Neusiedlersee and its status over time

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1 Neusiedlersee and its status over time Bundesministerium für Land- und Forstwirtschaft, Umwelt und Wasserwirtschaft Federal Ministry for Agriculture, Forestry, Environment and Water Management Republic of Austria, BMLFUW, Department VII (Water), Unit VII/2, p.a. Stubenring 1, A Wien. Contact Person: H. Fleckseder, Tel June 2002

2 Development in time and results achieved Neusiedlersee ('Lake Neusiedl') is a shallow 'steppe' lake situated in the east of Austria. It is shared with Hungary (in Hungarian it is called 'Fertö to'). Its total size is 321 km 2 (at defined water level), out of which more than half is reeded (at some places 3 to 5 km in width). Its theoretical renewal time is 1 year. The Figures 1 to 3 show its location, - via an aerial photograph - part of the lake in its landscape, and the increase of the reeded area in the period 1872 till Figure 1: Location of Neusiedlersee / Fertö to in the east of Austria, shared between Austria and Hungary. Neusiedlersee was formerly without any outflow, and due to this extremely large fluctuations of its wetted surface (nil to large) took place. In its history it had tried out completely several times. For the last time this was in the 19 th century. Then the lake area was also drained on purpose, in order to gain agriculturally productive land. Due to the relatively high natural soil salt content this undertaking proved to be quite unsuccessful. Neusiedlersee is a very shallow lake (av. depth 1.2 m). A fluctuation in the water level of +/- 1.0 cm means a change in the wetted lake surface of up to +/- 3 km 2. Thus the lake was / is not only endangered by nutrient inputs, but also by its scarce water balance. In order to stabilise the size of the reed-belt and to maintain the lake as a water body alive the outflow regulation for the weir (situated on Hungarian territory) is adapted to the hydrologic regime (in a specific year the ruling is based on the precipitation of a series of preceding years). The main surface water input is via River Wulka, followed by other catchments, but the lake receives also substantial groundwater inputs. A further increase of the reed-belt in the recently quite dry years would drastically endanger the water balance (due to an increase in evapotranspiration).

3 Figure 2: Aerial photograph of Neusiedlersee, from west to east, between Mörbisch and Rust. Vineyards are located in the forefront of the shot. Figure 3: The increase in size of the reed-belt between 1872 and Due to the fine grain-size in its sediments and also due to its shallowness the lake is quickly mixed by wind action and naturally turbid (= small values for transparency!). This thus also means that the phytoplankton growth is limited by the impossibility of solar radiation to utilise the full water column; what grew will quickly become incorporated into the bottom silt. The occurrence of large blooms is thus limited by natural conditions. Its temperature fluctuations are large (up to 28 o C in summer, whereas under 'normal winter conditions' its surface is usually frozen). It holds a high natural salt content (> 1 g/l), and thus also a high ionic content. In the late 1950s and early 1960s - caused by the decline of income to be gained in agriculture - an ongoing urbanisation process of this formerly strongly agricultural part of Austria started. Linked with this centralised water supply systems were introduced, and this in turn led to sewerage in the communities. The study of the status of lakes and their development in time was in Austria till the middle of 1970s predominantly work undertaken at the academic level. The study of the eutrophication process of Neusiedlersee was thus initiated by colleagues from academia. They observed in the early 1970s a quick eutrophication process, see the Figures 4 and 5. Please note that the 'initial value for totp' in 1970 is very likely wrong; this maybe due to a very limited number of samples or also a not fully adequate analysis. A

4 sharp increase in totp (with values up to 150 mg/m³) took place till the end of the 1970s, followed by a steady decline. Something relatively similar is observed for chlorophyll-a. The values fluctuate from year to year Ges-P (mg/m³) Chl-a (mg/m³) Fig. 4: Total P (yearly averages) over time in the open water body of Neusiedlersee Fig. 5: Chl-a (yearly averages) over time in the open water body of Neusiedlersee The driving forces for the eutrophication process were sewerage without adequate waste water treatment, a wrong policy in regard to non-indigenous fish (the eel was introduced in the late 1960s, in order to feed on cyprinids; it is now banned), and also intensification in agricultural practices. The discontinuation of the harvesting of reed - which in the past was used in large quantities in the construction industry - and its increased decay in the reed-belt may also have contributed. Wastewater treatment plants with N- and P-removal started to operate by 1975 and somewhat later (P-removal by simultaneous precipitation; N-removal by nitrification/denitrification that was initially mainly introduced to save energy in aeration and to assure appropriate sedimentation in the final clarifiers of the activated sludge plants). The erection of these plants was justified simply on the basis of the precautionary principle. It was further possible to show that the total cost increment for that part of wastewater treatment that refers to the removal of nitrogen and phosphorus is relatively small compared with the total cost of sewerage and wastewater treatment (incl. sludge handling and sludge disposal). A further removal of nutrients from wastewater treatment plants (e.g. filtration) is presently not planned. There is one exception: The wastewater treatment plant at Neusiedl (at the northern shore) has been discontinued; the sewage is pumped up to Parndorf and further transferred, treated jointly and finally discharged into River Leitha. Buffer strips on arable land adjacent to the creeks in the catchments of the lake were also introduced. The agricultural produce in the drainage area of Neusiedlersee is predominantly of arable origin, and with no substantial conversion into meat or dairy products. The input of particulate organic material and soil-bound P to a large extent takes place during rain events, via River Wulka. It was possible to show that a single large runoff (caused by a flood with a recurrence interval of once in five years) can carry half of a

