Long‐term allelopathic control of phytoplankton by the submerged macrophyte Elodea nuttallii

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Allelopathic effect of the aquatic macrophyte, Stratiotes aloides, on natural phytoplankton

1. A survey of different Dutch Stratiotes stands showed that the density of phytoplankton (except cyanobacteria) was always higher outside S. aloides than between the rosettes of S. aloides. Analyses of water samples revealed that nutrient limitation was unlikely to have caused the lower phytoplankton biomass in the vicinity of S. aloides. 2. An in situ incubation experiment in the Danube Delta, Romania, indicated allelopathic activity against phytoplankton in S. aloides stands. The growth rate of natural phytoplankton populations exposed to water from S. aloides stands was significantly lower than that of populations that had not been in contact with S. aloides exudates. 3. A laboratory microcosm experiment showed a significantly lower phytoplankton biomass in treatments with S. aloides exudates. Nutrient concentrations and the light intensity were high enough that the lower phytoplankton biomass could not be explained by nutrient or light limitation.     

Snail communities increase submerged macrophyte growth by grazing epiphytic algae and phytoplankton in a mesocosm experiment

The relationships between producers (e.g., macrophytes, phytoplankton and epiphytic algae) and snails play an important role in maintaining the function and stability of shallow ecosystems. Complex relationships exist among macrophytes, epiphytic algae, phytoplankton, and snails. We studied the effects of snail communities (consisting of Radix swinhoei, Hippeutis cantori, Bellamya aeruginosa, and Parafossarulus striatulus) on the biomass of phytoplankton and epiphytic algae as well as on the growth of three species of submerged macrophytes (Hydrilla verticillata, Vallisneria natans, and one exotic submerged plant, Elodea nuttallii) in a 90‐day outdoor mesocosm experiment conducted on the shore of subtropical Lake Liangzihu, China. A structural equation model showed that the snail communities affected the submerged macrophytes by grazing phytoplankton and epiphytic algae (reduction in phytoplankton Chl‐a and epiphytic algal abundance), enhancing the biomass of submerged macrophytes. Highly branched macrophytes with high surfaces and morphologies and many microhabitats supported the most snails and epiphytic algae (the biomass of the snail communities and epiphytic algae on H. verticillata was greater than that on V. natans), and snails preferred to feed on native plants. Competition drove the snails to change their grazing preferences to achieve coexistence.     

Allelopathic inhibition of epiphytes by submerged macrophytes

The hypothesis that epiphytes are more vulnerable to allelochemicals released by submerged macrophytes than phytoplankton was tested by measuring growth and photosystem (PS) II activity of three common epiphytic algae and cyanobacteria in coexistence with Myriophyllum spicatum using dialysis tubes. Results were compared with earlier experiments on planktonic species. Contrary to the planktonic species, the tested epiphytes, the green algae Stigeoclonium tenue, the diatom Gomphonema parvulum and the cyanobacterium Oscillatoria limosa, were not significantly inhibited by M. spicatum. Growth and PS II activity of O. limosa were even significantly enhanced by M. spicatum, but this effect disappeared under phosphorus-deficiency due to the allelopathically induced inhibition of the alkaline phosphatase activity or phosphorus leakage by the macrophytes. My findings of a lower vulnerability of epiphytes against allelopathic substances of submerged macrophytes are supported by results of a literature survey.     

Allelopathic inhibition of phytoplankton by exudates from Stratiotes aloides

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.     

