Interactions between allelopathic properties and growth kynetics in four freshwater phytoplankton species studied by model simulations

We chose four species of freshwater phytoplankton: the chlorophyceans Ankistrodesmus falcatus, Chlamydomonas reinhardtii and Selenastrum capricornutum, and the cyanobacteria Oscillatoria sp. in order to study their competitive abilities for nitrate and their allelopathic properties. We parameterized models of nitrate uptake and growth with laboratory experiments. According to them, the species were ranked (from the best to the worst competitors): S. capricornutum, C. reinhardtii, A. falcatus and Oscillatoria sp. C. reinhardtii and Oscillatoria sp. were previously reported as allelopathic. In the present work, Oscillatoria sp. was allelopathic only against A. falcatus. However, none of our species was sensitive to C. reinhardtii. Additionally, we found an unknown allelopathic effect of A. falcatus against Oscillatoria sp. Our findings point out the high specificity of allelopathic interactions. With these data, we constructed a model of interspecific competition for nitrate, including allelopathic interactions. By performing model simulations, we studied how three factors influence the outcome of competition: relative abundance of competing species, resistance to allelopathy, and nitrate concentration. Our simulations showed that the initial ratio of species abundances will significantly determine the outcome of competition. If the worst competitor was the allelopathic species, the more it needs to outnumber the competing species, unless it is very sensitive to allelopathy (not defended). Nitrate has an important influence, showing a non-intuitive outcome of competition experiments at low nitrate concentrations, where the worst competitor (allelopathic species) wins competition in the majority of cases, whereas at intermediate concentrations, the better competitor dominates except for unfavorable ratios of abundances. With the increased amounts of nitrate, conditions again favor the worst competitor (the stronger allelopathic species). Despite the potential for two species coexistence showed by previous theoretical analysis of systems was similar to ours, our simulations did not detect this outcome. We hypothesized that this is due to the strong allelopathic effect of Oscillatoria sp.     

The coevolution of two phytoplankton species on a single resource: Allelopathy as a pseudo-mixotrophy

Without the top-down effects and the external/physical forcing, a stable coexistence of two phytoplankton species under a single resource is impossible — a result well known from the principle of competitive exclusion. Here I demonstrate by analysis of a mathematical model that such a stable coexistence in a homogeneous media without any external factor would be possible, at least theoretically, provided (i) one of the two species is toxin producing thereby has an allelopathic effect on the other, and (ii) the allelopathic effect exceeds a critical level. The threshold level of allelopathy required for the coexistence has been derived analytically in terms of the parameters associated with the resource competition and the nutrient recycling. That the extra mortality of a competitor driven by allelopathy of a toxic species gives a positive feed back to the algal growth process through the recycling is explained. And that this positive feed back plays a pivotal role in reducing competition pressures and helping species succession in the two-species model is demonstrated. Based on these specific coexistence results, I introduce and explain theoretically the allelopathic effect of a toxic species as a ‘pseudo-mixotrophy’—a mechanism of ‘if you cannot beat them or eat them, just kill them by chemical weapons’. The impact of this mechanism of species succession by pseudo-mixotrophy in the form of alleopathy is discussed in the context of current understanding on straight mixotrophy and resource-species relationship among phytoplankton species.     

Resisting annihilation: relationships between functional trait dissimilarity,assemblage competitive power and allelopathy

Allelopathic species can alter biodiversity. Using simulated assemblages that are characterised by neutrality, lumpy coexistence and intransitivity, we explore relationships between within‐assemblage competitive dissimilarities and resistance to allelopathic species. An emergent behaviour from our models is that assemblages are more resistant to allelopathy when members strongly compete exploitatively (high competitive power). We found that neutral assemblages were the most vulnerable to allelopathic species, followed by lumpy and then by intransitive assemblages. We find support for our modeling in real‐world time‐series data from eight lakes of varied morphometry and trophic state. Our analysis of this data shows that a lake's history of allelopathic phytoplankton species biovolume density and dominance is related to the number of species clusters occurring in the plankton assemblages of those lakes, an emergent trend similar to that of our modeling. We suggest that an assemblage's competitive power determines its allelopathy resistance.     

