Ecology of still waters
Although the still waters of lakes and ponds are virtually a closed system, this does not mean they are unchanging. All show changes in populations and diversity over time, but the fluctuations become greater as the volume of water becomes smaller. A large lake is a stable habitat, and although subject to thermal stratification, shows relatively little change in water temperature or volume over the year. By contrast, a small pond is subject to great fluctuations. A hot summer can dry it out, a heavy rainstorm flood it, and oxygen can easily fall to a critical level. Animal populations change rapidly, not only from season to season, but also from one year to the next.
An algal bloom can appear overnight if conditions are right. The higher plants in general have a greater tolerance to fluctuations, but any permanent change in conditions will be followed by a change in the type of vegetation.
Ecology of streams and rivers
If a stream or river is studied at several points along its course, different types of plants and animals will be found corresponding to the different water velocities. In the upper reaches no free floating plants are found, and the only vegetation that can survive are algae, mosses and liverworts growing in crevices between the rocks.
The first vascular plants to appear are those that can root between rocks, but remain totally submerged and lie close to the stream bed where friction with the rock reduces the water velocity. Gradually as the velocity lessens the amount of vegetation increases, itself slowing the water flow and trapping silt, until eventually it becomes similar to pond vegetation. A similar gradation is found across the width of a river.
Moving water presents the same anchorage problem for animals as for plants, and in fast-flowing water only small bottom-dwelling organisms can survive, sheltered between the stones. The caddis-fly larvae builds cases of pebbles which not only act as camouflage but also protect their soft bodies from being buffeted against stones. Other organisms are flat in shape so they can keep within the narrow band of slower moving water close to the stream bed.
Characteristics of water plants
Reproduction
Only the simplest fresh water plants are aquatic in origin. All others, from mosses and ferns to the flowering plants, are land plants which have undergone a kind of ‘reverse evolution’ to colonise aquatic habitats. Unlike algae, which require free water for sexual reproduction, higher plants, with only very rare exceptions, require their flowering parts to reach the air for successful pollination and seeding. This presents such problems that most of these plants flower and set seed far less regularly and less abundantly than land plants and depend, instead, on vegetative reproduction for survival and spread. Annuals are rare because of the flowering problem and most which do occur are not strictly aquatic but are, rather, plants of open muddy habitats.
Structure
Non-emergent aquatic plants have soft stems which lack a cuticle and so allow the absorption of water, dissolved gases and salts directly over all of their submerged surfaces. They are strong and flexible enough to withstand water currents, but because they lack strengthening tissue many cannot survive drying out.
Floating-leaved plants have stomata only on their upper surfaces, where they are exposed to the air. The upper surfaces are waxy so rain or splashes of water will run off. Stems contain large air spaces to retain oxygen, which may be in short supply in the water and altogether absent from bottom sediments. Submerged leaves are usually finely divided, to offer little resistance to water flow, and also creating a greater surface area for gaseous exchange and mineral absorption. Some plants such as Ranunculus aquatilis have typical laminar aerial leaves, but finely divided submerged leaves.
Over-wintering
Various adaptations allow perennials to survive the winter. Some, such as the starworts (Callitriche spp), simply sink to the bottom to await the spring. Others, including the waterlilies and most emergent species, have stout perenniating underground stems or ‘rhizomes’ where food is stored as starch and from which new shoots arise in spring. Still other species, such as frog-bit (Hydrocharis morsus-ranae), produce ‘winter-buds’, or ‘turions’ which sink to the mud upon decay of the parent plant, remaining there until spring when they surface and develop into new adults.
