Download C.S. Reynolds Physical Properties of Water Relevant to Limnology

5 Physical Properties of Water Relevant to
Limnology and Limnetic Ecology
COLIN S. REYNOLDS
5.1
INTRODUCTION
Despite being relatively abundant and ubiquitous
on the planet, and as familiar as it is important to
the support of living organisms, there are several
quite anomalous physical properties of water that
turn out to be crucial to its suitability as a habitat
for aquatic organisms. It is, perhaps, important
that these are emphasised. The purpose of this
short chapter is, therefore, to highlight those special characteristics of water relevant to the subject
matter of the book and to furnish some generalised
explanations to account for them.
The fact that water is liquid at all, at least over a
wide range of normal temperatures, is, for example, crucial to the persistence of a steadily fluid environment. The fact that it is also resistant to rapid
temperature change is a function of the high specific heat of water. When it does cool sufficiently to
solidify, water freezes first at its surface: this has
the curious and unique effect of preserving liquid
water beneath a veneer of ice, rather than simply
solidifying right through, which is how most other
substances behave below their melting points.
Surface ice insulates the deeper water from further
heat loss to the atmosphere, preserving a relatively
equable liquid environment for aquatic organisms
in regional climates otherwise at times too hostile
to support much terrestrial life.
The density of water is unexpectedly high for
what is, ostensibly, a low-molecular-weight substance. Liquid water is approximately 800 times
more dense than air. Since the cells of most ani-
mals and plants also contain a great deal of water, it
follows that they are much more nearly isopycnic
with the aquatic medium. This Archimedean effect means that the necessity of mechanically robust supporting structures is much less pressing.
Aquatic organisms whose evolution has been confined to water (from amoebae to blanket weeds)
manage with only enough stiffening to maintain
their integrity in turbulence fields.
For a liquid of low molecular weight (it is
manifestly more resistant to flow, for example,
than petroleum [or gasoline]), water is also
relatively viscous, and at about 1 ¥ 10-3 N s m-2,
is roughly 50 times more viscous than air. Ignoring gravity, an object requires proportionately
more effort to progress through water than air.
At its edges, where mutual attraction between
water molecules acts unilaterally, a powerful
surface tension normally exists. However, the
air–water interface is not merely an exploitable
habitat for neustic plants and animals. For many
small invertebrates it is a formidable, sometimes
fatal, trap.
These properties arise through curiosities in
the molecular structure of water. In addition, its
suitability as a medium relates to its specific heat,
its transparency and its solvent properties. These
topics are addressed below. It is not intended
that the article should provide a comprehensive
overview of the physical properties. The intention
is simply to give a background to the anomalies
and to state how important they are to the biology
of lakes. More detailed expositions are given in
108
c. s. reynolds
Hutchinson (1957) and in Lampert & Sommer
(1993).
5.2
MOLECULAR STRUCTURE
Many of the physically anomalous properties of
water arise as a consequence of the structure and
behaviour of water molecules. The entity comprising two atoms of hydrogen and one atom of oxygen,
with a molecular weight of c.18 daltons, exists in
the gaseous phase (water vapour). The covalent
O–H bonds are formed by the sharing of the single
electron of either hydrogen with the six in the
outer ring of the oxygen atom. The distance from
the centre of the hydrogen nucleus to the centre of
the oxygen nucleus is 96 pm. The angle formed between the two bonds is approximately 105° and
not the 90° predicted from theory. The reason for
this is the mutual electrostatic repulsion of their
charges. In turn, this leaves the molecule itself
with a polarity, on one side (the hydrogen side) a
weak positive charge, and on the other (the oxygen
side) a weak negative charge. (It is this polarity that
is exploited in a microwave oven: an electric field
of alternating polarity causes the water molecules
to vibrate sufficiently to generate significant fluxes of heat.) The polarity gives to the water molecules a certain attraction one for another: when
molecules occur in liquid phase, polarity permits
them to polymerise into much larger aggregates,
held together by hydrogen bonds, with the general
formula (H2O)n. It is this phenomenon which underpins many of the anomalous physical properties of water.
The incidence of larger molecules raises the
freezing and the boiling points. Attraction among
molecules increases the viscosity of the liquid
phase, and their aggregation raises the density of
the liquid. An important feature of the clusters of
molecules is that they are dynamic, frequently reforming and breaking apart. The number of molecules in clusters broadly decreases with increasing
temperature, but the relationship is probabilistic
and, thus, empirically predictable: both the density and the viscosity of pure water (Fig. 5.1) are
robust functions of its temperature.
