Chironomid stratigraphy in the shallow and eutrophic Lake Søbygaard, Denmark: chironomid–macrophyte co-occurrence

1. A sediment core from the shallow, hypertrophic Lake Søbygaard (mean depth ∼1 m; [TP] 310 μg P L⁻¹) was analysed for subfossil remains to reconstruct chironomid community changes in relation to the succession and disappearance of aquatic macrophytes. 2. Species composition in the 1.10 m core indicates a succession from a 'naturally' eutrophic state to a hypertrophic state during recent centuries. Radiometric dating (²¹⁰Pb) of the uppermost 20 cm of the sediment core (∼1932–93) indicates that sediment accumulation rate had doubled in recent decades. 3. Changes in chironomid assemblages were in close agreement with changes in both diatoms and macrophyte remains in the same core. Distinct changes in chironomid communities reflect the eutrophication process and macrophyte succession through Chara , Ceratophyllum and Potamogeton dominance to the present state, with complete loss of submerged vegetation and dominance by phytoplankton. 4. The co-occurrence and relationship between aquatic macrophyte diversity and recent subfossil chironomid assemblages were assessed from an additional 25 Danish lakes. There was good agreement between the macrophyte and chironomid-based lake groupings. Overall, a significant difference ( P <0.001) was found in chironomid assemblages among lakes in different macrophyte classes. In a pair-wise comparison, the poorly buffered mesotrophic lakes and the alkaline eutrophic lakes had significantly different chironomid assemblages. 5. Chironomid taxa commonly reported to be associated with macrophytes ( Cricotopus , Endochironomus and Glyptotendipes ) were shown also to be indicators of highly productive lakes lacking abundant submerged vegetation.     

Ecological influences on larval chironomid communities in shallow lakes: implications for palaeolimnological interpretations

1. To correctly interpret chironomid faunas for palaeoenvironmental reconstruction, it is essential that we improve our understanding of the relative influence of ecosystem variables, biotic as well as physicochemical, on chironomid larvae. To address this, we analysed the surface sediments from 39 shallow lakes (29 Norfolk, U.K., 10 Denmark) for chironomid head capsules, and 70 chironomid taxa (including Chaoborus) were identified. 2. The shallow lakes were selected over large environmental gradients of aquatic macrophytes, total phosphorus (TP) and fish communities. Redundancy analysis (RDA) identified two significant variables that explained chironomid distribution: macrophyte species richness (P < 0.001) and TP (P < 0.005). Generalised linear models (GLM) identified specific taxa that had significant relationships with both these variables. Macrophyte percentage volume infested (PVI) and species richness were significant in classifying the lake types based on chironomid communities under twinspan analysis, although other factors, notably nutrient concentrations and fish communities, were also important, illustrating the complexities of classifying shallow lake ecosystems. Lakes with plant species richness >10 all had relatively diverse (Hill’s N2) chironomid assemblages, and lakes with Hill’s N2 >10 all had TP <250 μg L⁻¹ and total fish densities <2 fish per m². 3. Plant density (PVI), and perhaps more importantly species richness, were primary controls on the distribution of chironomid communities within these lakes. This clearly has implications for palaeoenvironmental reconstructions using zoobenthos remains (i.e. chironomids) and suggests that they could be used to track changes in benthic/pelagic production and could be used as indicators of changing macrophyte habitat. 4. Measuring key biological gradients, in addition to physicochemical gradients, allowed the major controls on chironomid distribution to be assessed more directly, in terms of plant substrate, food availability, competition and predation pressure, rather than implying indirect mechanisms through relationships with nutrients. Many of these variables, notably macrophyte abundance and species richness, are not routinely measured in such studies, despite their importance in determining zoobenthos in temperate shallow lakes. 5. When physical, chemical and ecological gradients are considered, as is often the case with palaeo‐reconstructions rather than training sets chosen to maximise one gradient, complex relationships exist, and attempting to reconstruct a single trophic variable quantitatively may not be appropriate or reliable.     

