THE PHYSICAL FEATURES AND BIOTA OF AQUATIC ECOSYSTEMS, AND THEIR THREATS
Introduction
The term ecosystem refers to a fundamental unit comprising of both living and non-living things that interact with each other, resulting in a stable system (lesson 1, 1).The term aquatic ecosystem refers to a natural unit consisting of biotic and abiotic things and whereby water is the main habitat. Aquatic ecosystems are classified into three types, namely, marine ecosystems, freshwater ecosystems, and the brackish water ecosystem. Freshwater ecosystems are classified into two, the lentil ecosystems that consist of static water and the lotic ecosystems that consist of flowing water. Thus, freshwater ecosystems include physical features such as ponds, rivers, lakes, swamps, springs, and streams. Marine ecosystems comprise physical features such as shallow seas and open oceans. Lastly, brackish aquatic ecosystems include physical features such as estuaries, salty marshes, mangrove forests, and mangrove swamps (Unit-8, 71).
Physical Characteristics of Fresh Water Ecosystems
Characteristics of Lentic ecosystems
Lentic ecosystems include ponds, swamps, lakes, pools marshes, and etcetera. Lakes are inland depressions containing static water, and they vary significantly in-depth and area. The penetration of light in lakes is often up to a certain depth, and this is affected by turbidity. Dissolved oxygen and temperature also varies with depth. Stagnant water has less dissolved oxygen hen compared to moving water since simply a moderately small water surface is in direct contact with air. The decay of organic matter occurs on the floor of the lake. There is a significant decrease in oxygen content with the depth of the lake. The temperature, oxygen, and sunlight degrees cause vertical zonation in lakes. These factors are indirectly responsible for lateral zonation because they affect the distribution of biota that are responsible for the horizontal zonation (life forms that exist in lakes). Shallow lakes don’t show thermal stratification since their water is well mixed, leading to uniform temperatures throughout, although those lakes that are more than 15m deep show noticeable stratification (Unit-8, 75).
Biota of Lakes
Sunlight penetration in lakes affects horizontal gradation, which refers to the distribution of living organisms in the waters. Different zones arise namely;
Littoral zone defined as the shallow water zone found near the lakeshore. In this zone, light penetrates to the floor of the lake, and rooted plants can grow only in this zone. The algae species remain attached to the plant surfaces, and together they are called periphyton. Rooted plants are classified into three types according to the zone of occurrence. First is the zone of emergent plants, which consists of plants whose stems and roots are submerged in water, but their upper stems and leaves protrude above the water surface. Examples of these plants include the arrowroots, cat’s-tail, among others. The zone plants with floating leaves follow this zone, and they have a wide photosynthesis area like the water lilies and the water shield plants. The third zone is the zone of submerged plants characterized by highly divided leaves that have overcome tearing by strong water currents to enable maximum absorption of nutrients since these plants have poorly developed roots such as the pondweed. The Littoral zone has animals such as herbivores, carnivores, and detritus (Unit-8, 78).
Physical Characteristics of Lotic Ecosystems
The upper course of a river is characterized by fast-flowing water, the presence of ‘V’ shaped valleys with unstable banks, more erosive energy that can move large stones, and knock them against the river beds and each other. The mature stage of a river (middle stage) is characterized by reduced river velocity along the foothill belt. However, the water still has enough velocity to transport sand, silt, and mud in suspension. At this stage, the river valley becomes more pronounced, forming ‘U’ shaped valleys. The old stage (lower river course) occurs in plains where the river meanders due to reduced water velocity forming ox-bow lakes and deltas. The deposition is evident at this stage (Unit-8, 84).
Biota ofLotic Ecosystems
Rapidly moving water hosts animals like riffle beetles, larvae, freshwater sponges, water limpet, and caddisflies. The sheltered microhabitat formed between rock fragments harbors the stoneflies and the dragons since they are adapted to this environment because they are flattened and have behavioral adaptation such as clinging to hard surfaces by instincts as well as orienting themselves to water currents. Annelids, flatworms, snails, cold water fishes, nekton, and other insect larvae are found beneath rocks where the water current is weak. Among the plant species found in this habitat are sessile algae. Rapidly moving water causes alt of erosion, and animals such as the zooplanktons are found here. Examples of these animals are crustacean and protozoa like water flies and copepods, water striders, backswimmers, diving beetles, and the water boatman (Unit-8, 86).
