Phosphorus Dynamics

Background

  • Nutrient: any element that an organism must take in to live, grow, and reproduce

    • Ex. Phosphorus (P) is used in the synthesis of DNA, RNA, and ATP

  • Macronutrients: C, H, N, O, P, S

    • Hydrologic = C and H (water)

    • Gaseous = C and N (main nutrients in atmosphere)

    • Sedimentary = P and S (mostly in rocks and soil)

Liebig’s Law of Minimum

  • Composition of Phosphorus

    • Biosphere = 0.03

    • Lithosphere = 0.08

    • Hydrosphere = ~0

    • Atmosphere = ~0

    • Demand/Supply (in water) = 80000

Phosphorus

  • No other nutrient has been studied more intensively in limnology

  • In contrast to N (with is many forms), the most significant for of inorganic P is: orthophosphate (PO43-)

Forms of Phosphorus

  • Particulate Phosphorus (>95%) - fraction that would be found on a filter, when water is filtered

    • a) P in organisms

    • b) Mineral P

    • c) Detritus P

  • Soluble (colloidal) Phosphorus (<3%) - soluble high molecular with compounds

  • XP (<1%) - soluble low molecular with organic P compounds

  • Biological Available Phosphorus is the dissolved inorganic forms (DIP) (<1%) - chemically reactive forms (e.g. PO43-)

  • Although PO43- is the most important form of P, and the form that algae use, it is difficult to measure, and so most limnological surveys use total phosphorus (TP)

    • TP range is from <1 μg/L to > 200 μg/L

    • most lakes in Canada are from ~10 - 50 μg/L

    • Note: we are talking about very low concentrations of TP that can cause big changes in lake systems

    • Remember: 1 μg/L = 1ppb

    • 1 ppb ~ ¼ tsp in an Olympic sized swimming pool

Lake Trophic State Classification

  • Oligotrophic = nutrient poor, low production

  • Mesotrophic = medium level of nutrients, medium production

  • Eutrophic = high level of nutrients, high production

Lake Productivity as a Function of TP

  • North American Lake Classification:

    • Ultra-oligotrophic, TP (μg/L) = <5

    • Olgio-mesotrophic, TP (μg/L) = 5 - 10

    • Mesoeutrophic, TP (μg/L) = 10 - 30

    • Eutrophic, TP (μg/L) = 30 - 100

    • Hypereutrophic, TP (μg/L) = >100

  • Regions in Ontario, that have many low nutrient lakes, tend to have lower TP boundaries for the 3 main lake categories:

    • Oligotrophic, TP (μg/L) = <10

    • Mesotrophic, TP (μg/L) = 10 - 20

    • Eutrophic, TP (μg/L) = >20

Eutrophication

  • Eutrophication: phenomenon of nutrient enrichment

  • Among limnologist, eutrophication is synonymous with increased growth of biota that is beyond what would have occurred in the absence of a perturbation

  • Cultural eutrophication: nutrient enrichment as a result of human activities, such as sewage inputs and/or agricultural run-off

  • Symptoms of Eutrophication:

    • Algal and cyanobacteria blooms

    • Excessive macrophyte growth

    • Deepwater oxygen depletions

Sources of Phosphorus

  • I. Allochthonous:

    • 1) Runoff (i.e. diffuse sources)

    • 2) Industry and municipalities (i.e. point sources like sewage)

    • 3) Weathering (slow)

    • 4) Precipitation

    • 5) Groundwater

  • II. Autochthonous:

    • 1) Fecal material from zooplankton and fish

    • 2) Macrophytes - can act as P pumps, take up P from sediments & release it into water. During die-back (at end of growing season) can release huge amounts of P

    • 3) From sediments (internal loading and the oxidized microzone)

Internal loading vs. External loading

  • Internal loading: the release of phosphorus from sediments into the water column, often due to processes like anoxic conditions

  • External loading: involves the influx of phosphorus from external sources such as agricultural runoff or wastewater discharges

Sediments

  • Sediments can be a significant source of P

    • closely related to the redox conditions (related to oxygen levels) at sediment-water interface

  • If you have oxidized microzone (i.e. oxygen at sediment-water interface), then you have a barrier to PO43- (orthophosphate) release from the sediments

  • If the sediment-water interface goes anoxic (reducing conditions), you lose the barrier of the oxidized microzone, and you get release of PO43- (orthophosphate) from sediments = internal loading

Iron

  • Iron (Fe) plays a key role

    • 2 forms: the oxidized form of Fe+++ (ferric) and the reduced form of Fe++ (ferrous)

  • If the sediment-water interface has an oxidized microzone, then iron is in ferric (Fe+++) form

    • Fe+++ and PO4- - - bind under oxic conditions

  • If the sediment-water interface goes anoxic, and the barrier of the oxidized microzone disappears, then iron is in the ferrous (Fe++) form

    • Fe++ no longer binds to PO4- - - , and so PO4- - - is released from sediments = internal loading

Proving the case for P

  • I. Empirical studies

    • e.g. studying the relationship between primary production and lakewater P concentration

  • II. Experimental studies

    • 1) Lab Studies

    • 2) Mesocosms

    • 3) Whole lake experiments

Luxury Consumption

  • Luxury consumption of P by algae = taking up excess P and storing it

Blue-green algae

  • Blue-green algae can store P in polyphosphate bodies

    • This ability allows them to thrive in nutrient-rich environments, potentially leading to harmful algal blooms and creating “whiting”

Is P always limiting?

  • Often P is limiting nutrient, but as waters becoming increasingly nutrient-rich, N can become a limiting nutrient