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The Phosphorus Cycle in Lakes

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Presentation on theme: "The Phosphorus Cycle in Lakes"— Presentation transcript:

1 The Phosphorus Cycle in Lakes
Aquatic Ecology


3 Phosphorus Why study P? Biomolecules ADP and ATP nucleic acids
phospholipids (cell membranes) apatite (bones)     

4 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 = µg/L, but can range depending on land use. Agriculture areas can reach 200 µg/L.

5 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

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

7 Lake Productivity Classification
Total Phosphorus mg/L Ultra-oligotrophic  <5 Oligotrophic 5-10 Mesotrophic 10-30 Eutrophic 30-100 Hypereutrophic  >100

8 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

9 The Thieving Baker 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? 2 1/4 cups sifted cake flour 2 teaspoons baking powder 1/2 teaspoon salt 1/2 pound Butter 2 cups sugar 4 large egg yolks 2 teaspoons vanilla 1 cup sour cream 4 large egg whites

10 2 1/4 cups sifted cake flour 2 teaspoons baking powder 1/2 teaspoon salt 1/2 pound Butter
2 cups sugar 4 large egg yolks 2 teaspoons vanilla 1 cup sour cream 4 large egg whites i.e. If you have plenty of everything else, then with only ½ teaspoon of salt, you can bake a cake.

11 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)

12 Point and Nonpoint sources

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

14 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

15 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

16 Internal P Loading 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.

17 external P loading phytoplankton  regeneration of P from sediments  decomposition  hypoxia

18 Bioturbation With Mayflies Without Mayflies J. Chaffin Physical resuspention by organisms living in oxic sediments may also increase the regeneration of Phosphorus from sediments into the overlying water

19 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

20 Wastewater Treatment 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. Addition of alum to precipitate P

21 Buffer Strips NRCS

22 Hypolimnion Aeration

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