European fishponds used for semi-intensive fish production can serve for refuges of water birds if the fish stocks are limited but the effects of other ecological factors on the ecosystem stability are seldom considered. The aim of this study is to elucidate causes of a marked decline of littoral stands dominated with Typha angustifolia in a hypertrophic fishpond forming a part of a valuable water-bird reserve. Field investigation and two experiments were conducted in order to separate the effects of mineral nutrient availability, redox condition, fish and geese. The physico-chemical characteristics of water and sediments confirmed hypertrophic conditions in the fishpond but a follow-up mesocosm experiment did not prove a negative effect of the sediment quality on the plant growth. On the other hand, a field enclosure experiment showed a significant negative effect on shoot density, which was attributed to grazing by geese. An additional damage to plant belowground parts was ascribed to large carp individuals, albeit in small numbers. The study points to a conservation dilemma caused by large numbers of geese destroying littoral stands in a fishpond nature reserve, which then looses its nesting function for other water-bird species.
European fishponds can serve as refuges for water birds if the fish stocks are limited, but the effects of other ecological factors on their ecological stability are rarely considered. The aim of this study is to determine the causes of marked dieback of littoral stands dominated by Typha angustifolia L. in a hypertrophic fishpond that is also a valuable water bird reserve. A field study and two experiments were conducted in order to separate the effects of mineral nutrient availability, redox conditions, fish and water birds. The physico-chemical characteristics of the water and sediments confirmed hypertrophic conditions in the fishpond, but a mesocosm experiment did not indicate it had a negative effect on plant growth. On the other hand, a field enclosure experiment showed that in sparse stands, unfenced parts had a significantly smaller shoot density than fenced parts. This was attributed to grazing by greylag geese (Anser anser L.). In addition, damage to belowground parts of plants were ascribed to large individuals of albeit a few large common carp (Cyprinus carpio L.). This study highlights a conservation dilemma as large numbers of geese destroy littoral stands in fishpond nature reserves, which then become unsuitable nesting sites for other species of water birds.
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