Photosynthetic characteristics of the submerged macrophyte Ceratophyllum demersum

The photosynthetic and growth characteristics of Ceratophyllum demersum L. were investigated under laboratory conditions which simulated those encountered in the plants' normal environment. The carbon fixation rate of C. demersum measured with ¹⁴C at light and carbon saturation at pH 8.0 was 4.48 mg C (g ash-free dry weight)⁻¹ h⁻¹. It was lower at pH 6.5 than at pH 8.0. The light use efficiencies in quiescent plants and actively growing plants were 6.3 and 8.7 × 10⁻⁹ kg CO₂ J⁻¹, respectively, with corresponding maximum photosynthetic rates of 2.67 and 4.36 mg C (g ash-free dry weight)⁻¹ h⁻¹. Photorespiration in actively growing plants consumed 24% of the carbon fixed. Incubation with DCMU demonstrated that about one-third was refixed. The optimum temperature for carbon fixation was 25°C. The C₃-photosynthetic pathway was the main operational route as indicated by the early photosynthetic products (largely C₃-acids) and the absence of Krantz anatomy and the chlorophyll a:b ratio (2.7). The maximum relative growth rates ranged from 0.025 to 0.041 g ash-free dry weight (g ash-free dry weight)⁻¹ day⁻¹ in the field (Lake Vechten, 1 to 3 m depth classes).     

The effect of different submerged macrophyte species and biomass on sediment resuspension in a shallow freshwater lake

Our study aim was to elucidate the effects of different species of submerged macrophytes and biomass levels on sediment resuspension. For this purpose experiments were conducted in four different enclosures (Potamogeton maackianus enclosure-PE, Vallisneria spinulosa enclosure-VE, manipulated enclosure-ME and aquaculture enclosure-AE). A sediment trap method was employed and the experiments were conducted from summer to winter in a shallow freshwater lake located in central China. A total of 813, 1277, 613 and 693 g DW m⁻² of sediment was resuspended in VE, AE, ME and PE, respectively. Our results showed that P. maackianus was more effective than V. spinulosa in restraining sediment resuspension. Macrophytes reached their maximum effectiveness of reducing resuspension at a certain species-specific biomass threshold above which biomass effects on resuspension were negligible. The threshold biomass was estimated as 300 g m⁻² for P. maackianus. Accordingly, within a lake management and aquaculture aspect, we conclude that as long as biomass does not fall below this threshold its consumption will not influence sediment resuspension. In the mid-lower reaches of the Yangtze River macrophyte coverage protects the lake sediment against adverse effects of monsoon wind; if the vegetation is eroded aquaculture sediment resuspension increases significantly.     

Decomposition of lignocellulose from a freshwater macrophyte by aero-aquatic fungi

Mineralization of uniformly radiolabeled [¹⁴C]lignocellulose and specifically radiolabeled [¹⁴C-lignin]lignocellulose from the freshwater sedgeCarex walteriana by five aero-aquatic fungi was investigated. The extent of mineralization varied among the five species from 2.2 to 4.2% for the lignin component and from 3.3 to 20.6% for the polysaccharide component. The extent of mineralization of both lignin and polysaccharide moieties by a mixed culture of the five fungi were generally markedly lower than by pure cultures, possibly due to the production of antimicrobial compounds.Spirosphaera foriformis, the most active strain in lignin as well as in polysaccharide mineralization, degraded ferulic acid faster than p-coumaric acid. Decomposition ofCarex walteriana lignocellulose by this strain resulted in decreased cinnamyl/vanillyl (C/V) and syringyl/vanillyl (S/V) ratios. Offprint requests to: M. Bergbauer.     

Oxygen transport in the submerged freshwater macrophyte Egeria densa planch. I. Oxygen production,storage and release