Salient Features and Biota of Marine Ecosystems
The seawater is salty with sodium chloride as the main salt. Other salts present arepotassium, calcium, and magnesium salts. Seawater has various kinds of wavesand tides that are produced by the pullof the sun and the moon. Sea currents are in continuous circulation through wind-driven surface currents and deep-sea currents leading to variations in sea temperature and salinity. Sea depth varies depending on the region whereit is found. Sealife extends to all depths though it is more on the continental shelf and islands. Seas are continuous bodies of water, but variations in temperature and salinity affect the free movement of marine organisms. Changes in water pressure vary with depth, and sinceorganismsare limited to the surface water where pressure is less and others are adapted to deepseawater (Unit-8, 88).
Threats to Freshwater Ecosystems
The freshwater ecosystems include the groundwater and surface water, the Lentic systems, which include the Ponds, the lakes, and the man-made water bodies. We also have the lotic systems, which include the rivers and the streams, and all these face various threats. For instance, the natural bonds are facing the threat of drainage, natural succession due to neglect and agricultural intensification. The ponds are also threatened by invasive species like the weeds, pollution by humans, recreational activities as well as climate change.
The freshwater lakes are threatened by urbanization, population growth, and industrialization. They are also threatened by acidification, Siltation, water abstraction, overexploitation, invasive species as well as climate change. The rivers are threatened by flow alterations and the construction of dams. Pollution is also a major threat to rivers as well as invasive species, which can deliberately be introduced or accidentally. Climate change affects rives by changing the flow patterns, river warming, and acidification. The coastal Estuaries are threatened by habitat destruction through human activities, overfishing, pollution from human activities like agriculture as well a climate change, which leads to alterations in temperatures. The same threats as the estuaries affect the rocky intertidal shore(lecture notes).
Threats to Marine Ecosystems
TheOpen Oceans are threatened by destruction of habitat is through overfishing. The wanton destruction of the fish ecosystem is worse in developing countries than in the developed ones. The open ocean has also been invaded by invasive species whose effect is the destruction of the native fish species. Pollution also has become a significant challenge to the ocean ecosystem. The pollution of the open sea is through persistent organic pollutants and also the inorganic ones like plastic. The organic pollutants have minimal effect on the sea, but the inorganic one like plastic causes high mortality of sea animals. Climate change also poses a threat to the aquatic ecosystem. For instance, climate change has led to acidification, and this has led to the dissolution of calcium carbonate, which adversely affects sea animals with shells. The Jellyfish is not affected by acidification, which implies that the population of the Jellyfish may increase exponentially while other species are declining, which may cause an imbalance in the open sea ecosystem (lecture notes).
The deep-sea bottomis threatened by pollution and climate change. Due to pollution, when deep-sea animals are studied, POPs and PCBs levels have been discovered to be high in these animals that reside on the ocean floor. The POPs and PCBs levels in deep-sea animals are high than in the coastal organisms. A colossal amount of plastics can and even micro-plastics can be found on the floor of the ocean, which may affect the deep-sea animals. Climate also leads to acidification, as we discussed earlier, and may have a devastating effect on the deep-sea organisms. Climate change has been intimated be the reason the deep sea may be warming up, which may affect this ecosystem(lecture notes).
References
TYPES OF ECOSYSTEMS: AQUATIC ECOSYSTEMS. Available at: http://www.egyankosh.ac.in/bitstream/123456789/16255/1/Unit-8.pdf. Retrieved on 29 April 2020.
Aquatic Ecosystems. Available at:https://www.marine.usf.edu/pjocean/packets/f00/nwq1.pdf. Retrieved on 29 April 2020.