Abstract
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.
Highlights
Free-floating macrophytes can have severe negative effects on aquatic ecosystems by forming dense floating mats
This study shows that the phosphorus concentration to which free-floating plants have been exposed in the past largely determines how the growth of a plant reacts to the actual phosphorus concentration
There are clear indications that A. filiculoides and L. minor/ gibba may perform luxury phosphorus uptake, whereas this is clearly absent in R. natans
Summary
Free-floating macrophytes can have severe negative effects on aquatic ecosystems by forming dense floating mats. [1, 2, 3, 4]) Among those studies, Kushari & Watanabe [1] found that species of the floating fern Azolla produced more biomass in P deficit conditions when the plants were previously held in enriched P conditions than when they were previously held in P deficit conditions. Kushari & Watanabe [1] found that species of the floating fern Azolla produced more biomass in P deficit conditions when the plants were previously held in enriched P conditions than when they were previously held in P deficit conditions This indicates that these species could perform a luxury P uptake that can be used later for growth. Since free-floating plants compete with each other for among others nutrients [6, 7], the previous phosphorus concentration may affect the competition between these plants
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