Diversity of freshwater fishes
Only a small portion (0.01%) of the surface water of the earth is freshwater, but these areas represent a variety of habitats, including swift-moving streams, deep glacial lakes, and ephemeral creeks. These freshwater habitats harbor diverse assemblages of fish, comprising more than 10,000 species in 23 orders. Much of this species richness is represented by Cypriniformes (2,662 species), Characiformes (1,343 species), Siluriformes (2,287 species), and Perciformes (2,185 species). The Amazon River alone is home to more than 1,300 species of freshwater fish, and more than 700 species of endemic haplochromine cichlids inhabit the East African rift lakes.
Distribution of freshwater fishes
On global and regional scales, the distribution of freshwater fishes is determined largely by historical circumstances. The world can be divided into distinct zoogeographic regions based on the distribution of organisms around the globe. Patterns noted at the global scale have been influenced over a long evolutionary time period by plate tectonics, including the movements and collisions of continental landmasses (continental drift). Major tectonic events played a large role in determining the families of fish that are present on a particular continent and that have the opportunity to become part of local fish assemblages. The isolation of fish caused by the separation of landmasses allowed for the diversification of species within major lineages.
Continental movements also affected climate, geologic, and drainage patterns across the landscape. The latitudinal location of landmasses influenced their susceptibility to glaciation during cooler periods of geologic history. During the Pleistocene epoch (11,000 to 1.8 million years ago), four major glacial periods resulted in the extirpation of fishes in areas covered by ice sheets, caused other species to move to nonglaciated refugia, and altered large-scale drainage patterns. These effects still influence the distribution of fish today, as many species no longer inhabit certain areas or are recolonizing portions of their previous range after seeking southern refugia during the Pleistocene.
Geologically, mountains that are pushed up from the collision of two landmasses can restrict the movement of fish. For example, fish assemblages on one side of the Appalachian Mountains are substantially different from those on the other side of the divide. Mountains and other geologic features typically form the boundaries of drainage basins; the evolution of fish that are isolated in distinct drainages leads to further diversification and variance within species over time. Within drainage basins, a number of factors are associated with patterns of fish diversity. Fewer species tend to occur in headwater streams, while more species inhabit downstream portions of the watershed. The size and variety of local habitat types also affect fish diversity, with the diversity of fish increasing with habitat area and internal variability. In addition to natural controls on the range and composition of fish communities, it is important to recognize that human activities, including deforestation, construction of dams, introduction of non-native species, and pollution, have influenced the distribution of fish in many regions of the world throughout the course of recent history.
Acting within this broader context, physical and chemical characteristics of the environment regulate the composition and diversity of fish species that inhabit freshwater habitats. In all aquatic systems, light penetration and water temperature determine physical conditions that fish encounter. Fish also must be adapted to tolerate chemical attributes of freshwater systems, such as salinity, oxygen, and pH. Local species assemblages and species distribution within a habitat largely reflect the preferences of fish for different physical and chemical conditions.
Light penetration directly and indirectly influences fish in freshwater habitats by warming the water, driving photosynthesis, and enabling visual activities. When light reaches the water surface, a small amount is reflected, and the remainder is absorbed as it enters the water column. Wavelengths of light are absorbed differentially with depth. Clear water in the upper few meters of the water column absorbs red wavelengths and converts the energy to heat. Only wavelengths between 400 and 700 nanometers can be used for photosynthesis, and these wavelengths penetrate deeper into the water column before they are absorbed. Light absorption is affected by particles and dissolved material in the water as well; for example, the presence of algae shifts absorption toward the green wavelengths. Light also enables fish to use vision to detect predators, prey, potential mates, and features of their habitat.