Raising the temperature of a liquid increases
the motion of the molecules: accordingly the
spaces separating them become, on average, larger,
and so the density of the liquid decreases. The behaviour of water is, however, anomalous in this respect. When water is frozen, the molecules are held
in a crystalline matrix: the density of ice just below
its freezing point is c.916.8 kg m-3. Upon melting
the molecules are able to move more closely together, giving a sharp rise in density to c.999.87 kg
m-3. As the temperature of the liquid water is
raised a little further, more individual molecules
break from the complexes and fall within the matrices: in this way, the same given number of molecules now occupies yet less space, and the density
of the water is further increased. Eventually, however, it is dismemberment of the clusters that predominates: as the temperature is raised further,
the main process operating is that of molecules
moving further apart. The liquid expands and its
density diminishes. Under a pressure of 1 atmosphere, pure water reaches its maximum density of
1000 kg m-3 at 3.94°C.
This behaviour explains not only why fresh
water at sea level achieves its greatest density
close to 4°C, but also why, under appropriate conditions, ice forms at the surface of a lake, so insulating the deeper water from further heat-loss. It is
also the explanation for the fact that, with every
degree above 4°C, the difference in density also becomes greater. Later chapters remind us that this
latter effect also enhances the mechanical energy
required to mix increasingly warmed surface waters with denser layers below. Such considerations
govern the whole sequence of cycles of density
stratification and mixing in lakes, of all morphometries and at all latitudes, and possess important consequences for the distribution of dissolved
nutrients and gases.
The presence of solutes lowers the temperature
of greatest density. When the salt content of water,
for instance, reaches 25 g kg-1, the temperature of
maximum density and the freezing point coincide
(at about -1.3°C). Increased hydrostatic pressure
also depresses the temperature of maximum density, by 0.1°C for every 10 bar of pressure. At a
depth of 1000 m, the temperature of maximum
109
Physical Properties of Water
(a)
(b)
1000
2.0
Absolute viscosity kg m–1 s–1 (x10–3)
Density kg m–3
999
998
997
1.5
1.0
996
0.5
0
10
20
0
30
10
20
30
Temperature °C
Fig. 5.1 Plots showing (a) the density and (b) the viscosity of water as a function of temperature. (Redrawn from
Reynolds, 1997; reproduced with permission of The Ecology Institute.)
density is 2.91°C. At 1700 m, in the world’s deepest
lake (Ozero Baykal = Lake Baikal), it is c.2.3°C.
The mutual attraction of water molecules resists their free movement and the ability of one
layer to slide over one another, if subjected to external forcing. Instead, a temperature-dependent
threshold is reached at which the flow is chaotic
and turbulent. In turn, momentum is dissipated
through a sequence of progressively smaller eddies
(the Kolmogorov Spectrum) until viscosity overwhelms the residual motion and re-establishes
order. The size of the smallest eddies rather depends upon the strength of the forcing and the
physical space available for its dissipation but, in
most lakes, the spectrum collapses in the range of
1–4 mm. This means that most of the phytoplankton, though transported freely through turbulent
layers, are not necessarily experiencing any direct
turbulence themselves. Their immediate environments are, thus, typically viscous, with all its implications on solute diffusion and locomotory
ease. Larger zooplankton, at least, exceed the size
of the smallest eddies. Those that they create
while feeding greatly increase the rate of encounters with food particles that are themselves entrained (Rothschild & Osborn 1988).
Additionally, molecular attraction generates
the powerful surface tension at the air–water interface (73.5 ¥ 10-3 N m-1 at 15°C). This is higher than
that of any other liquid save mercury. For small
plants and animals, whether they flourish in it or
flounder, surface forces exceed gravitational attraction. Surface tension supports not merely the
microorganisms of the neuston but, often, a flux of
110
c. s. reynolds
dry particulate deposit carried in the air. Animals
as robust as Gerrid pond-skaters and Gyrinid
whirligig beetles are also able to run about on a still
water surface as if it were a solid. Powerful surface
forces also resist the formation of very small bubbles, save when surfactants are present to reduce
the surface tension. This means that gas concentrations in water tend to reach supersaturation of
the solubility product before the gas comes out of
solution.
Molecular reorganisation associated with state
transitions in water is also relevant to the equability of aquatic environments. The so-called ‘latent
heat of evaporation’ of water (that is the energy
converted in raising liquid water to the gaseous
state) is 2.243 MJ kg-1. Compounding what is already a high specific heat (4.186 kJ kg-1 K-1), it is
evident that water is consumptive of solar
radiative fluxes without commensurate and lifethreatening rises in temperature, so long as the surface is open to a dry atmosphere. Under full
tropical sunlight (delivering up to 900 W m-2) and
with minimal surface reflectance, the potential
rate of temperature increase of the top millimetre
of a water column approaches 0.2 K s-1 (i.e.
