For freshwater aquatic systems, the biome concept must be modified.
• Ecologists from the land and the sea have independently developed concepts and descriptive terms for ecological systems.
• The biome concept was created for terrestrial ecosystems in which the dominant vegetation's growth form reflects climatic conditions.
• The dominant physical factors in aquatic systems, however, are depth, water temperature, flow rate, oxygen and nutrient concentrations, and the structural attributes of aquatic organisms do not differ significantly in relation to these factors.
• As a result, aquatic "biomes" do not exist in the same sense that they do in terrestrial ecosystems. Because the producers in many aquatic systems are single-celled algae, which do not form “vegetation” with a distinct structure, defining aquatic biomes based on vegetation would be impossible.
• As a result, physical characteristics such as salinity, water movement, and depth have been used to classify aquatic systems.
• Streams and rivers, lakes, wetlands, estuaries, and oceans are the most common types of aquatic environments, and each of these can be further subdivided based on a variety of factors.
1. FLOWING WATER: STREAMS AND RIVERS
• Wherever rainfall exceeds evaporation and excess water drains from the land, streams form.
• Streams join together to form rivers as they grow in length.
• Stream and river systems are frequently referred to as lotic systems, a term that refers to flowing freshwaters in general.
• The river continuum concept is based on the continuous change in environments and ecosystems from small streams at a river system's headwaters to the river's mouth.
• As one moves downstream, the water flows more slowly, becomes warmer, and more nutrient-rich; ecosystems become more complex and productive.
• Within small streams, ecologists distinguish between riffles, where water flows quickly over a rocky substratum, and pools, which are deeper, slower-moving stretches of water.
• In riffles, the water is well oxygenated, whereas in pools, silt and organic matter tend to accumulate.
• Both areas are unproductive because the nutrients required for life are washed away in riffles, while oxygen and sunlight are in short supply in pools.
• Streams, in general, lack the diversity and richness of life found in other aquatic systems.
• Algae and other photosynthetic organisms have low productivity near the headwaters of rivers, where small streams are often shaded and nutrient-poor.
• Seasonal flooding and elevated water tables often affect the riparian zone of terrestrial vegetation that surrounds streams.
• Leaves and other organic matter that falls or washes into streams from the surrounding vegetation support much of the food web of headwater ecosystems.
• Allochthons refers to organic material that enters the aquatic system from the outside. The more organic material a river has, the more of it is homegrown, or autochthonous.
• Rivers become wider, slower moving, more nutrient-laden, and more exposed to direct sunlight as they progress down the river continuum.
• Algae and plants flourish in the river because of the nutrients and sunlight.
• Rivers, on the other hand, become increasingly clogged with sediments washed in from the land and carried downstream.
• High turbidity in the lower reaches of silt-laden rivers, caused by suspended sediments, can block light and reduce production.
• This downstream drift must be balanced by upstream animal movement, production in the upstream portions of the system, and input of allochthonous materials to keep a fluvial system in a steady state.
• The terrestrial biomes that surround aquatic ecosystems interact with them. Streams receive runoff, groundwater, and organic matter from the land around them, as we've seen.
• Both aquatic and terrestrial environments are home to a wide range of organisms. For example, many frogs and salamanders have aquatic larval and terrestrial adult stages.
• Some terrestrial animals eat organisms that grow in streams and lakes, effectively transporting nutrients from aquatic to terrestrial environments.
• In contrast, many aquatic larval organisms, such as mosquitoes, feed on terrestrial organisms. While the aquatic and terrestrial biomes have distinct borders, organisms freely cross them, and the borders themselves shift as rivers rise and fall, extending onto and retreating from floodplains.
• Any change in the flow of water in a locic system is extremely sensitive.
• In India alone, several dams of various sizes interrupt stream flow. These dams are built to control flooding, provide irrigation water, or generate electricity.
• Dams change the rate of flow, the temperature of the water, and the sedimentation patterns. Water behind dams usually warms up, and bottom habitats become clogged with silt, destroying fish and other aquatic organisms' habitat. Large hydroelectric dams frequently release water downstream with low dissolved oxygen concentrations.
• The use of dams for flood control alters the seasonal flood cycles that are necessary for the maintenance of many types of riparian habitats on floodplains.
