• P is separated into several forms in the analytical process primarily through mechanical filtration.

• The orthophosphate anion (PO₄^-3) is the form available for uptake.

• The result from the undigested filtrate fraction is referred to as "soluble reactive phosphorous" (SRP) and is analogous to dissolved inorganic phosphorous (DIP).

DIP + DOP = TDP

PIP + POP = TPP

TDP + TPP = TP

SRP --> DIP    (DOP is usually small)

SRP --> TDP

TPP = TP - SRP

• TP ranges from 5 mg/L (5 ppm) in sewage effluent (most of which is SRP) to as little as 0.005 mg/L (5 ppb) in remote oligotrophic lakes.

• A detection level of at least 0.002 mg/L (2 ppb) is mandatory in lake research.

• Fig. 4.1 shows the aquatic phosphorous cycle.

• The ultimate source of P to aquatic ecosystems is from phosphate rock.

• P is utilized through plant uptake of DIP associated with photosynthesis, chemosynthesis, and decomposition.

• All organisms require P for metabolism and structure.

• Photosynthesis is mostly responsible for the uptake of DIP.

• Macrophytes and bacteria can also remove DIP from the water.

• Phytoplankton and bacteria are consumed by animal grazers, who in turn are consumed by predators.

• A fraction of DIP can enter the organic pools (DOP, POP) though excretion and death.

• DOP and POP can be recycled to DIP by bacteria.

• PIP and POP can settle into the sediments, and eventually converted to DIP.

• DIP can be released from the sediments.

• Sedimentation of dead phytoplankton and fecal pellets from phytoplankton and zooplankton result in loss of P from the open water.

• Physico-chemical conditions at the sediment-water interphase determine the release of P back to the water column.

4.1.3  SEDIMENT-WATER INTERFACE PROCESSES

• The exchange of P between sediment and water depends on several factors, acting separately or together:
  
  
  • redox potential

  • pH

  • water exchange, as it affects diffusion and transport

  • temperature, as it affects microbial activity

  • relative fractions of P in the sediment that are bound with iron/aluminum, calcium and organic matter.

• The relative importance of these factors varies with depth and degree of thermal stratification.

• At the onset of thermal stratification, dissolved oxygen (DO) declines in the hypolimnion due to microbial decomposition of organic matter.

• As DO approaches 0, reducing conditions prevail and iron in the surficial sediments is reduced from its ferric form (Fe³⁺) to is ferrous form (Fe²⁺).

• P, that was bound to the hydroxy complexes of ferric iron is now solubilized and released into the intersticial pore water, and is available for diffusion into the overlying, anoxic water.

• The rate of diffusion is a function of the concentration gradient in SRP between the intersticial pore water and the overlying water.

• The hypolimnetic P content increases more or less linearly throughout the stratified period.

• Rates of release attributed to the iron redox process are variable.

• Values as high as 52 mg/m²/day with mean of 16 mg/m²/day have been reported.

• When the lake destratifies in the autumn, the whole water column and surficial sediments are replenished with DO.

• Ferrous iron is then oxidized to the ferric state and P is once again sorbed in the hydroxy complexes and returned to the sediments (Fig. 4.2).

• Under oxic conditions, the solubility of iron is controlled by pH.

• At pH= 6, the solubility of ferric iron is minimal, and P can be effectively removed from the water column.

• With increasing pH, the solubility of iron increases and P is released from the sediments.

• High photosynthetic rates in eutrophic lakes can increase pH to 10, which produces high rates of release of P from the sediments.

• High pH due to photosynthesis can maintain high P in the water column.

• Temperature can be important in the release of P from sediments.

• The role of temperature is related to the stimulation of bacterial activity.

• Large release of P from sediments during summer can be attributed to iron-redox.

• In Lake Trummen in Sweden, removal of 1-m of rich sediment quickly resulted in recovery of the lake.

• Fig. 4.3 shows the redox potential in sediments and overlying water in two English lakes.

• Fig. 4.10 shows a hypothetical steady-state model of phosphorous cycling in a lake.

• Conclusions regarding sediment as source or sink of P:
  
  
  • Sediments are nearly always a sink for P.

  • Sediments can act as significant sources during a portion of the year.

  • Whether the sediment-released P actually reaches the photic zone and is available for algal uptake is a significant issue.

  • So long as sediments reach the hypolimnion, they are technically a source of P.

  • There is ample evidence to support the statement that some sediment-released P is transported to the epilimnion.

4.2  NITROGEN

• The nitrogen cycle (Fig. 4.11) is very complex.

• Two large biological source and sink processes occur with N that do not occur with P.

• There are:
  
  
  • The microbial fixation of N₂ from the atmosphere.
    
  • The return of nitrogen to the atmosphere through denitrification.

• The processes of nitrification (oxidation of ammonia) and denitrification may occur without biological mediation but at a slow rate.

• Microorganisms greatly speed up these processes.

• Nitrogen is most abundant as N₂, constituting 78% of the atmospheric gases.

• Nitrate (NO₃^-) is the form of nitrogen that can be used by plants.

• Its concentration varies from a trace when productivity is high, to 1 mg/L when not used.

