MAP Technical Reports Series No. 106 UNEP

or,
- 17 -
SO 4 2- [S] S2 -
SO 4 2- + 2 H2O SH 2 + 2 OH - + (2 O 2)
Sulphide ion reacts easily with bivalent iron to form FeS 2. The presence of hydrogen
sulphide always indicates highly reducing conditions. Hydrogen sulphide is not produced as long
as nitrate is available. Sulphide ion is rapidly oxidized in the presence of oxygen, but likely not
by nitrate reduction-denitrification.
Iron. The redox reaction of trivalent-bivalent iron, i.e.
Fe 3+ Fe 2+
plays an important role in regulating PO 4 availability (Stumm and Morgan, 1981). Trivalent iron
oxide-hydroxide complexes that are formed under oxic conditions adsorb multiple amounts of
PO 4 ions. The oxidized iron flocs that are deposited on sediments form a barrier layer, which
prevents the return flow of phosphorus to supernatant waters. Under reducing conditions this
barrier is broken down, and phosphorus, as well as that fraction of iron that does not eventually
become bound as iron sulphide, can move freely and return to the water phase. This process
is known as phosphorus release from sediments. Its magnitude varies, but under anoxic
conditions it can be as high as 10 mg P/m 2 .day.
3.2.2.4 Kinetics aspects
Among the kinetic aspects mention should be made of the rules that govern nutrient
uptake and growth kinetics in phytoplankton. Nutrient uptake is regulated by Michaelis-Menthen
kinetics, according to which the uptake rate, µ, is proportional to the concentration at low nutrient
concentrations, while rates reach an upper limit, µ max, with increasing concentrations.
Mathematically, this is expressed as
µ = µ max M/(k M + M),
M being the concentration of the nutrient in question (nitrogen, phosphorus, silica, etc.),
and k M the half-saturation constant.
Growth is regulated in a similar fashion. At concentrations of M > k M, growth is not limited by the said nutrient.
However, under natural conditions, growth, and accordingly yield, will simultaneously depend on
several nutrients, as well as on other factors (light, temperature), that act in concert. The
combination of all the factors, and the species specific genetically determined maximum growth
rate, regulate the potential and actual growth of phytoplankton. Accordingly, some species may
be able to produce algal blooms within a few days, while the growth of other species always
remains modest.
Michaelis-Menthen kinetics and corresponding growth relationships are important
concepts used in mathematical simulation models. Pertinent treatments of this subject are to
be found e.g., in Chapra and Reckhow (1983), Strasskraba and Gauch (1983), Jørgensen
(1988), a.o.
3.3 Relevance to eutrophication
Eutrophication stimulates primary production, i.e. aquatic plant life (algae and
macrophytes) as defined in the previous chapter. The processes that lead to visible