Can allelopathically active submerged macrophytes stabilise clear-water states in shallow lakes?

Abstract

Inhibition of phytoplankton by allelochemicals released by submerged macrophytes is supposed to be one of the mechanisms that contribute to the stabilisation of clear-water states in shallow lakes. The relevance of this process at ecosystem level, however, is debated because in situ evidence is difficult to achieve. Our literature review indicates that allelopathically active species such as Myriophyllum, Ceratophyllum, Elodea and Najas or certain charophytes are among the most frequent submerged macrophytes in temperate shallow lakes. The most common experimental approach for allelopathic interference between macrophytes and phytoplankton has been the use of plant extracts or purified plant compounds. Final evidence, however, requires combination with more realistic in situ experiments. Such investigations have successfully been performed with selected species. In situ allelopathic activity is also influenced by the fact that phytoplankton species exhibit differential sensitivity against allelochemicals both between and within major taxonomic groups such as diatoms, cyanobacteria and chlorophytes. In general, epiphytic species apparently are less sensitive towards allelochemicals than phytoplankton despite living closely attached to the plants and being of key importance for macrophyte growth due to their shading. Light and nutrient availability potentially influence the sensitivity of target algae and cyanobacteria. Whether or not additional stressors such as nutrient limitation enhance or dampen allelopathic interactions still has to be clarified. We strongly propose allelopathy as an important mechanism in the interaction between submerged macrophytes and phytoplankton in shallow lakes based on the frequent occurrence of active species and the knowledge of potential target species. The role of allelopathy interfering with epiphyton development is less well understood. Including further levels of complexity, such as nutrient interference, grazing and climate, will extend this ecosystem-based view of in situ allelopathy. ZusammenfassungDie Inhibition von Phytoplankton durch Allelochemikalien aus submersen Makrophyten ist einer der potentiellen Mechanismen, die zur Stabilisierung von Klarwasserzusta¨nden in Makrophyten-dominierten Flachseen beitragen. Die Relevanz dieses Prozesses auf Ö kosystemebene ist jedoch umstritten, da der in situ Nachweis schwierig ist. Unsere Literaturu¨bersicht zeigt, dass allelopathisch aktive Arten wie Myriophyllum, Ceratophyllum, Elodea und Najas sowie bestimmte Characeen zu den ha¨ufigsten submersen Makrophyten in Flachseen der gema¨ßigten Breiten geho¨ren. Der experimentelle Nachweis allelopathischer Effekte auf Phytoplankton erfolgte bisher u¨berwiegend durch Pflanzenextrakte oder aufgereinigte Substanzen. Ein endgu¨ltiger Beweis erfordert jedoch zusa¨tzlich Experimente unter in situ Bedingungen, publ. in: Basic and Applied Ecology, 9, 2008, pp. 422-432 Konstanzer

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