Hl. Golterman, THE LABYRINTH OF NUTRIENT CYCLES AND BUFFERS IN WETLANDS - RESULTS BASED ON RESEARCH IN THE CAMARGUE (SOUTHERN FRANCE), Hydrobiologia, 315(1), 1995, pp. 39-58
Wetlands, especially in the Mediterranean area, are subject to severe
eutrophication. This may upset the equilibrium between phytoplankton p
roduction in undesirable quantities and a quantitatively desirable mac
rophyte production. In order to manage this equilibrium, a quantitativ
e knowledge of nutrient input and fluxes is essential and the role of
sediments in these processes must be understood. This knowledge can be
useful even for agriculture, e.g. rice cultivation, where optimal uti
lization of fertilizers can lead to an economic benefit.In this articl
e different aspects of nutrient cycles are discussed in view of approa
ching a sufficiently precise quantification. The nutrient input balanc
e of the Camargue was therefore measured which showed that the input o
f nutrients with the irrigation water, taken from the river Rhone, rou
ghly equals the quantity of fertilizers added. Phytoplankton growth ca
n be approached reasonably with the Monod model, although there are st
ill many practical problems, such as the influence of the pH on P upta
ke and the problem of measuring P uptake in the field. The situation i
s worse for macrophyte growth; quantitative data are scarce and studie
s have often been carried out with unrealistic nutrient concentrations
or without addressing the influence of the sediment. This influence c
an also include negative factors, such as high concentrations of Fe2+,
H2S or FeS, but cannot yet be quantified. The nitrogen cycle in wetla
nds is dominated by denitrification. Most wetlands have sediments with
high concentrations of organic matter, therefore with a large reducin
g capacity. Besides this process, we have shown that denitrification c
an also be controlled by FeS. In the Camargue sediments this denitrifi
cation is mediated by bacteria from the sulfur cycle; this appeared to
be the major pathway. It was shown that a stoicheiometric relation ex
ists between nitrate reduced and sulphate produced. The influence of t
he temperature was quantified and appeared to be stronger at high orga
nic matter concentrations than at lower ones. Denitrification with FeS
means that the bacteria use nitrate also for their N demands, while t
his is not necessarily the case during denitrification with organic ma
tter. Mineralization of macrophytes is a much slower process than that
of phytoplankton, probably because of their high C/N ratio. We could,
however, not confirm the general assumption that the addition of nitr
ogen stimulates this mineralization. On the contrary, we found that tw
o amino acids both with a C/N ratio of 6 had different mineralization
rates. The amino acid composition of dead macrophytes and the C/N rati
o may be of equal importance. Unlike nitrogen, phosphate is always str
ongly adsorbed onto sediments. The two mechanisms of the adsorption of
inorganic phosphate onto sediments, i.e. the adsorption onto Fe(OOH)
and the precipitation of apatite, have been quantified. The adsorption
of phosphate onto Fe(OOH) can be satisfactorily described with the Fr
eundlich adsorption isotherm: P-ads = A(o-P)(B). The adsorption coeff
icient A depends on the pH of the system and the Ca2+ concentration of
the overlying water and can be quantified preliminarily by A = a.10((
-0.416pH)).(2.86 - (1.86.e(-Ca2+))). B can be approached by 0.333, wh
ich means the cube root of the phosphate concentration. The second mec
hanism is the solubility of apatite. We found a solubility product of
10(-50) for hard waters. The two mechanisms are combined in solubility
diagrams which describe equilibrium situations for specific lakes. Th
e conversion of Fe(OOH) to FeS has a strong influence on phosphate ads
orption, although the partial reduction of Fe(OOH) approximate to P by
H2S does not release significant quantities of phosphate. Even after
complete conversion to FeS only a small part of the bound phosphate wa
s released. Besides the two inorganic phosphate compounds, we establis
hed the existence of two organic pools, one soluble after extraction w
ith strong acid (ASOP), the other one with strong alkali. The first po
ol is probably humic bound phosphate, while the larger part of the sec
ond pool was phytate. The ASOP was remineralized during the desiccatio
n of a Camargue marsh; this drying up oxidized FeS, thus improving the
phosphate adsorption and decreasing the denitrification capacity. It
can, therefore, be an important tool for management. The phytate was s
trongly adsorbed onto Fe(OOH), which explains the non-bioavailability
towards bacteria. The fact that the sediment phosphate concentration c
an be approached by multiplying the relevant sediment adsorption const
ant with 3 root o-P concentration has the consequence that much larger
quantities of phosphate accumulate in the sediments than in the overl
ying water. This means that even if the phosphate input is stopped, th
e eutrophication will only be reversed very slowly, and not at all, if
the shallow waters in wetlands have no through flow - as is often the
case in many marshes in Mediterranean wetlands.