Dissolved GasesDissolved Gases

Important GasesImportant Gases 6 important gases are 6 important gases are

dissolved in lakes, dissolved in lakes, streams, seasstreams, seas

NitrogenNitrogen OxygenOxygen Carbon dioxideCarbon dioxide MethaneMethane Hydrogen sulfideHydrogen sulfide AmmoniaAmmonia All have important All have important

functions, but differ in functions, but differ in behavior, originbehavior, origin

Air Provides Some GasesAir Provides Some Gases

Atmosphere has Atmosphere has enough nitrogen enough nitrogen (78%), oxygen (21%), (78%), oxygen (21%), and carbon dioxide and carbon dioxide (0.03%) to serve as (0.03%) to serve as primary sourceprimary source

Others present only in Others present only in trace amounts in trace amounts in atmosphereatmosphere

Other Gas SourcesOther Gas Sources

MethaneMethane - anaerobic - anaerobic breakdown of breakdown of plants/animalsplants/animals

Hydrogen sulfideHydrogen sulfide - - chemical/bacterial chemical/bacterial transformationstransformations

AmmoniaAmmonia - - breakdown of breakdown of nitrogenous materials nitrogenous materials by bacteria, some by bacteria, some animalsanimals

How much gas is dissolved in How much gas is dissolved in water at any given time?water at any given time?

Dependent on several Dependent on several factors:factors:

Solubility factorSolubility factor

PressurePressure

TemperatureTemperature

SalinitySalinity

Solubility FactorSolubility Factor

Not all gases Not all gases dissolve in water to dissolve in water to same extentsame extent

Some gases Some gases dissolve very easily dissolve very easily in water, some in water, some dissolve very littledissolve very little

Pressure (atmosphere)Pressure (atmosphere)

Amount of gas Amount of gas absorbed by water is absorbed by water is proportional to its proportional to its partial pressurepartial pressure in the in the atmosphere (conc. = atmosphere (conc. = solubility factor X solubility factor X partial pressure)partial pressure)

Altitude decreases Altitude decreases saturation levelsaturation level by by ~1.4% per 100 m~1.4% per 100 m

TemperatureTemperature

Solubility of gas in Solubility of gas in water decreases as water decreases as temperature risestemperature rises

Generalization - cold Generalization - cold water can hold more water can hold more gas in solution than gas in solution than warm waterwarm water

Nearly linear Nearly linear relationship within relationship within normal range of normal range of natural water natural water temperaturestemperatures

SalinitySalinity

Presence of various Presence of various minerals in solution minerals in solution lowers the solubility of lowers the solubility of gasesgases

Generally disregarded Generally disregarded in limnology because in limnology because freshwaters have freshwaters have salinity near zerosalinity near zero

SalinitySalinity

Oceans (salinity of Oceans (salinity of 3.5%) have reduced 3.5%) have reduced gas saturation values gas saturation values of ~18-20%of ~18-20%

Saline pools/lakes Saline pools/lakes can have much higher can have much higher salinities (5-6 X ocean salinities (5-6 X ocean values)values)

Important Important consideration here for consideration here for gas solubilitiesgas solubilities

Relative SaturationRelative Saturation

Relation between existing solubility (amount of Relation between existing solubility (amount of gas present) and the equilibrium content gas present) and the equilibrium content expected at same temperature and partial expected at same temperature and partial pressurepressure

Can be less, or more (supersaturation)Can be less, or more (supersaturation)

OxygenOxygen

Abundant and dissolves readily in waterAbundant and dissolves readily in water Needed for respiration by organisms and for Needed for respiration by organisms and for

complete breakdown of organic mattercomplete breakdown of organic matter Relatively easy to measureRelatively easy to measure

OxygenOxygen

1/4 as abundant as nitrogen in atmosphere, but 1/4 as abundant as nitrogen in atmosphere, but twice as solubletwice as soluble

Solubility of oxygen Solubility of oxygen increasesincreases as temp. as temp. decreasesdecreases, salinity , salinity decreasesdecreases, and pressure , and pressure increasesincreases

OxygenOxygen

Two sources for Two sources for oxygen in lakesoxygen in lakes

AtmosphereAtmosphere

Photosynthesis Photosynthesis

AtmosphereAtmosphere Diffusion across air-water Diffusion across air-water

interface and down into interface and down into water columnwater column

Years to reach depth of 5 Years to reach depth of 5 mm

Wind-driven waves and Wind-driven waves and currents distribute oxygen currents distribute oxygen to lower levels to lower levels

Too much agitation can Too much agitation can prevent water from prevent water from becoming supersaturatedbecoming supersaturated

PhotosynthesisPhotosynthesis

Most oxygen in Most oxygen in standing watersstanding waters is by- is by-product of product of photosynthesisphotosynthesis

Phytoplankton Phytoplankton contribute mostcontribute most

Rooted macrophytes, Rooted macrophytes, attached algae, attached algae, benthic algae mats are benthic algae mats are chief producers in chief producers in shallow lakes, lake shallow lakes, lake marginsmargins

