Dissolved Gases

Important Gases 6 important gases are dissolved in lakes, streams, seas 6 important gases are dissolved in lakes, streams, seas Nitrogen Nitrogen Oxygen Oxygen Carbon dioxide Carbon dioxide Methane Methane Hydrogen sulfide Hydrogen sulfide Ammonia Ammonia All have important functions, but differ in behavior, origin 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 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 Others present only in trace amounts in atmosphere

Other Gas Sources Methane - anaerobic breakdown of plants/animals Methane - anaerobic breakdown of plants/animals Hydrogen sulfide - chemical/bacterial transformations Hydrogen sulfide - chemical/bacterial transformations Ammonia - breakdown of nitrogenous materials by bacteria, some animals Ammonia - breakdown of nitrogenous materials by bacteria, some animals

How much gas is dissolved in water at any given time? Dependent on several factors: Dependent on several factors: Solubility factor Solubility factor Pressure Pressure Temperature Temperature Salinity Salinity

Solubility Factor Not all gases dissolve in water to same extent Not all gases dissolve in water to same extent Some gases dissolve very easily in water, some dissolve very little 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) 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 Altitude decreases saturation level by ~1.4% per 100 m

Temperature Solubility of gas in water decreases as temperature rises Solubility of gas in water decreases as temperature rises Generalization - cold water can hold more gas in solution than warm water Generalization - cold water can hold more gas in solution than warm water Nearly linear relationship within normal range of natural water temperatures Nearly linear relationship within normal range of natural water temperatures

Salinity Presence of various minerals in solution lowers the solubility of gases Presence of various minerals in solution lowers the solubility of gases Generally disregarded in limnology because freshwaters have salinity near zero Generally disregarded in limnology because freshwaters have salinity near zero

Salinity Oceans (salinity of 3.5%) have reduced gas saturation values of ~18-20% Oceans (salinity of 3.5%) have reduced gas saturation values of ~18-20% Saline pools/lakes can have much higher salinities (5-6 X ocean values) Saline pools/lakes can have much higher salinities (5-6 X ocean values) Important consideration here for gas solubilities Important consideration here for gas solubilities

Relative Saturation Relation between existing solubility (amount of gas present) and the equilibrium content expected at same temperature and partial pressure 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) Can be less, or more (supersaturation)

Oxygen Abundant and dissolves readily in water Abundant and dissolves readily in water Needed for respiration by organisms and for complete breakdown of organic matter Needed for respiration by organisms and for complete breakdown of organic matter Relatively easy to measure Relatively easy to measure

Oxygen 1/4 as abundant as nitrogen in atmosphere, but twice as soluble 1/4 as abundant as nitrogen in atmosphere, but twice as soluble Solubility of oxygen increases as temp. decreases, salinity decreases, and pressure increases Solubility of oxygen increases as temp. decreases, salinity decreases, and pressure increases

Oxygen Two sources for oxygen in lakes Two sources for oxygen in lakes Atmosphere Atmosphere Photosynthesis Photosynthesis

Atmosphere Diffusion across air-water interface and down into water column Diffusion across air-water interface and down into water column Years to reach depth of 5 m Years to reach depth of 5 m Wind-driven waves and currents distribute oxygen to lower levels Wind-driven waves and currents distribute oxygen to lower levels Too much agitation can prevent water from becoming supersaturated Too much agitation can prevent water from becoming supersaturated

Photosynthesis Most oxygen in standing waters is by- product of photosynthesis Most oxygen in standing waters is by- product of photosynthesis Phytoplankton contribute most Phytoplankton contribute most Rooted macrophytes, attached algae, benthic algae mats are chief producers in shallow lakes, lake margins Rooted macrophytes, attached algae, benthic algae mats are chief producers in shallow lakes, lake margins

Loss of Oxygen Physical - change in temperature, pressure Physical - change in temperature, pressure Biological - most important - respiration by plants, animals, bacteria (decay processes) Biological - most important - respiration by plants, animals, bacteria (decay processes) Other - methane bubbles rising from sediments through water column Other - methane bubbles rising from sediments through water column

