Aquatic ecosystem

• Marine ecosystem

      Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine waters cover two-thirds of the surface of the Earth. Such places are considered ecosystems because the plant life supports the animal life and vice-versa. See food chains.

             Marine ecosystems are very important for the overall health of both marine and terrestrial environments. According to the World Resource Center, coastal habitats alone account for approximately 1/3 of all marine biological productivity, and estuarine ecosystems (i.e., salt marshes, sea grasses, mangrove forests) are among the most productive regions on the planet. In addition, other marine ecosystems such as coral reefs, provide food and shelter to the highest levels of marine diversity in the world.

           Marine ecosystems cover approximately 71% of the Earth's surface and contain approximately 97% of the planet's water. They generate 32% of the world's net primary production. They are distinguished from freshwater ecosystems by the presence of dissolved compounds, especially salts, in the water. Approximately 85% of the dissolved materials in seawater are sodium and chlorine. Seawater has an average salinity of 35 parts per thousand (ppt) of water. Actual salinity varies among different marine ecosystems.
 
          Marine ecosystems can be divided into the following zones: oceanic (the open part of the ocean where animals such as whales, sharks, and tuna live); profundal (bottom or deep water); benthic (bottom substrates); intertidal (the area between high and low tides); estuaries; salt marshes; coral reefs; and hydrothermal vents (where chemosynthetic sulfur bacteria form the food base).
          
         Classes of organisms found in marine ecosystems include brown algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks. Fish caught in marine ecosystems are the biggest source of commercial foods obtained from wild populations.

         Environmental problems concerning marine ecosystems include unsustainable exploitation of marine resources (for example overfishing of certain species), marine pollution, climate change, and building on coastal areas.

•  Coral reef  ecosystem

       Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. These polyps secrete hard carbonate exoskeletons which support and protect their bodies. Reefs grow best in warm, shallow, clear, sunny and agitated waters.

       Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on Earth. They occupy less than one tenth of one percent of the world ocean surface, about half the area of France, yet they provide a home for twenty-five percent of all marine species, including fish, molluscs, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales in other areas.

         Coral reefs deliver ecosystem services to tourism, fisheries and shoreline protection. The annual global economic value of coral reefs has been estimated at $US375 billion. However, coral reefs are fragile ecosystems, partly because they are very sensitive to water temperature. They are under threat from climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, overuse of reef resources, and harmful land-use practices, including urban and agricultural runoff and water pollution, which can harm reefs by encouraging excess algae growth.

• Pond Ecosystem

 

        The organisms inhabiting a pond ecosystem include algae, fungi, microorganisms, plants and fishes. These organisms can be further classified as producers, consumers and decomposers, based on their feeding habit. The energy in a pond ecosystem flows from the producers to the consumers. Decomposers, on the other hand consume dead organisms by decomposing them. Let’s look into the habitats and food chain in a pond ecosystem.

 Habitats in a Pond Ecosystem - There are mainly four habitats in a pond ecosystem, namely shore, surface film, open water and bottom water habitats.

Shore Habitat: The organisms inhabiting this habitat vary depending upon whether the shore is rocky, sandy or muddy. In case of rocky shores, plants might not be able to grow, whereas in muddy or sandy or mixed type, plants like grasses, algae and rushes can be present along with organisms such as earthworms, protozoa, snails, insects, small fishes and microorganisms.

Surface Film Habitat: Surface film habitat, as the name suggests implies to the surface of the pond. In general, insects like water striders and marsh traders, organisms that are free-floating and those that can walk on the surface of water inhabit the surface habitat. They nourish on the floating plants, dead insects, and sometimes, feed upon each other.

Open Water Habitat: Open water habitat is inhabited by fishes and the plankton (tiny organisms). Both phytoplankton such as algae and zooplankton such as insect larvae, rotifers, tiny crustaceans and invertebrates are present in this habitat. Fishes feed on plankton.

Bottom Water Habitat: Depending upon whether the pond is shallow or deep water, the bottom habitat varies. For example, if a pond is shallow and has sandy bottom, organisms like earthworms, snails and insects inhabit the bottom, whereas if the pond is deep and has muddy bottom, microorganisms, flatworm, rat-tailed maggot and nymphs of dragonflies mostly inhabit the bottom habitat.

Food Chain in a Pond Ecosystem - Food chain in a pond ecosystem is divided into three basic trophic levels, namely the first, second and third trophic levels. The first trophic level is represented by the producers or the autotrophs; for example, phytoplankton and plants. They prepare their own food with the help of energy from sunlight through the process of photosynthesis. The second trophic level is characterized by the herbivores such as insects, crustaceans and invertebrates inhabiting the pond and which consume the plants. The third and the topmost trophic level comprises of the carnivores, especially the fishes, which can feed on both plants and the herbivores of the first and second trophic level respectively.

       
           In addition to the three trophic levels, there are saprotrophic organisms, commonly known as decomposers, which are located at the bottom of the food chain. Decomposers, mostly the bacteria and fungi are very important in the nutrient cycle as all the organic matter from the dead and decayed organisms is converted into carbon dioxide and nutrients such as nitrogen, phosphorus and magnesium. These nutrients are generated in such a way that they can be readily used by algae and plants for production of food to be consumed by the herbivores. Furthermore, the carnivores consume the producers and herbivores. Thus, the flow of energy is maintained in a pond ecosystem.

Our ecosystem