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The aquatic macrophyte Stratiotes aloides Linnaeus, which has recently received attention in studies on allelopathy, has been shown to suppress phytoplankton growth. In the Netherlands, S. aloides often co-occurs with floating filamentous algae. However, filamentous algae are generally absent in close proximity to S. aloides, resulting in gaps in filamentous algae mats. We analyzed whether those gaps may be caused by allelopathic substances excreted by S. aloides or by nutrient depletion. We studied in a field survey the colonization of natural S. aloides by filamentous algae and determined in situ nutrient concentrations in natural S. aloides stands. To analyze the relative importance of allelopathy and nutrient competition in the interaction between S. aloides and filamentous algae, we carried out field experiments. Introduction of artificial (non-allelopathic) plants in natural S. aloides stands enabled us to compare the colonization by filamentous algae of both Stratiotes sp. and artificial plants. The filamentous algae were absent in close vicinity to S. aloides. Significantly lower concentrations of ortho-phosphate and potassium were observed close to S. aloides as compared with the filamentous algae. In the field experiments the artificial plants were rapidly colonized by filamentous algae, mainly Cladophera Kützing and Spirogyra Link, while all natural plants remained free of such algae. Additionally, most nutrient concentrations did not significantly differ in the proximity of artificial or natural stands of S. aloides. The concentrations of the major growth-limiting nutrients, phosphate and nitrate, were significantly higher and nonlimiting in natural Stratiotes stands. Our main conclusion is that, although allelopathic interactions between S. aloides and filamentous algae do occur under natural conditions, nutrient competition between the two can also be an important factor.

Submerged aquatic macrophytes are important in shallow aquatic ecosystems because they stabilize the macrophyte-dominated state by increasing water transparency in various ways. One of these is the excretion of allelopathic substances inhibitory to phytoplankton, but it is still controversial whether this mechanism can be important in the field. We developed a model that describes phytoplankton growth including the effects of two different macrophytes ( Chara sp. and Stratiotes aloides ), which are both known to excrete allelopathic substances. With this model we analysed singular and combined effects of shading (1), sediment resuspension (2), competition for nutrients (3) and allelopathy (4) on the development of phytoplankton. Both macrophyte species had a considerable effect on phosphorus levels in the water. Shading of phytoplankton by Chara sp. was estimated to be negligible. This macrophyte species has a growth form close to the sediment. The allelopathic activity of Chara sp. caused a phytoplankton growth reduction of ca. 5–10%, while the most pronounced effect seems to be related to reduction of resuspension. S. aloides floats on the water surface during spring and summer. Therefore, it has a potentially high shading effect. Additionally, this species has a relatively strong allelopathic capacity, sometimes resulting in a reduction of phytoplankton between 50 and 80%. The model suggests that the relative contribution of allelopathy in situ is low or negligible for charophytes. Only for macrophyte species that have a high allelopathic potential, like S. aloides , it may be a significant effect.

The allelopathic activity of the aquatic macrophyte, Stratiotes aloides, was determined with laboratory experiments. Active compounds exuded in the medium or present in plant tissue were extracted using standard procedures and solid phase extraction (SPE). The activity towards various cyanobacteria and chlorophytes was tested in two different bioassay systems using agar plates and liquid cultures of phytoplankton. Extracts and exudates of S. aloides affected phytoplankton growth. SPE-enriched exudates and enriched water from a natural Stratiotes stand caused inhibition of target species, however, also some controls were active. Phytoplankton species exhibited differential sensitivity to extracts of S. aloides. We observed inhibitory and stimulatory effects on phytoplankton. In general, more cyanobacteria than other phytoplankton species were inhibited, and the inhibition of cyanobacteria was stronger. In most cases, nutrient (P or K) limitation of Synechococcus elongatus and Scenedesmus obliquus decreased the sensitivity of these species towards allelochemicals from Stratiotes aloides, except for P-limited cultures of Scenedesmus. The allelopathically active compound(s) from Stratiotes are moderately lipophilic and most likely no phenolic compounds. Our results indicate that allelopathy (besides nutrient interference and shading) might also account for the low phytoplankton and filamentous algae densities in the vicinity of Stratiotes plants, at least during certain phases of the life-cycle of Stratiotes.

The allelopathic potential of exudates from the aquatic macrophyte Stratiotes aloides on the growth of phytoplankton was investigated. A selection of phytoplankton species, occurring in habitats similar to that of Stratiotes, was used: two cyanobacterial strains (toxic and non-toxic Microcystis aeruginosa), one green alga (Scenedesmus obliquus) and one eustigmatophyte (Nannochloropsis limnetica). The results indicate allelopathic effects of Stratiotes on phytoplankton in six of the eight cases, expressed in an extended duration of the initial biovolume doubling time. The overall inhibitory effect (8–51%) was strain-specific for the two cyanobacteria. We also studied the effect of irradiance on the allelopathic potential of exudates from Stratiotes. Irradiance influenced the response of Scenedesmus only. The inhibitory effect of Stratiotes exudates on the growth of this green alga was stronger at 35 µmol m-2 s-1 than at 105 µmol m-2 s-1. We conclude that Stratiotes has allelopathic effects on phytoplankton, and that irradiance can, but does not always determine the extent of the allelopathic inhibition. In our experiments, the sensitivity of cyanobacteria to Stratiotes exudates was not higher than for other phytoplankton strains, but within cyanobacteria, the toxic strain was more sensitive than the non-toxic one. [KEYWORDS: Allelopathy ; Exudate ; Growth inhibition ; Initial biovolume doubling time ; Irradiance ; Lag phase]