Lecture 4 Aquatic Habitats

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
BIO 412 Ecology

Harned Hall 302  phone 963 - 5782

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