Groundwater Management Strategies to Improve Surface Water
265
Although the concentrations ofP04-P in some wells were high, the annual load
is only approximately 60 kg, 50% of which is concentrated at the upstream end
(Wasserpark), however. Maximum flow is during spring and summer, the
minimum from September to November.
While identified sources of phosphorus should, of course, be cleaned up where
feasible, the total load is actually small compared to the volume of the lake and
should not be responsible for poor water quality in a stable system. In 1987, the
biomass of macrophytes in Alte Donau was estimated at 720 000 kg dry weight,
with a content of approximately 900 kg of phosphorus (Dokulil et al. 1997). It is
clear that these macrophytes could - and obviously did - easily buffer much
higher inputs of phosphorus. The minimum of phosphorus stored in macrophytes
was reached in 1995 with only 3 kg. Under these conditions, an input of 60 kg,
equivalent to approximately 15 Ilgri, contributes significantly to eutrophication.
5 Relationships Between Surface Water Quality and
Hydrological Regime
If the input of nutrients by groundwater is not a significant factor for poor water
quality, how can the hydrological regime influence water quality? Maitland and
Morgan (1997) identified the turnover period of water in a lake and the fluctuation
of water levels as important influences on the ecology of a lake that can be used
for management purposes. For the Alte Donau, three main effects that depend on
the hydrological regime were identified: (1) favourable conditions for the growth
of macrophytes, (2) renewal and dilution of surface water with a high nutrient load
by filtered groundwater with low load and (3) temperature.
The importance of littoral macrophytes, including fringing reeds as well as
submersed macrophytes, for the ecological robustness of lakes has been studied in
many instances (e.g. Ostendorp 1995; Rietz et al. 1995). Macrophytes have a
double effect on water quality: they consume nutrients from the water, and they
compete with plankton, thus supporting better transparency of water. To initiate
growth, they need appropriate sediment in an area of the lake bottom, which is
sufficiently illuminated. By the depth of water, the hydrological regime controls
the size of the area where water plants can grow. To achieve maximum area, the
water level should be low in spring, allowing illumination of large bottom areas.
Once growing, the water level can slowly rise, still having the plants exposed to
light at least in the upper parts. The rising water level inundates additional areas
for subsequent colonisation by water plants (Fig. 10).
Bank-filtrated groundwater from the Danube has low concentrations of
nutrients and does not carry phytoplankton, which could contribute to increased
turbidity if input by surface transfer, even if nutrient concentrations are as low as
in the Neue Donau.
265
Although the concentrations ofP04-P in some wells were high, the annual load
is only approximately 60 kg, 50% of which is concentrated at the upstream end
(Wasserpark), however. Maximum flow is during spring and summer, the
minimum from September to November.
While identified sources of phosphorus should, of course, be cleaned up where
feasible, the total load is actually small compared to the volume of the lake and
should not be responsible for poor water quality in a stable system. In 1987, the
biomass of macrophytes in Alte Donau was estimated at 720 000 kg dry weight,
with a content of approximately 900 kg of phosphorus (Dokulil et al. 1997). It is
clear that these macrophytes could - and obviously did - easily buffer much
higher inputs of phosphorus. The minimum of phosphorus stored in macrophytes
was reached in 1995 with only 3 kg. Under these conditions, an input of 60 kg,
equivalent to approximately 15 Ilgri, contributes significantly to eutrophication.
5 Relationships Between Surface Water Quality and
Hydrological Regime
If the input of nutrients by groundwater is not a significant factor for poor water
quality, how can the hydrological regime influence water quality? Maitland and
Morgan (1997) identified the turnover period of water in a lake and the fluctuation
of water levels as important influences on the ecology of a lake that can be used
for management purposes. For the Alte Donau, three main effects that depend on
the hydrological regime were identified: (1) favourable conditions for the growth
of macrophytes, (2) renewal and dilution of surface water with a high nutrient load
by filtered groundwater with low load and (3) temperature.
The importance of littoral macrophytes, including fringing reeds as well as
submersed macrophytes, for the ecological robustness of lakes has been studied in
many instances (e.g. Ostendorp 1995; Rietz et al. 1995). Macrophytes have a
double effect on water quality: they consume nutrients from the water, and they
compete with plankton, thus supporting better transparency of water. To initiate
growth, they need appropriate sediment in an area of the lake bottom, which is
sufficiently illuminated. By the depth of water, the hydrological regime controls
the size of the area where water plants can grow. To achieve maximum area, the
water level should be low in spring, allowing illumination of large bottom areas.
Once growing, the water level can slowly rise, still having the plants exposed to
light at least in the upper parts. The rising water level inundates additional areas
for subsequent colonisation by water plants (Fig. 10).
Bank-filtrated groundwater from the Danube has low concentrations of
nutrients and does not carry phytoplankton, which could contribute to increased
turbidity if input by surface transfer, even if nutrient concentrations are as low as
in the Neue Donau.