Invasion by mats of free-floating plants is among the most important threats to the functioning and biodiversity of freshwater ecosystems ranging from temperate ponds and ditches to tropical lakes. Dark, anoxic conditions under thick floating-plant cover leave little opportunity for animal or plant life, and they can have large negative impacts on fisheries and navigation in tropical lakes. Here, we demonstrate that floating-plant dominance can be a self-stabilizing ecosystem state, which may explain its notorious persistence in many situations. Our results, based on experiments, field data, and models, represent evidence for alternative domains of attraction in ecosystems. An implication of our findings is that nutrient enrichment reduces the resilience of freshwater systems against a shift to floating-plant dominance. On the other hand, our results also suggest that a single drastic harvest of floating plants can induce a permanent shift to an alternative state dominated by rooted, submerged growth forms.D ense mats of free-floating plant have an adverse effect on freshwater ecosystems because they create anoxic conditions that strongly reduce animal biomass and diversity (1). Invasions by introduced exotic species are partly responsible for the increase of floating plant dominance. The problems caused by Eichhornia crassipes, Pistia stratiotes, and Salvinia molesta are notorious: they hamper fish production and navigation in tropical regions around the world (2-4). However, eutrophication is likely to have boosted the spread of free-floating plants, too. In temperate climate zones, it is known that dense beds of duckweeds (Lemnaceae) and small, floating water ferns (Azollaceae) are a symptom of high-nutrient loading in small water bodies, such as ponds and canals (5, 6). Just as in the case of tropical plant beds, the dark and anoxic conditions under thick duckweed cover leave little opportunity for animal or plant life (1).The dependence of free-floating plants on high nutrient concentrations in the water is an obvious consequence of their growth form. They have no direct access to the sediment pool of nutrients, and they have a large portion of their leaf surface exposed to the atmosphere rather than to the water, thereby reducing the possibility of taking up nutrients other than carbon through their leaves. By contrast, rooted submerged macrophytes may take up a large part of their nutrients from the sediment (7, 8) and also use their shoots effectively for nutrient uptake from the water column (9, 10). Although floating plants are obviously superior competitors for light, submerged plants may affect the growth of free-floating plants through a reduction of available nutrients in the water column. Competition is likely to be especially strong for nitrogen. Although phosphorus availability in the water column can be reduced because of uptake by submerged macrophytes (11), many studies show unaltered or even increased ortho-P levels after increased macrophyte cover (12-15). By contrast, submerged nitrogen co...

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The authors worked up a material collected during 14 years at 20 sites of the Hungarian forestry and agricultural light‐trap network system. In this study they examined the effect of the polarization and intensity of the reflected moonlight on the abundance of 5 lepidopteran and 2 coleopteran species in the trap catches. The material consisted of 55,003 specimen. The relative abundances of the insects in catches per day were calculated for each swarming time, and these were arranged by species and flights according to moon phases and the catches of the corresponding days of lunations were meaned. In this way, the mean relative abundances (MRA) vs. moon phase, characteristic for the species, were calculated. Then these daily MRAs of the 7 species were meaned, these general MRAs (GMRAs) served as a basis of the further analysis. In the next step, from the GMRAs power spectra were calculated; the period times belonging to their extreme values were 14.5, 30, 10, 7.4, and 3.3 d, respectively. We calculated the correlation coefficient between the GMRAs and the polarization rate (PR) of the moonlight (r = + 0.87). Together with these, the theoretical RAs of insects caught at new moon divided by at full moon, were calculated, by using probability methods and data of the illumination conditions (light emission) of the Jermy‐type light‐trap, the Moon and the night sky. It was concluded that the calculated period time of 14.5 d was related to the polarization rate of the moonlight. The highest trap catches coincided with the maximal polarization rates. From the 30 day's period it was postulated that the intensity has a modificator effect. At new moon and at full moon (both phases emitting unpolarized light) the number of insects caught is influenced by the conditions of light intensity in the surrounding of the trap, which determine also the distance of attractiveness. So the moonlight effects both the extension of the collecting area and flight activity of the insects by the cyclic changes of light intensity and polarization rate. Therefore the number of the insects caught is influenced by the lunar cycle. Zusammenfassung Zur Wirkung der Mondphasen und der Intensität des polarisierten Mondlichtes auf Lichtfallen‐Fänge Fünf Lepidopteren‐ und zwei Coleopteren‐Arten mit insgesamt 55 003 Individuen, die während 14 Jahren in 20 auf forst‐ und landwirtschaftlichen Flächen in Ungarn verteilten Lichtfallen gefangen worden waren, wurden daraufhin untersucht, ob ihr Auftreten Beziehungen zur Mondphase und zur Intensität des polarisierten Mondlichtes erkennen läßt. Es wurde die mittlere relative Abundanz (MRA) der 7 Arten in Beziehung zur Mondphase berechnet. Sodann wurden die täglichen MRAs bestimmt und diese generellen MRAs (GMRAs) als Grundlage für die weitere Analyse verwendet. Der nächste Schritt bestand in der Berechnung der Mächtigkeitsspektren aus den GMRAs. Die zu den Extremwerten gehörenden Perioden betrugen 30, 14, 5, 10, 7, 4 und 3,3 d. Der zwischen den GMRAs und der Polarisationsrate (PR) des Mondlichtes bestehe...

1. The asymmetric competition for light and nutrients between floating and submerged aquatic plants is thought to be key in explaining why dominance by either of these groups can be stable and difficult to change. 2. Although the shading effect of floating plants on submerged plants has been well documented, the impact of submerged plants on floating plants has been poorly explored hitherto. 3. Here, we used laboratory experiments to examine how submerged plant (Elodea nuttallii) alter nutrient conditions in the water column and how this affects the growth of floating plants (Lemna gibba). 4. We demonstrate that, at higher nutrient concentrations, Lemna is increasingly likely to outcompete Elodea. 5. Under low nutrient concentrations (0.1-2 mg N L )1 ) Elodea can strongly reduce the growth of Lemna. Growth of floating plants virtually stopped in some of the experiments with Elodea. 6. Extremely reduced tissue N, Mn, chlorophyll and elongated roots indicated that the growth inhibition of Lemna by Elodea was predominantly caused by the latter's impact on the nutrient conditions for floating plants. 7. These results strengthen the hypothesis that submerged plants can prevent colonization of a lake by floating plants.