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Summary1. Dominance of free-floating plants such as duckweed is undesirable as it indicates eutrophication. The objectives of this study are to investigate whether the onset of duckweed dominance is related to weather conditions by analysing field observations, to evaluate the effect of different climate scenarios on the timing of duckweed dominance using a model and to evaluate to what extent nutrient levels should be lowered to counteract effects of global warming. 2. To analyse the onset of duckweed dominance in relation to weather conditions, duckweed cover in Dutch ditches was correlated with weather conditions for the period 1980-2005. Furthermore, a model was developed that describes biomass development over time as a function of temperature, light and nutrient availability, crowding and mortality. This model was used to evaluate the effects of climate change scenarios and the effects of lowering nutrients. 3. The onset of duckweed dominance in the field advanced by approximately 14 days with an increase of 1°C in the average maximum daily winter temperature. The modelled biomass development correlated well with the field observations. Scenarios showed that expected climate change will affect onset and duration of duckweed dominance in temperate ditches. Reducing nutrient levels may counteract the effect of warming. 4. Synthesis and applications. Global warming may lead to an increase in the dominance of free-floating plants in drainage ditches in the Netherlands. The expected reductions in nutrient-loading to surface waters as a result of different measures taken so far are likely not sufficient to counteract these effects of warming. Therefore, additional measures should be taken to avoid a further deterioration of the ecological water quality in ditches.

We studied the effect of waves on submerged macrophytes and hypothesized that exposure to large wave forces can hamper seedling establishment. In an indoor experiment in cylindrical mesocosms we tested whether large wave forces indeed inhibited the establishment of Chara globularis and Potamogeton pusillus from the propagule bank. We mimicked the effect of wave forces by generating a circular flow that caused resuspension of the sediment. Four treatments were applied, consisting of different repetition frequencies of resuspension events. Emergence and early growth of both species were monitored over 8 weeks. The resuspension treatments significantly reduced the emergence of both species, by 91% and 45% on average for Chara sp. and P. pusillus, respectively. We analysed field observations on the two species in the lakes of the IJsselmeer area in the Netherlands to evaluate whether wave forces may also inhibit establishment of macrophytes in the field. The field data seemed to support the hypothesis as both species hardly occurred in areas where a large bottom shear stress had occurred in spring, according to simulations with the SWAN wave model. The calculated maximum bottom shear stress correlated well with the occurrence of both Chara sp. and P. pusillus in the field. Regressions showed that this effect of wave forces was additional to the effect of light availability. Our study indicates that large wave forces may inhibit the establishment of macrophytes in large lakes. Reducing large wave forces can therefore potentially promote macrophyte development in these large lakes.

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Nutrients can determine the outcome of the competition between different floating plant species. The response of floating plants to current phosphorus levels may be affected by previously experienced phosphorus concentrations because some species have the ability to store excess phosphorus for later use. This might have an impact on their competition. Here, we investigate the effect of previous and actual phosphorus concentrations on the growth rate of free-floating plant species (Azolla filiculoides, Lemna minor/gibba and Ricciocarpus natans)and the effect of phosphorus history on the competition between L. minor/gibba and A. filiculoides and between L. minor/gibba and R. natans. As expected, plant growth was lower when previously kept at low instead of high phosphorus concentrations. Growth of L. minor/gibba and A. filiculoides with a phosphorus rich history was comparable for low and high actual phosphorus concentrations, however, internal phosphorus concentrations were significantly lower with low actual phosphorus concentration. This indicates that both species perform luxury phosphorus uptake. Furthermore, internal P concentration in Azolla and Lemna increased within two weeks after a period of P deficit without a strong increase in growth. A. filiculoides in a mixture with L. minor/gibba grew faster than its monoculture. Morphological differences may explain why A. filiculoides outcompeted L. minor/gibba and these differences may be induced by phosphorus concentrations in the past. Growth of L. minor/gibba was only reduced by the presence of A. filiculoides with a high phosphorus history. Growth of L. minor/gibba and R. natans in mixtures was positively affected only when they had a high phosphorus history themselves and their competitor a low phosphorus history. These observations clearly indicate that phosphorus history of competing plants is important for understanding the outcome of the competition. Therefore, actual and previously experienced phosphorus concentrations should be taken into account in future studies dealing with competition between plants.