Wet plants are there for a reason

A herbivorous grass carp, which can eat its own weight in vegetation daily, is removed from the fenced-off area of the lake to allow the native reed-grass to return and control nutrient levels.

Zoltan is surrounded by tins of raw corn and fly larvae, cans of Borsodi beer, and his Trabant playing Hungarian hits from the 70s. Nestled at the side of Malom Lake, he waits two hours before a tug on his translucent line drives him to his rod. The next ten minutes are spent pulling the unwilling victim ashore, presenting the 33 cm grass carp to envious friends, throwing him back in, and opening a fresh Borsodi. The fish returns to what it was doing – eating.

Grass carp, an alien species from Asia, were introduced to Malom Lake in the 1970s because of their appetites – they can eat their own weight in vegetation daily. Requests for the fish came from local fishing associations to clear the lake of reed-grass vegetation to allow more space for fishing and rowing.

In record time, the foreigners ate just about everything. “Fish like grass carp are added to about50% of the 1000 small lakes in Hungary,” says Sándor Tatár, a representative of the local NGO Tavirozsa. “The result is that nearly half of the lakes have been virtually cleared of vegetation.”

The fish’s impacts on Malom Lake were too much on top of earlier impacts. From the 1920s, landscaping and dredging to make way for new housing developments destroyed large areas of plant life. Waste cesspits were also dug that leached pollution into the groundwater and lake. The remaining lake plants were barely able to handles the increasing nutrient pollution. “But once the grass carp started eating, the lake’s self-cleaning capacity ended and nutrient pollution skyrocketed,” says Tatár. By 1980, large algal blooms appeared. Water quality deteriorated and fish reproduction decreased.

In 1985, Malom Lake was given national protection status. Despite new laws against adding foreign fish species, fishing associations continued to stock the lake with grass carp. In 1996, a new sewage treatment plant was built near the lake for the local town of Veresegyház and neighbouring villages. Plant capacity was over-used, however, to the point that concentrations of nutrients discharged from the treatment plant were above permitted levels and leached into the lake system. Bacteria levels increased sharply including toxic cyanobacteria and coliform bacteria resulting in human symptoms such as allergic reactions, fever and vomiting.

“Water quality at the sand beach became catastrophic,” says Tatár. Having attracted some 3000 to 4000 people daily in the past, beach numbers went down by 90% after 1990.

Tavirozsa founded. In response, some local residents united to form the Tavirózsa NGO in 1996. With funding from the Hungarian government, the NGO assessed local water quality and biological factors in the lake and surrounding Sz˝odrákosi Creek catchment area of 132 sq km. The creek runs north through Veresegyház and its three lakes including Malom Lake before draining into the Danube River above Budapest.

In 2006, with help from the UNDP-GEF Danube Regional Project (DRP) Small Grants Programme, Tavirozsa purchased equipment to test water in three lakes. Monitoring found that big rains in April and May caused significant nutrient pollution to the lakes because of the city’s poorly combined sewage system. The NGO notified local and regional authorities who came to test the water themselves. “But they didn’t test bacteria or algae,” says Tatár.

In early August, Tavirozsa measured algae and cyanobacteria chlorophyll and found counts to be double acceptable limits, and by August 18 counts measured four times the government limit. The Hungarian health authority ANTSZ finally appeared on the scene, but only at the end of the bathing season, and they failed to measure all parameters as required by law. “The government didn’t want to send out bad news during top season,” says Tatár.

Goodbye grass carp, hello reed-grass. DRP funds were also used by the NGO to implement a demonstration wetland rehabilitation project at the top end of one of the city’s three lakes, Pamut Lake. “We support the project because it will bring the reed-grass back, which will help bring back some valuable local fish species that have almost disappeared,” said Gusztáv Kiss, leader of Pamut Lake’s fishing association.

Following a baseline environmental assessment in the spring of 2006, the small lake area was fenced off and all grass carp were removed. Rooted and floating native reed-grass with high nutrient removal capacities were collected from nearby lakes and then manually added to the site. “We are confident that the new reed-grass will help improve water quality and fish habitat,” says Tatár.

The next step is to test the demonstration site water in the future to prove that quality improved. Based on that evidence, Tatár hopes to secure a larger project using the same strategy to restore all three lakes starting in 2007. “It’s a good idea to have all three lakes included,” says Kiss. “Grass carp might return to our lake otherwise, for example carried over by birds.” Tatár also wants to ensure that pollution from the local treatment plant stops soon.

“With small funds, one can improve the natural self-cleaning capacity of wetland areas,” he says. “The bigger problem is getting local support. At Pamut Lake, we were able to convince the fishing association there to accept the project. But the other two lakes each have their own association, and they’re not convinced yet. They still prefer their grass carp and open water space to the reed-grass, clean water, clean beach and healthy ecosystem. Now we’re working to change their perceptions.”

Paul Csagoly is a communications specialist for the UNDP/GEF
Danube Regional Project, and a writer on European environmental
issues since 1996.

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