Expected results of the sub-project

By assessing the pollution status of wetlands and exploring the ecosystem impacts of climate change and human impact, our research contributes to the operation of the tourism industry in the case of reservoirs on Lake Velence. It provides an action plan to avoid (blue) algal blooms experienced in recent years, defines the reference conditions of lakes that the environmental protection action plans should aim to achieve. By raising societal awareness of the speed of changes in the lake ecosystems, our goal is to identify and raise awareness of the path to a safe, livable environment.

The problems of the lake and its feeding reservoirs

Lake Velence is even shallower than Lake Balaton, making it highly sensitive to extreme weather. This year, after a relatively cold spring, the lake warmed up quickly, so not only the high water temperature but also the rate of warming could have caused problems. In 2021 and 2022 the lake has been characterized by critically low water levels that developed during the summer months. 

https://www.portfolio.hu/gazdasag/20220727/kritikus-a-velencei-to-vizszintje-legalabb-150-metert-kell-gyalogolniuk-a-strandoloknak-558247

https://index.hu/belfold/2022/07/26/velencei-to-vizszint-aszaly-tortenelmi-melypont-kiszaradas-szarazsag/

https://hungarytoday.hu/lake-velence-dried-out-replenished-from-reservoir/

https://www.szekesfehervar.hu/emelik-a-velencei-to-vizszintjet-matol-megnyitjak-a-patkai-viztarozo-zsilipjeit

For Lake Velence, as for other lakes, it is extremely difficult to determine the exact spatial distribution of oxygen saturation. The dissolved oxygen content of the water can vary considerably in different parts of the lake, but can also be different at the same point at different depths. Furthermore, it varies during the day, with a minimum usually occurring in the early hours of the morning. Pathological examination of the fish mortalities observed last month also showed that they were caused by a lack of oxygen.

https://sokszinuvidek.24.hu/mozaik/2021/09/13/110-mazsa-hal-elpusztult-velencei-to/

https://www.origo.hu/tudomany/20210824-velencei-to.html

Until recent years, the water quality of the reservoirs was characterized by a dynamic between the winter and summer seasons, with low algal counts in the nutrient-rich water in  winter and hypertrophic water quality in the summer due to the increasing algal blooms. In recent years, the hypertrophic condition has persisted for most of the year and the algal composition has changed, with the dominance of blue-green algae, which produce toxins dangerous to the marsh vegetation and reed beds of the lake.

Replenishment of the lake with excess water from reservoirs was hampered by the poor water quality of Lake Zámoly and Lake Pátka due to extensive fish farming. One critical problem is water quality deterioration and low oxygen saturation. Particularly the feeding canals have poor water quality due to shortage in water supply, partly due to decreasing rainfall in these years and partly because of the retention of waters to feed several fishponds north of the lake. Plans to restore the water level and maintain it have been elaborated recently. But we know very little about the aquatic ecosystem composition these restoration interventions should target. The aim of this project is to contribute towards restoration targets by reconstructing the lake’s biodiversity and ecosystem composition over the last 500 years and determine baseline composition.

Currently empty Zámolyi reservoir (Photo by Mihály Temesi)

We also aim to establish cooperation with the Central Transdanubian Water Management Directorate and the Danube-Ipoly National Park and to carry out surface sediment sampling in the western sub-basin of Lake Velence and its feeding water reservoir, lake Zámoly. Discussions are still ongoing to determine the exact sampling location and to obtain permits.

Further information

http://www.ovf.hu/hu/korabbi-esemenyek/a-velencei-to-oxigenhaztartasanak-problemai-es-a-halpusztulas/1

https://www.feol.hu/helyi-kozelet/2022/05/velencei-to-van-megoldas-a-szakember-szerint

Field work for coring Lake Velence and Pátka

By exploring the pollution status and the ecosystem impacts of climate change and human impact, our research aim is to contribute to the operation of the tourism industry in the case of Lake Velence and one of its reservoir Lake Pátka. Sampling of the lake sediment and water is the first and essential part of the research. The field work for coring the sediment of Lake Velence and Pátka reservoir took place between 15-16 September 2022 near Agárd, in collaboration with the colleagues of the Central Transdanubian Water Management Directorate (Agárd) and the Faculty of Water Sciences (University of Public Service, Baja). The water level in Lake Velence was the lowest ever measured (54 cm) due to reduced winter season precipitation in the last years and the severe 2022 summer drought. Both groundwater tables and incoming stream flow was at a minimum in September (Fig 1). The two lakes had poor water quality, with particularly high electric conductivity and pH (Table 1) due to shortage in water supply and partly due to decreasing rainfall in these years.

Figure 1 - Location of the gravity cores in Lake Velence and Pátka reservoir (a); teasing of the sampling boats off the harbor, normal paddles could not be used due to the low water level (b)

Lake Velence

Our first sampling site was VTLT-1 (abbreviation of Lake Velence Lányi Tisztás 1), the second VTLT-2 (abbr. of Lake Velence Lányi Tisztás 2) (Figure 1). Both cores were taken from the centre of Lányi Tisztás (Lányi Clearing) from opposite sides of our boat. To our best knowledge, and according to the colleagues of the Nature Conservation Service, this part of the Lake was not disturbed by mud dredging after 1958. However the sediment stratigraphy shows a distinct black horizon on top of which relatively fresh organic lake mud has accumulated suggesting sediment perturbation (Figure 2). Two ~30-cm sediment cores (Table 1) were retrieved from Lányi Tisztás (Fig 2). We also visited another clearing in the western subbasin of the lake, called Zagyi-tisztás (Zagy Clearing), where we managed to retrieve, two ~50-cm sediment cores (VTZT-1, VTZT-2), the first from its south-west corner, near the fringing reed-bed, the second from its northern edge using a gravity corer from a boat (Fig. 3).

                                          Figure 2 - Cores from Lányi Tisztás (Lányi Clearing) (VTLT-1 and -2)

The first information about the cores and the water data measured on site is summarized in Table 1.

Table 1 - Location and water-quality measured at the core points in Lake Velence, western sub-basin.

Pátka Reservoir

The third and fourth cores were taken from the feeding water reservoir of Lake Velence, called Pátka Reservoir. Our third sampling site is Pátkai-t-1 (abbreviation of Lake Pátka Core 1), the fourth site is Pátkai-t-2 (abbr. of Lake Pátka Core 2) that was taken from the other side of our boat. The first core was ~45-cm , while the second ~60-cm (Fig 3). Both  were taken from the southern part of the reservoir using a gravity corer and a boat.

                                          Figure 3 - Cores from Lake Pátka (Pátkai-t-1 and -2)

The first information about the cores and the water quality data measured at the site are in Table 2.

Table 2 - Location and water-quality measured at the core points in Pátka reservoir

Planned analyses for all of the cores

• Radiometric dating of the sediment cores by 210Pb/137Cs

• Pollen analysis - surface vegetation cover - reconstruction of vegetation changes, climate reconstruction

• Water quality (pH, dissolved oxygen content, conductivity)

• Geochemical and trophic state analyses (chlorophyll-a, major and trace elements, TOC, C/N, TN, N-isotope analysis)

• Chironomid analysis

• Cladocera analysis

• Diatom analysis

• environmental DNA to obtain algal flora transformation through time

                                                         Figure 4 – Part of the coring team on Lake Velence