of plants and invertebrates that thrive in temporary waters and some are unusual and of great interest. Though each pond represents only a small part of the landscape, the tendency for them to occur in clusters, linked formerly by only lightly used land, created a distinctive 'patch' ecology, particularly valuable for amphibians. Many ponds, being fed by ground water and thus isolated at the surface, are fishless and amphibian tadpoles do not easily coexist with predatory fish.
The number of ponds present in the 19th century in the UK has now been more than halved and at present rates of loss, there will soon be few left in the agricultural landscape. The reason is partly that they have been filled in because farming practices have become more intensive and ponds get in the way of large machines, whilst cattle grazing, as part of rotations in the use of land on mixed farms, has been replaced by continuous arable cultivation. Partly it is because stock is now watered through pipelines to drinking troughs and the ponds have not been maintained. With time they inevitably silt up. The distribution of newts and frogs in the UK closely echoes the distribution of ponds and their loss is one of the reasons for declining amphibian populations.
Whilst ponds in the agricultural landscape are declining, however, the numbers in gardens have been increasing. The grander garden ponds are the shallow mirror lakes designed by the landscape architects of the 18th and 19th centuries to reflect the magnificence of houses often built on the profits of the slave trade, ruthless mining or rabid industry, and from which any wild growth of plants was rapidly removed lest it sully the message of complete control (Fig.1.7). The more modest recent ponds are those of many suburban gardens. Garden ponds are generally very small and have hard edges of paving rather than soft and sedimented edges that will support a plant community. Assiduous owners will grow their plants in submerged pots, and spurn the uncontrolled spread of rhizomes in accumulating leaves and sediment. Goldfish in one or other variety may have been introduced in over-large numbers, and the water is held in with a plastic or rubber liner. The former puddled clay or later concrete linings are too vulnerable to piercing or cracking and modern polymers have taken over. Leaves will be cleared out every winter and plastic nets guard against hungry birds. Nonetheless invertebrates and sometimes amphibians will colonise and provide uncontrolled interest. But it is the rarely managed or neglected garden pond that is of the greatest interest. It will acquire characteristics of its older sisters in the fields and its even older siblings in the tundras and boglands, and among them the world of freshwater ecology, indeed of all ecology, and its significance in the functioning and future of our planet, can be opened up.
(Continues…)
Excerpted from "Ponds and Small Lakes"
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Table of Contents
Editor's preface v
Preface vi
About the author vii
About Naturalists' Handbooks viii
1 Ponds 1
2 Living in freshwater 12
3 The littoral 25
Key A Traditional key to kingdoms of organisms 36
Key B Contemporary key to kingdoms of organisms 38
Key C Pragmatic key to groups of microorganisams 43
Key D Algae visible, at least en masse, to the naked eye 48
Key E Periphyton, both attached to surfaces, and free living 54
Key F Protozoa 65
Key G Freshwater invertebrates 80
4 Plankton 84
Key H Common phytoplankton genera in ponds 93
5 Catchments, nutrients and organic matter 107
6 The ecological development of ponds and lakes 126
7 Food webs and structures in ponds 148
8 Problems with ponds and small lakes 164
9 Ponds and the future 184
10 Bibliography and further information 197
Index 206