Context‐dependent induction of allelopathy in plants under competition

Some plants use allelopathy to compete against neighbouring plants, and the ability to induce allelopathic compound production in response to competition is hypothesized to be adaptive, as plants can save costs of metabolite production in the absence of competitors. However, whether plants induce allelopathy has rarely been explored so far. We studied the inducibility of polyacetylenes – putative allelopathic compounds in Solidago altissima – in response to competition. Polyacetylenes were found in natural soil surrounding S. altissima patches within the range of concentration known to inhibit competitor growth. Individual S. altissima plants with higher polyacetylene concentration in roots suppressed the growth of the competitor plants more, suggesting that root polyacetylene levels proximate plants’ allelopathic capacity. Competition induced polyacetylenes in a context‐dependent manner: Whereas introduced Japanese and Australian populations of S. altissima had higher constitutive concentration of polyacetylenes than the native North American populations, inducibility was observed only in Australian plants, where the population is still at an early stage of invasion. Also, induction became more prominent under nutrient depletion, where enhanced allelopathy may be particularly beneficial for suppressing a competitor's exploitative capacity. Finally, we found weak evidence for a tradeoff between constitutive and induced polyacetylenes. The observed patterns suggest that allelopathic plants could respond to competition by inducing allelochemical production, but the benefit of such plasticity may vary across time and space. Shifts in competitor communities in introduced range over time may shape plant's plastic responses to competition, while variation in resource availability may alter competitive environment to influence the degree to which plants induce allelopathy.     

Biological control of phytoplankton by the subtropical submerged macrophytes Egeria densa and Potamogeton illinoensis: a mesocosm study

1. In temperate regions, submerged macrophytes can hamper phytoplankton blooms. Such an effect could arise directly, for instance via allelopathy, or indirectly, via competition for nutrients or the positive interaction between submerged macrophytes and zooplankton grazing. However, there is some evidence that the positive interaction between submerged macrophytes and zooplankton grazing is less marked in warmer regions, where the interaction is less well studied, and that negative effects of higher water plants on phytoplankton biomass are weaker. 2. We carried out two consecutive mesocosm experiments in Uruguay (subtropical South America) to study the effects of two common submerged macrophytes from this region (Egeria densa and Potamogeton illinoensis) on phytoplankton biomass, in the absence of zooplankton grazing. We compared phytoplankton development between different macrophyte treatments (no macrophytes, artificial macrophytes, real Egeria and real Potamogeton). We used artificial macrophytes to differentiate between physical effects (i.e. shading, sedimentation and competition with periphyton) and biological effects (i.e. nutrient competition and allelopathy). 3. In Experiment 1, we found no evidence for physical effects of macrophytes on phytoplankton biomass, but both macrophyte species seemed to exert strong biological effects on phytoplankton biomass. Only Egeria affected phytoplankton community structure, particularly tempering the dominance of Scenedesmus. Nutrient addition assays revealed that only Egeria suppressed phytoplankton through nutrient competition. 4. We performed a second mesocosm experiment with the same design, but applying saturating nutrient conditions as a way of excluding the effects of competition for nutrients. This experiment showed that both macrophytes were still able to suppress phytoplankton through biological mechanisms, providing evidence for allelopathic effects. Our results indicate that both common macrophytes are able to keep phytoplankton biomass low, even in the absence of zooplankton grazing.     

The nutrient-load hypothesis: patterns of resource limitation and community structure driven by competition for nutrients and light

Resource competition theory predicts that the outcome of competition for two nutrients depends on the ratio at which these nutrients are supplied. Yet there is considerable debate whether nutrient ratios or absolute nutrient loads determine the species composition of phytoplankton and plant communities. Here we extend the classical resource competition model for two nutrients by including light as additional resource. Our results suggest the nutrient-load hypothesis, which predicts that nutrient ratios determine the species composition in oligotrophic environments, whereas nutrient loads are decisive in eutrophic environments. The underlying mechanism is that nutrient enrichment shifts the species interactions from competition for nutrients to competition for light, which favors the dominance of superior light competitors overshadowing all other species. Intermediate nutrient loads can generate high biodiversity through a fine-grained patchwork of two-species and three-species coexistence equilibria. Depending on the species traits, however, competition for nutrients and light may also produce multiple alternative stable states, suppressing the predictability of the species composition. The nutrient-load hypothesis offers a solution for several discrepancies between classical resource competition theory and field observations, explains why eutrophication often leads to diversity loss, and provides a simple conceptual framework for patterns of biodiversity and community structure observed in nature.     