• Dams also fragment river systems and isolate populations by disrupting the natural movement of aquatic organisms upstream and downstream. As a result, lotic systems are among the most vulnerable biomes to habitat change.
2. STANDING WATER: LAKES AND PONDS
• Non-flowing water distinguishes lentic systems, which include lakes and ponds.
• In any type of depression, lakes and ponds can form. They range in size from small, temporary rainwater pools a few centimeters deep to Russia's Lake Baikal, which has a maximum depth of 1,740 meters (about a mile) and holds about one-fifth of the world's fresh water.
• Glacier retreat leaves behind gouged-out basins and blocks of ice buried in glacial deposits, which eventually melt, forming many lakes and ponds.
• The Great Lakes of North America were formed in glacial basins and were covered in thick ice until 10,000 years ago.
• Lakes can also form in geologically active areas like Africa's Great Rift Valley, where vertical shifting of blocks of the earth's crust creates basins in which water collects.
• Oxbow lakes, which are broad bends of the former river cut off by shifts in the main channel, can be found in broad river valleys like those of the Mississippi and Amazon rivers.
• Although an entire lake can be considered a biome, it is usually divided into several ecological zones, each with its own set of physical characteristics.
• The littoral zone is the shallow zone around the edge of a lake or pond where rooted vegetation like water lilies and pickerelweed can be found.
• The limnetic (or pelagic) zone is the open water beyond the littoral zone, where the producers are floating single-celled algae, or phytoplankton.
• Light penetration and the formation of thermally stratified layers of water can also be used to divide lakes vertically (the epilimnion toward the surface and the hypolimnion at depth).
• The benthic zone is made up of sediments at the bottom of lakes and ponds that provide habitat for burrowing animals and microorganisms.
• Lakes and ponds do not last forever. Each year, small temporary ponds can dry up, sometimes multiple times during the season. Most small temperate lakes formed as glaciers receded will gradually fill in with sediment until no open water remains.
• The previously aquatic ecosystem will gradually transform into a terrestrial ecosystem, first as a wet meadow, then as the region's natural terrestrial biome.
• Wetlands, which are areas of land with saturated soil and vegetation specifically adapted to such conditions, are where aquatic and terrestrial communities frequently collide.
• Swamps, marshes, and bogs are examples of freshwater wetlands, while salt marshes and mangrove wetlands are examples of wetlands associated with marine environments.
• The Okavango Swamp of Botswana, the Everglades of southern Florida, and the Pantanal of Brazil, Bolivia, and Paraguay—at 195,000 km2, the world's largest wetland—are examples of wetlands that range in size from vernal pools formed after spring rains to vast areas of river deltas.
• Most plants that grow in wetlands can tolerate low levels of oxygen in the soil; in fact, many of them are specialized for anoxic conditions and grow nowhere else.
• Wetlands are also important habitat for a variety of animals, including waterfowl and the larval stages of many fish and invertebrates that live in open waters.
• Hurricanes and other storms wreak havoc on coastal areas, but wetlands protect them. Wetland sediments are natural water purifying plants because they immobilize potentially toxic or polluting substances dissolved in water.
• Wetlands, unfortunately, take up space as well, and have been cut, drained, and filled to obtain wood products, develop new agricultural lands, and accommodate ever-increasing urban and suburban sprawl.
• Estuaries form at river mouths, particularly where the outflow is partially encircled by landforms or barrier islands.
• Estuaries are unique in that they contain both fresh and salt water.
• Furthermore, they receive an abundance of nutrients and sediments carried downstream by rivers.
• In the shallow waters of the estuary, the rapid exchange of nutrients between the sediments and the surface supports extremely high biological productivity.
• Estuaries are often surrounded by extensive tidal marshes at temperate latitudes and mangrove wetlands in the tropics, as they are sediment deposition areas.
• Due to a combination of high nutrient levels and lack of water stress, tidal marshes are among the most productive habitats on the planet.
• They add organic matter to estuarine ecosystems, which support abundant populations of oysters, crabs, fish, and the animals that eat them.
MARINE AQUATIC SYSTEMS: CLASSIFIED BY WATER DEPTH
• The oceans cover the majority of the earth's surface.
• Underneath the ocean's surface lies a vastly complex realm containing a wide range of physical conditions and ecological systems.
• Temperature, salinity, depth (which influences light and pressure), currents, substrata, and, at the ocean's edge, tides all contribute to variation in marine environments.
