Chemistry of Lake Blankensee
Blankensee is a shallow-water lake with an average depth of 1.60m. Its nutrient cycle is characterised by the following general processes:
- Bioactivity of organisms
- Chemical and especially organismic transport
- Rhythm of circulation, wind movement
- Exchange with the atmosphere, rainfall, inflow and outflow, adsorbtion into and desorption from particles
- Sedimentation
Substances such as N or P that are easy to quantify were referred to early on in characterising the balance of material, and are still referred to in order to determine the trophic level.
The main substances involved in the nutrient cycle are: Carbon, oxygen, nitrogen, phosphorous, sulphur, calcium, magnesium, sodium and potassium, as well as iron and manganese.
In quantity terms, carbon has the largest share in the metabolism. It is present in organic and inorganic forms, as well as in solution and particulate compounds. The main reactions are photosynthesis – the composition of organic substance – and mineralisation – the total decomposition.
CO2 is produced by respiration, and consumed by assimilation. During assimilation (extraction of CO2) the pH rises (see data); during respiration, CO2 production falls.
The photosynthesis reaction takes the following general scheme:
Ca(HCO3)2 |
<- -> |
Ca2+ 2HCO3- |
nHCO3- + 2nH2O |
-> |
(CH2O)n + nH2O + nO2 + nOH- |
This raises the pH.
Together with oxygen this forms the main share of the metabolism in the water.
Oxygen input is provided by atmospheric influx, photosynthesis and influx. Consumption takes place through respiration, decomposition and mineralisation of organic substances, and loss to the atmosphere.
So the less oxygen it receives and the more intense the output of the heterotrophic organisms, the worse the oxygen balance of a body of water becomes. This has two consequences: flowing water with powerful movement and limited depth have a better oxygen balance than standing bodies of water and the input of organic substances into the water worsens its oxygen balance. The smaller the oxygen input from the atmosphere, the more long-term is this result, therefore it is worst for standing bodies of water on still days.
The production of oxygen by photosynthesis can on an annual average be up to 6g O2 m-2 Day-1 in eutrophic lakes. This biogenic oxygen production plays an important part in the self-cleaning system of bodies of water.
The illustration shows saturated oxygen concentrations in winter and spring, an increase in the concentration during the summer during maximum primary production, and a drop that coincides with the decline of primary production. Whilst the chlorophyll-a content has almost reached its minimum already, there are still relatively high concentrations of filtratable substances. This indicates a high content of necrotising organic particulate material, which leads to an increased consumption of oxygen. The oxygen measurements also clearly show this.
Normally oxygen is transported from the atmosphere into the depths by vertical water movements. This means that with each full circulation oxygen-rich water reaches the bottom of the body of water and oxygen is almost evenly distributed. Things become more complicated when the lake is stratified. Then, the question is whether the lake is oligotrophic or eutrophic.
The illustration shows the vertical distribution of oxygen in lakes during the summer stagnation:
1: in an oligotrophic lake
2: in a eutrophic lake
T: Temperature pattern
Occasionally there is stratification in the Blankensee without a temperature gradient.
In the day, oxygen is added via photosynthesis, whilst at night oxygen deficits develop. This daily pattern reveals the phototrophic and heterotrophic bioactivity in the water.
Nitrogen
Nitrogen mainly occurs in the inorganic compounds ammonium, nitrite and nitrate, as well as organically-bonded in proteins. Input takes place via the atmosphere, N2 fixation, influx, and through effluent and protein decomposition. Inorganic nitrogen is taken up by plants and incorporated into organisms via the food chain. Excretion by organisms and their decomposition releases nitrogen again. Its conversion through nitrification and denitrification is microbially controlled.
The ammonium (ammonification) arising from the decomposition of protein is in a chemical balance with ammonia and if there is a adequate supply of oxygen, is oxidised first to nitrite and then to nitrate in the next nitrification step. In an anoxic hypolimnion, nitrate is then reduced to molecular nitrogen.
| Ammonification: |
(NH2)2CO +2H2O <--> NH4+ +NH3 + HCO3- |
| Nitrification: |
2NH4+ +3O2 +2 H2O -> 2NO2- + 4 H3O+
2NO2- + O2 -> 2NO3- |
| Denitrification: |
NO3- +2H -> NO2- + H2O
NO2- + 2H -> NO + H2O
2NO + 2H -> N2O + H2O
N2O + 2H -> N2 + H2O |
It is very important that the ammonia and nitrite phase is only brief, as both substances have a serious toxic effect on the water. The nitrate respiration that takes place during denitrification represents a fundamental cleaning strategy for the water to eliminate nitrate.