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

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.     

Harmful algal blooms of allelopathic microalgal species: The role of eutrophication

The ability of certain harmful algal species to produce and release chemicals that inhibit the growth of co-occurring phytoplankton species, here considered as allelopathy, is closely associated with competition for limiting nutrient resources. Many phytoplankton cells are known to release elevated amounts of organic compounds under nutrient limitation. Eutrophication alters the nitrogen-to-phosphorus balance and, when nutrient availability is unbalanced, nutrient limitation may result. Algal species that can compete successfully for available growth-limiting nutrient(s) have the potential to become dominant and form blooms. The stress conditions imposed by the shifted nutrient supply ratios can, in some algae, stimulate production of allelochemicals that inhibit potential competitors. Thus, under cultural eutrophication, altered nutrient (N, P) ratios and limiting nutrient supplies can stimulate increased production of allelochemicals, including toxins, by some algal species and accentuate the adverse effects of these substances on other algae. Future investigation on the characterization of the chemical compounds involved in the allelopathic process are needed to advance the study of the mode of action of phytoplankton allelochemicals.     

DOES ALLELOPATHY CONTRIBUTE TO CYLINDROSPERMOPSIS RACIBORSKII (CYANOBACTERIA) BLOOM OCCURRENCE AND GEOGRAPHIC EXPANSION?1

Cylindrospermopsis raciborskii (Wo?osz.) Seenayya et Subba Raju is a planktonic filamentous cyanobacterium whose sudden worldwide proliferation and ability to produce toxins are a reason for concern. In this paper, we suggest that its ecological dominance might be explained by antagonistic interaction with other phytoplankton species due to production of allelopathic metabolites. To test this hypothesis, experiments were run with exudates of natural phytoplankton and C. raciborskii strains isolated from Lagoa Santa, a small natural lake in southeastern Brazil, where this species has become dominant in recent years. The exudates were added to different algal species obtained from the same environment and maintained in culture. After 24 h incubation, PAM fluorometry was used to compare control and treatment photosynthetic responses (relative electron transport rate) to the dissolved extracellular products. Results indicate that most of the target species were sensitive to C. raciborskii exudates, which showed strong inhibitory effects on their photosynthetic activities. These results provide evidence that allelopathy may offer a competitive benefit to C. raciborskii and contribute to its stable dominance in Lagoa Santa. A potential allelopathic advantage could also help to explain the geographic expansion of this species at midlatitudes.     

Morphological and Morphometrical Variations of Selected Rotifer Species in Response to Predation: a Seasonal Study of Selected Brachionid Species from Lake Xochimilco (Mexico)

Allelopathy of filamentous green algae (FGA) has been less studied than that of macrophytes. Little Budworth Pool, Cheshire, UK is a small, shallow, clear-water lake with high TP concentrations, very high NO_3-N concentrations, only moderate phytoplankton density, high FGA growth (mainly Spirogyra sp.) and no submerged plants. Experiments were carried out to test the possible allelopathic effects of Spirogyra on the phytoplankton of this lake and on a submerged plant Elodea nuttallii. Changes in phytoplankton growth, phytoplankton species dynamics and species composition were apparently not influenced by allelopathy of live or decaying Spirogyra. A shift from diatom (Cyclotella sp) – cryptomonad (Chroomonas acuta and Cryptomonas erosa) dominance to Chlorococcales (Micractinium pusillum, Monoraphidium contortum and Scenedesmus opoliensis) – Volvocales (Chlorogonium elongatum and Pandorina morum) dominance was recorded in both control and FGA treatments, suggesting an effect of nutrient enrichment. Nutrient concentrations and differences in competitiveness among phytoplankton species can also explain differences in their growth rates in Spirogyra filtrate. Spirogyra also did not influence apex number per plant, shoot length or growth rate of E. nuttallii. This FGA species probably cannot control phytoplankton or E. nuttallii growth in nutrient rich conditions through allelopathy.     

