Marine Biology (2+1)

Temperature

Water temperatures in estuaries are more variable than in the nearby coastal waters. In part, this is because there is usually a smaller volume of water in an estuary and a larger surface area; therefore, it heats up and cools down more rapidly under prevailing atmospheric (climatic) conditions (fjords, being deep and with a large volume, do not show this variation). Another reason for the variation is freshwater input. Freshwater in rivers and streams is more subject to seasonal temperature change than seawater. Rivers in the temperate zones are colder in winter, and warmer in summer than adjacent seawater. When these freshwaters enter, they alter the temperature. As a result, estuarine waters are colder in winter and warmer in summer than surrounding coastal waters. The time scale is of interest in that with the changing tides, a given point in the estuary will show large temperature variation as a function of the difference between seawater and river water temperature.

Temperature also varies vertically. The surface waters have the greatest temperature range, and the deeper water the smallest. In salt wedge estuaries, this vertical temperature difference also reflects the fact that surface waters are dominated by freshwater,whereas the deeper waters may be completely or predominantly marine.