Oxygen exchange in the Egeria densa Planch. (Hydrocharitaceae) lacunar system was investigated using Clark-type oxygen electrodes to monitor root and shoot oxygen release into a bicompartment apparatus. An average root O₂ loss rate of 2.97±0.21 (s.e., n = 50) μl O₂ h⁻¹ cm⁻² root surface area was found, which was of a similar magnitude to values reported by other authors.Root O₂ loss was 4–5 times higher in the light than in the dark, and was approximately 60% higher with still water around the shoot than with a slow, continuous water flow across the shoot surface. Furthermore, both these root O₂ loss rates were reduced when the lower portion of the stem was darkened to simulate self-shading. Under illumination, the boundary layer and stem wall resistances caused thet stem lacunar oxygen partial pressure to be up to 5 kPa higher than in the water.Measurements of lacunar dimensions in Egeria allowed calculation of the oxygen gradient required to drive the observed transport rate. Simple gas-phase diffusion down a gradient of 5.49 × 10⁻² cm³ O₂ cm⁻³ m⁻¹ is sufficient to account for the oxygen transport rates in the experimental material.The ecological significance of the results is discussed, and a schematic model describing the diffusive transport of oxygen in the Egeria lacunar system is developed.     

Effect of temperature on submerged macrophyte litter decomposition within sediments from a large shallow and subtropical freshwater lake

In shallow aquatic systems, the majority of organic matter mineralization occurs in the sediments. Several factors including temperature control mineralization rates, however, the underlying causes of the effects are not well understood in subtropical lakes. In this study, we determined the influence of temperature on organic matter degradation by taking sediments from four sites in a subtropical large shallow freshwater lake, and monitoring organic matter composition and enzymes in microcosm experiments at five temperatures from 5 to 40°C. Following a three-month incubation, it was found that the mineralization of submerged plants in sediments was strongly influenced by temperature. Removal efficiency of total organic carbon in sediments ranged from 4.3 to 22.6% at 5°C, and reached 46.7–55.5% at 40°C. In addition, the removal efficiency of organic matter and the relative recalcitrant carbon decomposition depended on sediment type. For sediments in the site located in the lake center, recalcitrant and labile carbon decomposition had equivalent responses to the different temperatures. For sediments with dominance of submerged macrophytes, the humic acids were low even at high temperature. Thus, the annual deposition of plant litter in sediments favored organic carbon decomposition rather than humification.     

Morphological responses of a submerged macrophyte to epiphyton

Epiphyton might have distinctive influence on the morphology of substrate macrophyte. In this article, we evaluate the influence of epiphyton on the morphological characteristics of their substrate submerged macrophyte, Potamogeton perfoliatus under two light intensities. The experiment was carried out for a period of 84 days in 12 glass aquaria under laboratory conditions. It was based on a 2 × 2 factorial design with epiphyton status (present or absent) and light intensity (200 or 80 μE m⁻² s⁻¹). Both epiphyton and light intensity had significant effects on the morphology and biomass allocation of the experimental plants. The average number of leaves, total length of newly recruited shoots and diameter of stems were greater in the epiphyton-free control plants than in the epiphyton-colonized plants under low light conditions. The plants with epiphyton allocated more biomass in their rhizomes and roots (% relative to total biomass basis) when compared to the control plants in both light intensities. There were also significant epiphyton–light interactions. The control plants under low light intensity showed higher internodal elongation in their main shoots when compared to the plants under high light intensity as an adaptation mechanism. Whereas the plants with epiphyton did not show such an adaptation. The new shoots of the control plants under low light intensity did not show any internodal elongation as observed in the main shoots. Furthermore, the length of the leaves of main shoots was larger in control plants with epiphyton and high light intensity than in plants with epiphyton and low light intensity, but such a variation was absent in the new shoots. We conclude that the long-term colonization by epiphyton and their shading effects induced the observed morphological changes in plants.     

Diurnal variation in light and carbon limitation of photosynthesis by two species of submerged freshwater macrophyte with a differential ability to use bicarbonate

SUMMARY.

