Transcript Phosphorus - Carleton College

Phosphorus
Announcements:
Proposals
Last week's question of the day
Two lakes with the same surface area and similar levels of
P & N are located within km of each other, but one
routinely experiences fish kills, and the other does not.
What characteristics of these lakes are likely influencing
this pattern?
The winterkill lake is shallow enough that the water does
not contain enough oxygen to sustain oxic conditions
during ice cover.
Phosphorus (P)
Forms
Dissolved:
orthophosphate PO43-, polyphosphates, LMW P esters
Particulate:
Organic P: nucleic acids, ATP, enzymes, etc
mineral forms absorbed to clay particles
(complexes with many things!)
Measured as
SRP (soluble reactive P) = phosphate plus some organic P
Total P = phosphate plus "all" organic P
Phosphorus
Unlike N, P does not directly partake in redox rxns,
although availability is regulated, in part, by redox
reactions.
Often the limiting nutrient in lake systems
(but not always true for riverine systems)
Phosphorus budgets
External Inputs (loading)
• Dry deposition
Outputs
• Export
• Surface flows
– Dissolved organic P
– P sorbed to soil particles
• Internal loading
Watershed contributions to P inputs
• Little P is transported to lakes if watershed is wellwatered and vegetated (P is in high demand)
• Lots of P is transported to lake in poorly-vegetated
areas (including ag)
Watershed contributions, con't
• Dominant bedrock in area determines overall P
availability
• (e.g. igneous rock has low P)
Internal P-cycling views over time
• Classical theory
– Proposed by Einsele & Ohle, Germany, and Mortimer, England, early
1940's
• Sulfur modifications
– Proposed by (Hasler and Einsele 1948)
• Modern theory - the role of bacteria
Classical view of internal P cycling
Under oxic conditions, P accumulates in sediments as:
• Insoluble FePO4 precipitates (Fe3+)
• PO43- strongly sorbs to iron aggregate oxyhydroxides
(e.g., FeOOH); aggregates accumulate on sediments
• In dead algae cells and other organic matter
Classical view - anoxic conditions
Internal Loading = Sediment or hypolimnetic release of P
• In anoxic conditions, FeOOH-PO4 and FePO4
complexes dissolve, releasing PO43- and Fe2+
• Sediment PO43- concentrations 5-20 times greater than
water column
• If water column remains oxic, Fe3+ precipitates and
aggregates on the sediment surface prevent released
PO43- from diffusing upwards from anoxic sediments
and/or entering the water column
The P-cycling model, updated
The role of sulfur
• Microbial reduction of SO42- yields S2• Sulfide forms FeS or FeS2 (insoluble)
• If enough Fe is removed, less P-Fe complexes are
formed and more P remains available
Sulfur uptake of Fe not important in lakes with low levels of
S (e.g., igneous rock watershed)
Modern model of P-cycling
Classical models assumed that
microbes indirectly affected P
cycling by utilizing dissolved
O2, NO3-, SO42-, Fe3+ and Mn4+
as electron donors and there
by affecting the solubility of
chemical species
versus
Modern models suggest that
microbes play an active part
in P-cycling
Why re-evaluate?
• Fe2+ and PO43- were not released simultaneously as
they should if process were completely chemical
• Observed that sediments less able to take up P when
sterilized with antibiotics, implying bacterial role
• In some lakes, P is not released when the hypolimnion
becomes anoxic, suggesting that sediment P content
and retention is not controlled only by O2
Roles of bacteria in P-cycling
• Bacteria release P during decomposition
– SRP directly into water column following cell lysis
– polyphosphate granules accumulated under aerobic
conditions
Important because between 10 and 75 % of potentially soluble
sediment P in microbes
• Iron reducing bacteria (use Fe3+ as electron acceptor)
are necessary to solubilize the Fe-P aggregates under
anoxic conditions
Other processes of P release
• Elevated pH
– may replace P absorbed to FeOOH flocs with OH-
• Benthic algae films
– may reduce P-release while photosynthesizing, and increase
P-release while respiring
• Turbulence (wind or gas bubbles)
– Allows dissolved P in anoxic sediments to bypass Fe floc
layer and pass directly to water column
Other processes of P release
• Bioturbation
– Introduces O2 into sediments
– In the process of resuspending sediments, release soluble P
• Rooted aquatic plants
– Release P that originated from
sediments
www.fishontario.com/articles/ carp-european-style/
Epilimnetic P-cycling
• Primarily cycling between bacteria and phytoplankton
• P forms in epilimnion
–
–
–
–
Particulate P
Reactive inorganic soluble PO43Low-molecular weight organic P compounds
High -molecular weight colloidal compounds
• Phytoplankton & bacteria have enzymes that help with
uptake of low molecular weight organic P
• Particulate P can be lost to sedimentation
Are lakes P-sinks or sources?
