About 70% of our planet is covered with water, in which less than 3% of all the water is freshwater and some are frozen in the polar ice caps. Although marine and fresh water ecosystem are very different in structure, all animals and plants in water must live either in or on water. All these living organism interact with each other at this liquid medium and these opportunities and physical constrains for live existing in water gives the opportunity of studying the aquatic ecosystem.
The Impact of Water
Water has a greater viscosity and density then air, it is able to absorb a huge amount of solar radiation with a little increase in temperature. Water gets cooled at a slower rate once it has been heated, even at a slower rate compared to land. Water organism are somehow shielded against rapid and massive changes in temperature. However, aquatic organisms may have to get used to the increase in the water temperature from the region of hot freshwater springs, to the freezing winters in temperate part of the world. Hot water is not as dense as cold water, and this can results in turnover and massive mixing in lakes. In streams and rivers, the depth of water and its flow rates will determine the biological structure of the communities. Before plants can survive in the fast flowing water, plant have to be specialized in structure and be attached firmly. In large water like oceans and lake, winds and tides can carry sediments and organisms over a very long distance.
Variety of Habitats
In freshwater and marine ecological unit, algae are the major photosynthetic plants. Most of them are microscopic. Algal blooms which are the large colonies of algae cells are usually seen and colonies of filamentous forms are observed easily in slow moving water and patches on rocks.
All plants that lives in water are primary producers and they have ability to fix carbon dioxide (CO2) into carbohydrates which makes them very important in aquatic food chain. These marine and fresh water organisms such as fish and other aquatic organisms including fungi, bacteria, mammals and birds are all related to terrestrial forms.
Freshwater ecosystem are very diverse and they includes ponds, lakes, rivers and streams having a wide range of flow rates, depth and water chemistry. Wetlands are also included in aquatic ecosystem and the depth and the flow rate of this water may vary with season. Wetlands are termed swamps, marshes, fens and bogs depending on the position of the water table, plant growing in them and water chemistry.