NUTRIENT CYCLING


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)

    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  (Redfield Ratio)
                                        C          H         O         N         P         S
                                      106       263     110       16         1         0.7
                                - Compared with available nutrient ratios
                                - 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
                                                                - 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 organic colloids; less quickly available
                        - Alkaline phosphatase enzyme mediates can be an indicator of P limitation
                    iii. PP – particulate phosphorus -- often largest percentage of P in lakes (>70%)
                       
                        - 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-
                     - BUT,  measurement procedure actually digests some organics, too

        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]
            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. 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
            decomposition of organic matter releases orthophosphate
            iv. 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

  NITROGEN CYCLING

I. Background
    A. N can exist in multiple oxidation states
         -3          0          +3          +5
     NH4     N2      NO2     NO3-
     reduced                          oxidized

    B. N is a basic component of protoplasm

II. Nitrogen cycle
    A. Global N cycle
global nitrogen cycle

    B. Aquatic N cycle
        1. Closed cycle
aquatic nitrogen cycle

        2. Example of inputs and outputs
      
        3. Human impacts

III. Reactions within the aquatic cycle
    A. NH4+ uptake by algae: ammonium uptake by phytoplankton

    B. Ammonification -- ammonium production through decomposition of organic matter: ammonification
                NH4OH toxic

    C. Nitrification -- NH4+ conversion to NO3- (oxidation; bacterial gain of energy)
nitrification part 1
nitrification part 2

    D. NO3- uptake by algae (assimilatory nitrate reduction): NO3- to organic-N(NH3)
assimilatory nitrate reduction

    E. Denitrification -- dissimilatory NO3- to N2 (reduction)
denitrification

    F. Nitrogen fixation -- N2 to organic-N(NH3); cyanobacteria

        Is very energy expensive -- 76 kcal/mole N

III. Nitrogen cycle and N limitation
    A. Patterns
        In most cases, P is limiting to algal growth in lakes
        N most often limiting to algal growth in oceans and estuaries

    B. How can you get N limitation?

Factors influencing N versus P limitation in aquatic systems

        1. Loss of fixed N -- denitrification

        2. Limits on N fixation
            a. light -- the N - N triple bond in atmospheric N2 is hard to break -- requires lots of light energy.
            b. trace elements are limiting -- iron and molybdenum needed to fix N
            c. too little phosphorus for N-fixing cyanobacteria to grow
            d. CO2 can be limiting  (only on short time scales)
 

 
 


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