• 1 Rates of photosynthetic oxygen evolution by Callitriche cophocarpa and Ranunculus peltatus in stream were measured on live occasions during the light period on 2 days at ambient light and ambient inorganic carbon, ambient light and saturating inorganic carbon, saturating light and ambient inorganic carbon, saturating light and saturating inorganic carbon and air-equilibrium inorganic carbon and ambient light.
• 2 Despite an ambient CO₂ concentration of about 220 μm , which is about ten times air-equilibrium, the concentration of inorganic carbon was more limiting than light on all the occasions that rates were measured. On average, rates of photosynthesis at ambient concentrations of CO₂ were about 130 and 425 μmol O₂ g^?1 DW h^?1 for C. cophocarpa and R. peltatus, respectively. These rates as a percentage of carbon saturated rates were only about 35% for C. cophocarpa and about 60% for R. peltatus. Ambient rates as a percentage of light saturated rates were about 80% for C. cophocarpa and about 95% for R. peltatus. Only in early morning and late evening where the photon irradiance was below 160 μmol m^?2 s^?1 was there evidence for slight light limitation.
• 3 Based on results from pH-drift experiments and from rates of photosynthesis as a function of CO₂ concentration in the presence and absence of HCO₃^?, C. cophocarpa was unable, but R. peltatus able to use HCO₃^? at an ambient HCO₃^? concentration of about 0·84 mm . The greater rates of photosynthesis at ambient CO₂ concentration and the lesser limitation by inorganic carbon shown by R. peltatus compared to C. cophocarpa was the result of HCO₃^?-use as laboratory experiments showed that R. peltatus performed similarly to C. cophocarpa if the HCO₃^? concentration was reduced to 60 μm .

     

Variability of invertebrate herbivory on the submerged macrophyte Potamogeton perfoliatus

1. Invertebrate herbivory was studied in twenty-eight populations of the submerged macrophyte Potamogeton perfoliatus in Danish streams and lakes in mid-June. All populations but one experienced invertebrate herbivory and loss ranged from 0 to 11.9% of leaf area among populations. Loss generally increased with leaf age towards the base of the plants, and young apical leaves were rarely damaged. 2. Herbivory loss was significantly higher in streams (mean 5.0%) than in lakes (mean 2.2%), but varied greatly among populations within the same stream or lake and was not correlated to physico-chemical site characteristics, size or density of plant population, or leaf N and P content. High levels of invertebrate herbivory were therefore not associated with certain types of streams or lakes. 3. High herbivore biomass relative to abundance of plants was conducive to high loss. In streams, the biomass of the trichopteran Anabolia nervosa accounted for 50% of the variability in loss. No single species appeared to be equally important in lakes, although loss was correlated to the biomass of the chrysomelid beetle Macroplea appendiculata. Obligate herbivores, such as lepidopteran larvae, apparently exerted little damage on P. perfoliatus, and leaf mining and channelization from specialist feeders were negligible. It is concluded that shredders acting as facultative herbivores were the most important invertebrate herbivores on P. perfoliatus in Danish freshwaters.     

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.     

Dynamics of submerged macrophyte populations in response to biomanipulation

1. A 6‐year study (1992–97) of changes in submerged vegetation after biomanipulation was carried out in the eutrophicated Lake Finjasjön, Southern Sweden. Ten sites around the lake were revisited each year. At each site five samples of above‐ground biomass were taken at 10 cm water depth intervals. An investigation of the seed bank at the 10 sites, and a grazing experiment where birds and large fish were excluded was also conducted. 2. Between 1992 and 1996, in shallow areas (water depth < 3 m), vegetation cover increased from < 3 to 75% and above‐ground biomass from < 1 to 100 g DW m^–2. Mean outer water depth increased from 0.3 to 2.5 m. Elodea canadensis and Myriophyllum spicatum accounted for > 95% of the increase in biomass and plant cover. The following year (1997), however, cover and above‐ground biomass decreased, mainly attributable to the total disappearance of E. canadensis. Secchi depth increased after biomanipulation until 1996, but decreased again in 1997. 3. Total and mean number of submerged species increased after biomanipulation, probably as a result of the improved light climate. However, after the initial increase in species number there was a decrease during the following years, possibly attributed to competition from the rapidly expanding E. canadensis and M. spicatum. The lack of increase in species number after the disappearance of E. canadensis in 1997 implies that other factors also affected species richness. 4. A viable seed bank was not necessary for a rapid recolonization of submerged macrophytes, nor did grazing by waterfowl or fish delay the re‐colonization of submerged macrophytes. 5. Submerged macrophytes are capable of rapid recolonization if conditions improve, even in large lakes such as Finjasjön (11 km²). Species that spread by fragments will increase rapidly and probably outcompete other species. 6. The results indicate that after the initial Secchi depth increase, probably caused by high zooplankton densities, submerged vegetation further improved the light climate. The decrease in macrophyte biomass in 1997 may have caused the observed increase in phosphorus and chlorophyll a, and the decrease in Secchi depth. We suggest that nutrient competition from periphyton, attached to the macrophytes, may be an important factor in limiting phytoplankton production, although other factors (e.g. zooplankton grazing) are also of importance, especially as triggers for the shift to a clear‐water state.     

