The labyrinth of nutrient cycles and buffers in wetlands: results based on research in the Camargue (southern France)

Abstract

Wetlands, especially in the Mediterranean area, are subject to severe eutrophication . T h i s m a y u p s e t t h e e q u i l i b r i u m b e t w e e n p h y t o p l a n k t o n p r o d u c t i o n i n u n d e s i r a b l e q u a n t i t i e s a n d a q u a n t i t a t i v e l y d e s i r a b l e m a c r o p h y t e p r o d u c t i o n . I n o r d e r t o m a n a g e t h i s e q u i l i b r i u m , a q u a n t i t a t i v e k n o w l e d g e o f n u t r i e n t i n p u t a n d f l u x e s i s e s s e n t i a l a n d t h e r o l e of sediments in these processes must be understood . This knowledge can be useful even for agriculture, e .g. rice cultivation, where optimal utilization of fertilizers can lead to an economic benefit .In this article different aspects of nutrient cycles are discussed in view of approaching a sufficiently precise q u a n t i f i c a t i o n . The nutrient input balance of the Camargue was therefore measured which showed that the input of n u t r i e n t s w i t h t h e i r r i g a t i o n w a t e r , t a k e n f r o m t h e r i v e r R h o n e , r o u g h l y e q u a l s t h e q u a n t i t y o f f e r t i l i z e r s a d d e d . Phytoplankton growth can be approached reasonably with the Monod model, although there are still many practical problems, such as the influence of the pH on P uptake and the problem of measuring P uptake in the field. The situation is worse for macrophyte growth ; q u a n t i t a t i v e d a t a a r e s c a r c e a n d s t u d i e s h a v e o f t e n b e e n c a r r i e d o u t with unrealistic nutrient concentrations or without addressing the influence of the sediment . This influence can also include negative factors, such as high concentrations of Fe e+ , H 2 S o r F e S , b u t c a n n o t y e t b e q u a n t i f i e d . The nitrogen cycle in wetlands is dominated by denitrification . Most wetlands have sediments with high concentrations of organic matter, therefore with a large reducing capacity. Besides this process, we have shown that denitrification can also be controlled by FeS . In the Camargue sediments this denitrification is mediated by bacteria from the sulfur cycle ; this appeared to be the major pathway . It was shown that a stoicheiometric relation exists between nitrate reduced and sulphate produced . T h e i n f l u e n c e o f t h e t e m p e r a t u r e w a s q u a n t i f i e d a n d a p p e a r e d t o b e stronger at high organic matter concentrations than at lower ones . Denitrification with FeS means that the bacteria use nitrate also for their N demands, while this is not necessarily the case during denitrification with organic matter .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 nitrogen stimulates this mineralization . On the contrary, we found that two amino acids both with a C/N ratio of 6 had different mineralization rates . T h e a m i n o a c i d c o m p o s i t i o n o f d ...

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