Lecture 3: Estuaries Environment PDF

Lecture 3 The Estuaries environment The estuarine region is defined as the water area where river water and sea water meet and mix. There are different ways of dividing estuaries, and these are based on different principles and can be summarized in three: 1)...

Lecture 3 The Estuaries environment The estuarine region is defined as the water area where river water and sea water meet and mix. There are different ways of dividing estuaries, and these are based on different principles and can be summarized in three: 1) Geomorphology 2) Water column stratification 3) Energy system We will discuss the division of river estuaries based on geomorphology. Berciaro (1967) was able to divide the types of estuaries into four: 1. River basin valleys: During the last glacial period, the sea level was about 100 meters lower than the current level, and the great rivers overflowed into the surrounding valleys, which are flat coastal areas. These areas at present represent the continental shelf area. After the sea surface level rose; the valleys that surrounded the rivers were flooded and turned into the estuaries that bordered them. 2. Estuaries with natural barriers: In some areas and on ocean coasts, quantities of sand are transported by water currents to the ground surface submerged in water, and a barrier is then built to prevent water from returning to the ocean. Within this area, water is held, which is shallow and sometimes large. In many cases, the opening through which this type of estuary connects with the ocean is rather narrow, which obstructs the process of water exchange with the ocean. Accordingly, the tidal movement in this type of estuaries is limited. 3. Fjord estuaries: They are deep estuaries and U-shaped These estuaries are surrounded by high mountainous heights. The area where the mouth of the river connects with the sea is characterized by being shallow or deep. In the first case, the shallowness is due to sediments that wash away after the melting of snow. An example of these estuaries are those found on the coasts of Norway, Alaska, and the coasts of British Columbia. 4. Delta-type estuaries: These types of estuaries are found at the mouths of large rivers such as the Mississippi River and the Nile River, and are formed by large quantities of suspended sediments carried by the river, which are deposited at the confluence of the river with the sea. The reason for the deposition of materials is the weakness of the current in the confluence area and the large area of the confluence area. Some Small bays and salt marshes in the area of this type of estuary result from the deposition of suspended materials. Response of organisms to changes in environmental factors in estuaries: The organisms that live in the estuarine environment are mainly coming from the sea, and most of the species that live in shallow marine waters on the continental shelf, use this area to raise their young before migrating to the high seas, although the numbers of animal species inhabiting river estuaries are small, the types of animals that can withstand changing environmental conditions are very numerous. In any environment, animals have two options in the event of environmental pressures: 1. Either they migrate to areas more suitable for living. 2. Or stay in the difficult environment and adapt to it. However, if the conditions and environmental pressures become very difficult, the animal will undoubtedly perish, as well as the environmental pressures. There are not only natural pressures arising from the ocean, but there are pressures that arise from competition that occurs between individuals of the same species or between individuals of different species that live in the same place for food and space. This is because each type of living organism has a known and specific range of tolerance to the influencing biotic and abiotic factors. Therefore, this condition is known as the lethal period, where the animal cannot live. An animal's response to any set of environmental factors may reflect the extent of physiological change that has occurred, this is called adaptation to its body, and if environmental factors change again, the animal tries to change its physiological state according to the changing conditions, and the animal’s ability to this change depends on genetic factors. Adaptation to different environmental factors The organisms that inhabit river mouths adapt and acquire resistance to some environmental factors if they exceed the limits that the organism can tolerate. Among the most important factors that the organism adapts to in terms of tolerance are: salinity, temperature, amount of oxygen, and drought.  1) Salinity The critical salinity boundary that separates populations of animals living in freshwater from marine water animals ranges between 5 and 8 parts per thousand (ppt). These limits in the degree of salinity were described for the first time in 1934 and were derived from a group of physiological phenomena that include the ability to live, grow, move, and osmotic regulation. For example, the concentration of body fluid for the number of animals that live in salt water decreases to 5 ppt before any serious harm occurs to the organism, but in the event that the concentration falls below the mentioned percentage, then the cells of the animal's body begin to have an imbalance in their chemical system, which depends on the various salt ions, and then the albumin present in the various tissues begins to change significantly in its composition. Several principles can be adopted regarding the response of estuarine organisms to changes in salinity, which are: 1) Organisms that live in estuarine areas that are exposed to a significant change in salinity are tolerance to salinity changes more than fully marine organisms. 2. Organisms that live in the intertidal zone tolerate salinity more than those that live in the deep seas. 3. Settler (fixed) organisms can tolerate a wider range of salinity than mobile organisms that can move from one place to another when the change in salinity begins. The degree of salinity tolerance varies between the different larval stages and adult individuals of a single species. For example, adult individuals of some marine crabs tolerate long periods of exposure to a salinity of 5 ppt, while the different larval stages of the same species cannot be exposed to salinities of less than 20 ppt. The degree of tolerance of larvae to salinity is also affected by the salinity of the water in which they were hatched. This was demonstrated through experiments conducted on the larvae of American oysters found in oceans, seas, and salty water. It was found that the tolerance of larvae that hatched in water with low salinity was much higher than those hatched in water with low high salinity, it was found that the genetic factor and the cytoplasmic properties of the cell play an important role in the organism’s tolerance to salinity.