Top-bottom investigation of water and nutrient fluxes to the coastal zone through a lowland river basin Eric Struyf Kris Bal Sofie Van Belleghem Patrick Meire

Introduction The accurate quantification of fluxes of water and fluxes of dissolved constituents throughout whole river basins is of major importance. This study can act as the working tool for river managers and modellers to anticipate future changes in hydrology due to climate change and development of vegetation throughout the Schelde basin (France, Belgium, The Netherlands)

Overview Focus on two research projects within the total framework of integrated river basin management 1.The effect of vegetation on drainage in upper river basin sub catchments 2.The possible effect of changing external forcing conditions (increasing discharge) on the eventual transport of water and nutrients towards the coastal area through estuarine ecosystems

Dendermonde Vlissingen Gent Temse Antwerp Belgian-Dutch border North Sea Schelde Dender Rupel Durme Channel Gent-Terneuzen North sea The Schelde estuary -Entire salt gradient -Highly eutrofied -Highly industrialized Nete sub-catchment Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Part 1 : The effects of mowing on the discharge capacity of lowland rivers

Increased nutrient fluxes towards the river stimulate plant growth : - discharge capacity will decrease - possibility that adjacent valuable land will flood

Pros and cons of watervegetation Pro : - Vegetation acts as a sieve for suspended solids and nutriënts - Vegetation acts as refugia areas for organisms during extreme discharge conditions. -Vegetation has a positive effect on the diversity of organisms within and outside a river Contra : - increases the resistance - often seen as a nuisance in waterways (perception)

This study -Partial removement of macrophytes in three different mowing patterns for three different macrophytes -Patterns with the most ecological potential This as a compromise between the drainage and life support function of waterways

Pattern 1 Pattern 3 Pattern 2 : two sides of macrophytes

In-situ determination of biomass

Streamvelocity within the patterns (ex-situ) vegetation Velocity is higher for pattern 2 (all species)

Open water Velocity in pattern 2 is slightly higher then in the other patterns

Determination of the resistance ex-situ Pattern 2 had the highest K m values especially in the higher discharge range K m = V*(R 2/3 *S 1/2 ) -1 Pattern influence

Species influence - species with their biomass at the watersurface have higher K m values

Conclusion -Pattern two has the lowest resistance despite of higher biomasses -Plant architecture influences the resistance : * species with high leaf areas (Callitriche sp.) and branched leaves (Ranunculus aquatilis) have high resistances * concentration of biomass at the surface will impede the water flow less -The patterns have a better relative drainage at higher discharges

Part 2: the effects of hydrological changes on nutrient fluxes throughout estuarine waters to the coastal zone Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Changes in hydrological conditions throughout the river shed eventually influence discharge conditions in estuarine waters and can result in: major changes in nutrient fluxes towards brackish and salt estuarine and coastal waters - major changes in nutrient fluxes towards brackish and salt estuarine and coastal waters - major changes in nutrient concentrations in the freshwater part of estuarine waters Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Why is nutrient flux towards and through estuarine waters so important? -Estuarine and coastal waters are the main fishery grounds on Earth -Diatom communities are fundamental in estuarine food webs and play an essential role in productivity -The optimal balance between diatom communities and less preferred fytoplankton communities (e.g green algae) is dependent on nutrient ratios within coastal waters - Increase in N and P fluxes to coastal waters can induce eutrophication (explosive growth of algae e.g. Phaeocystis) and subsequent negative effects as oxygen depletion, fish kills, foam production... Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Picture taken from: Eutrophication Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

This case study : a period characterised by steadily increasing discharges in the Schelde estuary -The period was taken as a reference period for possible major future changes in hydrological conditions - Vegetation in upper catchments - Global change - Urbanisation Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Discharge from freshwater part towards brackish and coastal waters increased 2 to 3 fold in between m 3.s m 3.s -1 Yearly average discharge in the freshwater zone of the estuary Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Increasing discharge apparent in both winter and summer: absolute discharge increase was higher in winter Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Increase in nitrate loadings Nitrate input from the freshwater to the brackish increased concurrently with discharge, ammonia loadings were stable Discharges in the freshwater estuary Dissolved nitrogen output doubled between tons tons Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Higher run-off Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press Non-point pollution (mainly agriculture) towards the estuary increased Discharged suspended solids towards the estuary: indicator for surface run-off Similar patterns in nitrate discharge and loss from agricultural soil, nitrate mainly originates from agriculture, ammonia relates to point-pollution. Estimates show a big increase tons tons Source: AWZSource: VMM

P and Si discharge Dissolved silica export tripled Phosphorus export was less influenced Hydrology changes can have major impact on nutrient ratios Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Nitrogen in the freshwater zone Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press In winter, nitrate concentrations increase with discharge, in summer they decrease. Relations were opposite for ammonia. winter summer winter The natural, seasonal cycle appears to be flattened out by increasing discharge

The amount of nitrate in the Schelde however is also influenced by the presence or absence of macrophytes in the upper sub- catchments (Nete) When macrophytes are present (Summer) total N concentrations are lowered. When the plants decay (Winter) N is released which causes an increased N flux towards the upper sub-catchment and eventually the schelde Data S. Van Belleghem

summer winter Silica in the freshwater zone Diatoms: summer - spring: uptake of DSi winter - autumn: no uptake With increasing discharge, summer concentrations approach winter concentrations Diatom communities appear not adapted to high discharges Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Impact on management Higher discharges appear to: - dilute pollutants and nutrients - increase nutrient loadings towards coastal zone Dilution effect masks the problem of increasing loadings from non-point pollution. Dilution could affect political and public perception about the effectivity of pollution reducing measures Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

Flemish government has drastically increased investments in point-pollution reduction in the last decade to REDUCE nutrient loadings. Due to hydrological changes, the nutrient loadings have increased over the same period. Source: VMM Struyf E., S. Van Damme, P. Meire 2004: Estuarine, Coastal and Shelf Science: in press

General conclusion * Global change will increase discharge. Consequently nutrient run-off towards coastal zones will increase. * Vegetation however increases the residence time of water due to higher resistances. Self purifying capacity of the river is restored through the stimulation of ecosystem processes. * Vegetation management is a tool which can buffer the fluctuations in discharge in the river basin.

Acknowledgements We like to thank L. Clement for analyzing our water samples. We especially like to thank Flanders Hydraulics and AWZ for their cooperation