Freshwater Ecosystems, Human Impact on emigration and development of new species without the intense competition and predation caused by well-established species, as is typical of the sea In contrast, shallow lakes disappear after only a few thousand years, because they are filled with particles eroded from the land and organic matter produced in the lake In essence, freshwaters provide many opportunities, which constantly come and go, but individual lakes lack the long-term stability of the sea that has lasted for billions of years This scenario should have stimulated the selection of species with rapid evolution and efficient means of dispersal allowing them to colonize new freshwaters as their original habitats disappear Indeed, apart from species of fish, molluscs, and crustaceans associated mainly with large ancient lakes (e.g., Lakes Baikal, Titicaca, and Victoria) and large ancient river systems (e.g., the Danube, Amazonas, and other tropical rivers), most freshwater species are widespread within and even among continents Freshwater microorganisms among the bacteria, algae, and protozoans are generally both locally abundant and cosmopolitan Most aquatic rooted plants are also widespread, though a few species are endemic because underwater dispersal restricts their spread Species Evolution and Richness in Freshwater As a result of the intimate contact, the long coastline, and the suitable transition zone, there has been a lively exchange of species from freshwaters to terrestrial environments and back again Plants have evolved from a special group of freshwater green algae (Charophyceae, Coleochaetales) and diversified under the highly variable conditions on land Insects have evolved from groups of arthropods in the transition between freshwater and land Amphibians and reptiles have evolved from special fish in freshwater and brackish wetlands Many evolutionary lines have been followed at different times and places Freshwaters have formed a corridor for the two-way dispersal of organisms between land and sea Freshwaters also share many taxonomic groups of algae, plants, and animals with the terrestrial environment and the sea, whereas divergences are stronger between the land and the sea Freshwater environments are surrounded by large surface areas of terrestrial and oceanic environments, which promote the emigration of marine and terrestrial species and their adaptation to freshwater environments Among major groups of land plants, several have secondarily returned to freshwaters, though few have reached the sea Among liverworts, true mosses, bog mosses, horsetails, and ferns, there are many freshwater species, but no marine representatives Among flowering plants, 1000–1400 truly aquatic freshwater species have been described from many plant families, illustrating that the secondary return from land to freshwater has occurred independently and repeatedly The return process continues today, with more than 4000 species living an amphibious double life in the transition between land and freshwater The sea includes only about 60 species of flowering plants with a restricted taxonomic diversity The diverse freshwater insects (445,000 species) live a double life as eggs, larvae, and pupae in water and as flying 571 imagines dispersing and mating on land Mayflies live for only a few days on land without taking food, whereas adult dragonflies live a long and active terrestrial life Freshwater insects, therefore, require a suitable environmental quality in both freshwater and adjacent terrestrial environments for their sustained survival and development, though most evaluations of habitat quality in relation to species composition and richness of insects focus only on the aquatic zone No phylum – the highest taxonomic entity of organism below the kingdom – is found exclusively in freshwaters, but all large phyla apart from the echinoderms are represented Virtually all types of photosynthetic organisms, including cyanobacteria, the great variety of algae, mosses, and vascular plants, are represented in freshwaters (Table 1) In contrast, the sea has very few species of gold algae, yellow–green algae, plants, and insects, and it lacks some groups of green algae (e.g., desmids and stoneworts) The sea outnumbers freshwaters only with respect to photosynthetic species among red algae, brown algae, and dinoflagellates The three main determinants of species richness for a specific type of environment are surface area, habitat heterogeneity, and the time history in terms of durability and evolutionary development Considering that freshwaters cover only a few percentage of the surface area relative to that of the sea and the terrestrial environment, and considering that inland surface waters mostly have a short life time, the freshwater biodiversity is surprisingly high and must arise from the high habitat variability among individually confined water bodies and the suitable conditions for emigration and establishment of species from the large, adjacent terrestrial and marine environments Taking the vertebrates as an example, there are about 10,000 named species of freshwater fish and 15,000 species of marine fish Virtually all birds and mammals depend on freshwater bodies for drinking, but a large proportion of the species also are dependent on freshwater lakes, streams, and wetlands for breeding and feeding In Europe, about 25% of the bird species and 11% of the mammal species live their entire life or part of it in freshwaters Table Estimated number of algal species in freshwaters and total number of all species in soils, freshwaters, brackish waters, and the seaa Taxon Freshwater species All species Cyanophyta (blue–green algae) 1500 Rhodophyta (red algae) 150 Chrysophyceae (gold algae) 190b Xanthophyceae (yellow–green algae) 550b Bacillariophyceae (diatoms) 5000 Phaeophyceae (brown algae) Few Cryptophyta 100 Dinophyta (dinoflagellates) 200 Euglenophyta 700b Chlorophyta (green algae) 7000b Zygnematophyceae (e.g desmids) 6000b Charophyceae (stoneworts) 400 a Values are presented for major taxonomic groups Freshwater groups c Marine groups b 2000 5000c 2000 600 10,000 2000c 200 2000c 800 8000 6000 400