The total weight or ‘biomass’ of life in any habitat is a measure of its productivity. Simple visual inspection of the type, variety and density of aquatic algae and of the higher plants growing at the water ’s edge gives a general idea of relative productivity.
Different waterways can then be compared on a scale from dystrophic (no or extremely low productivity) through oligotrophic (low productivity) and mesotrophic (medium productivity) to eutrophic (high productivity).
The soft, acid waters and sterile, stony or gravelly shores of the north and west support a sparse, calcifuge vegetation and are in the range from dystrophic to oligotrophic. A typical highland loch has a marginal band of diatoms, filamentous algae and clumps of moss between the seasonal high and low water marks. Water lobelia (Lobelia dortmanna) dominates the shallows, while in slightly deeper water the quillwort (Isoetes spp) extends to 2-3m (7-10’) depth interspersed with water milfoil (Myriophyllum spp) on gravelly beds or green algae on finer sediments. Pondweed (Potamogeton spp) may flourish to a depth of 3-3.6m (10-12’) where the bottom is richly organic. Beyond this there are only diatoms. The total number of species is small, except in the shallow lochs of the machair (calcareous grasslands) of the western coast and islands. Many Highland lochs are very steep-sided with less than 10% of the water area shallow enough to support rooted plants.
Lowland ponds and lakes, by contrast, usually have a rich flora, and fall within the mesotrophic to eutrophic range. Reedswamp communities, described later in this chapter, flourish in the shallow margins. Further out are floating-leaved plants such as the white and yellow waterlilies (Nymphaea alba and Nuphar lutea), the water crowfoots (Ranunculus spp), common also in flowing waters, and broad-leaved pondweed (Potamogeton natans). Many other species occur as submerged rooted plants, as do the water milfoils and Canadian pondweed or water thyme (Elodea canadensis). The latter was introduced to Britain in 1842, spread rapidly to choke waterways across the southern part of the country and, although less dominant now than at its peak, still requires frequent management in lowland ponds and rivers. Free-floating plants include frog-bit, its leaves looking like small water- lilies, hornwort (Ceratophyllum demersum), unusual in that it flowers under water and is pollinated by drift, and duckweed (Lemna spp), which often carpets the surface with its tiny leaf-like stems.
Duckweed forms the ‘scum’ of foul, deoxygenated ponds with a high organic content, where it may help to purify waters unsuitable for other species. Although often subject to control measures, duckweed rapidly regains dominance wherever conditions are favourable.
The historical evolution of ponds and lakes can be studied by sampling undisturbed bottom sediments. Seasonal changes show up in the colour or size of particles washed down by inflow streams, with light (low productivity) winter sediments alternating with dark (high productivity) summer bands. The increasing thickness and organic content of the dark layers shows increasing productivity over time. Microscopic analysis reveals the development of the surrounding vegetation, which releases its pollen each year to float and then sink into the ooze. Since we know the climate and soils which various species prefer today, we can use pollen analysis to deduce changing conditions over past millennia.
Streams and rivers relate much less conveniently to a simple productivity scale than do ponds and shallow lakes. This is because their currents continually scour the channel and flush out nutrients. However, many streams and rivers have shown a recent trend towards eutrophication, as nitrates from excess fertiliser and wastes of various types find their way into watercourses. The smallest farm pond or the largest river system can be degraded in this way. Only some springs and the streams and tarns of mountains and ungrazed moorland are likely to remain unaffected.
Eutrophication
Eutrophication is a natural process in which water becomes laden with nutrient salts leached out of the land by streams and formed by the decay of organic matter in the water. It is a gradual process unless altered by man’s interference. The natural rate depends, amongst other factors, on the nature of the catchment area and the pond’s depth, and shallow lowland ponds may evolve from oligotrophic to eutrophic in a few decades. Eventually, eutrophic ponds undergo succession to dry land conditions. Trouble is caused when eutrophication takes place too quickly, as a result of extra nutrients being added to the water by pollution from sewage, fertiliser run-off and detergents. These nutrients may cause a sudden growth of algae, either in gelatinous masses, filaments or single cells, which can cover the surface in a thick mat. The oxygen produced by the algae is mainly lost at the surface, and the ‘bloom’ shades all life beneath. This prevents submerged plants photosynthesising and deoxygenation of the lower level results, damaging both plant and animal life. Eventually the algal population collapses due to nutrient shortage or a change in temperature or light and the pond becomes further deoxygenated by the mass of decaying vegetation. Fish and other organisms may die.
Algal blooms occur in naturally eutrophic water in optimum nutrient and weather conditions, but the effect is not serious if the ecosystem is in balance. If algal bloom is cleared to improve water flow or navigation, the subsequent burst of growth of submerged vegetation thriving in the improved conditions may cause further problems.
Zonation and succession
The edge of any productive water body will show different zones of vegetation, according to the depth of water. A gently shelving edge will have wider and more complex zones than one which drops steeply to deep water.
Examine a productive pond over the years and it becomes clear that each of the vegetation zones shown in the diagram gradually moves toward the middle, in the process of ‘ecological succession’. Rooted plants trap ooze between their stems; when they die and decompose they further thicken the bottom sediments which build up and reduce water depth. This in turn allows other plant species to invade and cause further drying. What is at present a few feet of open water covered in water lilies may in a matter of decades become a reedswamp, dry for part of the year. What was once a reedswamp may now be covered in willow scrub or alder carr. What is now damp scrubland may become in time a young pine or oak wood.
Wetlands range all along the continuum between open water and dry land conditions. With the exception of acid bogs, they show an inherent and short term instability which makes even eutrophying ponds seem slow to change by comparison. It is as if a take-off point is reached once emergent vegetation covers the open water. Wetlands management should always ask this question first: Should succession be allowed to continue naturally, or should we intervene to maintain the status quo, or should we attempt to either speed up or reverse the natural succession?
On open water bodies, ecological succession may be controlled by removing aquatic and reedswamp species. If plants are dug out at the roots, a great deal of accumulated silt and organic muck is removed as well, increasing the water ’s depth and further retarding succession. Some control programmes, e.g. the raking of duckweed (Lemna spp) or algal blooms or the top-cutting of rooted vegetation, have no direct effect on the rate of succession, although they may be a means of ‘harvesting’ excess nutrients taken up by the plants, thereby slowing eutrophication.
In wetlands areas, succession is usually controlled through scrub or carr clearance, sometimes followed by raising the water table through excavation or flooding. This is the main management task on many lowland wetlands reserves, particularly where fires, drainage or extraction of water on site or in the surrounding area has progressively lowered the water table and encouraged the quick invasion of shrub species.
In both aquatic-reedswamp and scrub control, it is essential to remember that the new habitats created by succession are valuable in their own right. In every case it is best to leave some areas free to develop and to maintain other areas as reedswamp, open carr or closed carr, even if the overall objective is to preserve the particular stage of succession deemed most valuable over most of the site.
Reversing succession by raising the water table via flooding or blocking the drainage ditches allows land managers to create new open water habitats within wetlands reserves. Levels need not be raised year-round, since the vegetation type depends largely on the spring and early summer levels during which time plants are establishing themselves. However, fluctuating levels usually create different plant communities from those of stable conditions, and effects of fluctuations on animal life may be extreme.
