A remarkably diverse assemblage of plants, animals, and microbes live in inland waters, with nearly all major groups of living organisms found in one sort of aquatic ecosystem or another. Nevertheless, no major group actually evolved in inland waters; all evolved either in the sea or on land, whence the biological invasion of inland waters eventually took place. The long period of time since this original invasion occurred, however, has allowed many important taxa of inland waters, such as different types of crustaceans, to evolve.
The only major groups of aquatic animals conspicuously absent from inland waters include the phyla Echinodermata, Ctenophora, and Hemichordata. Several other major groups of aquatic animals, as well as plants, are markedly less diverse in inland waters than they are in the sea: Notable among the animals are the phyla Porifera (sponges), Cnidaria, and Bryozoa (moss animals) and among the plants are Phaeophyta (brown algae) and Rhodophyta (red algae). The reason these groups did not invade as successfully as other groups is uncertain, but presumably they were less able to cope with lower salinities and reduced environmental stability. Major groups of the inland aquatic biota that are derived from terrestrial ancestors are insects and macrophytes other than large algae.
Whatever their origins, the invading biota needed to develop many adaptations to the special physicochemical features of inland waters. For those abandoning a marine environment the primary adaptation was a physiologic one that would permit survival in a considerably less saline, more dilute external medium. For terrestrial biota, the most necessary adaptations were those that would allow the organism to exist in a medium of significantly greater density and viscosity that also contained less oxygen. Many other adaptations were required to meet the challenges that particular features of a given aquatic environment posed. Thus, in running waters adaptations were needed that prevented an organism from being washed downstream; in highly saline lakes, a concentrated external medium was the challenging environmental feature; and in temporary waters, the main obstacle was to survive the dry phase. The adaptations themselves are many and varied and include those of physiology (e.g., osmoregulatory abilities), structure (e.g., flattened bodies of fauna living in running waters), behaviour (e.g., burrowing to avoid dehydration), and ecology (e.g., development of life cycles that accord with the occurrence of seasonally unfavourable conditions).
The biota of almost all inland saline waters did not evolve directly from marine ancestors but instead primarily from freshwater forms. Only a few forms appear to be of terrestrial derivation, and a few organisms in inland waters located near coasts are of marine origin. Although at first this evolutionary pathway may not seem obvious, it can be explained easily. Organisms that survive under greater environmental stress tend to have a greater ability to adapt than those that do not. Marine environments are considered less stressful than freshwater environments; hence, organisms from fresh waters are better able to adapt to the extremely stressful environment of inland saline waters.
Population and community development and structure
All types of inland aquatic ecosystems have well-defined structures and processes that are similar in general aspects but differ in particular details throughout the biosphere. Thus, as is true of marine and terrestrial ecosystems, almost all inland aquatic ecosystems have three fundamental trophic levels—primary producers (algae and macrophytes), consumers (animals), and decomposers (bacteria, fungi, small invertebrates)—that are interconnected by a complex web of links. Energy passes through these trophic levels primarily along the grazer and detrital chains and is progressively degraded to heat through metabolic activities. Meanwhile, the essential elements follow pathways that cycle between these biotic components and the abiotic components of the ecosystem (see biosphere: The organism and the environment: Resources of the biosphere: Nutrient cycling). A few inland aquatic ecosystems such as hot springs and highly saline lakes have conditions so inimical to life that biological diversity is restricted, trophic levels are correspondingly simple, and energetic and biogeochemical processes are compressed.
Organisms within the trophic network are arranged into populations and communities. In deep, freshwater lakes the primary producers (plants) are found either at the shallow edges of the lake (emergent, submerged, or floating macrophytes) or free-floating within its upper layers (microscopic algae, cyanobacteria, and photosynthetic bacteria of the plankton community) (). Plants are found only in the photic zone—the upper portion of the lake where photosynthesis occurs, also called the trophogenic zone. In this zone the production of biochemical energy through photosynthesis is greater than its consumption through respiration and decomposition. Animals and decomposers are found in both the photic and aphotic zones. In the aphotic zone, also called the tropholytic zone, the consumption of energy exceeds its production. The zones are demarcated by a plane of compensation at which primary production and consumption are equivalent. This plane varies diurnally and seasonally with changes in light penetration. The major biological communities of deep freshwater lakes are shown in . Included are the plankton, which contains tiny floating plants (phytoplankton) and animals (zooplankton) as well as microbes (see marine ecosystem: Marine biota: Plankton); the shoreline macrophytes; the benthos (bottom-dwelling organisms); the nekton (free-swimming forms in the water column); the periphyton (microscopic biota on submerged objects); the psammon (biota buried in sediments); and the neuston (biota associated with surface film). These organisms differ enormously in size, ranging from less than 0.5 micrometre (0.00002 inch) to greater than 1 metre (3.28 feet). They also vary in composition, structure, function, adaptations, and spatiotemporal location. Significant taxonomic differences also occur across continents; for example, the fish species of freshwater lakes in Africa are not the same as those of similar ecosystems in North America, nor are the plankton of lakes in Australia the same as those of lakes in Asia.
The populations and communities of inland waters other than freshwater lakes are similarly complex but markedly different in all except their fundamental ecological roles. Even major ecological and biogeochemical processes are quite distinct in different sorts of inland waters. In streams, for example, plankton populations are absent and much energy is derived allochthonously (from outside the stream). The processing and transport of essential elements follow a downstream sequence. Hypotheses attempting to explain ecological processes in running waters include the concept of the river continuum, which explains differences in lotic communities according to the changing ecological factors along the river system. Nutrient spiraling is another concept invoked to explain the cycling of nutrients while they are carried downstream. For large rivers of variable hydrology, the flood pulse concept has been instructive. This concept regards seasonal or occasional flood events as important ecological phenomena determining the biology of the river.