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The Monod model describes the relationship between growth rate and ambient nutrient concentration, the Droop model focuses on internal nutrient resources as the driving factor. Both were applied mainly to explain phytoplankton dynamics in lakes or in experimental cultures. Our test plants were two species of duckweeds – Lemna minor L. and Spirodela polyrhiza (L.) Schleiden sampled from 18 natural stands situated in 6 different water bodies. Plants were grown outdoor in original lake water or in mineral media of varying N and P concentrations (0–21 mg N-NO3 L-1 and 0–1853 μg P-PO4 L-1 for L.minor and 0–4.2 mg N-NO3 L-1 and 0–371 μg P-PO4 L-1 for S. polyrhiza). Moreover, we analysed concentrations of mineral forms of N and P in lake water and tissue nutrient concentrations in plants. Tissue N of both plants was significantly correlated with ambient inorganic nitrogen sources, no such relationship was observed for tissue P. The growth rate of both plants measured under experimental outdoor conditions was better explained by tissue N and P variability (the Droop model) than by the external nutrient availability (the Monod model). The latter also failed to fit the growth rate of both plants in artificial mineral media with a decreasing gradient of N and P concentrations. The plants grew at the expense of internal N and P resources which remarkably declined during 9-day long experiments. Calculated minimum tissue contents (11.19 ± 1.11 mg N g-1 and 0.97 ± 0.07 mg P g-1 in L. minor and 6.10 ± 1.85 mg N and 1.25 ± 0.37 mg P g-1 in S. polyrhiza) show that the latter species would be a superior competitor under N limiting conditions and the former – under P limitation. We confront obtained results with literature data on N uptake kinetics and postulate that the luxury consumption of nutrients and plant growth dependent mainly on internal N and P resources might be an adaptation of duckweeds to varying habitat conditions typical of astatic water bodies.
The effect of methylene chloride, chloroform, and carbon tertrachloride on the following aquatic organisms: Pseudomonas fluorescens, Scenedesmus quadricauda, Lemna minor, Daphnia magna, and Lebistes reticulatus, was investigated. Pseudomonas fluorescens bacteria were found to be most sensitive test organisms, and a toxic effect towards them was observed just at the concentration of 100 µg/l of each agent. Studies on degradation in distilled water, river-water, and aqueous ecosystem revealed moderate stability in aqueous medium of compounds under investigation. Chloroform and carbon tetrachloride were detecable in river-water still after 40 days in concentrations toxic to aqueous ecosystem. More rapid degradation of methane halogen derivatives in river-water than in distilled water indicated the presence of microorganisms living in water from natural source, and being accessory to the decomposition of these agents. Degradation in the presence of adapted microflora took, as a general rule, a slower cource. Decrease of temperature to 6°C inhibited considerably the degradation rate of the investigated contaminants. The reduction of methane halogen derivatives content during 21-day exposure in aqueous ecosystem came up to 73%. Moreover, these compounds were found not to have biocumulative properties, and not to be toxic - in concentrations up to 300 µg/l - to aqueous biocenosis homing the experimental ecosystem.
This work presents the dynamics of E. fetida (Sav.) earthworm populations during vermicomposting of duckweed (Lemna minor L.) biomass in small containers, and provides properties of the vermicomposts produced. An experiment was conducted under laboratory conditions (in darkness, at an average temperature 25±5oC, with substrate moisture 70-75%). Test pots (3 replications for each duckweed treatment) were filled with one litre of garden soil, into which 100 individuals of E.fetida, of known biomass, were introduced per pot. Duckweed was fed to earthworms regularly, in two treatments: (1) duckweed + + cattle manure (1:1), and (2) duckweed only. Earthworm number and biomass of tested populations were determined after 4 months of vermicomposting, and it was found that an average number of E.fetida in containers with duckweed and manure was 121±5 ind./container with a total biomass of 25.8±1.1 g. Populations in pure duckweed were significantly smaller (p<0.05), with 57±6 ind./container and a total biomass of 9.8±1 g. Cocoon production was also different across treatments. Populations in duckweed alone produced 55±13 cocoons /per container, significantly less (p<0.05) than the 231±37 cocoones when manure was added. Duckweed vermicomposts were odourless and had good granular structure. Chemical characteristics of both vermicompost types (with or without a manure supplement) were desirable. Content of macroelements in duckweed vermicomposts was high, whereas microelements, cadmium and lead were within the permitted levels, making these vermicomposts extremely useful in environmental reclamation, including agriculture. The manure addition was important for characteristics and chemical content of duckweed vermicomposts. The vermicomposts produced from duckweed and manure contained more ash, N, P, K, Mg, Zn, Cu, Ni, Cr, Cd and Pb.
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