About two-thirds of the Earth’s surface is covered with water. Oceans are the largest ecosystems existing on Earth. Life exists in various forms in the oceans. Dead remains of plants and animals, food scraps and droppings falling into the oceans make them rich in nutrients.
Ocean bed is not plain. It has huge mountains, trenches, ridges and volcanoes. This makes the oceans comprise many different habitats. But, the oceans do not contain a large variety of species. Just 20 percent of the species existing on Earth live in the oceans.
The oceans teem with a variety of life forms.
Rivers and lakes
Rivers, lakes and waterfalls are freshwater ecosystems. They are ever-changing. They either change according to the seasons or by themselves as they go through various stages. Some of the natural processes, like erosion and weathering, determine the water level in these areas. Rivers give birth to lakes when they change their course.
Sometimes, when too much silt and soil deposit around water bodies, they often change into dry lands. Small rivers and ponds appear only in monsoons and get dried up in summers. Such rivers are called non-perennial rivers. On the other hand, big rivers which contain water
throughout the year are called perennial rivers.
Rivers in their course, give birth to waterfalls, streams, lakes and ponds.