Coupled Biogeochemical Cycles Need to Be Acknowledged in Optimal Eutrophication
Management
Background: Water protection measures have focused on the reduction of phosphorus loading and
have largely ignored the effects of measures on the fluxes of terminal electron acceptors 1 despite
their impact on the cycling of phosphorus. For example, iron in bottom sediments plays a major role
in binding of mobile inorganic phosphorus. Yet, the cycling of iron is linked to other electron acceptors
and compounds formed in their reduction. Therefore, the cycles of both iron and phosphorus are
intimately coupled to the electron acceptors. The coupling may have a marked influence on the
design of water protection measures, those targeting agricultural phosphorus load, in particular.
Objective: The main aim of TEAQUILA project was to analyze how the agricultural water protection
measures can be implemented cost-efficiently when we take into account the effects of measures
on different phosphorus forms and iron in a phosphorus limited system. Simultaneously, we took into
account the coupled biogeochemical cycles in the receiving system and focused on the optimal
weighting between reduction of dissolved phosphorus and particulate phosphorus.
Results and discussion: In our study we have shown that land use controls the fluxes of electron
acceptors, the import of which to water courses is much higher from the fields than the other landcover classes. Based on our laboratory experiments, the release of soil-bound P in anaerobic
conditions was linked to the electron acceptors and the trophic status of the system; more soil-bound
phosphorus is released in a eutrophic sulfate-rich system than in an oligotrophic one.
Optimization highlighted that water protection measures should be chosen so that the binding
capacity for phosphorus transported from the fields is maintained in the bottom sediments of the
receiving system. It is always more effective to reduce the load of dissolved inorganic phosphorus
than particulate phosphorus when combatting eutrophication. In other words, the optimal
eutrophication management allows markedly higher unit costs for the reduction of dissolved
phosphorus than for the particulate phosphorus. Weighting between these two phosphorus forms
are yet dependent on the characteristics and trophic state of the recipient system. We should put
even more weight in reducing dissolved phosphorus in a eutrophic than in an oligotrophic lake. The
bottoms of an oligotrophic system retain phosphorus well due to the high pool of reactive iron in the
sediments. Hereby the additional iron carried by the eroded soil is not that important in binding of
phosphorus. The sediment of a eutrophic lake retains poorly phosphorus and therefore the reactive
iron coming with eroded soil is more “scarce” compared to sediment in good condition in oligotrophy.
Therefore, the entry of dissolved phosphorus into the system needs to further limited, whereas the
fluxes of particulate matter carrying iron can be loosened.
More information:
Pirkko Kortelainen,Finnish Environment Institute
Kari Hyytiäinen, Natural Resource Institute Finland
TEAQUILA-Consortium webpages are found from: www.syke.fi/hankkeet/teaquila
1
The compounds consumed in respiration are named terminal electron acceptors. The term comes from that in respiration reaction
organic matter is oxidized to carbon dioxide and it donates an electron to the electron acceptor which is thus reduced. The electron
acceptors commonly present in the nature are oxygen, nitrate, manganese and iron oxides and sulfate. Terminal electron acceptors
regulate the mineralization of organic nutrients back to the biogeochemical cycles in nature.