Oxygen Distribution Distribution changes as lake goes through seasonal temperature cycle Distribution changes as lake goes through seasonal temperature cycle Orthograde distribution during spring, fall turnovers in dimictic lake Orthograde distribution during spring, fall turnovers in dimictic lake Clinograde distribution during thermal stratification Clinograde distribution during thermal stratification

Another distribution Extreme clinograde - permanently meromictic lakes, anaerobic hypolimnion Extreme clinograde - permanently meromictic lakes, anaerobic hypolimnion

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 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

Daily variation in oxygen concentrations O 2 rises during day, declines at night O 2 rises during day, declines at night The greater the plant biomass, the greater the magnitude of the cycle The greater the plant biomass, the greater the magnitude of the cycle

Daily variation in oxygen concentrations

Seasonal variation in oxygen concentrations O 2 high during summer growing season, low in late-summer when plants die O 2 high during summer growing season, low in late-summer when plants die May produce anoxia and die-offs of animals (summerkill) May produce anoxia and die-offs of animals (summerkill)

Seasonal variation in oxygen concentrations O 2 also may be low during winter in ice- covered lakes O 2 also may be low during winter in ice- covered lakes Reduced light transmission, respiration only - Winterkill of animals Reduced light transmission, respiration only - Winterkill of animals

Carbon Dioxide CO 2 increasing in concentration in atmosphere CO 2 increasing in concentration in atmosphere High solubility X > O 2 High solubility X > O 2 Follows solubility laws (pressure, temp.) Follows solubility laws (pressure, temp.) Many sources other than atmosphere: rainwater, runoff, groundwater, respiration, decomposition in sediments Many sources other than atmosphere: rainwater, runoff, groundwater, respiration, decomposition in sediments

Carbon Dioxide CO 2 behaves much differently than other gases once it dissolves in water CO 2 behaves much differently than other gases once it dissolves in water Exists in equilibrium with many additional forms of carbon Exists in equilibrium with many additional forms of carbon

CO 2 + H 2 O = H 2 CO 3 H 2 CO 3 = HCO H + HCO 3 - = CO H + Carbonic acid bicarbonate carbonate

CO 2 + H 2 O = H 2 CO 3 = HCO H + = CO H + Putting it all together Sensitive to changes in pH Low pH - left side dominates High pH - right side dominates

CO 2 + H 2 O = H 2 CO 3 = HCO H + = CO H + Putting it all together Addition of CO 2 via respiration pushes equilibrium to right and lowers pH Removal of CO 2 via photosynthesis pulls equilibrium to left and raises pH

CaCO 3 + H 2 CO 3 = Ca (HCO 3 ) 2 Ca (HCO 3 ) 2 = CaCO 3 + H 2 O + CO 2 In most natural lakes, CO 2 combines with alkali metals or alkaline earth metals to form carbonates, bicarbonates marl Free CO 2 Aggressive CO 2 dissolves CaCO 3 and drives equation to left Photosynthesis pulls equation to the right

CO 2 + H 2 O = H 2 CO 3 = HCO H + = CO H + Buffer System Ca (HCO 3 ) 2 = CaCO 3 + H 2 O + CO 2 Little change in pH despite additions of lots of acids or base, as long as supply of carbonates & bicarbonates holds out CaCO 3 + H 2 CO 3 = Ca (HCO 3 ) 2

CO 2 Distribution

Nitrogen Exists in many different forms in natural freshwater systems Exists in many different forms in natural freshwater systems A major nutrient that affects the productivity of aquatic systems A major nutrient that affects the productivity of aquatic systems

Nitrogen Dissolved gas - N 2 Dissolved gas - N 2 Ammonia - NH 3 NH 4 + Ammonia - NH 3 NH 4 + Nitrite - NO 2 - Nitrite - NO 2 - Nitrate - NO 3 - Nitrate - NO 3 - Dissolved organics Dissolved organics Amino acids Amino acids Polypeptides Polypeptides Proteins Proteins Sources: atmosphere, rain, runoff, groundwater ** Sources: atmosphere, rain, runoff, groundwater ** Losses: water outflow, adsorption to sediments, dinitrification by bacteria Losses: water outflow, adsorption to sediments, dinitrification by bacteria

