Properties of Water - Freshwater Ecology: Concepts and Environmental Applications - page 17

a lake in the summer, and much cold weather is required to freeze the sur-
face of a lake. High heat capacity buffers water against rapid changes in
temperature. Thus, aquatic organisms generally do not experience the
rapid temperature swings experienced by terrestrial organisms.
A high heat of vaporization means that a considerable amount of en-
ergy is needed to evaporate water. We take advantage of the heat of va-
porization by perspiring; the evaporation of the moisture cools the skin
(takes away energy). Lakes and streams are also cooled by evaporation.
Another aspect of water that is important is surface tension . The high
surface tension of water results from hydrogen bonding, which pulls wa-
ter into a tight surface at a gas-water interface. Several organisms, such as
water striders, take advantage of this surface tension to walk on the sur-
face of water. Some lizards ( Basiliscus and Hydrosaurus ) also run across
the surface of water using the support of surface tension (Vogel, 1994). The
influence of surface tension also comes into play when water droplets form
spheres. Finally, surface tension leads to capillary action, the ability of wa-
ter to move up narrow tubes. Capillary action is important in forming the
capillary fringe (the moist zone in sediments immediately above ground-
water) because water creeps up the narrow spaces between sediment par-
ticles. Wetland plants with leaves above the water surface also use capil-
lary action to move moisture up their stems to their leaves.
RELATIONSHIPS AMONG WATER VISCOSITY, INERTIA, AND
PHYSICAL PARAMETERS
Viscosity is the resistance to change in form, or a sort of internal fric-
tion. Inertia is the resistance of a body to a change in its state of motion.
Water viscosity increases with smaller spatial scale, greater water move-
ment, and lower temperature. Inertia increases with size, density, and ve-
locity. These facts are underappreciated but very biologically and physi-
cally relevant to aquatic ecology. Consequences of these physical properties
include, but are not limited to (i) why fish are streamlined, but microscopic
swimming organisms are not; (ii) why the size of suspended particles cap-
tured by filter feeding has a lower limit; and (iii) why organisms in flow-
ing water can find refuge near solid surfaces. Aspects of these features of
life in aquatic environments can be discussed conveniently using the
Reynolds number (Re). This number can quantify spatial- and velocity-
related effects on viscosity and inertia. The effects of viscosity and inertia
and other properties of water on organisms have been described eloquently
and in greater detail (Purcell, 1977; Denny, 1993; Vogel, 1994), but I at-
tempt to describe water's physical effects briefly, using the Reynolds num-
ber as the basis of the discussion.
Relative viscosity increases and inertia decreases as the spatial scale be-
comes smaller. Viscosity increases because the attractive forces between in-
dividual water molecules become more important relative to the organism.
Thus, the influence of individual water molecules is greatest when organ-
isms are small or the space through which water is moving is small. I dis-
cuss the individual components of Re, inertial force and viscous force, and