Freshwater fish are ectotherms, and their internal temperature follows that of the surrounding water. Fish partition habitat space based on thermal gradients to avoid harmful temperatures as well as to take advantage of those that are optimal for a variety of physiological functions, including feeding, growth, and reproduction. Thus, seasonal movements and spawning are regulated strongly by temperature. Temperature varies on a geologic timescale, and shifts in the geographic distribution of fishes have been associated with major historical climatic changes. Temperature also varies locally and over short timescales, including diel and seasonal cycles, in freshwater aquatic systems. As light energy is converted to heat, the top portion of the water column warms first and to the greatest extent. Finally, latitude, altitude, and the velocity of water influence temperature. Due to large differences in water velocity, temperature patterns differ markedly in lakes versus running waters.
Although rivers erode and transport some salts from geologic formations within the watershed, most bodies of freshwater lack the high concentration of dissolved salts that characterizes seawater. Exceptions occur in some desert areas of the southwestern United States and northern Mexico as well as in the Rift Valley of East Africa. In these areas, minerals accumulate when streams flow through underlying geologic salt formations or when evaporation leaves behind high concentrations of salts. While diverse fish communities inhabit these mineral-rich waters, most freshwater fishes cannot adapt physiologically to life in saline waters. Regulating internal salt concentrations poses a challenge to most fishes living in freshwater environments. Because salts are more concentrated inside their bodies than in the water, osmotic and diffusion processes work to bring in water and remove salts. To counter this situation, freshwater teleosts excrete large quantities of dilute urine and transport salts back into their blood using chloride cells. While this adaptation enables fish to osmoregulate in freshwater, the internal retention of salts also stresses most freshwater fish if they are exposed to saline conditions.
The concentration of oxygen in freshwater has serious implications for fish presence and distribution in a given area, and anoxia can result in the death of individuals. Oxygen enters water via diffusion from air at the water surface. Turbulence increases the surface area of water, such that moving waters contain more oxygen than stagnant waters. In addition, photosynthesis of plants, respiration of plants and animals, and the oxidation of organic materials drive diel changes in oxygen concentrations. Oxygen solubility in water is correlated negatively with water temperatures, and higher temperatures reduce dissolved oxygen levels. At the same time, fish metabolic rates and oxygen consumption levels increase with temperature, such that low oxygen conditions in warm water are particularly stressful for fish. To survive in low-oxygen waters, more than 40 genera of fish possess some capacity to breathe oxygen from the air; most of these species live in tropical freshwater habitats, where high temperatures
and high rates of decomposition reduce the dissolved oxygen in the water.
The relative acidity or alkalinity of a water body is measured as its pH. Hydrogen ions, which increase the acidity of water, are produced when carbonic acid dissociates from dissolved carbon dioxide in the water or from rainfall. The free hydrogen ions can be neutralized by carbonate minerals and buffered by calcareous compounds in geologic features surrounding bodies of freshwater. In poorly buffered systems, photosynthesis also can remove hydrogen ions and increase the pH of the water body. Metabolic functions of fish require pH within a certain range, and most fish cannot tolerate pH levels outside a range of approximately 4.0–10.0. High or low pH can be detrimental to reproductive success, gill function, and oxygen transport. Acidic pH appears to be most deleterious to fish. Acidic water dissolves metals, such as aluminum, that can be toxic to fish. In addition, the abundance and diversity of species, particularly of invertebrates that are eaten by freshwater fish, decline as water becomes acidic.
Major freshwater habitats
The variety of niches created by variation in physical and chemical factors within freshwater environments contributes to the great diversity of freshwater fishes. Many of these specific niches are organized within several major freshwater habitats. Most freshwater fishes inhabit streams, rivers, and lakes. Some fish prefer areas of swift-moving water in high mountain streams, others live at deep depths in lakes, and still others thrive in stagnant ponds.