5 year's load of totp of River Wulka. 'Input loads' for totp into the lake are quite difficult to estimate and are thus not undertaken on a routine basis. This is due to the transport of totp over time (as not every storm event can be sampled), and as the lake's boundary is a somewhat open system. Detailed and rather long studies during the 1980s ('Gesamtprojekt Neusiedlersee') did not allow to find 'the truly limiting nutrient'. Figures 4 and 5 indicate that chl-a and totp correlate to some extent. One can further observe that both totp and chl-a vary from year to year. Nevertheless the lake describes as yet an ongoing process of re-oligotrophication. The variations in totp in Fig. 4 are due to variations in - nonpoint-source input via River Wulka, - the P-release in the reed-belt, as well as - the transfer of water from the reed-belt into the open water body of the lake. The variations in Fig. 5 for chl-a are due to - the variation in totp as well as - the dynamics of climatic forces (temperature; solar radiation during the growth season; occurrence of storms / strong winds during the growth season determining the turbidity / transparency). The discharges from wastewater treatment plants are controlled by a steady selfmonitoring from the side of the plant operators, linked with an external supervisory monitoring from the side of administration. This very quickly and effectively allows to overcome any shortcomings in plant operation. Neusiedlersee is - according to the Austrian experience - presently classified as a mesotrophic shallow steppe lake, with ongoing signs of improvement. A part of Neusiedlersee and Seewinkel (i.e. at its east shore, and into the adjacent landscape) was the first national nature park in Austria recognised by IUCN. The 'Neusiedlersee region' holds indigenous fauna and flora, and it offers also an important refuge for migrating birds. Main conclusions from the Austrian point of view and relating to the issue of eutrophication of lakes in a pan-european context Conclusions emerging from the situation of Lake Neusiedl: Values of totp that are indicative for a eutrophic status for other types of lakes will in a shallow lake of the type of Neusiedlersee not necessarily lead to a mass-accumulation of phytoplankton, with a subsequent decay and consumption of dissolved oxygen leading to a strong under-saturation of dissolved oxygen. Conclusion: A future EU eutrophication classification of lakes must also reflect lakes like Neusiedlersee as an own type of lake. Neusiedlersee had reached an unfavourable trophic status which is now improving markedly. This process can quite well be described by a long time-series of observations and their adequate interpretation. The values for totp vary from year to year, and the ones for Chlorophyll-a even more. For both these parameters a tolerable percent-

6 age value of a base value is from the Austrian point of view not a good indicator for description of the direction towards which the trophic status of a lake is heading. Nevertheless the lowering of the discharges of P (from point sources as well as nonpoint sources) proved to be an essential element in helping to foster the trophic status of Neusiedlersee. Compared with the overall cost of sewerage and wastewater treatment P-removal from urban waste water is a quite minor cost increment. The Secchi depth measurement is for naturally turbid lakes - like Neusiedlersee - not an appropriate measure for their trophic status. In shallow non-turbid lakes it even may fail completely due to the insufficient depth when one wishes to apply this principle of measurement. Further generalised conclusions: A future pan-european assessment of eutrophication processes in lakes will have to define what 'totp' in lakes comprises (way of sampling, including sample fractionation and sampling frequency; analytical methods). For all parameters chosen as criteria the sampling frequency and their distributions over the year will be an important issue. Similar issues will have to be discussed in regard to rivers discharging to lakes. A future fixing of 'allowable deviations from a high resp. good status', expressed as a percentage value of e.g. 'totp', is from the Austrian point of view a 'tricky issue'. If such approach is part of a future EU guidance relevant percentage values of selected parameters must be tailor-made in such a way that they fully reflect the 'internal variations' of the respective type of lake. In the case of lakes where algae growth is truly limited by phosphorus the application of the precautionary principle contained in the Directive 91/271/EEC should properly be maintained. It may also be meaningful to expand this basic principle of keeping reactive phosphorus away from the lake beyond the numeric limits and values set in this Directive.