Effect of macrophyte community composition and nutrient enrichment on plant biomass and algal blooms

Submerged freshwater macrophytes decline with increasing eutrophication. This has consequences for ecosystem processes in shallow lakes and ponds as macrophytes can reduce algal blooms under eutrophic conditions. We hypothesize that the productivity of submerged vegetation, biomass change under eutrophication and the suppression of algal blooms may be affected by macrophyte community composition. To test our hypothesis, we established three macrophyte community types in 36 fishless experimental ponds: one dominated by the oligotrophic species Chara globularis, one dominated by the eutrophic species Potamogeton pectinatus and a diverse vegetation which became co-dominated by Elodea nuttallii and C. globularis, and we fertilized half of the ponds.The macrophyte communities produced different amounts of biomass and they responded differently to fertilization. The community dominated by Potamogeton produced the lowest overall biomass, but was not affected by nutrient addition. The communities dominated by Chara and co-dominated by Elodea and Chara produced more than four-fold the amount of biomass produced in Potamogeton communities under oligotrophic conditions, but were strongly negatively affected by nutrient addition.Phytoplankton abundance did not differ significantly among the plant community types, but showed large variation within community types. There was a significant negative relationship between spring macrophyte biomass and the probability of summer algal blooms. The occurrence of algal blooms coincided with low daphnid densities and high pH (>10).We conclude that the macrophyte community composition, characterized by the dominant species, strongly affected the amount of biomass production as well as the short-term response of the vegetation to nutrient enrichment. Macrophyte community composition had no direct effect on algal blooms, but can affect the occurrence of algal blooms indirectly as these occurred only in ponds with low (<100 g/m² DW) spring macrophyte biomass.     

Spatial distribution of epiphytic chironomid larvae in a shallow macrophyte-dominated lake: effect of macrophyte species and food resources

Four submerged macrophytes were analysed with their respective assemblages of chironomid and algal taxa. Studies were made bimonthly, from January to November, in a shallow macrophyte-dominated lake. The periphytic algal community was represented by chlorophytes, diatoms and cyanobacteria; diatoms showed the highest relative abundance on the macrophyte species during the study. Algal biomass was affected by the season, and in January the highest values were found on Stratiotes aloides L., Potamogeton lucens L., Myriophyllum spicatum L.; in July on Chara aculeolata Kütz. The lowest algal biomass, on most of the macrophytes studied, was found in September. Larvae of Dicrotendipes sp., Diplocladius cultriger Kieffer, Endochironomus albipennis gr., Endochironomus impar (Walker), Glyptotendipes sp. and Paratanytarsus austriacus Kieffer were dominant within the chironomid community. The spatial distribution of chironomids between macrophytes species was related to periphytic algae biomass and certain environmental variables (temperature, water transparency, biomass of chlorophytes and diatoms). Analysis of chironomid diet and the results of canonical correspondence analysis confirmed this relationship. The strongest chironomid-algae relationships were observed on S. aloides and P. lucens.     

Shifts in aquatic macrophyte abundance and community composition in cottage developed lakes of the Canadian Shield

In this study, we examined how the biomass and species composition of aquatic plant communities relates to cottage development of Canadian Shield lakes. Within the North Kawartha Region of Ontario, we sampled the macrophyte communities at two water depths (0.5 m and 1.5 m) in lakes (n = 12) having a range of cottage densities (0-23 cottages km⁻¹ of shoreline). Across all lakes, 39 species were found, with individual lake richness ranging from six to ten. Macrophyte biomass decreased with increasing cottage density, irrespective of depth (ANCOVA dev’t*depth p = 0.925). In contrast, only the shallower depth showed a relationship between cottage development and richness and diversity; highly developed lakes had three or fewer species and diversities less than 1.5. There was also a shift in structural plant type from floating leaf and emergent on undeveloped lakes to submersed and submersed low-lying on developed lakes. Ordination analysis demonstrated that cottage development (and to a lesser extent, lake area) was strongly correlated (p = 0.05) with community species composition in southern Ontario lakes. Our results thus demonstrate that the management of cottage development should minimize the loss of biomass and species richness of aquatic plants given the likely negative effects of these alterations on other taxa in littoral zones and foodwebs in lake ecosystems.     

