Photosynthetic and morphological traits control aquatic plant distribution according to light stress
Résumé
The eutrophication of aquatic systems resulting from human activities frequently leads to phytoplankton blooms, which decrease water transparency and therefore limit light availability for aquatic plants. However, several submerged macrophytes are able to grow despite the light competition pressure exerted by phytoplankton. Photosynthetic performance and plant morphology may be key traits allowing plant species to tolerate phytoplankton-induced light limitation in eutrophic systems. The aim of this study was to explore how morphological and photosynthetic traits allow submerged macrophytes to withstand the stress induced by decreased light in eutrophic systems. Morphological and photosynthetic traits were measured in 7 species of submerged macrophytes collected from mesotrophic to hypertrophic shallow lakes (Dombes, Ain, France). Then, we determined whether these traits were related to species occurrence in lakes according to depth and water turbidity. This study highlighted 2 strategies potentially resulting in a trade-off between morphological and photosynthetic traits. The first strategy was tolerance to reduced light and was characterized by a higher investment in leaf area and photosynthetic efficiency (initial slope of photosynthetic activity measured at low light intensities), allowing plants to use low amounts of light energy. The second strategy was to avoid the regions of reduced light and was characterized by a greater investment in vertical growth and maximum photosynthesis at high light intensities. The morphological and physiological traits favoured in turbid lakes corresponded to the avoidance strategy with a low compensation point, allowing plants to grow to the photic zone to reach the water areas where light is at a maximum. Small species exhibiting a light stress tolerance strategy may be maintained in turbid lakes subjected to disturbances, offering a window of opportunity for successful recruitment and reproduction.