Lecture 4 Aquatic Habitats
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The gall on the stem of a hackberry leaf (cut open)
was made by an insect related to bugs (you can see them inside the
gall). They are probably a family and live together until they disperse
in the fall |
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Harned Hall 302
phone 963 - 5782
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Overview - Link to Course
Objectives
Hydrologic
Cycle
cycling of water from atmosphere
to Earth to atmosphere that is driven by radiant energy from the sun
- Precipitation
- Atmosphere to Earth
- Interception
- the rain that falls on something other than the soil or the surface
of lakes, rivers and oceans
- Some of the intercepted
water is directly returned to the atmosphere by evaporation
- Some precipitation Infiltrates
the soil to become Soil Moisture
and Groundwater
- note that we separate
soil moisture from groundwater - soil moisture is available to plants
but groundwater is too deep to be accessible
- both soil moisture and
groundwater flows into lakes and streams but does so slowly compared
to runoff
- flows into Aquifers
- Permeable layers of rock surrounded by impermeable rock so that
it is like an underground tank that can hold the water for 1000s of
years
- Runoff
- the excess water that, during heavy precipitation, does not infiltrate
the soil but flows into surface water (lakes, etc.)
- runoff can be damaging
to soils because it can erode soils
- runoff can be increased
by humans because many surfaces we make prevent infiltration (houses,
roads, sidewalks)
- Evapotranspiration
- Earth to Atmosphere
- Evaporation
- loss from surface of bodies of water and from directly from soil
- rate depends on relative
humidity (water in air divided by amount of water in saturated air) which
depends, in turn, on temperature as the amount of water needed to saturate
air increases with temperature
- Transpiration
- loss from surface of plants through the stomata in leaves
- Global
Water Budget
- Reservoirs of water, from
largest to smallest: Oceans (by a huge factor), Polar Ice and Glaciers,
Groundwater, Lakes, Soil Moisture, Atmosphere (water vapor)
- There is far more water
in the polar ice and glaciers than in the groundwater, lakes, soil,
and atmosphere summed together
- Loss of polar ice and
glaciers may change budget and may change climate
- Flows of water, from largest to smallest:
Evaporation from ocean, Precipitation onto ocean, Precipitation onto land,
Transpiration and Evaporation from Land, River discharge into oceans
- More water evaporates from oceans that
falls back onto oceans, so that the oceans supply 1/3 of the precipitation
that falls on land
Water's
Physical Properties
Remember that water is a polar molecule
and that liquid water is held together by the hydrogen bonding that results
from that polarity
- Specific heat
- energy needed to raise
one gram of a material 1° C, very high for water due to cohesion.
Water bodies can absorb lots of energy before they heat much, which means
that water bodies usually cool the land surrounding them in summer and warm
it during winter, as they slowly release heat
- Latent heat
- the energy needed to either free water vapor from the surface of water (latent
heat of evaporation) or to free water from ice (latent heat of freezing) -
both of these are large due to cohesion
- Cohesion/Adhesion
- force of hydrogen bonds binding water molecules to one another (cohesion)
or to hydrophilic surfaces (adhesion)
- Surface Tension
- effect on outer layer of water molecules due to pull from water on one side
versus much weaker pull of atmosphere on the other side. Can be a barrier
to small organisms.
- Viscosity
- resistance of water to motion of a body through it - high for water due
to cohesion and an important consideration for animals that move through water
- Buoyancy
- water is a dense material. Objects in water sink or float based on
their average density. Objects with a greater density than water sink,
those with a lesser density float. The ability of objects to float on
water is referred to as buoyancy. Those objects with a density equal
to the water surrounding them are neutrally buoyant and do not tend to rise
or fall in the water column. Remember that it is the average density that
counts and something will float even though it has portions denser than water
if that is more than balanced by portions that are less dense. Floating
objects displace a volume of water equal to their total weight.