0.2°C s-1), nominally sufficient to bring the water
to the boil in 5 to 6 minutes.
Several processes prevent this from happening.
These include transmission of heat to the
water layer below and the air layer above.
Water is, however, a poor conductor of heat
(<0.006 J cm-1 K-1 s-1), so dissipation is actually
greatly facilitated by turbulence induced by windor gravitational forcing. Most important is that
the same heat flux consumed in vaporising
0.4 g m-2 s-1 from the surface (that is, a layer 0.4 mm
in thickness every second) would fail to raise
the water temperature at all.
Just how much water can be evaporated is a
function of the heat flux and the atmospheric
saturation. Other factors being equal, the fluxes
are accelerated by wind action, which refreshes the
air above water and prevents saturation; at the
same time the surface area over which heat is lost
to the atmosphere is increased.
Differences in air and water temperature are
material to the determination of the net direction
of heat exchanges. For the limnologist disinterested in the complex quantitative derivation of evaporative and other heat fluxes, it is usually enough
to know that, except under conditions of vapour
saturation of the atmosphere, generally, there can
be net warming of the surface water when the air
temperature is greater than that of the water, and
net cooling when the water is warmer. Note that
condensation of vapour from warm, moist air on to
a cool water surface also adds heat to the water, at
the rate of 2.243 MJ kg-1 of condensate.
5.3
THE TRANSPARENCY
OF WATER
Despite being almost colourless, even pure water
is severely restrictive to the passage of the photosynthetically active wavelengths of the solar flux
(roundly, from 400 to 700 nm, and almost coincidental with ‘visible’ radiation). Selective absorption at particular wavelengths compounds this
effect: it is most powerfully evident to underwater
divers, as they see the fading light with increased
depth becoming more clearly composed of blue or
green wavelengths. In waters moderately to heavily stained with humic acids, or charged with fine
particles, the attenuation of light underwater soon
becomes prejudicial to maintenance of net plant
production and, thus, to the quality and quantity of
dependent aquatic communities.
The conceptual complexities of hydrological
optics constitute a large and difficult subject area,
the theoretical development of which is beyond
the scope of this chapter. The key work is still that
of Preisendorfer (1976) but the treatment in Kirk
(1994) is perhaps the more accessible. The following section is concerned mainly with those features of light penetration in lake waters which
affect their biological function.
The properties of the underwater light field are
due, in part, to the nature of light and, in part, to
inherent properties of the water. ‘Light’, in fact,
refers to the visible wavelengths of the solar electromagnetic flux, specifically those in the radiation band, 400–700 nm. Electromagnetic energy
occurs in indivisible units known as quanta, or, at
Physical Properties of Water
least within the visible waveband, as photons.
They travel, at great velocity (c, approximately 3 ¥
108 m s-1), along characteristically wave-like pathways, each of distinctive length (l) and frequency
(n), where ln = c. The energy, e, thus carried varies
with the frequency (and inversely with the wavelength), the relation being:
e = hn = hc l
(5.1)
where h is Planck’s constant (6.63 ¥ 10-34 J s). It can
be seen that for any given wavelength the quantum flux and its energy equivalent are intercalculable. Note, too, that, within the visible spectrum
alone, light at the red end of the spectrum (700 nm)
contains little over half the energy of that at the
blue extreme (400 nm). The energy content and the
flux density of a broad waveband of mixed radiation are not readily interconvertible, owing to the
multiplicity of l values. However, the approximate conversion of 2.77 ¥ 1018 quanta s-1 W-1
applied to solar radiation within the visible
spectrum, and under a wide range of meteorological conditions, is reliable to within a few percentage points (Morel & Smith 1974). Even under the
most favourable combination of the maximum
intensity of perpendicular solar radiation to the
upper atmosphere (averaging 1352 W m-2, the socalled solar constant), and the minimal absorption
through a dry cloudless atmosphere, no more than
c.900 W m-2 reaches the surface of the sea or a
lake: the visible fraction (nominally, about 45%
but, through selective absorption, nearer 50% at
ground level) rarely exceeds 450 W m-2, or roundly
1.25 ¥ 1021 photons m-2 s-1. Dividing through by
Avogadro’s number (6.023 ¥ 1023), the derivation
(0.0021 mol photons m-2 s-1) accurately predicts
maximum light measurements reported in the
literature.
Not all the light reaching the surface of a lake
necessarily penetrates the water column. A proportion is reflected, mainly as a function of the
angle of incidence of the light rays (c.2% when the
sunlight strikes perpendicular to the water surface; <5% at angles >30°, but increasing steeply towards 100% at progressively lower incidences).