Lakes are standing bodies of water surrounded by land, with small outflows relative to their internal volume. Lakes form in a variety of ways, including tectonic movements, volcanic activity, and glacial action. They also may originate as portions of rivers or bays that are cut off from the adjacent water body over time by deposition or sediment movement. Lakes receive inputs of water from the drainage basin, precipitation, and groundwater. These inputs are balanced by outflows of water to rivers, evaporation, and seepage into groundwater. The largest freshwater lake in the world is Lake Superior, which covers a surface area of 31,700 sq mi (82,103 km2). Lake Baikal holds the largest volume of freshwater—14,292 cu mi (23,000 km3). The 20 largest lakes contain over 67% of the total water in lakes worldwide, indicating that most lakes are small and shallow.
Ecological processes in many lakes are influenced greatly by a vertical temperature gradient that develops as sunlight warms the upper portion of the water column. Surface water warms when incoming radiation is absorbed and converted to heat. In temperate lakes the top portion of the water column heats during the summer, but temperature declines with depth. The warm surface water and cold bottom water are separated by the thermocline, a transition zone at the depth of greatest temperature change. Because water is most dense at 39.2°F (4°C) and becomes lighter by either cooling or heating, vertical temperature gradients and patterns of stratification vary with seasons and latitude. In cool portions of the temperate zone, differences in water temperature are minimal as the surface water of a lake warms to 39.2°F (4°C) in the spring; at this time, the lake mixes from surface to bottom. As warming continues, the lake stratifies in the summer, with a warm surface layer and cool deep waters. When the lake cools in the fall, stratification again breaks down. Reverse stratification occurs in the winter, however, as ice forms; colder, less dense water under the ice is suspended over warmer, more dense water around 39.2°F (4°C). The temperature gradient and stratification in lakes varies with latitude. In warmer temperate areas, the reverse stratification in winter does not occur, since ice rarely forms at these latitudes.
While it is common in temperate regions, this seasonal pattern of stratification driven by temperatures is not seen in all lakes. Some crater lakes may never stratify, because geothermal activity warms the deepest waters and minimizes temperature differences within the water column. Most tropical lakes stratify and mix on a daily basis. Annual variation in solar energy is minimal in the tropics, and daily changes in air temperature can establish and break down water column stratification. Wind is another factor that strongly affects stratification of many tropical lakes; wind adds kinetic energy to the lake and increases heat loss in surface waters. Some shallow tropical lakes, such as Lake Victoria, mix once a year when temperatures are lowest and winds are most persistent. Other deep tropical lakes may remain permanently stratified. For example, Lake Tanganyika reaches a depth of 4,823 ft (1,470 m), but the kinetic energy from wind cannot mix the waters below 820–984 ft (250–300 m). Polar or high-altitude lakes also may be stratified permanently if they remain frozen throughout the year.
Stratification of lakes caused by temperature gradients has two major effects on biological components of the lake ecosystem. First, stratification restricts mixing of nutrients within the lake to the area above the thermocline, unless wind or another turbulent force physically disturbs the lake waters. Thus, nutrients and other organic materials that enter the lake cannot be used to support production after they sink below the thermocline. Primary productivity of the lake is enhanced when vertical stratification breaks down. Spring blooms of plankton are common because sunlight for photosynthesis and nutrients from bottom depths are both available. In addition to affecting primary productivity, stratification can lead to oxygen depletion below the mixed zone, which affects the vertical distribution of many aquatic species. Oxygen is supplied in lakes by exchange with the atmosphere or from photosynthesis of green plants, both of which occur only in the upper portion of the water column. Organic matter eventually sinks to deeper waters below the photic and mixed zones, where it consumes oxygen through respiration and decomposition. The extent of oxygen depletion is greatest in highly productive lakes that are stratified for long periods of time.
Although abiotic factors establish the physical and chemical template of habitat conditions in lakes, biotic components and interactions also structure the ecology of lakes. Primary producers, such as phytoplankton, algae, and plants, form the basis of the food chain in lakes. As explained earlier, however, primary productivity follows seasonal patterns based on the availability of light and nutrients in the water column. Zooplankton graze on phytoplankton, but much biomass from primary producers eventually settles to the lake bottom, where it provides food for benthic detritivores, including insects, oligochaetes, and mollusks. Predation by fish has a strong effect on food webs in lakes. Although fish fill a wide variety of feeding niches, many species of fish, particularly as juveniles, consume large quantities of zooplankton. Pelagic fishes in lakes are typically strong swimmers that capture crustaceans and insects as the dominant component of their food supply, while others are predatory piscivores.