• Depends on lake characteristics…
Lakes with oxic hypolimnia
• Usually have what type of characteristics?
Large hypolimnia that holds large mass of O2 during
stratification
Low productivity
Lakes with oxic hypolimnia
• P sink
– Store about 2x as much external P load than lakes with
anoxic hypolimnia
– P stored in sediments increases exponentially with water
residence time (WRT)
• Retained P = 1/(1+sqrt(WRT))
• Deep lakes with WRT > 25 yrs often retain 70-90 % P
input permanently in sediments
Lakes with anoxic hypolimnia
• Usually have what types of characteristics?
Relatively small hypolimnia
Short(er) water residence times
High external P load
High productivity
Lakes with anoxic hypolimnia
• P-source
• Experience high internal P loading
– Significant amounts of P is not stored permanently in
sediments
A current model of P cycling
Epilimnion
Littoral zone
Hypolimnion
Profundal sediments
Anthropogenic eutrophication
Increases of nutrients with human activities
Total P
Nitrate
Inhabitants per km2 (# rivers)
Total N
% agriculture in river drainage (# rivers)
The detergent wars 1969-1970
Detergent foam from a fountain in front of the National Gallery of Art, Wash.,
DC, in 1959, when nonbiodegradable detergents were in common use. Under
gov't pressure, the detergent industry developed biodegradable detergents in
But detergents still contained phosporus…
• In 1969, greater than %50 of phosphorus in municipal
waster was from detergents (in the form of
polyphosphates)
• Huge increase since 1949 in the powdered detergents
used in washing machines
• Role of P in detergents was primarily to soften water
• Powerful U.S. Soap & Detergent Industry strongly
resisted change
Evidence
Lake Erie
Importance of P: whole lake manipulations at
ELA
N+P+C
• P necessary for high algal
blooms
• C not necessary (fixing CO2
from atm is enough)
• A photo is worth many words
N+C
See Shindler et al 1973 Can. J. Fish. Res. Bd. 30:1
• My power detergent now says: "biodegradable anionic
and nonionic surfactants (followed by lots of unspecified
ingredients)…. Contains less than 0.5% phosphorus by
weight"
P remediations
Lake Washington, Seattle, WA
Lake Washington wastewater diversion
Chlorophy
ll-a1967 nutrient diversion
completed
Total
P
1963 nutrient diversion begun
(~28%)
See Edmondson and Lehman 1981 Limnology and
Lake Washington wastewater diversion
• One of the first U.S. studies to demonstrate the
feasibility and impact of reducing secondary and
primary effluent sources
• Succeeded in part due to Lake Washington's
– very deep basin that during stratification remained
oxygenated, even at the peak of eutrophication
– rapid flushing rate (short WRT)
– primarily urban and forested watershed
Other methods of remediation
•
•
•
•
•
•
Point source reductions (Lake Washington)
Sewage treatment plants
Diversions
Use natural or constructed wetlands to absorb nutrients
Buffer strips
Precipitate water column P
Add aluminum sulfate (alum) or ferric chloride to precipitate P as AlPO4,
FePO4 or Fe(OOH)PO4
• Dredge P-rich sediments
• Withdraw hypolimnetic water that is high in dissolved P
• Hypolimnetic aeration
Question of the day:
• Explain why Lake Washington's watershed, morphology
and flushing rate influenced recovery from nutrient
loading. WHY are these characteristics important?
• Under what conditions (lake characteristics) would
simply reducing P-inputs not work? Why not?