There are essentially two methods of elimination:
NO3- -> NO2- -> N2O -> N2
-> NH2OH -> NH3
A balance arises, depending on the pH: NH3 + H+ -> NH4+
As shown in the phase diagram, only part is present as NH3. This can however reach quite toxic concentrations.
It is easy to see that the total nitrogen (TNb) can increase as a result of biomass production, but that toxic NH3 concentrations are reached above all due to increase in temperature and the pH value.
Phosphorous
Phosphorous occurs in bodies of water almost exclusively as orthophosphate. In organisms it mainly occurs as polyphosphate. Phosphorous can divide into different fractions, depending on the compound: easily soluble, or bioavailable orthophosphate, dissolved or colloidal organic phosphate and particulate phosphate, all fractions together make up total phosphate. Particulate phosphorous can be chemically bonded or adsorbed in particles. With the help of phosphatase, algae and bacteria succeed in releasing bioavailable orthophosphate. Solutions of inorganic phosphorous compounds occur only in a few µg/l in non-anthropogenically-affected waters. As an essential nutrient for the primary producers, phosphorous is therefore the minimum factor far more often than nitrogen and the eutrophication of lakes is primarily due to the increase in phosphate.
In the trophogenic layer, phosphate is taken up from photoautotrophic organisms and conveyed into the food chain. Only part of this particulate phosphorous sediments with necrotising organisms, far and away the majority is already transformed in the euphotic zone and taken up again. (Small P cycle)
An increase in total P with constant orthophosphate content indicates one source. As no major influxes were found, it must be assumed that there has been “lake-internal” fertilising from the sediment as a consequence of remobilisation of phosphate. The fate of the phosphate deposited in the sediment is closely linked with oxidation and reduction processes. Phosphate is adsorbed under aerobic conditions into sediment particles or absorbed into iron hydroxide:
FE(OOH) +H3PO4 -> Fe(O-HPO4) +OH
Fe(III) hydroxophosphate is insoluble, as long as the redox potential in the sediment is greater than 0.2 V or the oxygen saturation is above 10%. In this case the upper layer of sediment also represents a barrier layer that retains insoluble P even longer.
Sulphur
Sulphur mainly occurs as sulphate in the water, in which form it is taken up by phytoplankton. The sulphur balance is mainly affected by the oxidation of hydrogen sulphide and the formation of sulphides, in particular ferrous sulphides, in the sediment.
We did not find any free H2S. The illustration shows a clear decline in the sulphate content with increase in primary production.
Iron and manganese
Iron (III) compounds are almost insoluble, so that iron only forms part of a solution under reduction conditions as iron (II) hydrogen carbonate, however this requires the oxygen content to be below 50%, with a high content of CO2 and a pH below 7.5. These conditions often occur in the hypolimnion of eutrophic lakes within the water/sediment contact zone. Together with iron and manganese, other heavy metals can also accumulate in the hypolimnion.
Sediment and nutrient cycle
In the contact area of the sediment surface and the water, precipitation, solution and exchange processes play a key part. Certain elements are primarily adsorbed and rereleased under special conditions. The reduction and oxidation conditions predominating in the water/sediment contact zone are decisive in this. Iron and phosphate ions, which dissolve in the deeper layers of sediment under anaerobic conditions, can be readsorbed in the aerobic sediment surface. This causes a relocation of material and an accumulation on the sediment surface. Under the changing redox conditions (e.g. overnight oxygen consumption) this can therefore lead to the mobilisation of phosphate, which then becomes available as orthophosphate again in the cycle within the body of water. In general this only affects inorganic phosphorous, as organically bonded phosphorous is present in a stable form and has to be decomposed microbially first.
Mobilisation of iron and manganese from the sediment can be clearly seen. This corresponds with the release of phosphate.