An assessment of the importance of emergent and floating-leaved macrophytes to trophic status in the Loosdrecht lakes (The Netherlands)

The potential importance of the six major emergent and floating-leaved macrophyte species in recycling of sediment phosphorus in the Loosdrecht lakes was studied. Representative plant samples were collected at the time of maximum biomass, and analysed for biomass and carbon, nitrogen and phosphorus contents. Species cover was determined by aerial photography.Total cover in the seven lakes studied ranged between 2 and 26 percent. For the four main species, biomass per unit area increased with lake trophic status. Consistent differences in C, N and P contents per unit biomass were not observed. Although cover values were small, significant amounts of C, N and P were contained in the macrophytes when compared with maximum sestonic content.Potential P loads from macrophyte decay were calculated. In Lake Loosdrecht, the P load represented 15 percent of current external P inputs. The potential importance of macrophyte decay to P recycling in the other lakes is greater.Decay of macrophyte species at the end of the growing season appears to affect autumnal nutrient and chlorophyll a levels in the water column of some lakes. The re-establishment of submerged species following lake restoration may increase the importance of this pathway in the lakes.     

Impacts of sediment type on the performance and composition of submerged macrophyte communities

To restore deteriorated lake ecosystems, it is important to identify environmental factors that influence submerged macrophyte communities. While sediment is a critical environmental factor for submerged macrophytes and many studies have examined effects of sediment type on the growth of individual submerged macrophytes, very few have tested how sediment type affects the growth and species composition of submerged macrophyte communities. We constructed submerged macrophyte communities containing four co-occurring submerged macrophytes (Hydrilla verticillata, Myriophyllum spicatum, Ceratophyllum demersum and Chara fragilis) and subjected them to three sediment treatments, i.e., clay, a mixture of clay and quartz sand at a volume ratio of 1:1 and a mixture at a volume ratio of 1:4. Compared to the clay, the 1:1 mixture treatment greatly increased overall biomass, number of shoot nodes and shoot length of the community, but decreased its diversity. This was because it substantially promoted the growth of H. verticillata within the community, making it the most abundant species in the mixture sediment, but decreased that of M. spicatum and C. demersum. The sediment type had no significant effects on the growth of C. fragilis. As a primary nutrient source for plant growth, sediment type can have differential effects on various submerged macrophyte species and 1:1 mixture treatment could enhance the performance of the communities, increasing the overall biomass, number of shoot nodes and shoot length by 39.03%, 150.13% and 9.94%, respectively, compared to the clay treatment. Thus, measures should be taken to mediate the sediment condition to restore submerged macrophyte communities with different dominant species.     

The response of two submerged macrophytes and periphyton to elevated temperatures in the presence and absence of snails: a microcosm approach

Global warming may affect snail–periphyton–macrophyte relationships in lakes with implications also for water clarity. We conducted a 40-day aquaria experiment to elucidate the response of submerged macrophytes and periphyton on real and artificial plants to elevated temperatures (3°C) under eutrophic conditions, with and without snails present. With snails, the biomass and length of Vallisneria spinulosa leaves increased more at the high temperature, and at both temperatures growth was higher than in absence of snails. The biomass of periphyton on V. spinulosa as well as on artificial plants was higher at the highest temperature in the absence but not in the presence of snails. The biomass of Potamogeton crispus (in a decaying state) declined in all treatments and was not affected by temperature or snails. While total snail biomass did not differ between temperatures, lower abundance of adults (size >1 cm) was observed at the high temperatures. We conclude that the effect of elevated temperature on the snail–periphyton–macrophyte relationship in summer differs among macrophyte species in active growth or senescent species in subtropical lakes and that snails, when abundant, improve the chances of maintaining actively growing macrophytes under eutrophic conditions, and more so in a warmer future with potentially denser growth of periphyton.     

Freshwater macrophyte communities in lakes of variable landscape position and development in northern Wisconsin,U.S.A.

