A comparative study of ecophysiological traits of the invasive species Lemna minuta Kunth and the native Lemna minor Linnaeus

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Paolacci, Simona
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University College Cork
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Invasive aquatic plants are a major threat to biodiversity and a considerable amount of money is spent on their management and control. In this study the invasiveness of the alien freshwater plant Lemna minuta was investigated. The performance of this species under different environmental conditions was tested and compared with the performance of the native species Lemna minor. Physiological and morphological parameters were used to quantify the performance of the two species and interpret the growth strategies adopted. In fully controlled conditions, L. minuta and L. minor were grown using different nutrient concentrations, different light intensities and in the presence of several physical and chemical stressors. In parallel, the presence and abundance of L. minuta, and L. minor was monitored for two years in natural freshwater ponds where the two species occur spontaneously. The observation that the water fern Azolla filiculoides co-occurs very often with Lemnaceae, led us to include it in monitoring of growth performance. Also investigated was the correlation between the presence and abundance of the three species and environmental factors of the waterbody such as water chemistry and canopy shade. A one year long outdoor experiment was carried out in order to investigate the performance of the three species throughout the seasons. The laboratory experiments showed that L. minuta has generally a higher growth rate than L. minor. L. minuta outgrows the native species at all the nitrate concentrations tested and at all the Ca/Mg ratios and concentrations tested. Only at very low concentrations of phosphate did L. minor outgrow the alien species. When exposed to stressors, L. minor tolerated low temperatures best, while L. minuta tolerated best high aluminium and copper concentrations, high temperatures and drought stress. It was concluded that the commonly accepted believe that competitive species grow faster, but are less able to tolerate stress, is not always correct. The outdoor experiment showed that, in the summer months, A. filiculoides and L. minor outgrow the native L. minor, but the native species is the first one to re-start its growth after the winter in accordance with the tolerance to low temperatures observed under laboratory conditions. The outdoor experiment showed that the fastest growing and most competitive species is A. filiculoides, followed by L. minuta. Yet, field monitoring showed that these two species were not able to exclude the native species in the wild. Survival of winter conditions and/or re-colonisation, together with the invasibility, need to be considered to explain this apparent discrepancy.
Invasive species , Lemna , Duckweed, , Azolla
Paolacci, S. 2016. A comparative study of ecophysiological traits of the invasive species Lemna minuta Kunth and the native Lemna minor Linnaeus. PhD Thesis, University College Cork.