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?
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