Using multivariate analyses, we evaluated the roles of landscape position and human development in the presence, composition, and abundance of freshwater macrophyte communities among 60 study lakes located in Vilas County, Wisconsin. These lakes varied in their position in a drainage network and surrounding land use as well as size, maximum depth, chemistry, and clarity. We used non-metric multidimensional scaling ordination, cluster analyses, and classification tree analyses to assess the multivariate relationships among macrophyte communities and environmental variables including an index of human development. Macrophyte communities in lakes high in the landscape were found to be dominated by isoetids while lakes low in the drainage network were dominated by elodeids due to the difference in bicarbonate concentrations. The presence of the human-introduced crayfish, Orconectes rusticus, had a high and significant correlation with the absence of macrophytes. An increase in human development correlated with a decrease in macrophyte abundance.     

Microhabitat influence on chironomid community structure and stable isotope signatures in West Greenland lakes

Most functional feeding types are represented within the species rich group of aquatic chironomids. Thus, we hypothesized that different lake types and microhabitats within lakes would (1) host specific chironomid communities and (2) that the individual communities would show specific δ ¹³C stable isotope signatures reflecting the prevailing origin of food source. To test our hypotheses, five lakes in southwest Greenland were investigated at a high taxonomic resolution and with detailed information on δ ¹³C signature of the chironomids and of individual microhabitats (macrophytes, sediment, stones, and profundal). We found that there was a significant difference in δ ¹³C between the chironomid assemblages of freshwater lakes and oligosaline lakes, while assemblages of the littoral microhabitats did not differ significantly. The δ ¹³C of chironomids reflected the wide variety of habitat signals, particularly in the freshwater lakes. Our results indicate that many chironomid taxa are ubiquitous and are found in several microhabitats, suggesting that they can adjust their feeding strategy according to the habitat. The implication is that chironomid assemblage composition has only limited use as indicator of littoral microhabitats in the Arctic. On the other hand, the δ ¹³C signature of fossil chironomids might have a potential as indicator of microhabitats in freshwater lakes.     

Plant community dynamics in a chain of lakes: principal factors in the decline of rooted macrophytes with eutrophication

Shoe Lake and East Graham Lake, part of a small chain of lakes in southeastern Michigan, USA, differ in nutrient loading and in the structure and productivity of their aquatic plant communities. A comparative study of species frequency and biomass distributions, nutrient contents, and responses to experimental nutrient enrichment and shading, was conducted to determine the principal factors controlling the macrophyte dynamics. A central objective was to address the question of why rooted macrophyte growth declines with eutrophication, and to test existing models designed to explain this phenomenon. In the more eutrophic Shoe Lake, diversity and productivity of rooted macrophytes were relatively low, restricted primarily by combined shading of phytoplankton, periphyton, and non-rooted macrophytes (principally Ceratophyllum demersum, along with Utricularia vulgaris and Cladophora fracta). In the less eutrophic East Graham Lake, lower nitrogen availability restricted the growth of all of these shading components, resulting in clearer water and higher productivity and diversity of rooted macrophytes. The macrophytes did not allelopathically suppress the phytoplankton in East Graham Lake. The results supported a direct relationship between nutrient loading, increasing growth of phytoplankton, periphyton and non-rooted macrophytes, and decline of rooted macrophytes.     

Relationship between epipelon,epiphyton and phytoplankton in two limnological phases in a shallow tropical reservoir with high Nymphaea coverage