Plant–plant interactions as a mechanism structuring plant diversity in a Mediterranean semi‐arid ecosystem

Plant–plant interactions are among the fundamental processes that shape structure and functioning of arid and semi‐arid plant communities. Despite the large amount of studies that have assessed the relationship between plant–plant interactions (i.e., facilitation and competition) and diversity, often researchers forget a third kind of interaction, known as allelopathy. We examined the effect of plant–plant interactions of three dominant species: the perennial grass Lygeum spartum, the allelopathic dwarf shrub Artemisia herba‐alba, and the nurse shrub Salsola vermiculata, on plant diversity and species composition in a semi‐arid ecosystem in NE Spain. Specifically, we quantified the interaction outcome (IO) based on species co‐occurrence, we analyzed diversity by calculation of the individual species–area relationship (ISAR), and compositional changes by calculation of the Chao‐Jaccard similarity index. We found that S. vermiculata had more positive IO values than L. spartum, and A. herba‐alba had values between them. Lygeum spartum and A. herba‐alba acted as diversity repellers, whereas S. vermiculata acted as a diversity accumulator. As aridity increased, A. herba‐alba transitioned from diversity repeller to neutral and S. vermiculata transitioned from neutral to diversity accumulator, while L. spartum remained as diversity repeller. Artemisia herba‐alba had more perennial grass species in its local neighborhood than expected by the null model, suggesting some tolerance of this group to its “chemical neighbor”. Consequently, species that coexist with A. herba‐alba were very similar among different A. herba‐alba individuals. Our findings highlight the role of the nurse shrub S. vermiculata as ecosystem engineer, creating and maintaining patches of diversity, as well as the complex mechanism that an allelopathic plant may have on diversity and species assemblage. Further research is needed to determine the relative importance of allelopathy and competition in the overall interference of allelopathic plants.     

Ingredients for protist coexistence: competition, endosymbiosis and a pinch of biochemical interactions

1. The interaction between mutualism, facilitation or interference and exploitation competition is of major interest as it may govern species coexistence. However, the interplay of these mechanisms has received little attention. This issue dates back to Gause, who experimentally explored competition using protists as a model [Gause, G.F. (1935) Vérifications expérimentales de la théorie mathématique de la lutte pour la vie. Actualités Scientifiques et Industrielles, 277]. He showed the coexistence of Paramecium caudatum with a potentially allelopathic species, Paramecium bursaria. 2. Paramecium bursaria hosts the green algae Chlorella vulgaris. Therefore, P. bursaria may benefit from carbohydrates synthesised by the algae. Studying endosymbiosis with P. bursaria is possible as it can be freed of its endosymbiont. In addition, C. vulgaris is known to produce allelochemicals, and P. bursaria may benefit also from allelopathic compounds. 3. We designed an experiment to separate the effects of resource exploitation, endosymbiosis and allelopathy and to assess their relative importance for the coexistence of P. bursaria with a competitor that exploits the same resource, bacteria. The experiment was repeated with two competitors, Colpidium striatum or Tetrahymena pyriformis. 4. Results show that the presence of the endosymbiont enables the coexistence of competitors, while its loss leads to competitive exclusion. These results are in agreement with predictions based on resource equilibrium density of monocultures (R*) supporting the idea that P. bursaria's endosymbiont is a resource provider for its host. When P. bursaria and T. pyriformis coexist, the density of the latter shows large variation that match the effects of culture medium of P. bursaria. Our experiment suggests these effects are because of biochemicals produced in P. bursaria culture. 5. Our results expose the hidden diversity of mechanisms that underlie competitive interactions. They thus support Gauses's speculation (1935) that allelopathic effects might have been involved in his competition experiments. We discuss how a species engaged both in competition for a resource and in costly interference such as allelopathy may counterbalance these costs with a resource-provider endosymbiont.     

