Upload
rabiya-nasir
View
183
Download
1
Tags:
Embed Size (px)
Citation preview
DISSOLVED GASES
RABIYA NASIR
DISSOLVED GASES IN SEAWATER
H2O: Universal Polar Solvent
What is seawater?
* Seawater is a solution of about 96.5% water and
* 3.5% dissolved salts. • The most abundant salt in seawater is sodium
chloride (NaCl). • Most elements on Earth are present in seawater. • Because these substances are dissolved, they
are in the form of ions (positive or negative
atoms).
gases
Most gases in the air dissolve readily in seawater at the ocean’s surface
Major gases found in seawater (in order of relative abundance)NitrogenOxygenCarbon dioxide
Important Gases 6 important gases are
dissolved in lakes, streams, seas
Nitrogen Oxygen Carbon dioxide Methane Hydrogen sulfide Ammonia All have important functions,
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 transformations
Ammonia - breakdown of nitrogenous materials by bacteria, some animals
How much gas is dissolved in water at any given time?
Dependent on several factors:
Solubility factor
Pressure
Temperature
Salinity
Solubility Factor Not all gases
dissolve in water to same extent
Some gases dissolve very easily in water, some dissolve very little
Pressure (atmosphere) Amount of gas
absorbed by water is proportional to its partial pressure in the atmosphere (conc. = solubility factor X partial pressure)
Altitude decreases saturation level by ~1.4% per 100 m
Temperature Solubility of gas in
water decreases as temperature rises
Generalization - cold water can hold more gas in solution than warm water
Nearly linear relationship within normal range of natural water temperatures
Salinity Presence of
various minerals in solution lowers the solubility of gases
Generally disregarded in limnology because freshwaters have salinity near zero
Relative Saturation Relation between
existing solubility (amount of gas present) and the equilibrium content expected at same temperature and partial pressure
Can be less, or more (supersaturation)
Oxygen Abundant and
dissolves readily in water
Needed for respiration by organisms and for complete breakdown of organic matter
Relatively 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 agitation can prevent water from becoming supersaturated
Photosynthesis Most oxygen in standing
waters is by-product of photosynthesis
Phytoplankton contribute most
Rooted macrophytes, attached algae, benthic algae mats are chief producers in shallow lakes, lake margins
Loss of Oxygen
Physical - change in temperature, pressure
Biological - most important - respiration by plants, animals, bacteria (decay processes)
Other - methane bubbles rising from sediments through water column
Oxygen Distribution
Distribution changes as lake goes through seasonal temperature cycle
Orthograde distribution during spring, fall turnovers in dimictic lake
Clinograde distribution during thermal stratification
Daily, seasonal variation in oxygen concentrations
The more plant material in a lake or pond, the more prone that system is to both daily and seasonal variations in dissolved oxygen content
Seasonal variation in oxygen concentrations
O2 high during summer growing season, low in late-summer when plants die
May produce anoxia and die-offs of animals (summerkill)
Seasonal variation in oxygen concentrations
O2 also may be low during winter in ice-covered lakes
Reduced light transmission, respiration only - Winterkill of animals
Carbon Dioxide
CO2 increasing in concentration in atmosphere
High solubility - 200 X > O2
Follows solubility laws (pressure, temp.) Many sources other than atmosphere:
rainwater, runoff, groundwater, respiration, decomposition in sediments
Carbon Dioxide
CO2 behaves much differently than other gases once it dissolves in water
Exists in equilibrium with many additional forms of carbon
CO2 + H2O = H2CO3 Carbonic acid
H2CO3 = HCO3- + H+ bicarbonate
HCO3- = CO3
2- + H+ carbonate
Putting it all together
CO2 + H2O = H2CO3
Sensitive to changes in pH
Low pH - left side dominates
High pH - right side dominates
= HCO3- + H+ = CO3
2- + 2H+
Putting it all together
Addition of CO2 via respiration pushes equilibrium to right and lowers pH
Removal of CO2 via photosynthesis pulls
equilibrium to left and raises pH
Buffer System
CO2 + H2O = H2CO3 = HCO3- + H+ = CO3
2- + 2H+
CaCO3 + H2CO3 = Ca (HCO3)2
Ca (HCO3)2 = CaCO3 + H2O + CO2
Little change in pH despite additions of lots of acids orbase, as long as supply of carbonates & bicarbonatesholds out
Nitrogen Exists in many
different forms in natural freshwater systems
A major nutrient that affects the productivity of aquatic systems
Nitrogen
Dissolved gas - N2
Ammonia - NH3 NH4+
Nitrite - NO2-
Nitrate - NO3-
Dissolved organicsAmino acidsPolypeptidesProteins
Sources: atmosphere, rain, runoff, groundwater **
Losses: water outflow, adsorption to sediments, dinitrification by bacteria
Nitrogen Cycle
Ammonia Readily
assimilated by plants
Nitrification by bacteria
Present in low concentrations in oxygenated waters
Ammonia Accumulates in
hypolimnion No
photosynthesis or nitrification
Release from sediments during anoxia
Nitrate Nitrates high in presence of
oxygen Nitrification Nitrates not assimilated
easily by plants Molybdenum needed to
reduce nitrate Poor abundance in igneous
basins
Nitrate
Denitrification to N2 only by anaerobic bacteria in hypolimnion
Nitrate:ammoniaCalcareous runoff 25:1Igneous runoff 1:1Sewage or fertilizer
1:10
Phosphorus
Total concentrations in unpolluted waters 0.01-0.05 mg/L
Sources:Rainfall (unpolluted <0.03 mg/L) (polluted
>0.1 mg/L)Groundwater ~0.02 mg/LSurface runoff - variable - often major
contributor to lakes (especially with pollutants)
Phosphorus
>90% of P in water is in form of organic phosphates or related materials in living things or their secretions
Great scarcity - limiting factor Rapid turnover of organic P between
living organismsBacteria, phytoplankton, zooplankton, others5-100 minutes, more rapid under deficiency
Phosphorus
In presence of O2, various forms of phosphates form complexes, chelates, and insoluble salts with several metal ionsE.g., calcium and ironInduce precipitation of P in oxygenated
waters
Confusing, interrelated terms Alkalinity
Hardness
Salinity
Alkalinity
Measure of buffering capacity of water
Carbonates and bicarbonates of alkali metals
Hardness
Calcium and magnesium salt content Temporary hardness - carbonates and
bicarbonates, can be removed by boilingPrecipitation of CaCO3
Ca(HCO3)2 = CaCO3 + H2O + CO2
Permanent hardness - sulfates, chlorides, other anions
SALINITY is the total quantity of dissolved inorganic solids in water. You can see here that the most abundant dissolved solids are chloride, sodium, and sulfate.
Salinity
Concentrations of Ca2+ Mg2+ Na+ K+ and HCO3
- CO32- SO4
2- Cl-
Plus other ionized components of other elements