To an animal, the most fundamental difference between living on land and living in water is the scarcity of oxygen. Oxygen is so poorly soluble in water that it is hardly surprising that no warm-blooded animals can live submerged for more than short periods.
Aquatic mammals, such as whales, and aquatic birds, such as penguins, all breathe air and thus obtain enough oxygen to sustain the high metabolic rate associated with a constantly warm body.
And surely the complexity of gills is related to the need for extensive surfaces for gas exchange with water. Most often gills are located where water can be readily pumped past them to maintain a constant supply of oxygen. The deadly effects of aquatic pollution can be traced to exhaustion of the oxygen supply.
Aquatic ecosystems also unusual in that most producers are microscopic. Since so many people disregard anything they can’t see, this may be why the structure of underwater communities escapes the understanding of many citizens.
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Microscopic producers dominate in water, because they have a large surface area compared with their volume. This ensures efficient absorption of nutrients. Furthermore, vascular tissue is unnecessary in this habitat, and large plahts are subject to damage by waves or current.
Finally, the high specific heat of water gives aquatic environments temperature stability unknown on land. Water is slow to warm and slow to cool.
Large bodies of water, especially, fluctuate little in temperature. Not only are aquatic organisms freed from the usual problems of temperature regulation, but density changes associated with minor temperature shift account for Lake Turnover and also affect ocean currents.
Remembering these facts can help us maintain the integrity of aquatic ecosystems. This is an important step toward ensuring stability of the biosphere.