Regime shift in microalgal dynamics: Impact of water level changes on planktonic and benthic algal biomass.

Whole-lake microalgal biomass surveys were carried out in Lake Balaton to investigate the seasonal, spatial, and temporal changes of benthic algae, as well as to identify the drivers of the phytobenthos. Phytobenthos was controlled mainly by light: the highest benthic algal biomass was in the shallow littoral region characterized by large grain size (sand) with good light availability but lower nutrient content in the sediment. During the investigated period, phytoplankton biomass showed a significant decrease in almost the entire lake. At the same time, the biomass of benthic algae increased significantly in the eastern areas, increasing the contribution of total lake microalgae biomass (from 20 % to 27 %). Benthic algal biomass increase can be explained by the better light supply, owing to the artificially maintained high water level which greatly mitigates water mixing. The decrease in planktonic algal biomass could be attributed to increased zooplankton grazing, which is otherwise negatively affected by mixing. As a result of the high water level, the trophic structure of the lake has been rearranged in recent decades with a shift from the planktonic life form to the benthic one while the nutrient supply has largely remained unchanged.

Optimization of assay conditions to quantify ECOD activity in vivo in individual Daphnia magna. Assay performance evaluation with model CYP 450 inducers/inhibitors.

Screening the activity of the cytochrome P450 (CYP450) mixed function oxidase system in aquatic invertebrates received seldom applications in ecotoxicology due to low baseline enzymatic activities characteristic for these organisms. In this study, an existing in vivo spectrofluorometric assay method based on quantifying the cytochrome P450 mediated conversion of 7-ethocycoumarin (EtC) used as substrate to the product 7-hydroxycoumarin (HCm) called: ethoxycoumarin-O-deethylase (ECOD) activity, initially applicable on pooled samples of Daphnia magna, was optimized for use on individual organisms. Optimal assay conditions have been established for as small as 3- and 6 days old individuals, and the limits of spectrofluorometric detection of HCm excreted by daphnids in the incubation media were defined. The modified assay was tested by screening the modulation of ECOD activity in daphnids following 24 h exposure to ß-naphthoflavone (ß-NF, reference CYP450 inducer) and to prochloraz (PCZ), a potent CYP450 inhibitor. Maximal ECOD activity levels in daphnids were recorded following 2 hours of incubation to 200 nM EtC. The limit of spectrofluorometric detection of HCm in the incubation media was 6.25 nM, achieved by more than 80% of three days old daphnids and all six days old individuals. Exposure of daphnids to ß-NF demonstrated a bell-shaped ECOD activity induction potential, while PCZ elicited partial (60%) inhibition of ECOD activity. This optimized in vivo ECOD activity assay may serve as a cost-effective tool to study the responsiveness of Phase-I metabolism in D. magna to toxic pressure and its applicability to other aquatic invertebrates is also worth for consideration.

Temperature related toxicity features of acute acetamiprid and thiacloprid exposure in Daphnia magna and implications on reproductive performance.

This study investigated the potential for elevated temperature to alter the toxicity of acetamiprid (ACE) and thiacloprid (Thia) in the ecotoxicity model Daphnia magna. The modulation of CYP450 monooxygenases (ECOD), ABC transporter activity (MXR) and incident cellular reactive oxygen species (ROS) overproduction was screened in premature daphnids following acute (48 h) exposure to sublethal concentrations of ACE and Thia (0.1-, 1.0 µM) at standard 21 °C and elevated 26 °C temperatures. Delayed outcomes of acute exposures were further evaluated based on the reproduction performance of daphnids monitored over 14 days of recovery. Exposures to ACE and Thia at 21o C elicited moderate induction of ECOD activity, pronounced inhibition of MXR activity and severe ROS overproduction in daphnids. In the high thermal regime, treatments resulted in significantly lower induction of ECOD activity and inhibition of MXR activity, suggesting a suppressed metabolism of neonicotinoids and less impaired membrane transport activity in daphnids. Elevated temperature on its own, caused a three-fold rise in ROS levels in control daphnids, while ROS overproduction upon neonicotinoid exposure was less accentuated. Acute exposures to ACE and Thia caused significant decreases also in the reproduction of daphnids, indicating delayed outcomes even at environmentally relevant concentrations. Both the cellular alterations in exposed daphnids and decreases in their reproductive output post exposures evidenced closely similar toxicity patterns and potentials for the two neonicotinoids. While elevated temperature elicited only a shift in baseline cellular alterations evoked by neonicotinoids, it significantly worsened the reproductive performance of daphnids following neonicotinoid exposures.

Dense macrophyte cover has significant structural and functional influence on planktonic microbial communities leading to bacterial success.

We intend to assess how macrophyte cover affects planktonic microbial communities by changing the physical and chemical environment, and how macrophyte-derived DOC affects the balance between autotrophy and heterotrophy/chemoorganotrophy in a shallow lake. The structure and production of phytoplankton and bacterioplankton in the open water of a large shallow lake and in the littoral zone were compared at two sampling stations with different macrophyte cover. According to the obtained results, uncoupling between bacterioplankton and phytoplankton was observed due to the high content of organic carbon of emergent macrophyte origin. While phytoplankton were regulated by TSS, bacterioplankton (in both heterotrophic and photoheterotrophic forms) were determined by dissolved organic carbon. As a result of these processes, the littoral and pelagic zones in the lake are completely separated from each other. In open water the autotrophic processes dominated, but at the sampling stations inside the reed belt, the metabolic processes shifted in the direction of chemoorganotrophy. Our results suggest that increase of macrophyte cover in shallow water bodies will increase the significance of microbe-based carbon pathways and weakens the efficiency of carbon transport from primary producers to higher trophic levels through the planktonic food chain.

