Az Eszterházy Károly Tanárképző Főiskola Tudományos Közleményei. 2004. Sectio Biologiae. (Acta Academiae Paedagogicae Agriensis : Nova series ; Tom. 25)

Makrai, L., Dulai, S., Polyánka, H., Ertli, T. and Lehoczki, E.: Monitoring of the Functional State of Beds of Common Reed (Phragmites australis) in Shallow Lake Balaton (Hungary) by Means of Chlorophyll Fluorescence Studies

Monitoring of Functional state of Reed.

.. 141 C0

2 assimilation were also carried out, in order to demonstrate the direct correlation between the two methods for reed investigations. Material and methods Area of study At the mean water level, Lake Balaton has a surface area of 594 km

2, a mean depth of 3.14 m and a water volume of 1.8 km

3. Half of its watershed of 5179 km

2 is drained by the River Zala through the Kis-Balaton Reservoir into Keszthely Bay. Lake Balaton has a deep littoral zone on the northern shore and a shallower one in the south. The macro vegetation also differs on the northern and southern shores. On the northern side, continuous stands of reed can be found, bordering the shoreline and extending to the deeper, open water. Along the southern shore, the vegetation is less developed as compared with that on the northern shore, and there are no continuous stands of reed. In an interpretation of the current reed-bed decay in Lake Balaton, the impact of the past 50 years cannot be ignored. From the second half of the 1950s, vaste areas of Lake Balaton became covered by reeds. Between 1958 and 1968, for example this reed-bed growth was approximately 500 ha (from 1376 to 1862 ha). From the 1960s onwards this expansion slowed down, and in the 1970s the process reversed and the reedy areas (reedboss) slowly diminished. Measurements showed a reduction of 10% in 1975, of 30% in 1987 and of a further 10% in 1993 (Virág, 1997). Thus, starting from 1968, the area of reed-beds decreased by 40%, from 1 862 to 1 129 ha. Expressed in a different way, 3.1% of the surface of the lake was covered by reeds in 1968, as compared with only 1.9% in 1993 (Kovács et al., 1994). At present, the reed areas extend along almost one-half (110 km) of the shoreline of Lake Balaton. A rapid eutrophication of the lake started at the end of the 1960s, and by the 1970, the south-western (Keszthely Bay) areas had become hypertrophic, mostly as a result of the excessive nutrient supply from the River Zala (Heródek, 1986). Collection of plant material Measurements were carried out along the shores of Lake Balaton in July and the first half of August in 1996, 1997 and 1998. For determination of the C0

2 assimilation and fluorescence induction, we gathered the plants together with their rhizomes in 1996. However it later emerged that the upper parts of the plant (approximately 60 cm, 6-7 nodes) were sufficient for these measurements. Each year, the samples were taken from the same locations during the early morning, or late evening. Samples were harvested from the waterward side of the reed-beds from a circles around 5 m in diameter. Until