NUTRIENT CYCLING AND PHOSPHORUS


I. General Principles of Nutrient cycling

    A. Energy versus nutrients
        Energy flows
        Nutrients cycle

    B. Closed system

example of closed system for nutrient cycling
 

        1. rate = cycles/time
            a. as rate increases, productivity increases
            b. total N or P versus the amount of inorganically available N or P
        2. pathways

            - In a closed system all the nutrients cycle within the system

    C. Open system

example for open system of nutrient cycling
                - Boundaries

        1. rate
        2. pathways
        3. residence time  time spent cycling before being lost from the system

            - residence time = amount of nutrient in the system/amount in output
            - nutrient use depends on recycling rate and residence time
            - inputs and outputs do not necessarily balance --
 

PHOSPHORUS
I. Importance --  Why study P?
    A. Biomolecules
        ADP and ATP, nucleic acids, phospholipids (membranes), apatite (bones and teeth)

    B. Limiting nutrient
        1. Theoretically most limiting nutrient

                                - 'Ecological stoichiometry' -- Ratio of elements in plankton and other organisms (oceanographer
                                   Redfield in the 1950's)
                                - Found an average phytoplankton composition of
                                        C          H         O         N         P         S
                                      106       263     110       16         1         0.7
                                - and compared with available nutrient ratios
                                - based on these comparisons he considered P to be the most limiting nutrient even though it is only
                                        ~1% organic matter
                                - BECAUSE the amount of P available to organisms is much less than the amount required
                                        relative to these other elements
                                - in freshwater, P is often 80,000 X less concentrated than the amount required by phytoplankton
                                - Also implies that if nothing else is limiting, then increasing P can theoretically generate >100X
                                        the weight of added P in algae

            2. Algal biomass versus total P

General relationship between total phosphorus in freshwater and algal biomass
            3.  Forms and Measurement of P
                - Total P = DIP + DOP + PP
                    i. DIP – (<5%) dissolved inorganic phosphorus -- PO43- polyphosphates
                    ii. DOP – dissolved organic phosphorus -- often associations of organic colloids; less quickly available
                        - Alkaline phosphatase enzyme mediates the release of P from these organic compounds; produced more when P is
                            limiting and can be an indicator of P limitation
                    iii. PP – particulate phosphorus -- often largest percentage of P in lakes (>70%) – nucleic acids (decompose slowly),
                        phosphate sugars, ATP (available more quickly)
                        - most P is in organic matter -- living or dead organisms;
                        - some particulate P is mineral P (not as bioavailable)
                        - phosphate adsorbed onto clays
            -Measurement of phosphorus
                    - soluble reactive phosphate (SRP) and scientists long thought this was PO43-
                    - Unreactive phosphate – break down dissolved compounds – thought was organic (SUP) + PO43-
                    - Particulate
                    - BUT, SRP is not PO4 3-; measurement procedure actually digests some organics

        4. P loading versus mean depth -- trophic state classification
Interaction of P loading and lake depth in determining lake trophic status
 
 
 
Lake Productivity Classification Total Phosphorus mg/L
Ultra-oligotrophic  <5
Oligotrophic 5-10
Mesotrophic 10-30
Eutrophic 30-100
Hypereutrophic  >100

        5. No gas phase
            i. phosphine (PH3) gas may be produced by bacterial action under strongly reducing conditions
            ii. spontaneously combusts
            iii. may be responsible for will-o'-the-wisps, moving lights over swamps and marshes

        6. Sources of P
            i. weathering of calcium phosphate minerals, especially apatite [Ca5(PO4)3OH] – is a slow process
            ii. mostly stored in marine deep ocean sediments
            iii. anthropogenic P is now often much greater than natural inputs of P in many watersheds -- sewage, urban runoff, agriculture,
                "cultural eutrophication”
                - ‘point source’ – sewage (treated or untreated), industry...
                - ‘nonpoint source’ – e.g., agriculture – animal waste, fertilizers
        7. Modes of Entry of P to aquatic systems
            i. Precipitation – dust in air
            ii. Groundwater –P adsorbs to soil particles
            iii. Surface runoff

        8. Decomposition and excretion
            i. biota persist due to well-developed, efficient recycling of P
            ii. P excreted by animals is rapidly taken up by algae and bacteria
            iii. often one major function of decomposition is the liberating of usable P
            iv.decomposition of organic matter releases orthophosphate
            v. Lack of oxygen due to decomposition actually feeds back and affects the availability of PO43- through some
                more redox reactions.

II. Redox reactions

        - P doesn't go through redox reactions itself, but it is influenced by the solubility of Fe, which changes due to its redox state
      

    A. Iron trap for P
        - In oxygenated waters, iron is present as Fe3+ (ferric)
        - At pH<7 you get reaction of ferric iron and phosphate to form vivianite -- vivianite
        - At pH> or equal to 7 you get formation of P containing ferricoxyhydroxides at high pH
        -                         stratified lake                                                                      day of turnover
iron, oxygen and orthophosphate profiles during summer stratificationiron, oxygen, and orthophosphate concentrations at the day of fall turnover
                               one week later
iron, oxygen and orthophosphate concentrations one week after turnover, during isothermal mixing

            - What happens?
                i. Fe2+ is converted to Fe3+ due to presence of oxygen
                ii. Fe3+ goes to Fex(OH)y(PO4)z , FeOH, and FePO4
            - "iron trap for P", less available for algae
            - Can be a critical point for eutrophication -- when hypolimnion becomes anoxic, then more P is released and that increases the P
                recycling and loading from within the lake as well -- contributes to increased eutrophication.
            - As long as hypolimnion remains oxic, any phosphate in sediments will be trapped by iron trap as it comes to the sediment
                surface, even if the sediments are anoxic.

    B. Sulfur trap for iron

Description of the sulfur trap for iron and its effects

        - If enough FeS precipitates you can remove enough Fe to get iron poor water and so at overturn more P is available for algal uptake
        - "Sulfur trap for iron"
        - increases phosphate release, because reduces the potential iron trap

SUMMARY OF REDOX EFFECTS ON PHOSPHATE CONCENTRATIONS:

        - Fe3+ conversion to Fe2+  releases PO43-
        - sulfur trap may lower iron concentrations enough to allow some phosphate to remain at overturn
        - All these reactions mediated by bacteria
 
 

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