Considerations on the influence of extreme events on the phosphorus transport from river catchments to the sea

Abstract

In this paper, results from rivers of different sizes in Romania, Hungary and Austria are presented. The paper shows the dynamics of extreme events and their contribution to the total P and suspended solids transported in these rivers. Special attention is paid to the influence of the size of the catchment and the event probability on the relative contribution of a single event to the total loads transported in the river. Further, the development of phosphorus loads along the Danube River at a flood event is shown. From the results it can be concluded that there is no immediate influence of high flow and flood events in upstream parts of the Basin on the transport of phosphorus from the catchment to the receiving Sea. Particle-bound phosphorus is mobilised from the catchment (through erosion) and the river bottom to a high extent at high flow events and transported at peak discharges to downstream, where retention by sedimentation of particles takes place. On the one hand this retention is a transport to flooded areas. In this case it can be considered as more or less long term retention. On the other hand sedimentation takes place in the riverbed, in case the tractive effort of the river is reduced. In this second case the P-pool in the sediments of the sedimentation area will be increased. If anaerobic conditions in the sediment appear, part of the phosphorus will be transformed to soluble ortho-phosphate and will continuously contribute to the phosphorus transport to the receiving sea. Part of the P-retained in the river sediment will be mobilised by resuspension at the next biggest high flow event. Altogether, these alternating processes of suspension, transport, export to flooded areas or sedimentation in the river bed with partly solution and partly resuspension at the next event decrease the share of the phosphorus transport during high flow events on the total loads transported in the more downstream parts of a catchments as compared to the more upstream parts. In the year of occurrence of an extreme flood event the P-transport of this year is dominated by the flood event. As an average over many years the contribution of high flow events to the total P-transport still may be between 7 and 20% in smaller catchments (around 1,000 km2). In a big catchment (e.g. river Danube) much smaller contributions of flood events on the total P-transport can be expected as an average over many years.

Long-Term Variability of Nutrients and Chlorophyll in the Chesapeake Bay: A Retrospective Analysis, 1985–2008

“…However, low flow conditions permit phytoplankton to grow in the regions where conditions are favorable, mainly in the shallow and landward regions (Harding et al 1986;Fisher et al 1999). The particle-reactive nature of phosphorus and the natural geochemical 'buffering' action in the aquatic ecosystems render the concentrations of ortho-PO 4 and TP in the Chesapeake Bay less responsive to precipitation and discharge events from the watershed (Zessner et al 2005). On the contrary, NO 3 is readily mobilized during strong hydrologic events (e.g., Goolsby et al 1999).…”

Enhancement of the MONERIS Model for Application in Alpine Catchments in Austria

“…Additional influential factors including geology, relief, and precipitation may significantly influence SS emissions, but cannot be considered because of lack of information. Furthermore, most of the monitoring data on SS loads in river systems are based on a monitoring network with 12 measurements per year, which is not sufficient to derive accurate yearly SS loads and related TP loads (ZESSNER et al, 2005b) and relate them to specific catchment properties. Improvement in this respect can be expected from the new monitoring network for SS in rivers, which is currently implemented in Austria (LALK, 2009).…”

“…Additionally, the observation of TP loads is much more difficult as for DIN. Even annual TP loads derived from monthly concentration measurements over a six year period are highly uncertain due to unsteady SS and TP transport (ZESSNER et al, 2005b) and the questions if flood events have been monitored or not. Finally, the modifications have been implemented into the MONERIS model for application to the whole Austrian territory.…”

Environmental Impacts of Tourism on Lakes

“…Moreover, as climate changes, extreme events in the drainage basin become increasingly important for nutrient transport and loading (e.g. Zessner et al 2005;Strauss and Staudinger 2007). Recently, more holistic approaches have been used to evaluate pressures including tourism on trans-boundary lake districts (Klug 2010).…”