Rivers and streams
Rivers and streams flow downhill in defined channels from headwater streams to main river channels to estuaries. The Nile is the longest river in the world—4,180 mi (6,727 km). The Amazon drains the largest area (nearly 2.3 million sq mi, or 6.0 million km2) and carries the greatest flow, with a total discharge at its mouth of 6.4 million cu ft per second (180,000 m3 per second).
The nature of streams and rivers is determined largely by their setting in the watershed. The drainage basin is the total area drained by a river system, including all of its headwaters and tributaries, and the number of fish species typically increases directly with the area of the drainage basin. Streams within the drainage basin can be classified at tributary junctions to determine their stream order, a measure that serves as a useful indicator of stream size, discharge, and drainage area. As stream size increases, so does the order; thus, the smallest streams are termed "first order," and the confluence of two first-order streams is identified as a "second-order" stream. The process continues toward the main river channel until it has been assigned the appropriate order. Each increase in stream order represents three to four times fewer streams, each of which is roughly twice as long and drains approximately five times the area of a stream of the next smaller order.
Streams and rivers are considered physically open systems, meaning that physical factors, such as width, depth, velocity, and temperature, change continually along their course from source to mouth. The "river continuum concept" emphasizes the continuity of the structure and function of river communities from headwaters to lowland portions of river channels. This concept uses stream order as its basis and suggests that changes in physical conditions, functional feeding groups, and species diversity occur dynamically and continuously along the gradient from upstream to downstream portions of rivers.
The discharge of water increases from headwaters to the main stem of rivers and determines the size and habitat features of the channels. Low-order streams tend to flow alternately through riffles, pools, and runs. Riffles are shallow,
high-gradient stretches where fast-moving water flows over rocky substrates and creates turbulence. Pools are deep, low-gradient areas through which water moves slowly. In runs, water flows rapidly but smoothly. Slopes decrease in higher-order streams and rivers, such that they flow smoothly through their channels. River channels meander along a sinuous course. At each meander, the river deposits materials on the inward portion of the curve, where velocity is slowest, but high water velocities erode the outer portion of the curve. Velocity also affects the type of substrate and presence of vegetation in particular areas of flowing waters. For example, large particles, such as gravel, are transported only by fast-moving water. Finer particles, such as sand and silt, form the substrate in areas where current is slower.
Biotic patterns in streams and rivers are heavily influenced by the outcomes of physical processes. Many species of fish require specific substrates for spawning, while other substrate types and flow patterns support aquatic vegetation. Vegetation and woody debris are important to freshwater fish as well; both offer cover from predators, spawning areas, and food-rich foraging sites. In addition, as discharge varies with seasons and precipitation, rivers may overflow their channels and flood riparian wetlands or floodplains, temporarily expanding the vegetated habitats available to freshwater fish. In the tropics, where precipitation primarily occurs during one or two rainy seasons throughout the year, numerous fish communities are dependent on these seasonal floods to expand available habitats, increase feeding opportunities, and mobilize nutrients within the river.
In temperate regions, primary production is necessary as the basis of the food chain in streams and rivers, and algae attached to the sediment surface are the predominant primary producers. Some fish, such as loaches (Homalopteridae) and catfishes (Loricariidae), consume algae directly by scraping it off rocks in the stream. More commonly, aquatic insects and crustaceans are relied upon as intermediaries between the algae and fish—invertebrates graze on algae in small streams, and fish feed on the aquatic invertebrates. As the stream gradient decreases, mosses, rooted plants, and filamentous algae become important primary producers upon which invertebrates and fishes feed. Detritus also may constitute a major part of the food chain, particularly in streams or rivers with extensive cover of streamside vegetation. Scavenging invertebrates and fish feed on the organic detritus. In medium-size to large rivers, members of the fish fauna engage in diverse feeding strategies, including herbivory, invertebrate feeding, piscivory, omnivory, and detritivory.