INTERACTIONS BETWEEN PULSED NUTRIENT SUPPLIES AND A PHOTOCYCLE AFFECT PHYTOPLANKTON COMPETITION FOR LIMITING NUTRIENTS IN LONG-TERM CULTURE1

The influence of periodic nutrient supplies and a photocycle on phytoplankton competition for limiting nutrients was examined using the diatoms Thalassiosira rotula Meunier (clone 411) and Chaetoceros sp. cf. vixvisibilis Schiller (clone 847). Chaetoceros sp. cf. vixvisibilis displaced T. rotula from ammonium-limited cultures under constant light irrespective of whether ammonium was supplied continuously, in 6 pulses.day^?1 or in a single daily pulse. In contrast, the species coexisted under the 14:10 h LD photocycle under either continuous or pulsed ammonium supplies with the relative abundance of C. sp. cf. vixvisibilis increasing as the interval between ammonium additions lengthened. Coexistence was not observed with either silicic acid or nitrate limitation. Chaetoceros sp. cf. vixvisibilis displaced T. rotula from both nitrate- and silicic acid-limited chemostat cultures and from semi-continuous cultures grown under the same photoperiod that produced coexistence with a daily pulse of ammonium. The presence of a photocycle was both necessary and sufficient to permit coexistence with ammonium limitation. Under continuous ammonium supply the photocycle may have induced a temporal separation of ammonium uptake between species, permitting sharing of the limiting nutrient and coexistence. In contrast, the species were shown to be in direct competition for the daily ammonium pulse. A competition model suggested that coexistence in this case arose from a balance between the species’ammonium uptake rates and their nitrogen demands for steady-state growth induced by the photocycle. The results indicate that variations in nutrient supply rates may contribute to the coexistence of phytoplankton species in the sea, but that the identity of the limiting nutrient and the influence of variations in other non-limiting resources play important roles in affecting the outcome of nutrient competition among planktonic algae.     

Toxic phytoplankton as a keystone species in aquatic ecosystems: stable coexistence to biodiversity

The effect of allelochemicals released by toxic species in plankton community is often taken into account to reveal plankton biodiversity. Using a minimal chemostat model we show that the interaction between toxic and non‐toxic phytoplankton species with changing competitive effects among species due to allelopathy helps to promote the stable coexistence of many species on a single resource and hence can solve the paradox of plankton. We emphasize toxic phytoplankton as a keystone species that strongly uncovers its allelochemicals on other non‐toxic phytoplankton and enhances the species persistence and diversity in aquatic ecosystems. In addition, we analyze the consistency of ecosystem functioning and species diversity using a number of approaches, such as sampling hypothesis with selection and complementarity effects, cascading extinction–reinvasion, and examining system dynamics at different enrichment levels and toxicity. Our results suggest that chemostats with one toxic and one or more nontoxic phytoplankton species can be used for the experimental verification of the stable coexistence of many species on a single resource in aquatic ecology.     

Inference of allelopathy is complicated by effects of activated carbon on plant growth

Allelopathy can play an important role in structuring plant communities, but allelopathic effects are often difficult to detect because many methods used to test for allelopathy can be confounded by experimental artifacts. The use of activated carbon, a technique for neutralizing allelopathic compounds, is now employed in tests for allelopathy; however, this technique also could produce large experimental artifacts. In three independent experiments, it was shown that adding activated carbon to potting media affected nutrient availability and plant growth. For most species tested, activated carbon increased plant biomass, even in the absence of the potentially allelopathic agent. The increased growth corresponded to increased plant nitrogen content, likely resulting from greater nitrogen availability. Activated carbon also affected nitrogen and other nutrient concentrations in soil media in the absence of plants. The observed effects of activated carbon on plant growth can confound its use to test for allelopathy. The detection of allelopathy relies on the difference between plant growth in medium with carbon and that in medium without carbon in the presence of the potentially allelopathic competitor; however, this difference may be biased if activated carbon alters soil nutrient availability and plant growth even in the absence of the focal allelopathic agent.     