Loss of OxygenLoss of Oxygen

PhysicalPhysical - change in temperature, - change in temperature, pressurepressure

BiologicalBiological - most important - - most important - respiration by plants, animals, respiration by plants, animals, bacteria (decay processes)bacteria (decay processes)

OtherOther - methane bubbles rising from - methane bubbles rising from sediments through water columnsediments through water column

Oxygen DistributionOxygen Distribution

Distribution changes as lake goes Distribution changes as lake goes through seasonal temperature cyclethrough seasonal temperature cycle

OrthogradeOrthograde distribution during spring, distribution during spring, fall turnovers in dimictic lakefall turnovers in dimictic lake

ClinogradeClinograde distribution during thermal distribution during thermal stratificationstratification

Another distributionAnother distribution

Extreme clinograde - permanently Extreme clinograde - permanently meromictic lakes, anaerobic hypolimnionmeromictic lakes, anaerobic hypolimnion

Daily, seasonal variation in Daily, seasonal variation in oxygen concentrationsoxygen concentrations

The more plant material in a lake or pond, The more plant material in a lake or pond, the more prone that system is to both daily the more prone that system is to both daily and seasonal variations in dissolved and seasonal variations in dissolved oxygen contentoxygen content

Daily variation in oxygen Daily variation in oxygen concentrationsconcentrations

OO22 rises during day, declines at night rises during day, declines at night

The greater the plant biomass, the greater The greater the plant biomass, the greater the magnitude of the cyclethe magnitude of the cycle

Daily variation in oxygen Daily variation in oxygen concentrationsconcentrations

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Seasonal variation in oxygen Seasonal variation in oxygen concentrationsconcentrations

OO22 high during summer growing season, high during summer growing season,

low in late-summer when plants dielow in late-summer when plants die May produce May produce anoxiaanoxia and die-offs of and die-offs of

animals (animals (summerkillsummerkill))

Seasonal variation in oxygen Seasonal variation in oxygen concentrationsconcentrations

OO22 also may be low during winter in ice- also may be low during winter in ice-

covered lakescovered lakes Reduced light transmission, respiration Reduced light transmission, respiration

only - only - WinterkillWinterkill of animals of animals

Carbon DioxideCarbon Dioxide

COCO2 2 increasing in concentration in increasing in concentration in

atmosphereatmosphere High solubility - 200 X > OHigh solubility - 200 X > O22

Follows solubility laws (pressure, temp.)Follows solubility laws (pressure, temp.) Many sources other than atmosphere: Many sources other than atmosphere:

rainwater, runoff, groundwater, respiration, rainwater, runoff, groundwater, respiration, decomposition in sedimentsdecomposition in sediments

Carbon DioxideCarbon Dioxide

COCO2 2 behaves much differently than other behaves much differently than other

gases once it dissolves in watergases once it dissolves in water Exists in equilibrium with many additional Exists in equilibrium with many additional

forms of carbonforms of carbon

CO2 + H2O = H2CO3

H2CO3 = HCO3- + H+

HCO3- = CO3

2- + H+

Carbonic acid

bicarbonate

carbonate

CO2 + H2O = H2CO3 = HCO3- + H+ = CO3

2- + 2H+

Putting it all togetherPutting it all together

Sensitive to changes in pH

Low pH - left side dominates

High pH - right side dominates

CO2 + H2O = H2CO3 = HCO3- + H+ = CO3

2- + 2H+

Putting it all togetherPutting it all together

Addition of CO2 via respiration pushesequilibrium to right and lowers pH

Removal of CO2 via photosynthesis pullsequilibrium to left and raises pH

CaCOCaCO33 + H + H22COCO33 = Ca (HCO = Ca (HCO33))22

Ca (HCOCa (HCO33))22 = CaCO = CaCO3 3 + H + H22O + COO + CO22

In most natural lakes, COIn most natural lakes, CO22 combines with combines withalkali metals or alkaline earth metals toalkali metals or alkaline earth metals toform carbonates, bicarbonatesform carbonates, bicarbonates

marl Free CO2

Aggressive COAggressive CO22 dissolves CaCO dissolves CaCO33 and drives equation to left and drives equation to left

Photosynthesis pulls equation to the rightPhotosynthesis pulls equation to the right

CO2 + H2O = H2CO3 = HCO3- + H+ = CO3

2- + 2H+

Buffer SystemBuffer System

Ca (HCOCa (HCO33))22 = CaCO = CaCO3 3 + H + H22O + COO + CO22

Little change in pH despite additions of lots of acids orLittle change in pH despite additions of lots of acids orbase, as long as supply of carbonates & bicarbonatesbase, as long as supply of carbonates & bicarbonatesholds outholds out

CaCOCaCO33 + H + H22COCO33 = Ca (HCO = Ca (HCO33))22

COCO22 Distribution Distribution

COCO22 Distribution Distribution

COCO22 Distribution Distribution

NitrogenNitrogen

Exists in many Exists in many different forms in different forms in natural freshwater natural freshwater systemssystems