The passage of light through a surface ruffled by
111
wind is, in consequence, extremely complex but,
when the sun is low in the sky, surface wind action
significantly enhances and extends the period of
the significant underwater light field.
Photons that do penetrate the water surface are
subject to absorption, though not without a degree
of prior scattering. Absorption refers to the process
by which molecules capture photons passing
nearby. The energy of the molecule increases by an
amount corresponding to the energy of the photon.
Short-wavelength quanta raise the electronic
energy of recipient atoms: electrons are lifted from
the ground state to a higher, ‘excited’ one (whence
they are potentially shed to constitute chemical
reductive power). The energy of longer-wavelength quanta is insufficient to drive more than
transitions among the orbits of individual electrons. Excitation is actually very brief (10-9 to 10-4
s, depending on wavelength). In this way, most of
the light energy absorbed by water is only briefly
involved in electron excitation and ends up either
as heat or as chemical energy. So far as lake ecosystems are concerned, the latter, when incorporated
into photosynthate, is the basis of all biological
production.
Even in pure water, absorption of photons is not
uniform across the spectrum (Fig. 5.2): photons
with wavelengths of about 400–480 nm are least
likely to be captured by water molecules; those
with wavelengths closer to 700 nm are 30 times
more likely to be absorbed. For this reason, water is
not colourless at all. Selective absorption in the red
leaves natural waters of high purity distinctly
blue-green in colour. However, the absorption
spectrum can be significantly modified by solutes,
especially those derived from humic and fulvic
substances arising in the oxidation (mainly) of
plant material present in, or on, the soils with
which water may come in contact (see Chapter 7).
Abundance of these substances is influenced by
the nature of the catchment, the type of vegetation
present and its refractory products, and seasonal
variations in hydrology. Low-latitude, acidic
forests, in areas of moderate rainfall and slow percolating drainage, as well as the peatlands of higher
latitudes, provide good examples of stained waters.
Kirk (1994) cites several Australian waters where
112
c. s. reynolds
1.0
Absorption coefficient (m–1)
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
Fig. 5.2 The absorption of visible light (375–725 nm) by
pure water. Drawn from data presented by Kirk (1994).
wavelength of the downwelling photons. The Secchi Disc, that simple instrument used by limnologists to compare the optical properties of lake
waters, both among lakes and, in any given water,
through time, is mainly sensitive to scatter. There
can be no exact conversion between Secchi Disc
measurements and profiles of light penetration,
although helpful approximate factors have been
proposed many times (but see, especially,
Preisendorfer, 1976).
Thus, attenuation of light underwater is roughly the sum of the absorption (which is wavelengthsensitive) and the scattering (which is not).
However, it is more usual to consider underwater
irradiance – the integral of the unabsorbed light,
whether scattered or not, usually referred to as
scalar irradiance – and the way in which this is diminished with depth. Scalar-irradiance meters
may be sensitive to the entire visible spectrum
(more or less, the photosynthetically active wavelengths) or, with appropriate filters, to selected
wavebands only. Other factors being equal (most,
especially, with respect to density and uniformity
of suspended particulates), scalar irradiance of
a given waveband is diminished exponentially
with depth, according to the Beer–Lambert
formulation:
I z 2 = I z1 ◊ exp - [el( z 2 - z1 )]
enhanced absorption of blue wavelengths leaves
them distinctly yellow in colour.
Although most of the photons entering water
are absorbed, many undergo prior scattering: even
in pure water, paths of electrons are deflected as
they bounce off molecules. Scattering of downwelling photons occurs in all directions, including
backwards. Whilst the light is not absorbed any
faster, net restriction of the total pathlength to the
upper layers before it is absorbed means that scattering does contribute to the enhanced probability
that absorption will occur relatively close to the
water surface.
The effect is considerably enhanced by suspended particles (seston) – inert mineral substances (such as clay, fine silt and other tripton),
planktonic algae and bacteria and their debris –
especially if these are a few orders larger than the
(5.2)
where Iz1 and Iz2 are the waveband-specific scalar
irradiances at two points in the water column, separated vertically by 1 m, and el is the wavelengthspecific coefficient of attenuation. Often, the
formulation relates an average coefficient to light
penetrating the surface (I0). Thus,
I z = I0 ◊ exp - [elz ]
(5.3)
Owing to differential absorption, attenuation of
white light becomes less steep as the fraction
penetrating to depth is increasingly made up of
wavelengths which are absorbed least rapidly.
