Covariance in water- and nutrient budgets of Dutch peat polders: what governs nutrient retention?

Water and nutrient budgets were constructed for 13 low-lying peat polders in the Netherlands that varied in elevation relative to sea level (-0.2 to -2.4 m below sea level), land use (7-70% of the total polder area covered by agriculture; largely dairy farming), and surface water prevalence (6-43%). Water balances were verified with chloride budgets and accepted when both met the criterion (total inflows - total outflows)/(total inflows) <0.05. Apart from precipitation and evapotranspiration (overall means 913 vs. 600 mm), in- and outlet (171 vs. 420 mm) as well as in- and outward seepage (137 vs. 174 mm) were important items in the water budgets. Nutrient budgets, however, were dominated by terms related to agricultural land use (~60% of all inputs, 90% of N-removal and 80% of P removal) rather than water fluxes (8% and 5% of N and P inputs; 6 and 18% of outputs). After agriculture (200 kg N h

Macrophytes, a key to understanding changes caused by eutrophication in shallow freshwater ecosystems

Eutrophication of shallow waters has often been associated with the decline of submerged macrophytes. In this study, a conceptual model as a working hypothesis for the mechanism responsible for this decline was used to generate the research topics. Aims of the study were to better understand the mechanism by testing parts of the hypothesis, and to compilate the results in a simulation model, a.o. for management purposes.The model hypothesis used suggests that periphyton development acted as a trigger for macrophyte decline, with a postponed phytoplankton reaction due to allelopathic growth limitation by substances excreted by the macrophytes.We selected one abundant macrophyte species, Potamogeton pectinatus L., and studied growth, photosynthesis and plasticity. Tuber size appeared to be an important factor for the plant that developed from it. Temperature and light influenced growth nonlinearly and interactively. With increasing age, maximal photosynthetic rate and respiration decreased. Light level during gro wth also affected photosynthesis. With respect to photosynthetic performance and stem elongation capacity, P. pectinatus can be characterized as 'average' when compared to other macrophyte species.Periphyton development on glass slides under eutrophic conditions in the laboratory was comparable to that in the field as found by other authors. Temperature distinctly affected the growth curves, whilst light did so for the attenuance-density curves of the experimental communities. In the studied light and temperature range, periphyton attenuated about 50% of the light within 3-4 weeks.All four tested freshwater snail species significantly removed periphyton from glass slides, whilst the two tested crustaceans did not. Differences in removal between and within species could be explained largely by differences in snail activity, snail size and taxonomic composition of the periphyton. Temperature effects were rarely found. From the plant's point of view, a population of Lymnaea peregra (Müll.) appears to be the best solution against periphyton accumulation. It has one of the highest removal rates and a constant, high activity and presence on macrophytes during the season.The existence of allelopathic effects of macrophyte exudates on phytoplankton growth was demonstrated clearly. When present, it was reflected in a growth limitation after one week of on average 10-15%. It could be different for different times of the season, different planktonic algae and different macrophyte species.In enclosures with different types of wall material (gauze and polythene), we found that polythene had the highest macrophyte biomass, the lake had the lowest and gauze was intermediate. This could be explained by improved light conditions in the polythene enclosures due to reduced seston and periphyton density, and by the absence of sediment-disturbing fish in both enclosure types. In another enclosure experiment, sticklebacks (Gasterosteus aculeatus L.) in field densities had a similar effect on zoo- and phytoplankton as the bream-dominated (Abramis brama L.) fish community in Lake Veluwe.The simulation model SAGA1 could reasonably well describe the seasonal vegetation development in Lake Veluwe under various light conditions. Simulations pointed out that in shallow water both periphyton shading and water turbidity are important in limiting plant development. In deeper waters the relative importance of periphyton is reduced. The present, seemingly stable situation in Lake Veluwe actually may well be very labile, since small deviations from the present conditions in the model resulted in large changes in simulated macrophyte biomass.For our conceptual model, we may conclude that the supposed role of allelopathy is now experimentally supported. Periphyton removal by freshwater snails, as well as the role of benthivorous and piscivorous fish, and possible effects of pesticides on periphyton grazers and zooplankton appear components worthwhile to incorporate. Finally, some illustrations are given of the use of the present study for the management of shallow lakes and submerged macrophyte beds

Waterplanten: bondgenoten bij het waterkwaliteitsbeheer? Een visie op de toekomst van het beheer van waterplanten vegetaties

Achtereenvolgens worden besproken: de gevolgen van eutrofiering voor een aquatische levensgemeenschap, de effecten van de huidige beheersmethoden en een model, dat de belangrijkste interacties van de levensgemeenschap beschrijf

Nitrogen source apportionment for the catchment, estuary and adjacent coastal waters of the Scheldt.

Using the systems approach framework (SAF), a coupled model suite was developed for simulating land-use decision making in response to nutrient abatement costs and water and nutrient fluxes in the hydrological network of the Scheldt River, and nutrient fluxes in the estuary and adjacent coastal sea. The purpose was to assess the efficiency of different long-term water quality improvement measures in current and future climate and societal settings, targeting nitrogen (N) load reduction. The spatial-dynamic model suite consists of two dynamically linked modules: PCRaster is used for the drainage network and is combined with ExtendSim modules for farming decision making and estuarine N dispersal. Model predictions of annual mean flow and total N concentrations compared well with data available for river and estuary (r² ≥ 0.83). Source apportionment was carried out to societal sectors and administrative regions; both households and agriculture are the major sources of N, with the regions of Flanders and Wallonia contributing most. Load reductions by different measures implemented in the model were comparable (~75% remaining after 30 yr), but costs differed greatly. Increasing domestic sewage connectivity was more effective, at comparatively low cost (47% remaining). The two climate scenarios did not lead to major differences in load compared with the business-as-usual scenario (~88% remaining). Thus, this spatially explicit model of water flow and N fluxes in the Scheldt catchment can be used to compare different long-term policy options for N load reduction to river, estuary, and receiving sea in terms of their effectiveness, cost, and optimal location of implementation