Inland waters form very diverse habitats
Inland water bodies vary in size, depth, amount and nature of solutes, water flow rate, the characteristics of the bottom, water temperature, and many other aspects. Their combinations form very different habitats inhabited by very different species. The number of species usually increases with the increase in the area or catchment basin of a water body, as larger bodies of water are likely to contain more diverse habitats, which increases diversity. However, in addition to the size of a water body, species richness is also affected by, for example, water temperature, water depth, the stability of the water level, and the attributes of the bottom . The biota of water bodies consists of very different forms: from bacteria and plankton invisible to the naked eye, various insects and molluscs, to fish, and plants several metres long.
The biota of watercourses
The biota of watercourses is always closely related to the bottom and banks of the water body. The most diverse habitats are the border areas of different environments (including land and water). Rivers and land share a long border and as the rivers are mostly shallow, they are the most species-rich type of inland water body .
The phytoplankton of Estonian rivers is quite rich in species – more than 400 taxa have been identified– but its biomass is rather small due to the rapid water exchange in rivers (except in the slow-flowing lower reaches of longer rivers) .
The typical micro-biota of rivers is formed by the periphyton, which consists of bacteria, algae, fungi, and small animals attached to the riverbed, underwater rocks or plants. Although only diatoms have been identified in Estonia from periphyton taxa, more than 580 taxa have been found – i.e. more than the species found in phytoplankton in total .
The species richness and number of macrophytes (large plants) varies depending on the flow rate, amount, and nutrient content of the water, and other factors. Of macrophytes, 124 species of vascular plants, 22 species of mosses, and 34 species of macroalgae are known. Plants visible to the naked eye and large algae are the most species-rich in the shallow sections of rivers that do not have tree cover .
The number of large invertebrate (invertebrates you can see) taxa in Estonian rivers is approximately 1,000. These mainly include benthic insects (e.g. lake flies, mayflies), arachnids, crustaceans (e.g. crayfish), molluscs (e.g. thick shelled river mussel), Nematozoa, flat and ringworms, sponges, and moss animals .
There are at least 45 species of fish in Estonian rivers (including Cyclostomata), which live here permanently or arrive here regularly, plus occasional visitors. Due to the diverse living conditions, there are more fish species in rivers than in lakes. In addition, migratory fish regularly move from lakes and seas to rivers and vice versa. The most common fish species are the stone loach, minnow, pike, roach, and perch .
Additionally, grass snake or wintering grass frogs can be found in the rivers. The most well-known birds associated with rivers are ducks, sandpipers, white-throated dippers, and kingfishers; among mammals, the beaver, otter, European and American minks, muskrat, water vole, and water shrew love the rivers. However, they can all also live in stagnant waters .
The biota of lakes
The lion’s share of lake phytoplankton is made up of diatoms, green and golden algae, and blue-green algae. The latter are, in fact, bacteria, whose proliferation or flowering can make the body of water toxic to humans. The spread of phytoplankton depends mainly on light, nutrients, water movement, and temperature. In winter, the abundance of phytoplankton is low, with the richest species composition occurring in summer .
The number of zooplankton is also usually highest in lakes in summer. Unicellular protozoa are the most abundant. Of the multicellular species, the most common taxa are rotifers (up to 200 different species), water fleas, and copepods (both approximately 70 species) .
The periphyton is found in the shallow water of the lakes almost everywhere where there is a suitable bottom and enough light. The periphyton contains both single-celled diatoms and green and red algae up to a metre long. The periphyton is an important oxygen producer near the bottom .
Macrophytes are represented in the lakes by flowering plants, mosses, and liverworts, but they also include charophytes and large red and green algae. More than 100 species of vascular plants and some twenty species of both mosses and charophytes have been found in Estonian lakes. Macrophytes are divided into four according to the type of growth. Submerged plants have all their parts under water (e.g. charophytes). Shore plants have only their roots and the lower part of the stem under water, but the leaves and flowers protrude from the water (e.g. reeds). Plants with floating leaves have roots and stems under the water and the leaves lie flat on the surface of the water (e.g. water-lily). Floating plants float freely, without being attached to the bottom, on the surface of the water (e.g. duckweeds) .
The most common habitants of the benthos of the lakes are molluscs, leeches, crustaceans, Oligochaeta, and many insect larvae . At least 1,000 species of large invertebrates can probably be found in Estonian stagnant waters .
There are about 30 fish species in Estonian stagnant waters (excluding foreign and alien species). The number of species depends primarily on the area of the lake. Species-rich fishing lakes include Lakes Mullutu, Suurlaht, Tamula, Vagula, Lahepera, Viljandi, and Saadjärv .
The birdlife of the lakes is diverse, but few of them nest directly on the water. The Podiceps build floating nests, while the nests of the marsh warblers are attached to the reeds. Many species visit lakes for in their search for food (gulls, Sterna, ducks, swans, Ardea, eagles) or during migration (geese) .
In addition to the water vole permanently living on the shores of the lake, the lakes often see beavers, muskrats, and otters arrived from the rivers. Of the amphibians, for example, the common toad spawns in the lakes.  Smaller ponds with clear water that get a lot of sunlight are an important breeding ground for many amphibians .
Last modified: 12.11.2021
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