• Concentrations above 1 mg/L are usually associated with artificial inputs (fertilization).

• Ammonia (NH₃) or ammonium (NH₄⁺), which is the principal form in water, becomes abundant in the absence of DO or in very enriched waters, but it is usually less abundant than nitrate.

• Plants often prefer ammonium to nitrate because it is in a more reduced form.

• Nitrification is the process by which NH₃ is transformed first into NO₂ and then into NO₃.

• The process occurs only under aerobic conditions.

• Organisms that normally perform the transformations are Nitrosomonas and Nitrobacter.

• The yield of energy by nitrification is rather low compared to other transformations in the nitrogen cycle.

2NH4+ + 3O2 --> 2NO2- + 2H2O + 4H+ + energy

2NO2- + O2 --> 2NO3- + energy

Denitrification

• Denitrification occurs only in the absence, or near absence, of oxygen.

• A common denitrifier is Thiobacillus denitrificans.

5S2- + 6NO3- + 2H2O --> 5SO42- + 3N2 + 4H+ + energy

• In denitrification, the bacteria reduce nitrate first to nitrite and then to molecular nitrogen or N₂O (nitrous oxide).

• This process removes nitrogen from ecosystems or wastewater.

• The necessary alternation of aerobiosis for nitrification, and anaerobiosis for denitrification has implications for management.

• Example: The Llanos de Mojos "camellones".

• Nitrogen fixation is an energy-consuming aerobic process carried on in aquatic environments by bacteria such as Azotobacter and Clostridium and by blue-green algae (Cyanobacteria) Nostoc and others.

• Nitrogen fixation can represent a significant input of N to an ecosystem.

• Measured rates are from 0.00004 to 0.072 mg/L/day.

• In Clear Lake, in California, nitrogen fixation contributed 43% of the annual N input.

• Because N fixation is an energy-demanding process, it becomes advantageous only when nitrate or ammonium are no longer available.

• N fixation also increases with productivity as nitrate is depleted.

• In a freshwater lake, the residence time of N tends to be longer than P, because P tends to be removed to the sediments.

• N, as nitrate or ammonium, is much more soluble than P.

• N is limited in enriched waters through denitrification, but not as much in waters of low to moderate enrichment.

• N fixation from the atmosphere occurs in aerobic environments.

• N fixation occurs only when ecosystem is depleted of alternate sources of N.

• N occurs in precipitation more than P.

• In phosphorous-poor watersheds, little amounts of P in rain can be important.

• Nitrate in rain is very common, having been transformed from N₂ to NO₃^- in the atmosphere by lighting.

• There are fewer sources of P than for N, and sedimentation is probably a more efficient remover of P than N in most aquatic ecosystems.

• In moderately enriched systems, P should be limiting.

• In highly enriched systems, the recycling of P and the loss of N through denitrification contribute to N being limiting.

4.3  SULPHUR

• S is almost never a limiting nutrient in aquatic ecosystems.

• The normal levels as SO₄^2- are more than adequate to meet plant needs.

• The sulphur cycle is shown in Fig. 4.12.

• Odorous conditions are readily created when waters are loaded with organic waste to the point that DO is removed.

• Then SO₄^2- is the electron acceptor used for the breakdown of organic matter.

• H₂S is produced, which has the smell of rotten eggs.

• If nitrate is available, N-reducing bacteria will dominate and odors will be minimal.

• Production of SO₄^2- does not persist in the presence of oxygen.

• SO₄^2- enters aquatic ecosystems through atmospheric deposition of sea salt and as a combustion product of fossil fuels, manifested as acid rain, as well as through natural weathering processes in the watershed.

• Carbon (C) comprises nearly 50% of the dry organic matter in living organisms.

• C is usually not limiting to growth.

• Fig. 4.13 shows the carbon cycle.

• The atmosphere is a source of C, as with N.

• Rate of input of CO₂ is dependent on the physical process of diffusion across the air-water interface.

• CO₂ currently (2010) comprises 0.0388% of the atmospheric gases.

• Carbon is present in aquatic ecosystems as CO₂, HCO₃^- and CO₃^2-.

• CO₂ diffuses into water from the atmosphere when the water is undersaturated, and from water to atmosphere when supersaturated.

• In eutrophic lakes, CO₂ is depleted to very low levels causing pH to rise to 10 or more.

• Photosynthesis and respiration are two major factors that cause a significant departure from equilibrium of the system with the atmosphere.

• As algae phosynthesize, depleted CO₂ can be replaced in water by the following two reactions:

H2CO3 --> CO2 + H2O

HCO3- --> CO2 + OH-

• If CO₂ is replenished from the atmosphere as fast as it is removed by algae, the pH will not change.

• If algae consume CO₂ faster than it can be replaced by diffusion from the atmosphere, H⁺ will decrease and the pH will rise. Algal uptake usually does exceed atmospheric resupply.

• The pH will decrease if CO₂ production through respiration is in excess of the CO₂ loss through diffusion to the atmosphere.

• Photosynthesis tends to be self-limiting; if CO₂ is reduced, the pH rises leading to more reduction of CO₂.