Disentangling causes of the dieback of littoral stands in an overloaded water-bird reserve: roles of eutrophication, fish and geese

Abstract 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.