Macrophytes and phytoplankton are recognized as having roles in determining alternative stable states in shallow lakes and reservoirs, while the role of periphyton has been poorly investigated. Temporal and spatial variation of phytoplankton, epipelon and epiphyton was examined in a shallow reservoir with high abundance of aquatic macrophytes. The relationships between algae communities and abiotic factors, macrophyte coverage and zooplankton density were also analyzed. Monthly sampling was performed in three zones of the depth gradient of the reservoir. Two phases of algal dominance were found: a phytoplankton phase and epipelon phase. The phase of phytoplankton dominance was characterized by high macrophyte coverage. Rotifera was the dominant zooplankton group in all the zones. Flagellate algae were dominant in phytoplankton, epipelon and epiphyton. Macrophyte coverage was found to be a predictor for algal biomass. Changes in biomass and species composition were associated with macrophyte cover variation, mainly the Nymphaea. In addition to the abiotic factors, the macrophyte coverage was a determining factor for changes to the algal community, contributing to the alternation between dominance phases of phytoplankton and epipelon. The macrophyte–phytoplankton–periphyton relationship needs to be further known in shallow reservoirs, especially the role of epipelon as an alternate stable state.

     

OPINION Manipulating lake community structure: where do we go from here?

SUMMARY. 1 More than 10 years experience with whole lake pelagic manipulation has suggested some general trends applicable to all freshwater pelagic communities and some specific trends related to lake depth.
2 Among the general trends is the observation that the trophic cascade is strongly damped. This means that changes in phytoplankton biomass can be assured only when the fish community is strongly manipulated.
3 Among the depth related trends is the observation that in shallow lakes, changes in fish community structure are more likely to have cascading impacts on phytoplankton than are changes in deep lakes.
4 In shallow lakes, fish removal frequently results in decreased turbidity which is associated with the development of dense macrophyte populations and significant reductions of algal standing stocks. The mechanisms involve: increased grazing by zooplankton, the removal of fish induced bioturbation and nutrient recycling, and direct and indirect macrophyte effects (shading, zooplankton refuges and competition for nutrients).
5 In shallow lakes, where planktivore biomass can be regulated and macrophyte development is acceptable, fish biomanipulalions are likely to result in reduced algal populations and improved water quality.
6 In deep lakes, where macrophytes are not as important, long-term effects of fish manipulations are strongly dependent upon the probability of non-grazable algal bloom development. This is determined by many factors (chemical, physical and grazer related) which modify the impact that grazers have on phytoplankton biomass.
7 In deep lakes, successful fish biomanipulations may only be effective when chemical and physical factors are altered to produce algal species compositions that permit strong top-down control of prey by predators.     

Factors influencing macrophyte metrics in Estonian coastal lakes in the light of ecological status assessment

Macrophytes and factors affecting their distribution were studied in 19 coastal lakes of Estonia. The aim of the study was to determine the factors influencing the distribution of macrophytes in coastal lakes and to assess the suitability of valid macrophyte metrics. Our hypothesis was that in coastal lakes most of the macrophyte distribution patterns are caused by lake-specific variables. Morphological, physico-chemical and catchment area characteristics of the lakes varied greatly. Lakes were in different development stages—lakes nearest to the Baltic Sea were younger and more influenced by brackish water and the furthest lakes were older with more freshwater. All that variability was reflected in macrophyte parameters. Factor analysis of environmental indices divided them into three groups—catchment area, morphometric and water chemistry factors. The first factor may be considered as a pressure and the other two as lake-type-specific factors. Lake catchment area parameters had an influence on Bolboschoenus maritimus, Chara tomentosa and Typha latifolia abundance. Morphometric parameters had an influence on the depth distribution of macrophytes and water chemistry factors on the abundance of helophytes. Current indicator species showed more variability associated with lake-specific factors than with changes in status or the influence of pressures.     

Is the island biogeography model a poor predictor of biodiversity patterns in shallow lakes?

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Macroinvertebrates associated with two submerged macrophytes,Lagarosiphon ilicifolius and Vallisneria aethiopica,in the Sanyati Basin,Lake Kariba,Zimbabwe: effect of plant morphological complexity