Water
and Light
Pure water absorbs light - the photons
combine with electrons and raise their energy, which can be lost from them as
thermal radiation or can be converted into kinetic energy (heat energy)
- Some sunlight is reflected from
surface of water
- more reflects as the angle between light
and water surface decreases (least at 90° to surface)
- Longer wavelengths absorbed more quickly (blue
last)
- Particles in water increase absorption
- Microalgae have pigments in their chloroplasts
that absorb specific wavelengths of light
- Deep waters have no light at all
- Organisms are often bioluminescent
Temperature
Profiles of Bodies of Water
Water becomes more dense as it cools
until it reaches its maximum density at 4°C
- water sinks as it cools
- ice is less dense than water
due to spacing of water molecules in ice crystal lattice
- bodies of water heat from the
surface where sun shines on them
- Thermal
Stratification of Standing bodies
of water (lakes, seas and oceans)
- Epilimnion
- wind-mixed upper layer
- Thermocline
- region of steep change in temperature which separates upper and lower
waters
- Hypolimnion
- lower, colder water (low of 4° C), does not mix with upper layer
- in Temperate regions, thermal stratification
may be seasonal
- as fall cools epilimnion to temperature
of hypolimnion, thermocline disappears and water is same density from
top to bottom
- winds can then mix entire water
column, as situation called the Fall Turnover
in lakes
- winter produces colder surface than
deep water and turnover halts
- spring brings heating until a second
turnover occurs (Spring Turnover)
- turnovers can mix nutrient-rich
water with nutrient-depleted water and result in spikes of algal productivity
during the turnover (until the thermocline is re-established and the epilimnion
is once again depleted of nutrients by algal growth)
- this cycle is not always found
- deep lakes and oceans - thermocline
simply moves up and down but does not disappear and wind does not mix
bottom water, just somewhat deeper water, when thermocline is at maximal
depth
- in tropics, thermoclines can be
virtually permanent
- Salt concentration also changes density
of water and haloclines can
be produced where bodies of water of different salt content contact one another
- often happens where fresh water
overlies deeper salty water
Water
as a Solvent
Water is a powerful solvent for
all hydrophilic substances due to adhesion of water molecules to the solute
molecules
- many minerals are soluble in
water and solubility (amount that can be dissolved before saturation is reached)
increases with temperature
- soils are source of mineral
ions found in water and so water composition reflects the region from
which it is drawn
- oceans are usually the
ultimate destination of dissolved ions as rivers add new ions but evaporation
leaves them behind
- oceans are getting more salty
through time
- NOTE - book uses symbol that
looks like ‰ and not the usual percent symbol, %. The single
0 in the denominator means hundredths (parts per hundred), the double
0s mean thousandths (parts per thousand). Lower concentrations use
abbreviations (ppm = parts per million, ppb = parts per billion, etc.)
- ocean water (usually called
salt water, although some lakes and seas are saltier) is 35‰ salt
or 35 parts salt per 1000 parts water (which is also 3.5%)
- Gases are also soluble in water
but solubility decreases with temperature so that, when saturated, warm water
has less oxygen, etc. than cold water
- they enter water across the
surface and diffuse to deeper
water or mix when the water mixes due to turbulence
- stratification of water bodies due to temperature
or salt can separate hypolimnion from its source of oxygen
- the ocean's deeper layers are oxygenated
due to input of oxygen-rich waters in northern latitudes, where surface
water rapidly cools and sinks to replace water that comes to the surface
in areas of upwelling
- there is an Oxygen Minimum Zone from 200
to 2000 meters depth due to loss of oxygen to respiration and decomposition
of material falling from surface waters
Acidity
Pure water has a pH of 7 but water
in nature is never pure
- Water dissolves ions from the
surfaces it contacts so that natural waters reflect their surroundings and
the pH can range from 2 to 12
- ocean water tends to be slightly
alkaline with a pH range of 7.5 to 8.4
- Surface waters absorb CO2
which combines with water and dissociates to acidify the water
- CO2
+ H2O
= H2CO3
= HCO3-
+ H+
= CO3--
+ H+
- carbon dioxide serves to
buffer surface waters
- some pollutants from oil and
coal burning (NO2
and SO3
in particular) combine with water to form acids in the atmosphere that can
precipitate as acid rain
- acid rain can acidify surface
waters and cause normally insoluble ions (such as aluminum) to dissolve
- these ions may cause damage
to the ecosystem (fish kills in lakes, tree deaths in forests)
Movement
of Surface Water
The level of surface waters is affected
by a variety of forces, some very local and some celestial, which results in
complex changes it the level of surface waters (here, oceans and large lakes)
Wind
- Waves are caused by wind blowing
across water
- the stronger the wind and the
longer the reach (distance across which the wind blows) the larger the waves
- tremendous energy in waves (water
is heavy and they can disturb water for thousands of miles)
- motion of water in a wave is
circular so that waves do not mix water very much
- in lakes, bays and seas winds
can pile up water on leeward side until either the piled mass becomes too
