Properties of Water - Freshwater Ecology: Concepts and Environmental Applications - page 28

SUMMARY

1. The uniqueness of water is related in large part to the polarity of the
molecule and the associated hydrogen bonding.
2. Water is an excellent solvent. Ions generally become more soluble as
water temperature increases and gases become less soluble.
3. Water is most dense at 3.98°C. Ice is significantly less dense than
liquid water. The variation of density with temperature is
characterized by nonlinear relationships.
4. Density decreases above 3.98°C, with a greater density decrease per
degree temperature rise as temperature increases.
5. Dissolved salts can increase density. A part per hundred difference in
salinity causes a greater density difference than a 50°C temperature
difference.
6. Hydrogen bonding of water leads to additional properties, including
high heat of fusion, heat of vaporization, heat capacity, and surface
tension.
7. Reynolds number (Re) is the ratio of inertia to viscous force and
describes how the properties of water vary with spatial scale and
water movement. Small organisms have low values of Re and little
inertia relative to the viscous forces they experience; the opposite is
true for larger organisms.
8. Biological activities, such as swimming, filter and suspension feeding,
and sinking and many other aspects of aquatic ecology are
constrained by properties of water that can be described by Reynolds
numbers.
9. Water movement can be molecular, laminar, or turbulent. Molecular
movement is also referred to as Brownian motion. Laminar flow is
caused by physical processes that predominate at smaller Reynolds
numbers and is more common close to solid surfaces or in the pores
of sediments. Turbulent flow commonly occurs in open water at
Reynolds numbers greater than about 5.
10. Evaporation and cooling of water vapor (the hydrological cycle
and gravity), wind, density differences, Coriolis effects, and
activities of organisms (particularly animals) are all factors that can
move water.

QUESTIONS FOR THOUGHT

1. Why is less of a temperature difference required for stratification
(stable layers of different density) to occur in tropical lakes than in
temperate lakes?
2. Why do bacteria generally sink more slowly than dead fish, even
when they both have approximately the same density?
3. If you wanted to simulate a 1- m-diameter spherical bacterium

m/s with a 1-cm-diameter sphere, how fast would
you have to move the sphere to achieve the Reynolds number that the
bacterium experiences?
4. Flow is slower in a small pipe than in a large pipe, given the same

swimming at 10