Physiological and metabolic alterations induced by commercial neonicotinoid formulations in Daphnia magna.

Neonicotinoid insecticides are widely used agents in agriculture to control a broad range of insect pests. Although use of neonicotinoid pesticides has resulted in the widespread contamination of surface waters, sublethal toxicity data of these products in relation to non-target aquatic biota are still poor. Therefore, the objective of this study was to assess the effects of two neonicotinoid pesticides with widespread use on the basic physiological functions: the thoracic limb activity and heart rate of Daphnia magna, and to screen for their potential to affect the cytochrome P450 monooxygenase system (ECOD activity) of daphnids. The considered pesticides were the acetamiprid- and thiacloprid based products Mospilan 20 SG and Calypso 480 SC, respectively. The dose-dependent variation in the three biological endpoints considered were assessed following 24 h exposures. The two neonicotinoid formulations elicited significant depression on the thoracic limb activity and heart rate of daphnids at doses close to the immobility thresholds of formulations (48h-EC50: Mospilan 20 SG = 190 mg L-1; Calypso 480 SC = 120 mg L-1), an effect mainly attributable to the overall drop in the general health status of the organisms. The alterations in the physiological traits were significant at exposures to 190 mg L-1 for Mospilan 20 SG and 48 mg L-1 for Calypso 480 SC. The dose related variation in the ECOD activity of daphnids exposed to the selected neonicotinoid formulations followed a biphasic pattern, with starting effective doses for Mospilan 20 SG of 6.3 mg L-1 (=1/20 of 48h-EC50 for Daphnia neonates), and for Calypso 480 SC of 0.034 mg L-1 (=1/4000 of 48h-EC50). Maximal ECOD activity (2.2 fold increase vs. controls) was induced by Mospilan 20 SG in daphnids exposed to 114 mg L-1 product (=48 h-EC20), and by Calypso 480 SC (1.8 fold increase) at 5.2 mg L-1 dose (=1/20 of 48 h-EC50). Our results outlined significant alterations in the physiological traits and ECOD activity in exposed daphnids at concentrations below the immobility thresholds (48 h-EC50) of the products used as benchmarks to rate their toxicity risks to aquatic biota. Therefore, we think our findings might deserve consideration in the environmental risk evaluation of these products.

The effects of interspecific interactions between bloom forming cyanobacteria and Scenedesmus quadricauda (chlorophyta) on their photophysiology.

Eutrophication and enhanced external nutrient loading of lakes and seas are most clearly reflected by increased cyanobacterial blooms, which are often toxic. Freshwater cyanobacteria produce a number of bioactive secondary metabolites, some of which have allelopathic properties, significantly influencing the biological processes of other algae, thereby affecting species composition and succession of the phytoplankton. The goal of this work was to investigate the influence of bloom-forming cyanobacterial exudates on the photophysiology of the green alga Scenedesmus quadricauda by chlorophyll fluorescence analysis. We were able to prove the effect of algal cell-free filtrates on the performance of S. quadricauda and demonstrate for the first time that the freshwater picocyanobacterium Cyanobium gracile has strong negative impact on the coexisting green alga. Neither the cyanotoxin (MYC, CYN and ATX) producing, nor the non-toxic strains showed any systematic effect on the production of S. quadricauda. Various strains of the cyanobacterium Cylindrospermopsis raciborskii inhibited the performance of the green alga independently of their origin. Our results urge further studies for a better understanding of the factors affecting the release of allelopathic compounds and the mechanisms of their effects on target organisms.

The ecotoxicological evaluation of Cylindrospermopsis raciborskii from Lake Balaton (Hungary) employing a battery of bioassays and chemical screening.

Ecotoxicity of four Cylindrospermopsis raciborskii strains (ACT 9502, ACT 9503, ACT 9504, ACT 9505) isolated from Lake Balaton (Hungary) was evaluated in four aquatic bioassays including the Thamnocephalus platyurus acute lethality test; Daphnia magna acute immobilization assay; D. magna feeding inhibition assay and Danio rerio embryo developmental toxicity assay, assisted by chemical screening for known toxins by HPLC-MS. For reference, we analyzed in parallel the toxin content and toxic effects of two previously characterized toxin-producing strains: the Australian cylindrospermopsin producer AQS C. raciborskii and the anatoxins producer Oscillatoria sp. PCC 6506. Bioassays were used to evaluate the overall toxicity of the hydrophilic bioactive metabolites pool synthesized by the selected cyanobacteria. Chemical screening has proven that the ACT C. raciborskii extracts investigated did not contained cylindrospermopsins and anatoxins. The relative toxicity of the ACT C. raciborskii aqueous extracts observed in each bioassay was comparable to the effects recorded for the anatoxins producer PCC 6506 strain while toxicity values (EC50/LC50) calculated for the AQS extract were in general one order of magnitude lower. Concerning sublethal effects of ACT C. raciborskii extracts to the D. rerio embryogenesis, the general morphological abnormality observed was a significant retardation of development. Overall, our results suggest that C. raciborskii populating Lake Balaton produce metabolites with significant bioactive potencies. Therefore, continued investigation of these unknown compounds is required.