large or the wind stops or slackens and the piled water will then flow back
towards windward side and set up a regular oscillation, a standing wave (because
it does not continue to move in one direction by moves back and forth across
a body of water) called a seiche
(from a Swiss-French term "to sway back and forth")
Tides
- the moon and sun both exert gravitational
pull on the Earth, including the oceans
- the effect of the Sun's and
Moon's gravitational pull is felt at center of mass of the solid Earth,
which is near its geometric center as the Earth is roughly a sphere
- the effect on water, a liquid
that is not a part of the solid Earth, is felt at the water's center of
mass, which is at the Earth's surface
- On the side of the Earth nearest
the Sun and Moon, the center of mass of the ocean water is closer to the Sun
and Moon than is the Earth's center of gravity and so the pull of the Sun
and Moon are greater on the water there since the force of gravity increases
as distance decreases
- On the side of the Earth farthest
the Sun and Moon, the center of mass of the ocean water is farther from the
Sun and Moon than is the Earth's center of gravity and so the pull of the
Sun and Moon are greater on the Earth than on the water on the far side and
the water bulges away from the Sun and Moon
- Thus the Sun and Moon each
cause two bulges of water that we call Tides,
one on the side of the Earth nearest them (because their gravity is pulling
harder on the water than on the Earth) and a second on the side of the
Earth farthest from them (because their gravity is pulling harder on the
Earth than on the water)
- the moon's effect is about
twice that of the Sun and so the Moon's bulge is most
noticeable
- the bulges (tides) move across
the Earth's surface as the Earth rotates, as the Moon revolves around the
Earth and as the Earth revolves around the Sun
- The Moon's tides occur about
every 12.5 hours (not 12 hours because the Moon is moving as the Earth
rotates)
- the Sun's tides add to or
subtract from the height of the lunar tides depending on whether they
are both in the same phase (bulging up or lying flat)
- when the Moon is in line
with the Sun (on either side of the Earth), that is at full and new moons,
the Sun's tides and the Moon's tides add up to produce larger than normal
tides called Spring Tides
- when the Moon is maximally
out of line with the Sun (a line drawn between the center of the Moon
and the center of the Earth is perpendicular with one drawn from the center
of the Sun to the center of the Earth), that is at second and third quarter
moons, the Sun's tides and the Moon's tides partially cancel one another
and produce smaller than normal tides called
Neap Tides
- Other factors cause variation
in tides
- the Moon does not circle
the Earth in the plane of the Earth's revolution about the Sun but at
an angle about 23° to it
- this results in the tides
being stronger at temperate latitudes than in the tropics
- local conditions, like the
depth of the basin, shape of the coast, and proximity of storm winds can
alter the height of tides
- Bay of Fundy, Nova Scotia,
Canada has spring tides up to 40 feet from low to high tide
- Atlantic has two high (and,
therefore, two low) tides per day
- Gulf of Mexico has only one
tide per day
- The presence of tides along coasts
has produced a unique habitat called the Intertidal Zone, where organisms
are alternately under water and exposed to air
- effect is most pronounced
on rocky coasts
- on sandy coasts the tidal
effect is lessened by the effect of the upper layers of sand
Estuaries
Estuaries
are the mouths of rivers where fresh and salt water mix
- vertical mixing of salt and fresh
water depends on tides and on the river flow, which can change seasonally
or in response to storms anywhere in the river's watershed
- fresh water is less dense,
so the incoming river flow tends to spread out on the surface of the estuary
- winds can mix the salt and
fresh water
- tides can send a surface
layer of dense ocean salt water up river so that the upper layer is more
dense than the lower layer
- this is an unstable situation
and the denser water will sink to the bottom and mix with the less dense
water that is coming to the surface, a form of mixing in estuaries called
Tidal Overmixing
- in general, salinity is greatest
at mouth of estuary and decreases as you proceed up river
- changes in the salt concentration
mean that the organisms in the estuary must either adapt to changing conditions
or must move to remain in water that has the correct salinity for them
- fish move with the tides,
clams and mussels close their shells, but many organisms must adapt their
physiologies to widely varying salt concentrations
- the changing salinity means the
estuaries are stressful habitats and the diversity of organisms is lower in
them than in the ocean off shore from the estuary and the river upstream from
the estuary
- the constant input of nutrients
from the river means that estuaries are very productive environments, with
lots of plant and animal growth
Terms
Hydrologic Cycle,
Precipitation, Interception,
Infiltration, Soil Moisture, Groundwater, Aquifers, Runoff, Evapotranspiration,
Evaporation, Transpiration, Global Water Budget, Specific heat, Latent heat,
Cohesion/Adhesion, Surface Tension, Viscosity, Bouyancy, Thermal Stratification,
Epilimnion, Theromcline, Hypolimnion, Fall Turnover, Spring Turnover, Halocline,
Diffusion, Turbulence, Seiche, Tide, Spring Tide, Neap Tide, Intertidal Zone,
Estuary, Tidal Overmixing
Last updated January 20, 2007