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What happens when you mix salt water from Puget Sound and fresh water from streams and rivers? You get an estuary – a fascinating and important transition zone between land and sea.
A special thanks to ohjoy! and her research that resulted in this eathcache.
Estuaries are transition zones between land and sea. They are found in sheltered bays, inlets, and lagoons where freshwater rivers and streams meet and mix with the salt water, forming a melting pot of organic and mineral nutrients. The nutrient-rich soup of the estuary nourishes plankton and plants, which in turn, nourish oysters, clams, crabs, salmon, and birds.
Freshwater runoff from surrounding hills and creeks makes its way to Mud Bay and dilutes its salinity, creating an estuary containing a nutrient soup ideal for growing plankton, the base of the food chain. Estuaries also provide transitional places for young fish such as salmon to adjust to ocean’s salinity.
Twice daily, tidal rhythms pull Puget Sound waters in and out of Mud Bay estuary. The word “estuary” comes from the Latin word “aestus,” meaning “tides.” The range of tides in this area can be more than 15’ depending on the season and phase of the moon.
An amazing diversity of shellfish and fish such as sole and flounder thrives in these rich mudflats, contributing to a complex food web. Microscopic larvae from crabs, clams and other shelled animals swim with other zooplankton and feed on tiny plants called phytoplankton. These plankton nourish mussels, ghost shrimp, marine worms and other animals that feed by filtering estuarine waters. Larger animals such as moon snails and crabs scavenge the mudflats for these smaller animals. These, in turn, serve as an important food source for birds and other predators such as us.
Estuaries are defined as semi-enclosed coastal bodies of water that have a free connection with the sea and within which seawater is measurably diluted by fresh water. The fresh-water sources for most estuaries are streams, rivers, and even groundwater for some areas.
Water circulation characteristics may also be used to classify different types of estuaries. The movement of water in estuaries is regulated by the ebb and flow of tides; differences in the density of water; and wind. Because most estuaries are influenced by lunar tides, the once-daily (diurnal) or twice-daily (semidiurnal) rise and fall of water results in a net flow out of the estuary. In the strictest sense, estuarine circulation usually refers to the residual water movement after the short-term tidal effects are removed. Thus, circulation is the time-averaged current in an estuary and is sometimes described as net current, nontidal flow, or tidal residual.
The density of water also plays a major role in the movement of water in estuaries. Density, which is the weight per unit volume of water, increases with increasing salinity and decreasing temperature. In an estuary, the lighter fresh water mixes with the heavier salt water from coastal waters and creates a gradient in water density in the estuary. As the fresh water gains salt, becomes heavier, and sinks, the resulting movement of water is known as gravitational circulation, and is caused by density and elevation differences between the fresh-water runoff and saltier coastal waters. In some estuaries, large differences in water temperatures can also drive gravitational circulation.
Rivers and streams bring fresh water into estuaries. The fresh-water travel time slows as it mixes with estuarine water. Thus, the water spends some time in the estuary before it gradually exits into coastal waters. The hydraulic residence time of an estuary is the time required to replace the equivalent amount of fresh water in the estuary by fresh-water inputs. In short, it is the time that a molecule of water spends in the estuary. Estimates of residence time are useful for calculating the movements and concentrations of dissolved substances, such as nutrients or pollutants, in the estuary. Of course, the residence times within a particular estuary vary depending on many factors, including fresh-water input, circulation, and bathymetry (depth characteristics).
Each estuary is unique with respect to physical, chemical, and biological characteristics, but estuaries share many common features. For example, rivers provide a continuous input of sediment into the estuary. Turbidity is a measure of the amount of suspended particles, which includes fine sediments, in the water. Circulation within the estuary redistributes the suspended sediment and a turbidity maximum usually is located in the region of the estuary where fresh water from the rivers contacts the more saline coastal waters.
Chemical interactions between the sediment particles and dissolved ions in seawater result in an attraction between particles that causes the particles to stick together (flocculate). The heavier and larger particles settle out of the water column and are deposited on the bottom. Over time, these deposits may fill navigation channels.
In summary, estuarine habitats benefit us because:
- they create a rich nursery environment for salmon and other species. Tens of thousands of birds, mammals, fish, and other wildlife rely on estuarine habitats to live, fee, and reproduce.
- acre for acre, they are among the most biologically productive ecosystems on earth.
- they serve as buffers, protecting shorelines from erosion and flooding.
- they filter pollutants, improving water quality.
To log your find for this earthcache, please do the following:
1. Send your answers to the following questions to me:
a) what was the water level at the time of your visit to this spot?
b) with the water at that level, what evidences do you see of estuarine habitats?
c) what evidences of wildlife did you observe?
d) what do the words on the monument mean to you?
2. Post your log.
3. Download a photo of yourself at the monument.
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