The Phosphorus Cycle in Lakes

Aquatic Ecology

Phosphorus

Why study P?• Biomolecules

– ADP and ATP– nucleic acids– phospholipids (cell membranes)– apatite (bones)

Forms of Phosphorus• Total P = DIP + DOP + PP

• DIP/orthophosphate – (<5%) dissolved inorganic phosphorus, major component of SRP

• PO43- polyphosphates

• DOP – dissolved organic phosphorus -- often organic colloids from living or decomposing org; less quickly available

• PP – particulate phosphorus -- often largest percentage of P in lakes (>70%)

• Algae, animals, detritus, suspended sediments• We usually measure soluble reactive phosphate (SRP) which is DIP

and some DOP. SRP is highest during mixing events (Spring epilimnion) before algae growth & lowest in summer epilimnion.

• TP is best indicator of lake’s nutrients– Avg concentrations of TP in lakes = 10-80 µg/L, but can range depending

on land use. Agriculture areas can reach 200 µg/L.

Phosphine Gas

• Poisonous gas (PH3) – produced by anaerobic bacterial decomposition of organic matter.

• Found in sewage treatment plants.• Will-of-the-wisps – phenomenon of burning

swamp gas (methane) igniting phosphine as it spontaneously oxidizes in air.

• Found in pesticides

Phosphorus and Lake Classification

The productivity of a lake is often determined by its P loading and its volume (mean depth)

Lake Productivity Classification

Total Phosphorus g/L

Ultra-oligotrophic  <5

Oligotrophic 5-10

Mesotrophic 10-30

Eutrophic 30-100

Hypereutrophic  >100

Limiting nutrient

• Theoretically, phosphorus is usually the most limiting nutrient in freshwater systems as determined by Ecological stoichiometry– Ratios of elements in plankton and other organisms

2 1/4 cups sifted cake flour2 teaspoons baking powder1/2 teaspoon salt1/2 pound Butter2 cups sugar4 large egg yolks2 teaspoons vanilla1 cup sour cream4 large egg whites

Suppose you were a baker and wanted to sabotage a rival baker by stealing supplies from his storehouse. You can carry 50 lbs. of any ingredient with you.

What do you steal in order to prevent him from making the most cakes?

The Thieving Baker

2 1/4 cups sifted cake flour2 teaspoons baking powder1/2 teaspoon salt1/2 pound Butter2 cups sugar4 large egg yolks2 teaspoons vanilla1 cup sour cream4 large egg whites

i.e. If you have plenty of everything else, then with only ½ teaspoon of salt, you can bake a cake.

Sources of Phosphorus• Weathering of calcium phosphate minerals, especially

apatite [Ca5(PO4)3OH] from sediments of ancient oceans. There are no important gaseous sources of P.

• Anthropogenic P is now often much greater than natural inputs of P in many watersheds– Sewage, agriculture, etc.

• Increased production of algae due to increased Anthropogenic P input is cultural eutrophication

• Anthropogenic P may come from – point sources (think of a pipe)– nonpoint sources (diffuse, like agriculture runoff)

Point and Nonpoint sources

thinkquest.org

Remember, when you're fertilizing the lawn, you MAY NOT just be fertilizing the lawn!Image courtesy of the Washington State Water Quality Consortium

Reservoirs of P

• Rocks - apatite• Soil – holds P• Guano – soluble organic phosphates• Atmosphere – wind-blown dust• Oceans – tectonic forces• Fresh water – Bonds w/ Ca+ & Mg+ & clays• Primary Producers – limiting factor• Consumers – calcium phosphate in bones

External vs. Internal P Loading

• “Loading” refers to input of a nutrient per unit time• External loading refers to sources outside the lake (as in

previous slides)• If all external sources of P were removed, a lake would

continue to grow algae for many years. This is because P is recycled within the lake. This recycling is termed Internal Loading

Internal P Loading

lakes.chebucto.org/DATA/PARAMETERS/TP/popup.html

• P may be recycled in the food web several times

– Phytoplankton are extremely efficient at absorbing any P that is released by excretion or decomposition

• Eventually P will be lost from lake either by outflow or by sedimentation to the lake bottom.

• P is bound in lake sediments under oxic conditions, but may be regenerated from sediments under anoxic conditions (iron and microbes play an important role)

• Deep lakes with oxic hypolimnia and long WRT may retain 70-90% of incoming P in the sediments

• Lakes with Anoxic hypolimnia retain only half as much P as lakes with oxic hypolimnia

• Therefore external loading may result in a positive feedback loop that multiplies eutrophication.

phytoplankton

decomposition

hypoxia

regeneration of P from sediments

external P loading

Bioturbation

Physical resuspention by organisms living in oxic sediments may also increase the regeneration of Phosphorus from sediments into the overlying water

J. Chaffin

Without Mayflies

With Mayflies

Phosphorus Remediation

• Eutrophication can be ugly: high algal biomass (sometimes toxic), hypoxia, fish kills, foul smells

• One answer is to reduce P loading by– Removing P from waste water– Diverting waste water Using natural or constructed wetlands to trap P– Using buffer strips to trap agricultural runoff– Using pumps to aerate the hypolimnion

Wastewater Treatment

www.defra.gov.uk

Addition of alum to precipitate P

Population equivalent (in waste-water monitoring and treatment) refers to the amount of oxygen—demanding substances whose oxygen consumption during biodegradation equals the average oxygen demand of the waste water produced by one person. For practical calculations, it is assumed that one unit equals 54 grams of BOD per 24 hours.

Buffer Strips

www.epa.gov/owow/nps/Section319III/OH.htm

NRCS

content.cdlib.org/xtf/data

Hypolimnion Aeration