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Investigation 3 – Concept Day








Watersheds: Investigation 3

Concept Day


In this Investigation, we will extend our discussion of the abiotic components of watersheds to include temperature and salinity. In introducing the relation of salinity to the oxygen capacity of water, we will briefly discuss estuaries, a common and important feature of many watersheds.

We will end with a very brief discussion of the salmon life cycle and consider the importance of dissolved oxygen concentrations in this process.



  • If oxygen gas was not able to dissolve in water, no animal life underwater would be possible. Aquatic organisms are not so different from land animals in their need for oxygen gas to survive. However, their method of extracting oxygen gas from their aquatic environment is necessarily different than that used by land animals when directly breathing air. One should note that, in evolutionary terms, the methods used by aquatic animals far precedes the methods animals use to breathe air on land.
  • It is important to note that, while you may be aware that fish have gills, you may not know that many other aquatic animals have gills as well. This slide depicts the gills of a mussel. All clams, oysters, and other bi-valves have very similar gills. In addition, crabs, lobsters, shrimp, and other crustaceans have gills as do many other animals.
  • Gills extract O2 from water and, equally important, expel CO2 from aquatic animal bodies. Carbon dioxide is a byproduct waste of aquatic animal metabolism just as it is for land animals.
  • In many respects, the gill is analogous to the alveoli of land animals’ lungs. In the lungs, oxygen-depleted air is pumped into the lung and enters the small, microscopic alveoli chambers. There, oxygen-depleted blood is brought into close contact with the air and O2 gas is absorbed while CO2 gas defuses into the alveoli to be exhaled. The “oxygenated” blood is then carried from the lung and throughout the body.
  • In gills, capillaries carrying oxygen-depleted blood absorb O2 and releases CO2 directly from and into the water that flows over the gill surface. The structure of gills assures that this surface area is as great as possible. The oxygen-rich blood then flows from the lung to provide body tissues with O2, essential for life.



  • This slide shows an idealized graph of the relationship between water temperature and the ability of water to hold dissolved oxygen.
  • As indicated, dissolved oxygen (DO) is often measured as mg/L (milligrams O2 per liter). This is the unit (mg/L) that you will measure using your oxygen meter in the lab.



  • The importance of water temperature and its content of dissolved oxygen is shown in this slide. These data show the amount of dissolved oxygen in a body of water, perhaps a lake or bay, during different months of the year. If temperature data were superimposed on this graph, we would find that the oxygen content of the water is “inversely proportional” to the temperature. That is, during the summer months, where dissolved oxygen is the lowest, water temperature is the highest.
  • Such a dissolved O2 profile is of use to fisheries biologists in that it helps explain why some fish are stressed in the summer months when dissolved O2 is low. Fish may migrate from areas of low dissolved O2 to maintain their metabolism at appropriate levels. Low dissolved oxygen levels in warm water summer months can be particularly acute for fish since their cold-blooded metabolism is typically much higher as their body temperature is increased by warm water. Therefore, at precisely the time when they need the most dissolved oxygen, the water contains the least.



  • This slide simply shows the major pieces of equipment that you will use in the Investigation 2 lab. As mentioned above, the oxygen meter on the right is calibrated in mg/L-dissolved oxygen (DO).




  • All water drained from the land surface that is not lost to evaporation eventually arrives at an ocean or sea. The places where this happens often create a very important environment know as an estuary.
  • In estuaries, freshwater flowing from a river comes into contact with saltwater from the ocean. Since the tide comes in and goes out twice a day, much of the time flowing river water confronts ocean water flowing directly against it. As might be expected with two such enormous forces acting against each other, the estuary is a place of dynamic changes, often depending not only on the time of day but on the season of the year as well.
  • An estuary is a segment of a river that experiences tidal effects. One of the effects of this tidal influence is that the river often significantly widens and slows down as it nears the ocean. The photograph in this slide is of the Columbia River estuary on the Oregon coast. On the Columbia, the tidal effect extends upstream some 60 river miles (97km). The estuary is many kilometers across at its widest. Slowed water-flow causes the formation of many small islands as materials carried by the river are deposited with reduced velocity and discharge rates. As we discussed in Investigation 2, this can result in the formation of a delta.
  • The mixing of fresh and saltwater, which modulates during the course of daily tidal cycles and with a monthly variation of high and low tides, results in a wide range of salinities at different spots in the estuary at different times. Such a mix of saltwater and freshwater is called “brackish” water, with salinities anywhere in between pure fresh and pure ocean water.
  • The multiplicity of salinity conditions and the tremendous amount of nutrients constantly delivered to the estuary makes it a place of abundant and diverse plant and animal life. Plants and animals that are normally strictly marine (saltwater) and those that are strictly freshwater may move in and out of estuaries with the tides. There is also a host of species found in no other ecosystem. Needless to say, estuaries are very important biological niches and must be protected.



  • This slide shows a graphic that simplifies the interaction of the point in an estuary where flowing freshwater meets ocean water. Ocean water, with its high salinity, is much denser than fresh river water. As a result, the less dense river water tends to float over the ocean water which dives down below it.
  • The photograph inserted into this graphic is once again taken at the Columbia River estuary. As seen, the river water (coming from the right) is much murkier with sediment than the bluer ocean water. Eventually, these two waters will mix, but the presence of such river “plumes” often can be seen to extend many kilometers into the ocean.



  • This slide shows the difference, in terms of dissolved oxygen, between freshwater and saltwater. You will perform an experiment in Investigation 3 lab in which you analyze both freshwater and saltwater dissolved oxygen concentration.
  • In our discussion of estuaries, the dissolved oxygen content of water of different salinities adds yet another level of complexity to the dynamic estuarine ecosystem system. 



  • This slide simply introduces us to a brief discussion of one of the very important commercial species of estuary ecosystems, salmon and trout.
  • While all fish, of course, require dissolved O2 to survive, salmon, particularly their eggs and very young forms, require a rather high O2 concentration.



  • This slide summarizes the life cycle of most salmon. To begin with, salmon eggs are laid in gravel-bottomed, freshwater streams leading to the ocean – often many, many kilometers from the ocean. Salmon chose a certain type of gravel that is large enough so the eggs can fall between the cobbles and pebbles and therefore be somewhat protected from predation.
  • It is imperative that adequate water circulation is provided for the salmon eggs and developing embryos. While adult salmon require dissolved O2 of about 8mg/L, salmon eggs require much closer to 11mg/L. For adequate water circulation to occur in the gravel in which the eggs are laid, there should not be much sediment present. This is why logging operations along streams in salmon country can severely damage salmon spawning beds. The deforestation caused by logging tends to result in the runoff of more dirt and clay into the watershed, which can smother otherwise good salmon spawning grounds.
  • Once salmon hatch they are first called fingerlings and then smolt. At the smolt stage, the salmon head downstream to the estuary and then into the open ocean where they feed and grow for a period anywhere from 1 to 4 years.
  • After growing to adult size and sexual maturity, salmon return to the same estuary they left and “run” upstream until they arrive at the exact same spawning beds in which they hatched, often to within meters! There they spawn and die almost immediately afterward.
  • How salmon know which of dozens of tributaries to turn into on the journey to their native spanning beds, and exactly where to stop, is not entirely clear but, of course, is a fascinating question.



  • This final slide shows a proud angler and an adult salmon. While the salmon sports fishery is large, the commercial salmon industry is also very important to local economies. Reports from Alaska alone indicate that over 700 million pounds of salmon were harvested commercially in 2014.