Nitrogen Cycle

Ammonia Readily assimilated by plants Readily assimilated by plants Nitrification by bacteria Nitrification by bacteria Present in low concentrations in oxygenated waters Present in low concentrations in oxygenated waters

Ammonia Accumulates in hypolimnion Accumulates in hypolimnion No photosynthesis or nitrification No photosynthesis or nitrification Release from sediments during anoxia Release from sediments during anoxia

Nitrate Nitrates high in presence of oxygen Nitrates high in presence of oxygen Nitrification Nitrification Nitrates not assimilated easily by plants Nitrates not assimilated easily by plants Molybdenum needed to reduce nitrate Molybdenum needed to reduce nitrate Poor abundance in igneous basins Poor abundance in igneous basins

Nitrate Denitrification to N 2 only by anaerobic bacteria in hypolimnion Denitrification to N 2 only by anaerobic bacteria in hypolimnion Nitrate:ammonia Nitrate:ammonia Calcareous runoff 25:1 Calcareous runoff 25:1 Igneous runoff 1:1 Igneous runoff 1:1 Sewage or fertilizer 1:10 Sewage or fertilizer 1:10

Nitrate Nitrogen loading by itself often does little to change lake productivity Nitrogen loading by itself often does little to change lake productivity Phosphorus more often limiting for plant growth than nitrogen Phosphorus more often limiting for plant growth than nitrogen

Phosphorus Total concentrations in unpolluted waters mg/L Total concentrations in unpolluted waters mg/L Sources: Sources: Rainfall (unpolluted 0.1 mg/L) Rainfall (unpolluted 0.1 mg/L) Groundwater ~0.02 mg/L Groundwater ~0.02 mg/L Surface runoff - variable - often major contributor to lakes (especially with pollutants) Surface 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 >90% of P in water is in form of organic phosphates or related materials in living things or their secretions Great scarcity - limiting factor Great scarcity - limiting factor Rapid turnover of organic P between living organisms Rapid turnover of organic P between living organisms Bacteria, phytoplankton, zooplankton, others Bacteria, phytoplankton, zooplankton, others minutes, more rapid under deficiency minutes, more rapid under deficiency

Phosphorus In presence of O 2, various forms of phosphates form complexes, chelates, and insoluble salts with several metal ions In presence of O 2, various forms of phosphates form complexes, chelates, and insoluble salts with several metal ions E.g., calcium and iron E.g., calcium and iron Induce precipitation of P in oxygenated waters Induce precipitation of P in oxygenated waters

Phosphorus Distribution

Confusing, interrelated terms Alkalinity Alkalinity Hardness Hardness Salinity Salinity

Alkalinity Measure of buffering capacity of water Measure of buffering capacity of water Carbonates and bicarbonates of alkali metals Carbonates and bicarbonates of alkali metals

Hardness Calcium and magnesium salt content Calcium and magnesium salt content Temporary hardness - carbonates and bicarbonates, can be removed by boiling Temporary hardness - carbonates and bicarbonates, can be removed by boiling Precipitation of CaCO 3 Precipitation of CaCO 3 Ca(HCO 3 ) 2 = CaCO 3 + H 2 O + CO 2 Ca(HCO 3 ) 2 = CaCO 3 + H 2 O + CO 2 Permanent hardness - sulfates, chlorides, other anions Permanent hardness - sulfates, chlorides, other anions

Salinity Concentrations of Ca 2+ Mg 2+ Na + K + and HCO 3 - CO 3 2- SO 4 2- Cl - Concentrations of Ca 2+ Mg 2+ Na + K + and HCO 3 - CO 3 2- SO 4 2- Cl - Plus other ionized components of other elements Plus other ionized components of other elements