While primary production from within the system forms the energy base in temperate streams, the large diversity of fish species and productivity exhibited in tropical streams is supported by organic matter from outside the system. Tropical streams and rivers generally flow through dense forested areas. Although large streams may be wide and open to the sun, small streams may be shaded completely by the forest canopy. Instead of relying on photosynthesis as an energy source, life in these streams depends on organic matter that enters in the form of leaves or detritus from the forest. The warm water of these tropical streams enhances colonization by bacteria and fungi, which break down the terrestrial organic matter. Some fish consume the detritus directly, but most species rely on decapod crustaceans and, to a lesser extent, insects as intermediate detritivores. In larger streams, fish are dependent on energy that enters during the rainy season. Rains produce an increase in suspended organic particles and terrestrial insects that are washed from the land into streams. The most important energy sources become available to fish inhabiting tropical rivers when the river floods adjacent areas of land. Fish then can exploit food resources in the form of decaying vegetation, seeds, fruits, and insects that are available on the floodplain.
Other freshwater habitats
Although streams, rivers, and lakes provide the most abundant and important habitats, fish inhabit other bodies of freshwater as well. As mentioned briefly earlier, such wetlands as river floodplain marshes, shoreline marshes along lakes, and deepwater swamps provide an expanded foraging and refuge area for fish when they become inundated with water. Wetlands are particularly important as nursery habitats for a variety of fish species. Fish also utilize extreme habitats, such as underground caves and desert streams. Unique adaptations enable fish to survive in these environments. Many fish in caves have reduced eyes, and some are blind; instead of relying on vision, enhanced chemosensory and tactile abilities allow them to locate food, mates, and living space. In ephemeral streams, fish survive periods without water by resting in mud or another substrate during the dry season, depositing eggs that do not hatch until water inundates the streambeds in the following year, and utilizing respiratory adaptations to breathe atmospheric oxygen.
Interconnections of freshwater habitats
While physical, chemical, and biological features of different freshwater habitats have been distinguished here, it is important to recognize that all aquatic habitats truly are interconnected. Rivers flow into and out of lakes, rivers and lakes may spill over into wetlands, and rivers and streams may even flow through underground caves in the midst of a surface route. These linkages form a continuum of habitats that often extends to estuarine or marine systems. In addition to these physical connections, biotic connections are important among freshwater habitats. Organisms may disperse between habitat types directly or via a vector. Furthermore, sustaining a food web in one habitat may be dependent on nutrient inputs from another portion of the aquatic system or from terrestrial uplands. Because of the high level of interconnections between aquatic habitats, an action that is detrimental to one component may prove harmful to a much larger system. Recognizing these interconnections is essential for understanding the ramifications of human activities on aquatic environments.
Allan, J. David. Stream Ecology: Structure and Function of Running Waters. New York: Chapman & Hall, 1995.
Cushing, Colbert E., and J. David Allen. Streams: Their Ecology and Life. San Diego: Academic Press, 2001.
Dobson, Mike, and Chris Frid. Ecology of Aquatic Systems. Essex, U.K.: Addison Wesley Longman, 1998.
Giller, Paul S., and Bjorn Malmqvist. The Biology of Streams and Rivers. Oxford: Oxford University Press, 1998.
Matthews, William J. Patterns in Freshwater Fish Ecology. New York: Chapman & Hall, 1998.
Payne, A. I. The Ecology of Tropical Lakes and Rivers. New York: John Wiley & Sons, 1986.
Bootsma, H. A., and R. E. Hecky. "Conservation of the African Great Lakes: A Limnological Perspective." Conservation Biology 7, no. 3 (1993): 644–656.
Katherine E. Mills, MS