WetzelLimCh.14 Phosphorus in Lakes and Streams

Phosphorus in Lakes and Streams: Comprehensive Study Notes

  1. Introduction to Phosphorus in Surface Water

Phosphorus (P) exists in various forms within surface waters, broadly categorized into particulate and dissolved fractions.

1.1 Particulate Phosphorus

This fraction encompasses P associated with solid matter greater than 0.45 µm and includes:

  • In living things: Phosphorus incorporated into the biomass of aquatic organisms such as algae, bacteria, zooplankton, and macrophytes.

  • In minerals or adsorbed to clays: Inorganic phosphorus bound to mineral particles (e.g., apatite, iron oxides) or adsorbed onto the surfaces of clay minerals.

  • Adsorbed to dead organics: Phosphorus associated with detritus and dead organic matter.

1.2 Dissolved Phosphorus

This fraction consists of phosphorus compounds that pass through a 0.45 µm filter and are readily available or become available over time. It includes:

  • Orthophosphates (PO₄³⁻): The inorganic, biologically available form of phosphorus. These are rapidly taken up by primary producers.

  • Polyphosphates: Chains of phosphate units, often found in detergents and can be hydrolyzed to orthophosphates.

  • Organic molecules with phosphorus: A diverse group of organic compounds containing phosphorus.

  • Esters of phosphate: Specific types of organic phosphorus compounds (e.g., DNA, RNA, phospholipids).

  1. Phosphorus Distribution in Lakes and Trophic States

The distribution of phosphorus varies significantly depending on a lake's trophic status and stratification patterns.

2.1 P in Stratified Lakes: Oligotrophic vs. Eutrophic

  • Oligotrophic Lakes (Very Low Productivity):

    • Characterized by low concentrations of both soluble phosphorus (Ps) and total phosphorus (PT) throughout the entire water column.

    • Oxygen and temperature profiles typically show less distinct stratification or high oxygen levels extend deep, preventing significant P release from sediments.

  • Eutrophic Lakes (High Productivity):

    • Exhibit higher concentrations of phosphorus, particularly in the hypolimnion (deeper, colder layer) during periods of stratification.

    • Soluble P (Ps) and Total P (PT) increase significantly with depth, especially under anoxic conditions, due to internal loading from sediments.

    • The deeper waters often become anoxic, driving P release.

2.2 Nutrient Profiles of African Rift Lakes

Case studies from Lake Tanganyika, Lake Malawi, and Lake Victoria illustrate these principles:

  • Lakes Tanganyika and Malawi: Both are deep, stratified lakes. They show very low Soluble Reactive Phosphate (SRP) in the oxygenated epilimnion, but SRP concentrations increase dramatically with depth in the anoxic hypolimnion, indicating considerable internal loading.

  • Lake Victoria (Comparison: 1961 vs. 1990):

    • 1961: Relatively oligotrophic conditions, with low SRP throughout the water column and oxygen extending to greater depths.

    • 1990: Exhibited significantly higher SRP concentrations, particularly in deeper waters, and shallower oxygen depletion (anoxia). This shift highlights the profound impact of increasing external P loading (eutrophication) on internal P cycling and anoxia.

  1. The Phosphorus Cycle

The aquatic phosphorus cycle involves continuous exchange among organisms, dissolved forms, and sediments.

  • Key Components: Macrophytes, Mud (sediments), Bacteria, Zooplankton, and Algae.

  • Cycling Pathways:

    • Algae absorb dissolved orthophosphate (PO₄³⁻).

    • Zooplankton consume algae.

    • Organisms die, decompose, and contribute to the organic matter in sediments (mud).

    • Bacteria decompose organic phosphorus in sediments, regenerating orthophosphate, which can then be released back into the water.

    • Macrophytes anchor in sediments, taking up phosphorus through their roots.

  • This highlights the dynamic nature of P cycling, with sediments serving as both a sink and a source of phosphorus.

  1. Factors Controlling Phosphorus Mobilization from Sediments

The sediment-water interface is a critical zone for regulating phosphorus availability in the water column.

4.1 Redox Potential (Oxygen Conditions)

  • Red Mud (Oxidized / Aerobic): Under oxidized conditions (O₂ present and high redox potential), phosphorus is typically bound to ferric iron hydroxides (Fe(OH)₃, often referred to as 'red mud').

  • Black Mud (Reduced / Anoxic): Under reduced conditions (O₂ absent and low redox potential), iron(III) hydroxides are reduced to soluble ferrous iron (Fe²⁺). This releases the bound phosphate into the interstitial water of the sediments and, subsequently, into the overlying water column (referred to as 'black mud').

    • Mortimer's Classical Experiment: Experimental sediment-water tanks demonstrated that in aerated chambers, redox potential (Eh) remained high, oxygen was present, and phosphate release was minimal. In contrast, anoxic chambers showed a sharp drop in Eh, oxygen depletion, and a significant increase in phosphate and ferrous iron concentrations in the overlying water. This confirmed the crucial role of redox status in P liberation.

4.2 Role of Sulfate-Reducing Bacteria (SRB)

Under anaerobic conditions, SRB contribute indirectly to phosphate release:

  • Iron-Reducing Bacteria (FeRB) directly reduce Fe(III) (in Fe(OH)₃-PO₄) to Fe(II) (Fe²⁺).

  • Sulfate-Reducing Bacteria (SRB) reduce sulfate (SO₄²⁻) to hydrogen sulfide (H₂S).

    • SO₄²⁻ → H₂S

  • The H₂S then reacts with Fe²⁺ to form highly insoluble iron sulfide (FeS (pyrite) ↓).

    • H₂S + Fe²⁺ → FeS ↓

  • By precipitating Fe²⁺ as FeS, SRB prevent the re-oxidation of Fe²⁺ back to Fe(OH)₃ that would re-bind phosphate. This maintains high dissolved PO₄³⁻ concentrations in the interstitial water, sustaining phosphate release into the water column.

4.3 pH and Epipelic Algae

Photosynthetic activity by epipelic microalgae (algae living on sediment surfaces) and bacterial communities can dynamically alter the microenvironment at the sediment-water interface:

  • Light conditions: Photosynthesis consumes CO₂ and produces O₂, leading to localized increases in pH and oxygen concentrations within millimeters of the sediment surface.

  • Dark conditions: Respiration consumes O₂ and produces CO₂, resulting in decreased pH and oxygen.

  • Impact on P: Elevated pH (due to photosynthesis) can promote phosphorus co-precipitation with calcium (e.g., calcite) or adsorption to metal hydroxides, potentially reducing P availability. Conversely, reduced oxygen in the dark can trigger P release.

4.4 Macrophytes Pumping P

  • Macrophytes, or aquatic plants, play a significant role in the internal cycling of phosphorus, often referred to as 'phosphorus pumping'.

  • Their root systems extend into the anoxic or sub-oxic sediments, taking up phosphorus that has been released from mineral complexes (e.g., iron-bound P under reducing conditions) or from decomposing organic matter.

  • This absorbed phosphorus is then translocated through the plant's vascular system to its leaves and other above-sediment tissues.

  • Upon senescence and decomposition, or through exudation, this phosphorus can be released into the overlying water column, effectively moving P from the sediment sink back into the active water column, making it available for other aquatic organisms like algae.

  • This process can thus contribute to internal phosphorus loading, particularly in shallow lakes where macrophytes are abundant.