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)

FormsDissolved:

orthophosphate PO43-, polyphosphates, LMW P esters

Particulate:Organic P: nucleic acids, ATP, enzymes, etcmineral forms absorbed to clay particles

(complexes with many things!)

Measured asSRP (soluble reactive P) = phosphate plus some organic PTotal 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

• Surface flows– Dissolved organic P– P sorbed to soil particles

Outputs • Export

• Internal loading

Watershed contributions to P inputs

• Little P is transported to lakes if watershed is well-watered 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 PO4

3- 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 SO4

2- 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 conditionsImportant 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 PO4

3-

– Low-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

Profundal sediments

Epilimnion

Littoral zone

Hypolimnion

Anthropogenic eutrophication

Increases of nutrients with human activities

% agriculture in river drainage (# rivers)

Inhabitants per km2 (# rivers)

Total PNitrate

Total N

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 1965.

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

• P necessary for high algal blooms

• C not necessary (fixing CO2 from atm is enough)

• A photo is worth many wordsN+P+C

N+C

See Shindler et al 1973 Can. J. Fish. Res. Bd. 30:1415-1440

• 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

Total P

Chlorophyll-a1967 nutrient diversion

completed

1963 nutrient diversion begun (~28%) See Edmondson and Lehman 1981 Limnology and

Oceanography 26:1-29

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?