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