An assessment of maintenance works and their impact on macroinvertebrate communities and long-term recolonization to small lowland watercourses.

Due to their small size and high anthropogenic pressure, small watercourses are particularly prone to severe siltation and are densely overgrown with macrophytes. Many of these watercourses are subject to regular maintenance works (RMW), consisting of seasonal desilting and vegetation clearance, in order to ensure unobstructed water flow. The aim of the study was to assess the impact of three types of maintenance works: dredging and mud removal (DMR), river channel vegetation removal (RCVR) and river bank vegetation removal (RBVR) on taxa species richness, macroinvertebrate density and the Shannon-Wiener diversity index, as well as their changes and long-term benthic recolonization one and two years after completion of the works. The study was carried out in 21 habitats on eight rivers in the European Central Plains Ecoregion. A total of 107 zoobenthic taxa were found at all sites, and their species composition was characteristic of highly hydrophytic waters with low hydrological and hydrochemical quality parameters. A significant decrease in macroinvertebrate taxa richness was observed one year after the works, as the average number of taxa had dropped from thirteen to eight, with a further fall to seven taxa two years after the RMW. The same was true for density, which had decreased from an average of 2496 to 786 individuals per square meter one year after the RMW, while, a gradual recolonization was recorded two years after the RMW, with an average density of 1295 individuals per square meter. The Shannon-Wiener index, which had averaged 2.528 before the RMW, also decreased, falling to 1.982 and 1.832 one and two years after. BACI statistical analyses showed that of the three types of maintenance work, desilting and bottom sediment removal had the largest negative impact, significantly reducing taxonomic composition (by an average of 53%), density (by an average of 43%), and ecological index values (by an average of 40%). Over-frequent maintenance can prevent macroinvertebrate populations from recovering, thus depleting the environment of valuable taxa, including those that provide food for fish and other vertebrates.

Photosynthetic efficiency of perennial ryegrass (Lolium perenne L.) seedlings in response to Ni and Cd stress.

Perennial ryegrass is a grass species used to establish lawns in urban areas where pollution is a major environmental problems. Cadmium (Cd) and nickel (Ni) contribute significantly to these pollutants and may cause photosynthetic limitation. The main objective of this work was to perform a comprehensive analysis of photosynthetic efficiency of perennial ryegrass seedlings under Cd and Ni stress. Some of the main indices of photosynthetic efficiency (prompt and delayed chlorophyll-a fluorescence signals and modulated reflectance at 820 nm) were compared with growth parameters. Two cultivars were tested: 'Niga' and 'Nira'. A decrease in photosystem (PS) II and PSI activity was observed. This was due to an increase in nonradiative dissipation of the PSII antenna, a decrease in PSII antenna size, or a decrease in the number of photosynthetic complexes with fully closed PSII RCs. Efficiency of electron transport was decreased. The effect on the modulated reflectance signal could indicate a restriction in electron flow from PSII to PSI. The correlation between photosynthetic efficiency parameters, such as Area, Fo, Fm, and Fv, and growth parameters, confirmed that some photosynthetic efficiency parameters can be used as indicators for early detection of heavy metal effects.

Structural and functional disorder in the photosynthetic apparatus of radish plants under magnesium deficiency.

Magnesium (Mg) is one of the significant macronutrients which is involved in the structural stabilisation of plant tissues and many enzymes such as PSII. The latter efficiency and performance were analysed, using chlorophyll (Chl) a fluorescence induction kinetics and microscopic images, to detect the changes in structure and function of photosynthetic apparatus of radish plants grown under Mg deficiency (Mgdef). Plants grown under Mgdef showed less PSII connectivity and fewer active primary electron acceptors (QA) oxidizing reaction centres than control plants. Confocal and electron microscopy analyses showed an increased amount of starch in chloroplasts, and 3,3'-diaminobenzidine (DAB)-uptake method revealed higher H2O2 accumulation under Mgdef. Prominent changes in the Chl a fluorescence parameters such as dissipated energy flux per reaction centre (DIo/RC), relative variable fluorescence at 150µs (Vl), and the sum of the partial driving forces for the events involved in OJIP fluorescence rise (DFabs) were observed under Mg deficiency. The latter also significantly affected some other parameters such as dissipated energy fluxes per cross-section (DIo/CSo), performance index for energy conservation from photons absorbed by PSII antenna until the reduction of PSI acceptors (PItotal), and relative variable fluorescence at 300µs (Vk). This work emphasises the use of chlorophyll fluorescence in combination with microscopic and statistical analyses to diagnose the effects of nutrients deficiency stress on plants at an early stage of its development as demonstrated for the example of Mgdef. Due to the short growth period and simple cultivation conditions of radish plant we recommend it as a new standard (model) plant to study nutrients deficiency and changes in plant photosynthetic efficiency under stress conditions.