Properties of Gas in Water

Oxygen Sources and Sinks

Oxygen Distribution (space & time)

Measuring Dissolved Oxygen

Measuring 1º Production and Respiration

Oxygen Dynamics & Budgets

Properties of a Gas in Water• Solubility of a gas in water is dependent on:

– Temperature (T): increase T will decrease solubility– Pressure (P): increase P will increase solubility– Salinity (S): increase S will decrease solubility

Saturation & Dissolved Oxygen

• Gas content for water continuously exposed to the atmosphere slowly equilibrates to a saturation constant (equilibrium solubility) based on temperature, pressure and salinity.

• Dissolved Oxygen (DO) is the amount of free O2 dissolved in water (not oxygen incorporated in other molecules).

• DO content in water can exceed the saturation constant. These super-saturated conditions are temporary, as excess DO will escape to the atmosphere of bubble out of solution (degas). Conditions may also be under-saturated; thereby the net flux of oxygen is into the water from the atmosphere.

O2 Sources and Sinks

• Sources (gains):– Atmosphere– Photosynthesis (PS)

CO2 + H2O + light energy → CH2O + O2

• Sinks (losses): – Atmosphere (at conditions of super saturation)– Aerobic Respiration (reverse of PS)– Microbial Chemosynthesis (often minor)– Abiotic chemical reactivity (often minor)

• DO content is the balance of sources and sinks; including that due to mixing with other water masses.– DO > 2 mg/L; aerobic; oxic– DO ≤ 2 mg/L; microaerobic; hypoxic– DO = 0 mg/L; anaerobic; anoxic

Oxygen Distribution• Effects of stratification of lakes:

– Transport through thermocline is diffusion-limited– High rates of heterotrophic activity in benthos as detritus is

consumed (in lakes with high primary production, this effect is higher due to increased “rain” of detritus and DOM).

– In amictic lakes, hypolimnion can eventually be depleted of oxygen; similarly meromictic lakes may have anoxic monomolimnion.

– In littoral zone high rates of photosynthesis by benthic macrophytes can create supersaturated conditions; similarly in eutrophic phytoplankton communities.

• Many temporal variations in DO can be attributed to diurnal cycles of photosynthesis and respiration. – In some habitats (especially eutrophic areas) DO can fluctuate

from super saturation to zero over the course of a single day.– Time of day greatly influences data; must be considered.

Oxygen Profile Interpretation

A) Orthograde:

Vary well mixed and/or oligotrophic lake

B) Clinograde:

Stable epilimnion with net PS; hypolimnion net respiration.

C) Positive Heterograde:

Light penetration to nutrient rich layer at thermocline;

D) Negative Heterograde:

Decomposition maximum at thermocline; net respiration.

Measuring Dissolved Oxygen• DO by Winkler Titration:

– The relevant chemical reactions occurring throughout the procedure are outlined below: 

Mn2+ + 2OH- + 1/2 O2 → oxygen-manganese complex + H2O (1)

oxygen-manganese complex + 4H+ + 2I- → I2 + Mn2+ + 2H2O (2)

I2 + 2Na2S2O3 → Na2S4O6 + 2NaI (3)

– Reaction Steps:

→ → →

Water sample #1 #2

#3) Add Na-thiosulfate until yellow; add starch indicator to enhance endpoint; continue titrating until clear (endpoint)

2) DO by electro-chemical probe:Voltage applied across cathode reacts with O2 and causes an electrical flow from the anode.

At cathode: O2 + 4H+ + 4e- → 2H2O

Measuring Dissolved Oxygen

OxyGuard Probe

Orion DO Probe

Net Primary Production(= Net Photosynthesis)

• Net Primary Production (NPP) is Gross Primary Production (GPP) minus Respiration (R).

• These are rate measurements and can be reported in units of mg DO/L/d, or these values can be converted to organic carbon equivalents, mg C/L/d. This conversion requires the atomic mass conversion and a photosynthetic quotient (PQ = +ΔO2 / - ΔCO2).

• NPP may be measured as:– changes in oxygen content (light/dark incubations; whole lake)– uptake of CO2 into biomass (using radioactive 14CO2).

Phytoplankton 1º Production by Light / Dark Incubations.

• Volumetric estimate of primary production is performed at depth intervals across the euphotic zone.

• These values are integrated over depth of the lake to derive an areal primary production.

•These data must be corrected for lake morphology.

• There is not the surface area of lake at each depth; so estimates must account for this difference. (See Cole Table 12-5 & Fig 12-5.)

• Lake morphology correction yields an areal phytoplankton primary production value that representing the average for any area of the lake.

• The lake morphology corrected average can be multiplied by lake surface area to yield total lake phytoplankton production.

•What about littoral zone benthic algea (periphyton) and macrophytes?

Whole Community Rates by Daily Oxygen Budgets

GPP

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R

NPP = GPP - R