do dan co2.pdf
TRANSCRIPT
Dissolved Gases
Important Gases
6 important gases are dissolved in water systems (Ex: Ocean)
Nitrogen Oxygen Carbon dioxide Methane Hydrogen sulfide Ammonia All have important
funcGons, but differ in behavior, origin
Air Provides Some Gases
Atmosphere has enough nitrogen (78%), oxygen (21%), and carbon dioxide (0.03%) to serve as primary source
Others present only in trace amounts in atmosphere
Other Gas Sources
Methane -‐ anaerobic breakdown of plants/animals
Hydrogen sulfide -‐ chemical/bacterial transformaGons
Ammonia -‐ breakdown of nitrogenous materials by bacteria, some animals
How much gas is dissolved in water at any given Gme?
Dependent on several factors:
Solubility factor Pressure Temperature Salinity
The solubility and saturaGon value for gases in sea water increase as temperature and salinity decrease and as pressure
increases. • Solubility is the ability of something to be dissolved and go into soluGon. • SaturaGon value is the equilibrium amount of gas dissolved in water at an exisGng
temperature, salinity and pressure. – Water is undersaturated when under exisGng condiGons it has the capacity to dissolve more gas.
Gas content is below the saturaGon value. – Water is saturated when under exisGng condiGons it contains as much dissolved gas as it can hold
in equilibrium. Gas content is at saturaGon value. – Water is supersaturated when under exisGng condiGons it contains more dissolved gas than it can
hold in equilibrium. Gas content is above saturaGon value and excess gas will come out of soluGon.
• The surface layer is usually saturated in atmospheric gases because of direct exchange with the atmosphere.
• Below the surface layer, gas content reflects relaGve importance of respiraGon, photosynthesis, decay and gases released from volcanic vents.
5-6 Gases in Seawater
Solubility Factor
Not all gases dissolve in water to same extent
Some gases dissolve very easily in water, some dissolve very li^le
Pressure (atmosphere)
Amount of gas absorbed by water is proporGonal to its par6al pressure in the atmosphere
AlGtude decreases saturaGon level by ~1.4% per 100 m
Temperature
Solubility of gas in water decreases as temperature rises
GeneralizaGon -‐ cold water can hold more gas in soluGon than warm water
Nearly linear relaGonship within normal range of natural water temperatures
Salinity
Presence of various minerals in soluGon lowers the solubility of gases
Generally disregarded in limnology because freshwaters have salinity near zero
Salinity
Oceans (salinity of 3.5%) have reduced gas saturaGon values of ~18-‐20%
Saline pools/lakes can have much higher saliniGes (5-‐6 X ocean values)
Important consideraGon here for gas solubiliGes
Oxygen
Abundant and dissolves readily in water Needed for respiraGon by organisms and for complete breakdown of organic ma^er
RelaGvely easy to measure
Oxygen
1/4 as abundant as nitrogen in atmosphere, but twice as soluble
Solubility of oxygen increases as temp. decreases, salinity decreases, and pressure increases
Oxygen
Two sources for oxygen in lakes
Atmosphere
Photosynthesis
Atmosphere
Diffusion across air-‐water interface and down into water column
Years to reach depth of 5 m Wind-‐driven waves and
currents distribute oxygen to lower levels
Too much agitaGon can prevent water from becoming supersaturated
Photosynthesis
Most oxygen in standing waters is by-‐product of photosynthesis
Phytoplankton contribute most
Rooted macrophytes, a^ached algae, benthic algae mats are chief producers in shallow lakes, lake margins
Photosynthesis: Your one-‐stop shop for all of your oxygen needs!
Carbon Dioxide (from air)
Water (from ground)
Oxygen (to air) Carbohydrate
(plant material)
Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2
Happy Rays of Sunshine
Happy Rays of Sunshine
CO2 O2
Aquatic plants and phytoplankton (single cell floating plants) release oxygen into the water as a product of photosynthesis
Phytoplankton (single cell plants) – are the base of the aquatic food web and provide most of the aquatic oxygen.
Submerged aquatic plants can provide shelter for young fish as well as house an abundant food supply.
Hypoxic
Anoxic
Normoxic
Habitat ClassificaGon Based on DO ConcentraGon
0 – 0.2 mg/L
0.2 – 2 mg/L
> 2 mg/L
Most fish need oxygen levels > 2.0 mg/L
AbioGc Factors That Affect DO ConcentraGon
• Temperature
• Water Clarity
• Current Velocity (Flow)
• Wind
Temperature • The warmer water is, the less DO it can hold – Think about opening a coke bo^le aher it sat a few hours on the dash of your car.
• Excess DO evaporates into the atmosphere!
100% DO Saturation
0 2 4 6 8
10 12 14 16 18 20
0 5 10 15 20 25 30 35 Temperature (C)
100%
Sat
urat
ion
Lave
l
Wind Stirs in atmospheric oxygen
Oxygen > 100% Saturation
Oxygen < 100% Saturation
Oxygen diffuses out of water column
Oxygen diffuses into water column
Water Column
Atmosphere
Oxygen Can Diffuse Out of or Into the Water Column
Current Velocity • The faster water flows, the more atmospheric oxygen is
mixed into the water.
Water Clarity
Amount of Sunlight Reaching Plants
The muddier the water is, the less light reaches the plants!