Two antifeedant lignans from the freshwater macrophyte Saururus cernuus

Two diarylbutane derivatives of dihydroguaiaretic acid have been isolated from emergent portions of the southeastern United States freshwater angiosperm Saururus cernuus L. (Saururaceae). Bioassay-guided fractionation of organic extracts of S. cernuus led to the compounds, sauriols A and B, in addition to five previously known lignoids. These metabolites deter feeding by the omnivorous crayfish Procambarus clarkii. The two lignans were identified by analysis of nuclear magnetic resonance and mass spectral data, and by comparison with spectral data of dihydroguaiaretic acid.     

Macroinvertebrate size-distributions of two contrasting freshwater macrophyte communities

SUMMARY.

• 1 Macroinvertebrates (>0.1 mg, fresh weight) were collected every 2 weeks for 18 weeks from weedbeds dominated by either the macroalga Chara or rooted plants (mostly Isoetes sp., Potamogeton filiformis, Nuphar variegatum and Myriophyllum exalbescens) in Narrow Lake, Alberta, Canada. Significant differences in total biomass, taxonomic composition, and size-structure of the macroinvertebrate community were found between the two weedbed types.
• 2 Total biomass of macroinvertebrates in the Chara beds (seasonal mean 26.4 g m^?2) was consistently higher than in the rooted-plant weedbeds (seasonal mean 7.6 g m^?2).
• 3 The macroinvertebrate community of the Chara beds was dominated by chironomids, anisopterans, gastropods and sphaerid clams whereas amphipods dominated the community in the rooted-plant weedbeds.
• 4 On average, the proportion of total biomass in the various size-classes (biomass size-spectrum) of the macroinvertebrate community in Chara beds varied very little between 1 and 512 mg. In rooted plant weedbeds, the spectrum showed a strong peak in the 4–8 mg size-class, a weak peak in the 32–64 mg size-class, and proportionately low biomass in the 128–256 mg and 256–512 mg size-classes. The slope of the normalized size-spectrum for the macroinvertebrate community of the Chara beds (-0.89) was significantly different from that of the rooted plant community (-1.11). The biomass of large organisms (>64 mg) was 10 times greater in the Chara beds than in the rooted-plant weedbeds.
• 5 The species composition of aquatic macrophyte beds can greatly influence the abundance, taxonomic composition, and size-structure of the littoral zone macroinvertebrate community. Therefore, any changes in plant species composition of weedbeds, through introduction of exotic plant species or introduction (or loss) of an efficient grazer (e.g. crayfish), have the potential to alter greatly the structure of the macroinvertebrate community in a lake, with unknown consequences to fish and waterfowl dependent upon these prey.

     

Allelopathic activity of Ceratophyllum demersum L. and Najas marina ssp. intermedia (Wolfgang) Casper

Hydrobiologia - We investigated the allelopathic activity of two submersed macrophytes with different growth forms and nutrient uptake modes, Ceratophyllum demersum and Najas marina ssp....