A major nutrient that A major nutrient that affects the affects the productivity of aquatic productivity of aquatic systemssystems

NitrogenNitrogen

Dissolved gas - NDissolved gas - N22

Ammonia - NHAmmonia - NH33 NH NH44++

Nitrite - NONitrite - NO22--

Nitrate - NONitrate - NO33--

Dissolved organicsDissolved organics Amino acidsAmino acids PolypeptidesPolypeptides ProteinsProteins

Sources: atmosphere, Sources: atmosphere, rain, runoff, rain, runoff, groundwater **groundwater **

Losses: water Losses: water outflow, adsorption to outflow, adsorption to sediments, sediments, dinitrification by dinitrification by bacteriabacteria

Nitrogen CycleNitrogen Cycle

AmmoniaAmmonia

Readily assimilated Readily assimilated by plantsby plants

Nitrification by Nitrification by bacteriabacteria

Present in low Present in low concentrations in concentrations in oxygenated watersoxygenated waters

AmmoniaAmmonia

Accumulates in Accumulates in hypolimnionhypolimnion

No photosynthesis or No photosynthesis or nitrificationnitrification

Release from Release from sediments during sediments during anoxiaanoxia

NitrateNitrate

Nitrates high in presence Nitrates high in presence of oxygenof oxygen

NitrificationNitrification Nitrates not assimilated Nitrates not assimilated

easily by plantseasily by plants Molybdenum needed to Molybdenum needed to

reduce nitratereduce nitrate Poor abundance in Poor abundance in

igneous basinsigneous basins

NitrateNitrate

Denitrification to NDenitrification to N22

only by anaerobic only by anaerobic bacteria in bacteria in hypolimnionhypolimnion

Nitrate:ammoniaNitrate:ammonia Calcareous runoff 25:1Calcareous runoff 25:1 Igneous runoff 1:1Igneous runoff 1:1 Sewage or fertilizer Sewage or fertilizer

1:101:10

NitrateNitrate

Nitrogen loading by Nitrogen loading by itself often does little itself often does little to change lake to change lake productivityproductivity

Phosphorus more Phosphorus more often limiting for plant often limiting for plant growth than nitrogengrowth than nitrogen

PhosphorusPhosphorus

Total concentrations in unpolluted waters Total concentrations in unpolluted waters 0.01-0.05 mg/L0.01-0.05 mg/L

Sources:Sources: Rainfall (unpolluted <0.03 mg/L) (polluted Rainfall (unpolluted <0.03 mg/L) (polluted

>0.1 mg/L)>0.1 mg/L) Groundwater ~0.02 mg/LGroundwater ~0.02 mg/L Surface runoff - variable - often major Surface runoff - variable - often major

contributor to lakes (especially with pollutants)contributor to lakes (especially with pollutants)

PhosphorusPhosphorus

>90% of P in water is in form of organic >90% of P in water is in form of organic phosphates or related materials in living phosphates or related materials in living things or their secretionsthings or their secretions

Great scarcity - limiting factorGreat scarcity - limiting factor Rapid turnover of organic P between living Rapid turnover of organic P between living

organismsorganisms Bacteria, phytoplankton, zooplankton, othersBacteria, phytoplankton, zooplankton, others 5-100 minutes, more rapid under deficiency5-100 minutes, more rapid under deficiency

PhosphorusPhosphorus

In presence of OIn presence of O22, various forms of , various forms of

phosphates form complexes, chelates, phosphates form complexes, chelates, and insoluble salts with several metal ionsand insoluble salts with several metal ions E.g., calcium and ironE.g., calcium and iron Induce precipitation of P in oxygenated watersInduce precipitation of P in oxygenated waters

Phosphorus DistributionPhosphorus Distribution

Phosphorus DistributionPhosphorus Distribution

Confusing, interrelated termsConfusing, interrelated terms

AlkalinityAlkalinity

HardnessHardness

SalinitySalinity

AlkalinityAlkalinity

Measure of buffering capacity of waterMeasure of buffering capacity of water

Carbonates and bicarbonates of alkali Carbonates and bicarbonates of alkali metalsmetals

HardnessHardness

Calcium and magnesium salt contentCalcium and magnesium salt content Temporary hardnessTemporary hardness - carbonates and - carbonates and

bicarbonates, can be removed by boilingbicarbonates, can be removed by boiling Precipitation of CaCOPrecipitation of CaCO33

Ca(HCOCa(HCO33))22 = CaCO = CaCO33 + H + H22O + COO + CO22

Permanent hardnessPermanent hardness - sulfates, chlorides, - sulfates, chlorides, other anionsother anions

SalinitySalinity

Concentrations of CaConcentrations of Ca2+2+ Mg Mg2+2+ Na Na++ K K++ and and HCOHCO33

-- CO CO332-2- SO SO44

2-2- Cl Cl--

Plus other ionized components of other Plus other ionized components of other elementselements