Hydrobiologia 392: 29–39, 1999.
H.L. Golterman (ed.), Sediment–Water Interaction 9.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
29
Quantification of P-flux through shallow, agricultural and natural
waters as found in wetlands of the Camargue (S-France)
H. L. Golterman
Association “Leiden-Camargue”, Station Biologique de la Tour du Valat, F-13200 Arles, France
Address for correspondence: Ch. de Tintarlot 5, F-13200 Arles, France
Key words: iron phosphate, calcium phosphate, o-phosphate, modelling, sediments, Camargue, wetlands
Abstract
The flux of phosphate in the aquatic ecosystems of the Camargue was modelled. The model developed for the
hydrological unit, the water basin of the Aube de Bouic, is based on the hydrology of the system, the reaction
equations of the adsorption of phosphate onto Fe(OOH) and the solubility product of apatite. The water basin
consistsofan inlet canal, theAubedeBouic,ricefieldsandnaturalmarshes,plusadrainagecanal throughwhichthe
excess water is pumped into the Etang de Vaccarès. Phosphate is introduced into ricefields either with the irrigation
water, taken from the Rhône, or as fertiliser. Most of the phosphate will be fixed on the ricefield soils either as iron-
bound or as calcium-bound phosphate, depending on the pH. The o-phosphate concentration will slowly increase
to an extent depending on the pH. From the ricefields water will flow towards the Etang de Vaccarès; the amount
of phosphate reaching the Etang de Vaccarès depends on the pH of the system and the depth of the water outlet
of the ricefields. In six different scenarios the amount of phosphate reaching the Etang de Vaccarès is calculated
dependingon the quantity of fertiliser, the depth of the water outlet and the pH. For the Etang de Vaccarès the model
predicts the present concentration of o-P relatively well and shows that the loading is considerable. This should
cause concern; the danger of serious eutrophicationis real. Part of the irrigationwater is used to refill or keep under
water the natural marshes in this hydrological unit, which without irrigation would dry out during summer. Again
high P-concentrations are found in the marshes, but mostly in the form of Fe(OOH)≈PorCaCO
3
≈P, the ratio of
which depends again on the pH of the sediments. The o-P concentration has remained low up to now.
Abbreviations: Fe(OOH)≈P = iron-bound phosphate; CaCO
3
≈P = calcium-bound phosphate; o-P
s
= saturation
value of o-P with apatite
Introduction
In Mediterranean wetlands, such as the Camargue
(Rhone Delta, France), the Ebro delta and the Parque
Doñana or Guadalquivir delta (Spain) and the Po delta
(Italy), agriculture and environmental protection have
to co-exist or even co-operate. These wetlands are fer-
tile areas, in the marshes of which nature protection
has to be active in order to maintain the presence of
important flora and fauna.
Nowadays, agriculturecannotbe profitablewithout
the input of large quantities of fertilisers. The quantit-
ies used are, however, larger than needed. The devel-
opment towards limited maximal quantities that can
legally be applied, as now found in some Northern
European countries, does not yet take place in the
Mediterranean countries. The excess fertiliser added
will gradually find its way into marshes and lakes. Al-
though there are now a large number of international
programmes concerning wetland protection, there is
no programme against over-utilisation of fertiliser.
The resulting input of fertilisers into shallow wa-
ters does not lead to eutrophication as rapidly as in
deeperwatersbecause mostof the incomingphosphate
will be adsorbed onto the sediments, and the danger
of eutrophication is not generally understood, or is
even denied. The reason for this is that phosphate is
strongly adsorbed onto the soil particles or sediments,
while nitrogen is often lost by denitrification (Golter-
man, 1995a). The adsorption of phosphate can now