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Copepods are important contributors to the zooplankton community in lakes. Being “sandwiched” between predators and resources, they are sensitive to changes in the environment. It has been proposed that the proportion of calanoids of total copepod abundance or biomass could be a valuable indicator of eutrophication. We investigated relationships between environmental factors and the abundance, biomass and size of calanoid and cyclopoid copepods as well as their proportions in summer in 68 Danish freshwater lakes (587 lake years) with contrasting nutrient levels and pH. When lake pH was < 6.0, mean lake depth and trophic state were the most important factors and calanoids completely dominated the copepod community. In shallow lakes with a mean depth < 2.5 m and with pH > 6.0, the proportion of calanoids in terms of biomass decreased substantially with increasing phosphorus and chlorophyll a concentrations but stayed around 50% at > 2.5 m depth irrespective of nutrient level. Time series of the lakes, recovering from eutrophication, confirmed this multi-lake pattern although the trajectory varied from lake to lake. We conclude that the proportion of calanoids in terms of biomass might be a valuable indicator of trophic state in shallow but not deep lakes and only when pH > 6.

Global changes (e.g., warming and population growth) affect nutrient loadings and temperatures, but global warming also results in more frequent extreme events, such as heat waves. Using data from the world&rsquo, s longest-running shallow lake experimental mesocosm facility, we studied the effects of different levels of nutrient loadings combined with varying temperatures, which also included a simulated 1-month summer heat wave (HW), on nutrient and oxygen concentrations, gross ecosystem primary production (GPP), ecosystem respiration (ER), net ecosystem production (NEP) and bacterioplankton production (BACPR). The mesocosms had two nutrient levels (high (HN) and low (LN)) combined with three different temperatures according to the IPCC 2007 warming scenarios (unheated, A2 and A2 + 50%) that were applied for 11 years prior to the present experiment. The simulated HW consisted of 5 &deg, C extra temperature increases only in the A2 and A2 + 50% treatments applied from 1 July to 1 August 2014. Linear mixed effect modeling revealed a strong effect of nutrient treatment on the concentration of chlorophyll a (Chl-a), on various forms of phosphorus and nitrogen as well as on oxygen concentration and oxygen percentage (24 h means). Applying the full dataset, we also found a significant positive effect of nutrient loading on GPP, ER, NEP and BACPR, and of temperature on ER and BACPR. The HW had a significant positive effect on GPP and ER. When dividing the data into LN and HN, temperature also had a significant positive effect on Chl-a in LN and on orthophosphate in HN. Linear mixed models revealed differential effects of nutrients, Chl-a and macrophyte abundance (PVI) on the metabolism variables, with PVI being particularly important in the LN mesocosms. All metabolism variables also responded strongly to a cooling-low irradiance event in the middle of the HW, resulting in a severe drop in oxygen concentrations, not least in the HN heated mesocosms. Our results demonstrate strong effects of nutrients as well as an overall rapid response in oxygen metabolism and BACPR to changes in temperature, including HWs, making them sensitive ecosystem indicators of climate warming.

Rapid climate changes may potentially have strong impacts on the ecosystem structure and nutrient dynamics of lakes as well as implications for water quality. We used a space-for-time approach to elucidate such possible effects by comparing data from 1656 shallow lakes (mean depth −1. Physicochemical variables showed significant seasonal differences, which can be attributed to different sunlight regimes and temperatures. The FL lakes had overall higher fish biomasses (notably in the littoral zone) but a substantially lower zooplankton biomass and body mass of microcrustaceans, a much lower zooplankton:phytoplankton biomass ratio (lower grazing on phytoplankton), and a markedly lower biomass of benthic invertebrates, indicating much greater control of consumers by fish in the FL lakes. Accordingly, the summer phytoplankton biomass was higher in the FL lakes. Cyanobacteria in summer were proportionally more important in the FL lakes at all TP levels, whereas the proportion of dinophytes, chrysophytes, and cryptophytes was higher in the DK lakes at low TP. Submerged macrophytes occurred at higher TP (>100 µg L−1) in the FL lakes, but coverage was higher in the DK lakes at low TP. We also found lower oxygen saturation in the nutrient-rich FL lakes than in the DK lakes, suggesting lower net ecosystem production in the FL lakes. We discuss our results within the framework of climate warming.

Antimony (Sb) occurs in the environment both naturally and anthropogenically. Numerous activities cause its anthropogenic accumulation and the release is mainly into the terrestrial environment. Therefore, it is crucial to investigate the fate of Sb in soils at the smelting sites where its deposition is often very high and the land needs to be cleaned up. However, little is known regarding the sorption behavior of environmentally found Sb oxidized states in the soils with different characteristics and compositions. The present study examined solubility and toxicity of Sb from the soils polluted due to the smelting operations. An inhibition of seed growth was used as an indicator of the toxic effect of Sb. Batch adsorption experiments were conducted over the pH range of 3–7 and in different equilibrium concentrations of Sb (III) as a form of the smelter release. The water soluble proportion of Sb was low (0.70–1.63%) compared to its deposition in the surface soils; however, extracted amounts caused a slightly reduced germination rate of wheat seeds with a lighter weight. Nevertheless, the soil soluble Sb concentration did not produce severe toxicity symptoms in the growth of roots and sprouts of the germinated seeds. The effects of Sb (III) and (V) forms were slightly different at the level of 5 mg·L− 1. In Sb (III) solution the smaller elongation of both sprouts and roots was observed as compared to the control samples while in Sb (V) solution only sprout lengths were shorter. Distribution coefficient (Kd) of Sb in soil water system was estimated to be 56 mL·g− 1 at pH values of 3 and 10–17 mL·g− 1 at pH of 6–7. Thus, neutral pH increases Sb mobility and its translocation to the plants that should be accounted of in agricultural production and remediation activities.

This current study examined the fate of antimony (Sb) in the soil environment and its association with soil humic acid (HA). It is anticipated that a significant proportion of Sb is retained in soil organic layer; therefore, the oxidation state of Sb is controlled by the soil HA to some extent which influences Sb solubility and ecotoxicity. Parent HA material as well as prepared HA–Sb (III) composites was investigated by determining elemental composition and by performing nuclear magnetic resonance (NMR) and X-ray absorption fine structure (XAFS) spectroscopy study in order to study Sb coordination to natural macro organic ligand. The Sb (III) binding to soil derived HA mainly contributed to its open chains through carboxyl and hydroxyl moieties as revealed by the 1H and solid state 13C NMR spectroscopy. The protons in carboxylic and hydroxylic groups (those proton signals are characteristic of HAs with different origins) disappeared in the HA–Sb composite; and relative changes were observed in aliphatic proton distribution between the HA samples with and without Sb. The overall patterns of 13C NMR spectra for the investigated samples were analogous to each other; moreover, it was estimated that the cyclic structure of the HA nucleus remained unchanged during Sb (III) association. Based on the absorption edge energy and coordination numbers, Sb oxidation state in a native soil was interpreted as pentavalent, meanwhile the HA–Sb composite contained both Sb (III) and Sb (V). It was shown that HA catalyzes Sb (III) oxidation to Sb (V) but the process was relatively slow. However, XAFS spectra sensitivity was limited when studying the HA–Sb composite that was prepared from the isolated soil HA fraction, so only data on the first shell Sb–O coordination were interpreted.