High predictability of direct competition between marine diatoms under different temperatures and nutrient states

The distribution of marine phytoplankton will shift alongside changes in marine environments, leading to altered species frequencies and community composition. An understanding of the response of mixed populations to abiotic changes is required to adequately predict how environmental change may affect the future composition of phytoplankton communities. This study investigated the growth and competitive ability of two marine diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana, along a temperature gradient (9–35°C) spanning the thermal niches of both species under both high‐nitrogen nutrient‐replete and low‐nitrogen nutrient‐limited conditions. Across this temperature gradient, the competitive outcome under both nutrient conditions at any assay temperature, and the critical temperature at which competitive advantage shifted from one species to the other, was well predicted by the temperature dependencies of the growth rates of the two species measured in monocultures. The temperature at which the competitive advantage switched from P. tricornutum to T. pseudonana increased from 18.8°C under replete conditions to 25.3°C under nutrient‐limited conditions. Thus, P. tricornutum was a better competitor over a wider temperature range in a low N environment. Being able to determine the competitive outcomes from physiological responses of single species to environmental changes has the potential to significantly improve the predictive power of phytoplankton spatial distribution and community composition models.     

A shared limiting resource leads to competitive exclusion in a cross-feeding system

Species interactions and coexistence are often dependent upon environmental conditions. When two cross-feeding bacteria exchange essential nutrients, the addition of a cross-fed nutrient to the environment can release one species from its dependence on the other. Previous studies suggest that continued coexistence depends on relative growth rates: coexistence is maintained if the slower-growing species is released from its dependence on the other, but if the faster-growing species is released, the slower-growing species will be lost (a hypothesis that we call ‘feed the faster grower’ or FFG). Using invasion-from-rare experiments with two reciprocally cross-feeding bacteria, genome-scale metabolic modelling and classical ecological models, we explored the potential for coexistence when one cross-feeder became independent. We found that whether nutrient addition shifted an interaction from mutualism to commensalism or parasitism depended on whether the nutrient that limited total growth was required by one or both species. Parasitism resulted when both species required the growth-limiting resource. Importantly, coexistence was only lost when the interaction became parasitism, and the obligate species had a slower growth rate. Under these restricted conditions, the FFG hypothesis applied. Our results contribute to a mechanistic understanding of how resources can be manipulated to alter interactions and coexistence in microbial communities.     

Competing Effects of Toxin-Producing Phytoplankton on Overall Plankton Populations in the Bay of Bengal

The coexistence of a large number of phytoplankton species on a seemingly limited variety of resources is a classical problem in ecology, known as ‘the paradox of the plankton’. Strong fluctuations in species abundance due to the external factors or competitive interactions leading to oscillations, chaos and short-term equilibria have been cited so far to explain multi-species coexistence and biodiversity of phytoplankton. However, none of the explanations has been universally accepted. The qualitative view and statistical analysis of our field data establish two distinct roles of toxin-producing phytoplankton (TPP): toxin allelopathy weakens the interspecific competition among phytoplankton groups and the inhibition due to ingestion of toxic substances reduces the abundance of the grazer zooplankton. Structuring the overall plankton population as a combination of nontoxic phytoplankton (NTP), toxic phytoplankton, and zooplankton, here we offer a novel solution to the plankton paradox governed by the activity of TPP. We demonstrate our findings through qualitative analysis of our sample data followed by analysis of a mathematical model.     

Searching for allelopathic effects of submerged macrophytes on phytoplankton—state of the art and open questions

Allelopathy, here defined as biochemical interactions between aquatic primary producers, has always been intriguing as a process explaining the dominance of certain plant or algal species over others. Negative chemical interference has been invoked as one of the steering mechanisms behind mutual dominance of either submerged macrophytes or phytoplankton in shallow eutrophic lakes. Yet, despite much effort, convincing evidence for allelopathic interactions in situ is still missing. Also, laboratory approaches often lack reality. Inspired by a series of talks at the Shallow Lakes 2005 meeting in Dalfsen, the Netherlands, we argue that there is circumstantial but strong evidence that allelopathic interference between submerged macrophytes and phytoplankton may indeed exist in aquatic ecosystems despite the problems associated with research in this field. We first discuss experimental approaches combining laboratory and field studies, based on examples presented at this meeting. We then discuss the impact of nutrient status of both producing and target organism and biotic factors such as herbivory or pathogens that might affect allelopathy. Further topics are the potential seasonality of effects and the species-specificity of certain allelochemicals. We conclude with some thoughts why a final proof for allelopathy in situ might remain difficult or even inaccessible in some cases, and why we nevertheless should not abandon this idea.     

Interaction between the macrophyte <Emphasis Type="Italic">Stratiotes aloides</Emphasis> and filamentous algae: does it indicate allelopathy?

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.