ABSTRACT
Water covers about 75% of Earth’s surface and occurs as part of a hydrologic cycle. Due to the vast surface area, evaporation from the oceans exceeds that from continents. In addition, the higher elevation of the land results in a greater precipitation rate as water vapor rises and cools (Figure 3.1). Thus, precipitation exceeds evaporation for terrestrial ecosystems, resulting in a surplus of freshwater on the continents. Also, the present con‘guration of the continents is widespread relative to the supercontinent Pangaea, and this has resulted in a more uniform distribution of fresh surface water across the planet. Small rivulets form from rain, contribute to streams, large rivers, and lakes, and ‘nally input into estuaries and the oceans. The greatest division in these aquatic environments is salinity. Falling on the continents from evaporative processes, the water is almost pure. In the oceans, however, the water is very salty, which prohibits its use for drinking by most terrestrial organisms, including humans (although some marine birds and other organisms have evolved in ways that allow them to use seawater directly). Accordingly, one of the major differences in aquatic environments, which can limit the distribution and abundance of ‘shes and divides the planet into two large aquatic environments, is the salinity of water. There are freshwater ecosystems and saltwater (marine) ecosystems. Most of this book is concerned with the abiotic and biotic attributes of these ecosystems and the adaptations of the ‘shes that live in them.