Vallisneria aethiopica and Lagarosiphon ilicifolius are common and abundant submerged macrophytes in Lake Kariba, Zimbabwe. The two species have distinct structural morphologies, with Vallisneria consisting of long ribbon-like leaves, while Lagarosiphon has filiform stems with numerous small alternate leaves. This study investigated the effect of these architectural differences between the two plant species on their epiphytic macroinvertebrate assemblages in the shallow inshore waters of Lake Kariba. Ten sites were sampled on three occasions between May and July 2005. A total of 56 macroinvertebrate taxa was collected, 48 from Lagarosiphon and 45 from Vallisneria. Generally, the two plant species were associated with similar macroinvertebrate communities, but the average abundances of most taxa, and thus the overall macroinvertebrate abundances, were significantly greater on Lagarosiphon. The main macroinvertebrate functional feeding groups found on both plant species were collector-gatherers, grazers and predators, all of which were significantly more abundant on Lagarosiphon. Although the macroinvertebrate assemblages associated with Vallisneria and Lagarosiphon generally consisted of the same taxa, there were distinct and significant differences between them, probably due to the architectural differences between the two submerged macrophytes.     

Distribution and growth of benthic macroinvertebrates among different patch types of the littoral zones of two arctic lakes

1. To evaluate the effect of habitat patch heterogeneity on abundance and growth of macroinvertebrates in arctic lakes, macroinvertebrate abundance, individual biomass, and potential food resources were studied in three patch types in two arctic lakes on the Alaskan North Slope near the Toolik Lake Field Station. An experiment was conducted to determine which sediment patch type supported higher growth rates for Chironomus sp., a commonly occurring macroinvertebrate. 2. Potential organic matter (OM) resources were significantly higher in both rock and macrophyte patches than in open‐mud patches. Total macroinvertebrate densities in both lakes were highest in rock patches, intermediate in macrophytes and lowest in open‐mud. The open‐mud patches also had lower species richness compared with other patch types. Additionally, individual biomass for one clam species and two chironomid species was significantly greater in rock patches than in open‐mud. 3. In a laboratory experiment, Chironomus showed two to three times greater mass increase in sediments from macrophyte and rock patches than from open‐mud patches. Rock and macrophyte experimental sediments had at least 1.5 × the percentage OM as open‐mud sediments. 4. Chlorophyll a appeared to be the best predictor for invertebrate abundances across all patch types measured, whereas OM content appeared to be the variable most closely associated with Chironomus growth. 5. Our results combined with previous studies show that the relationships between macroinvertebrate community structure, individual growth, and habitat heterogeneity are complex, reflecting the interaction of multiple resources, and biotic interactions, such as the presence or absence of a selective vertebrate predator (lake trout, Salvelinus namaycush).     

Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.     

Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales. 2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance. 3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L^?1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes. 4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.     

Water table and species identity outweigh carbon and nitrogen availability in a softwater plant community

Performance of aquatic macrophytes is driven by many environmental factors, and a major challenge is to understand how aquatic macrophyte communities are structured in various environments. In softwater lakes in Western Europe, hydrological state (submersed/emersed), carbon dioxide and ammonium levels and species interactions are considered as driving forces in structuring amphibious plant communities. In this study we aimed at evaluating the relative importance of these factors for four species in a competitive neighbourhood. Softwater lake habitat was simulated during one growing season in laboratory conditions, mimicking water level fluctuation, photoperiod and temperature. Artificial communities consisted of small populations of four softwater macrophyte species: Luronium natans, Baldellia ranunculoides ssp. repens, Eleocharis multicaulis and Hydrocotyle vulgaris. These communities were subjected to two levels of carbon dioxide and ammonium. Additionally, monocultures of Baldellia and Eleocharis were grown at a higher nutrient level combination in order to measure their competitive response in a community. Time (hydrological state) and species identity turned out to be the only consistently significant factors determining community composition. Plant performance was clearly species-dependent, while carbon dioxide and ammonium did not have major effects. The competitive response was significant in both Eleocharis and Baldellia. Competition intensity was highest in the emersed state. Carbon dioxide had a supplementary effect on the within-species performance in Luronium, Baldellia and Eleocharis, with high carbon dioxide level mainly resulting in more flowers and more stolons. Community outcomes and competitive responses in aquatic macrophytes appear difficult to predict, because of mixed life strategies and morphological and functional plasticity. We conclude that hydrological state was the only important environmental factor. The identity of the species that were present—implying species interactions—largely determined community outcome.