Possible causes for the decline of the water soldier (Stratiotes aloides L.) in the Netherlands

Abstract

Forty-five locations in peaty lowland areas, where Stratiotes aloides L. was studied in August/September 1980, were studied again in February and August/September 1992. Water and sediment pore water samples were collected and vegetation surveys were made. Furthermore, nutrient analyses were made of differently aged tissues of plants collected in 1992. Stratiotes had disappeared from 24 of the 45 locations. Analyses revealed that this decline could most probably be attributed to changes in the hydrological conditions. As a result of modern agricultural practice ground water levels are generally lowered, especially in summer. To prevent the peat from drying out, sulphate enriched alkaline water is let in. Sulphate reduction and concomitant iron sulphide precipitation, as well as decreased iron inputs due to decreased ground water influence, appear to result in decreased iron levels and increased sulphide levels in sediment pore water. As a result plant growth appears to become iron limited while roots die due to sulphide toxification. Furthermore, phosphate release from sediments is enhanced due to decreased iron levels in sediment pore water. Sixteen of the 24 locations, where Stratiotes had disappeared since 1980, were dominated by non-rooting species that covered almost the entire water surface. In 5 of the locations where Stratiotes was still present, non-rooting species had become co-dominant and appeared to overgrow the Stratiotes stands. Finally ammonium levels had strongly increased in most of the locations where Stratiotes aloides had disappeared. We, therefore, suggest a multiple environmental stress hypothesis including light competition by non-rooting species, iron limitation, sulphide toxicity and ammonium toxicity as possible causes for the decline of Stratiotes in the Netherlands, at least in the last decade.