Loss of Oxygen
Physical -‐ change in temperature, pressure
Biological -‐ most important -‐ respiraGon by plants, animals, bacteria (decay processes)
Other -‐ methane bubbles rising from sediments through water column
Daily variaGon in oxygen concentraGons
O2 rises during day, declines at night The greater the plant biomass, the greater the magnitude of the cycle
Daily AquaGc Oxygen Cycle
Sunrise Noon Sunset Midnight Midnight
Diss
olve
d O
xyge
n
Sunshine Moonshine Moonshine
Seasonal variaGon in oxygen concentraGons
O2 high during summer growing season, low in late-‐summer when plants die
May produce anoxia and die-‐offs of animals (summer kill)
Seasonal variaGon in oxygen concentraGons
O2 also may be low during winter in ice-‐covered lakes
Reduced light transmission, respiraGon only -‐ Winterkill of animals
Oxygen tends to be abundant in the surface layer and deep layer bo^om, but lowest in the pycnocline.
• Surface layer is rich in oxygen because of photosynthesis and contact with the atmosphere.
• Oxygen minimum layer occurs at about 150 to 1500m below the surface and coincides with the pycnocline.
– Sinking food parGcles se^le into this layer and become suspended in place because of the greater density of the water below.
– The food draws large numbers of organisms which respire, consuming oxygen.
– Decay of uneaten material consumes addiGonal oxygen.
– Density difference prevents mixing downward of oxygen-‐rich water from the surface or upwards from the deep layer.
• The deep layer is rich in oxygen because its water is derived from the cold surface waters which sank (convect) to the bo^om. ConsumpGon is low because there are fewer organisms and less decay consuming oxygen.
• Anoxic waters contain no oxygen and are inhabited by anaerobic organisms (bacteria).
5-6 Gases in Seawater: O2
Carbon Dioxide
CO2 increasing in concentraGon in atmosphere High solubility -‐ 200 X > O2
Follows solubility laws (pressure, temp.)
Many sources other than atmosphere: rainwater, runoff, groundwater, respiraGon, decomposiGon in sediments
Oceanic carbon cycle Solubility of CaCO3 dependent on pressure, temperature and mineral type
Solubility ↑ at larger depths and lower temperatures
Carbon Dioxide
CO2 behaves much differently than other gases once it dissolves in water
Exists in equilibrium with many addiGonal forms of carbon
CO2 + H2O = H2CO3
H2CO3 = HCO3- + H+
HCO3- = CO3
2- + H+
Carbonic acid
bicarbonate
carbonate
CO2 + H2O = H2CO3 = HCO3- + H+ = CO3
2- + 2H+
Sensitive to changes in pH
Low pH - left side dominates
High pH - right side dominates
CO2 + H2O = H2CO3 = HCO3- + H+ = CO3
2- + 2H+
Addition of CO2 via respiration pushes equilibrium to right and lowers pH
Removal of CO2 via photosynthesis pulls equilibrium to left and raises pH
Sunrise Noon Sunset Midnight Midnight
CO2+H2OH2CO3H++HCO3-2H++CO3
-
An increase in CO2 causes an increase in H+ pH
Sunshine Moonshine Moonshine
pH L
evel
CO2 + H2O = H2CO3 = HCO3- + H+ = CO3
2- + 2H+
Oceanic carbon cycle
Below saturation levels: ���- CaCO3 decreases due to dissolution���- increases due to settling Depth at which CaCO3 is not preserved:��� CCD (Carbonate Compensation Depth)
Carbon dioxide is of major importance in controlling
acidity in the sea water. • Major sources of carbon dioxide are
respiraGon and decay.
• Major sinks are photosynthesis and construcGon of carbonate shells.
• Carbon dioxide controls the acidity of sea water.
– A soluGon is acid if it has excess H+ (hydrogen) ions and is a base if it has excess OH-‐ (hydroxyl) ions.
5-6 Gases in Seawater
– pH is related to the amount of CO2 dissolved in water because it combines with the water to produce carbonic acid which releases H+ ions. • CO2 + H2O ←→ H2CO3 ←→ H+ + HCO3
-‐←→ H+ + CO3
-‐2
– H2CO3 is carbonic acid, HCO3-‐ is the bicarbonate
ion and CO3-‐2 is the carbonate ion.
– Changing the amount of CO2 shihs the reacGon to either the right or leh of the equaGon. • Adding CO2 shihs the reacGon to the right and produces more H
+ ions making the water more acid.
• Removing CO2 shihs the reacGon to the leh, combining H+ ions with carbonate and bicarbonate ions reducing the acidity.
5-6 Gases in Seawater
ConGnue
– Dissolved CO2 in water acts as a buffer, a substance that prevents large shihs in pH.
– DissoluGon of carbonate shells in deep water results because cold water under great pressure has a high saturaGon value for CO2 and the addiGonal CO2 releases more H+ ions making the water acid.
– Warm, shallow water is under low pressure, contains less dissolved CO2 and is less acidic. Carbonate sediments are stable and do not dissolve.
Take your Gme
• h^p://vimeo.com/65512340
Relevance: Oceans play key role in carbon cycling
Ocean-CO2 = 50-60 * atmospheric-CO2
• Surface ocean pH is already 0.1 lower than preindustrial values. How many magnitude the increasing [H+] ??
• By the end of century pH 0.3 – 0.4 units lower
Phytoplankton (single cell plants) – Acidification
Hypoxia in the Gulf of Mexico
Hypoxic waters
Image from Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC, January 2003