Transcript Oxygen Dynamics & Budgets

Oxygen Dynamics & Budgets
Properties of Gas in Water
Oxygen Sources and Sinks
Oxygen Distribution (space & time)
Measuring Dissolved Oxygen
Measuring 1º Production and Respiration
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 undersaturated; 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
oxygen-manganese complex + 4H+ + 2I- → I2 + Mn2+ + 2H2O
I2 + 2Na2S2O3 → Na2S4O6 + 2NaI
#3) Add Na-thiosulfate until
yellow; add starch indicator to
enhance endpoint; continue
titrating until clear (endpoint)
– Reaction Steps:
Water
sample
#1
→
#2
→
(1)
(2)
(3)
→
Measuring Dissolved Oxygen
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
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
NPP = GPP - R
R
Relative DO concentration
GPP