Hobbies And Interests
Salinity
Salinity varies vertically and horizontally in the water of estuaries for several reasons. First of all, freshwater from rivers and streams flows only one-way into estuaries, while salt water flows in and out with the tide. Secondly, salt water is denser, thus heavier, than fresh water so it sinks, making the water at the bottom of the estuary generally saltier than at the surface. Finally, water at the mouth of a river, the point where it mixes with the sea, tends to have higher salinity than at any point upstream, especially at high tide. So it follows, that salinity at the mouth then decreases at low tide, when the salt water of the sea recedes. During normal conditions, freshwater flows downstream at the surface and sea water flows upstream under the fresh water.
Changes in Salinity
The salinity of seawater is about 35 percent, while fresh water ranges from 0.065 to 0.30 percent. The concentration of dissolved salts in stream water increases incrementally as it mixes with sea water. However, in dry climates and during summer in many climates, evaporation of water from the estuary is sometimes greater than the flow of fresh water into the estuary, causing an increase in salinity. On the other hand, heavy rainfall during a hurricane and snow melt in spring cause a decrease in salinity. Finally, salt water contains less dissolved oxygen than freshwater because oxygen becomes less soluble in water as salinity increases. Since aquatic organisms depend on dissolved oxygen in water for respiration, this becomes an important fact.
Temperature
Water temperature in estuaries fluctuates on both a daily and seasonal basis. The sun heats the water and high tide can cause an increase or decrease in estuarine temperature depending on the season. The upper layer of water is cooler in winter and warmer in summer compared with the dense bottom layer of water. When water temperature increases, less oxygen dissolves in the water compared with cooler temperatures and the amount of oxygen in the water directly affects the condition of an estuarine ecosystem. Most organisms have a temperature range they tolerate best and deviation from this range stresses the population. Seasonal changes in temperature and oxygen level impact the habitat of estuarine species.
Nutrient Traps
Floods and strong winds cause sea and fresh water to mix in the estuary ecosystem. Depending on the season, incoming tides can cool or warm the water in an estuary. Mixing waters of different salinity or temperatures creates something scientists call a nutrient trap. Tides bring nutrients and oxygen into estuaries. Fresh water that flows into the estuary reduces the nutrient level of the estuary, with the exception of phosphorus. Circulation of heavier salt water and lighter freshwater creates a counter current that traps nutrients in the estuary instead of sweeping them out with the tide. Plants in marshes and mudflats then pump nutrients from the sediment on the bottom of the estuary up to the surface of the water. Phytoplankton, small microscopic plants, then move upstream in the estuary and reproduce in the nutrient-rich water.
Organisms
Estuarine organisms must be able to tolerate some level of salinity as well as changes in salinity and temperature. During the longer period of daylight in summer, water temperatures warm and phytoplankton migrate to the surface. Some species, called sessile organisms, remain in one place, typically at the bottom of the estuary. These species thrive only within a limited range of salinity. Other bottom-dwellers such as oysters and crabs can withstand some fluctuations in salinity, although their populations decrease during periods of decreased salinity. An increase or decrease in salinity causes a decline in population due to the negative impact on reproduction and growth. Estuarine fish generally spawn in the lower-salinity water upstream and the larvae move downstream as they mature. Increased salinity upstream threatens the ability of young fish to survive.