• Many marine ecologists divide the ocean into zones based on depth. Between the highest and lowest tidal water levels, the littoral zone (also known as the intertidal zone) is exposed to air on a regular basis.
• The littoral zone's ecological conditions change rapidly as the tide flows in and out.
• The sharp zonation of organisms based on their ability to tolerate the stresses of terrestrial conditions, to which they are exposed to varying degrees depending on their position within the intertidal range, is a common result.
• The neritic zone extends beyond the range of the lowest tidal level to depths of about 200 m, which correspond to the continental shelf's edge. Because the sunlit surface layers of water are close enough to the nutrients in the sediments below that strong waves can move them to the surface, the neritic zone is generally a high-productivity zone.
• The seafloor drops rapidly beyond the neritic zone to the oceanic zone's great depths.
• Nutrients are scarce, and production is severely restricted.
• The benthic zone is the seafloor beneath the oceanic zone.
• Vertically, both the neritic and oceanic zones can be divided into a superficial photic zone with enough light for photosynthesis and an aphotic zone with no light.
• Organic material raining down from above is the main source of food for organisms in the aphotic zone. Kelp forests, for example, which grow in shallow, fertile waters along continental coasts, provide habitat for a diverse range of marine life.
• Large swaths of shallow polar seas are covered in pack ice, which seals the air–water interface and raises water salinity by excluding salts from the ice.
• Hydrothermal vents are deep-sea environments dominated by the input of hot water laden with hydrogen Sulphide, which provides the reducing power used by chemosynthetic bacteria to fuel high productivity in the otherwise sterile abyssal environment, resulting in a dim, salty environment with no wave disturbance.
• Each terrestrial and aquatic biome's physical characteristics define the environments to which its inhabitants have adapted in form and function.
• Ecological specialization and the resulting limits to the distributions of organisms and populations are based on the close association between organisms and their environments over evolutionary time.
• Adaptations, on the other hand, reflect not only these physical factors in the environment, but also the many interactions that organisms have with other organisms and individuals of their own species.
1. Coral reefs are similar to tropical rain forests in terms of biological production and diversity of their inhabitants, whereas the open ocean has been compared to a desert due to its low productivity.
2. Corals that build reefs can be found in shallow waters of warm oceans, where water temperatures are usually above 20°C all year.
3. Coral reefs frequently surround volcanic islands, where nutrients eroding from the rich volcanic soil and deep-water currents forced upward by the island's profile feed them.
4. Corals are doubly productive because photosynthetic algae in their tissues produce the carbohydrate energy that allows them to grow at such a rapid rate.
5. Furthermore, the complexity of the structure built by corals over time provides a diverse range of substrata and hiding places for algae and animals, making coral reefs one of the world's most diverse biomes.
6. Coral bleaching is a phenomenon that occurs when the algal symbionts of corals are killed by rising sea surface temperatures in the tropics.
7. These biomes' survival is now in jeopardy. Physical conditions in other marine biomes support unique forms of life and ecosystem characteristics.
HUMAN INPUTS INTO FRESH-WATER BIOMES
1. Human activities produce a variety of inputs that can dramatically alter the quality and ecological functioning of freshwater biomes of all types.
2. The intimate connections between terrestrial and aquatic biomes are further demonstrated by these inputs and their effects.
3. When various gases produced by the combustion of fossil fuels, particularly Sulphur dioxide and nitrogen oxides, combine with atmospheric moisture to form sulfuric and nitric acids, acid rain occurs.
4. This acidified precipitation enters lakes and streams, lowering the pH to as low as 4, well below many organisms' tolerance limits. Acidified waters lose plant life and algae, and the low pH disrupts fish and other aquatic animals' normal reproduction. The ecosystem as a whole could collapse in the worst-case scenario.
5. The addition of limiting nutrients to aquatic ecosystems, such as phosphorus, is known as eutrophication.
6. These nutrients could come from agricultural runoff carrying sewage, industrial wastes, fertilizers, or animal waste.
7. A sudden influx of nutrients could boost production dramatically, but it could also disrupt ecosystem function by favoring certain organisms over others.
8. The abundance of organic material encourages the growth of rapidly expanding populations of decomposing bacteria, but the process depletes the oxygen available to other organisms in the water.