By implementing the WFD into national law, the monitoring station network was expanded along the rivers to measure nutrient concentrations among other parameters on a regular basis. Monitoring nutrient concentrations (especially nitrogen (N) and phosphorus (P)) is one method to evaluate water quality, since their concentration levels provide information on the trophic status. Nu-trient loads, however, provide information on the contribution of a river to the nutrient balance of a system, which is, for example, to the eutrophication of a sea. But nutrient loads cannot be measured directly (Zweynert 2008). They are the product of concentration of a substance and discharge. Very often, the nutrient load is presented without discussing the way it has been obtained.
There are different sampling strategies and calculation methods available to obtain yearly in-stream nutrient loads (Kronvang & Bruhn 1996, Zessner et al.
2008, Zweynert 2008), depending on the availability of data and the considered substance. For most stations continuous discharges are available but nutrient concentrations are sampled monthly or fortnightly, which might lead to under-or overestimation of nutrient loads when nutrient peaks are missed under-or sam-pled respectively. Zessner et al. (2008) compared continuous measurements with differently frequent samplings for nitrate (N O3-N) and total phospho-rus (T P) at the Danube, showing that the wide spread method according to OSPAR (2008), based on 24 nutrient samplings per year under consideration of daily discharges, leads to deviations of around 12 respectively 25 % of the reference load.
Uncertainties regarding the calculation of nutrient loads have to be considered when evaluating the nutrient retention in floodplains.
Whereas loads can be calculated from measurements of discharge and concen-tration, emissions from the catchment into the river can only be modelled.
Therefore several nutrient emission models (for N and P) were developed since the 1990’s to visualize the effect of catchment characteristics on the river. Some of them also model the effect of measurements to meet the goals of the WFD, namely reducing nutrient loads. Considerable effort was undertaken in carry-ing out different measurements in the catchments of the EU to reduce nutrient emissions into surface waters. After having improved the quantity and perfor-mance of WWTP (Behrendt et al. 2002), the main source of nitrogen emissions can be attributed to diffuse emissions. For nitrogen and also for phospho-rus the potential of further reductions by technical solutions is cost intensive and the effects are limited. Instead, regarding cost effectiveness of measures, new options are highlighted by several authors (Constanza et al. 1997, Gren 1993, Meyerhoff & Dehnhardt 2007, Mitsch & Gosselink 2000) which is the naturally given function of wetlands and thus riparian floodplains acting as nutrient sinks when intact and connected to the river system. But floodplains with natural flooding regimes belong to the most threatened ecosystems in the world (Brunotte et al. 2009, Opperman et al. 2009) and the land-use form wetland, based on biotop types, is rare on floodplains (Brunotte et al. 2009).
1.3 Wetlands and floodplains
Wetlands are transition zones between aquatic and land ecosystems with an excessive supply of water, either driven by groundwater, by surface water,
by rainwater, or by a combination. Defining wetlands is complex (Mitsch &
Gosselink 1993). There is no agreed on definition on wetlands neither world-wide nor in Europe (Maltby et al. 2009, Mitsch et al. 2009). Even if the WFD demands a prohibition of regression for aquatic ecosystems and con-nected wetlands (matter and hydrology), no definiton of wetlands is provided within the Framework. Different definitions are found in Mitsch & Gosselink (1993), stating, that most definitions include three main components: presence of water, either at the surface or within the root zone, unique soil conditions and vegetation adapted to wet conditions. The definition given by the Ram-sar Convention Secretariat (2006) is very broad and even includes rivers and lakes up to a depth of 6m. Difficulties in providing an exact definition of wet-lands derive from the fact, that wetwet-lands aggregate a wide range of different ecosystems and habitats (Maltby et al. 2009) - including artifical wetlands.
Country specific nomenclatures for wetland types exist. Europeans differen-tiate wetland types, whether the soil is peat forming or not (for an overview see Hofmeister (2006)). Peat results from water logging conditions in the soil, when organic compounds cannot be mineralized completely and accumulate.
In contrast, this fact is not considered in the United States (Mitsch et al. 2009).
Temporal and spatial dynamics are high, leading to changing conditions (e.g.
changing water levels due to seasonal or sudden environmental settings). The differentiaton between wetlands and riparian floodplains remains unclear be-cause the dynamics of water levels (surface flow and groundwater) are also high in floodplains, allowing different biotopes to coexist on a floodplain. Riparian floodplains or floodplains can be defined as ”the surface or strip of relatively smooth land adjacent to a river channel, constructed by the present river in its existing regime and covered with water when the river overflows its banks”
(Hamilton 2009). Others differentiate between the morphologic (historic) and the active floodplain, whereby the active or recent floodplain is defined as the floodplain extent which is at least inundated once in hundred years (Brunotte et al. 2009).
This study assumes inundated floodplains to act as a ”wetland”. Hereby, the wetland is characterized by its hydric soil properties regardless of organic or anorganic soil content. The river itself is excluded from the inundated floodplain.