Nevertheless, it is quite usual to find extinction
coefficients quoted in the literature fitted to the
near-surface attenuation of scalar irradiance in
the full photosynthetically active spectrum. The
wavelength specificity is omitted from the
equation:
113
Physical Properties of Water
I z = I0 ◊ exp- ez
(5.4)
e ª (ln I z - ln I0 ) z
(5.5)
whence,
It is recognised that the influence of the different
sources of water supplied collectively to natural
lakes, on the transparency of the recipient systems, is profound. Reported coefficients of vertical
attenuation of photosynthetically active radiation
(e) in the world’s clearest lakes varies from 0.06 to
0.18 m-1. At the other extreme, drainage from
catchments characterised by slow plant decomposition may be stained brown by humic derivatives
(as in several Australian reservoirs; e ª 1.5–
2.0 m-1), while eroding soils and exposures of unconsolidated deposits may charge the water with
clouds of fine yellow or whitish clay (reports place
e between 8 and 20 m-1; resuspension of earlier deposition may contribute substantially to the suspended load at times). The comparison between
these influences is scarcely more striking than at
the confluence of the (‘black water’) Rio Negro and
the (‘white water’) Solimões rivers where they
meet at Manaus, Brasil, to form the lower Amazon.
Where the capacity of chemical nutrients can support it in large concentrations, planktonic biomass
itself lends great turbidity to the water: in extremes, absorption by photosynthetic pigments
can largely account for values of e up to c.8 m-1.
In these ways, the hospitability of lake waters to
the support of further photoautotrophic biomass
can be just as easily constrained by the penetration
of light or, at least, by the factors resisting it, as by a
more commonly acknowledged scarcity of available nutrients (see Reynolds 1997).
5.4
IMPACTS UPON LAKES AND
THEIR LIFE
The behaviour of water masses impounded in lake
basins is governed, on the one hand, by the driving
environmental variables (solar heat income, mechanical stirring by wind and gravitational forcing
and hydrological exchanges; availability of vital
solutes, released from the catchment) appropriate
to the location (latitude, altitude, aspect, proximity to the ocean, climate and catchment criteria),
and, on the other, by the limits to reactivity imposed by the physical properties of water. For instance, the extent, rate and frequency of mixing of
lakes represent compromises between the energy
of forcing and the inertia incumbent upon high
density, high viscosity, poor heat conductance and
the buoyant resistance engendered by the coefficient of thermal expansion. These relationships
determine if, when and for how long lakes undergo
thermal stratification (see Chapters 2 and 6). They
interact with the flux of nutrients exported from
the catchment or reprocessed in the lake (see
Chapter 4) and with the scalar light gradient in determining the proportion of the solar day during
which entrained pelagic organisms can be exposed
to daylight (see Chapter 9). All the main biota of
lakes (see Chapters 10–16) are directly influenced
by the Archimedean properties of water, and the
stresses incumbent upon its movement; the lives
of all of them are touched by the interaction of
turbulence and viscosity; all respond to the
changes in water temperature and all are sensitive
to gas content.
It is easy to become excited by and, in time, to be
specialist in certain types of organisms, of microhabitats, or even of selected lakes. Theorists may
become fascinated by the behaviour of aquatic
ecosystems and their comparability with terrestrial counterparts (I have argued as much elsewhere: see Reynolds, 1997) but it is essential still
to recognise that the characteristic constraints of
systems themselves frequently relate to those imposed by the physical environment which they inhabit. So far as lakes and limnologists are
concerned, the truism is inescapable: the properties of the limnetic environment are substantially
governed by the unusual physical properties of
water itself.
REFERENCES
Hutchinson, G.E. (1957) A Treatise on Limnology, Vol. 1,
Geography, Physics, Chemistry. Wiley, New York,
1016 pp.
114
c. s. reynolds
Kirk, J.T.O. (1994) Light and Photosynthesis in Aquatic
Ecosystems. Cambridge University Press, Cambridge,
528 pp.
Lampert, W. & Sommer, U. (1993) Limnoökologie. Georg
Thieme Verlag, Stuttgart, 440 pp.
Morel, A. & Smith, R.C. (1974) Relation between total
quanta and total energy for photosynthesis. Limnology
and Oceanography, 19, 591–600.
Preisendorfer, R.W. (1976) Hydrological Optics. United
States Department of Commerce, Washington, DC,
382 pp.
Reynolds, C.S. (1997) Vegetation Processes in the Pelagic: a Model for Ecosystem Theory. Ecology Institute,
Oldendorf, 372 pp.
Rothschild, B.J. & Osborn, T.R (1988). Small-scale turbulence and plankton contact rates. Journal of Plankton
Research, 10, 465–474.