Inhibition of photosystem II (PSII) electron transport as a convenient endpoint to assess stress of the herbicide linuron on freshwater plants

Effects of the herbicide linuron on photosynthesis of the freshwater macrophytes Elodea nuttallii (Planchon) St. John, Myriophyllum spicatum L., Potamogeton crispus L., Ranunculus circinatus Sibth., Ceratophyllum demersum L. and Chara globularis (Thuill.), and of the alga Scenedesmus acutus Meyen, were assessed by measuring the efficiency of photosystem II electron flow using chlorophyll fluorescence. In a series of single-species laboratory tests several plant species were exposed to linuron at concentrations ranging from 0 to 1000 μg l−1. It was found that the primary effect of linuron, inhibition of photosystem II electron flow, occurred with a half-lifetime of about 0.1 to 1.9 h after addition of linuron to the growth medium. The direct effect of the herbicide on photosynthesis appeared to be reversible. Complete recovery from the inhibition occurred with a half-lifetime of 0.5 to 1.8 h after transfer of linuron treated plants to linuron free medium. The EC50,24h of the inhibition of photosystem II electron transport by linuron was about 9–13 μg l−1 for most of the macrophytes tested. For S. acutus the EC50,72h for inhibition of photosystem II electron flow was about 17 μg l−1 for the free suspension, and 22 μg l−1 for cells encapsulated in alginate beads. In a long-term indoor microcosm experiment, the photosystem II electron flow of the macrophytes E. nuttallii, C. demersum and the alga Spirogyra sp. was determined during 4 weeks of chronic exposure to linuron. The EC50,4weeks for the long-term exposure was 8.3, 8.7 and 25.1 μg l−1 for E. nuttallii, C. demersum and Spirogyra, respectively. These results are very similar to those calculated for the acute effects. The relative biomass increase of E. nuttallii in the microcosms was determined during 3 weeks of chronic exposure and was related to the efficiency of photosystem II electron transport as assessed in the different treatments. It is concluded that effects of the photosynthesis inhibiting herbicide on aquatic macrophytes, algae and algae encapsulated in alginate beads can be conveniently evaluated by measuring photosystem II electron transport by means of chlorophyll fluorescence. This method can be used as a rapid and non-destructive technique in aquatic ecological research. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ecotoxicological threshold levels of a mixture of herbicides (atrazine, diuron and metolachlor) in freshwater microcosms

Twelve indoor, plankton-dominated, freshwater microcosms (600 l) were used to study the effect of a mixture of herbicides on structural and functional aspects of these ecosystems. The EC50, 72 h values of the most susceptible standard test alga Selenastrum capricornutum (EC50, atrazine=54 μg l−1, EC50, diuron=15 μg l−1, EC50, metolachlor=56 μg l−1) were used as a starting point for the dosage applied in the microcosms (dosages: 0, 0.01, 0.03, 0.1, 0.3, 1× EC50). The microcosms were exposed to chronic levels for 28 days and subsequently monitored for 4 more weeks. The following effects were observed: (1) direct effects became apparent from an initial drop in photosynthesis efficiency, pH and oxygen concentration and a decrease in the abundance of several phytoplankton taxa at the 0.3 × EC50 treatment level and higher. (2) Fourteen days post application an increase in the abundance of several phytoplankton taxa (Chlamydomonas sp. and Stephanodiscus/Cyclotella) was observed; oxygen concentrations recovered while alkalinity, conductivity and total inorganic nitrogen were elevated. (3) Effects on fauna were minor. Daphnia galeata showed a decreasing trend and the cyclopoid copepods an increasing trend at the end of the experiment. Multivariate statistical analyses demonstrated no effects of any treatment level on the zooplankton community. Effects were reported for the phytoplankton community at dose levels of 0.3 × EC50 and higher. On species level the most sensitive taxon was Chlorophyceae coccales. For this taxon a NOEC at the dose level of 0.01 × EC50 was calculated. This effect however was relatively small in magnitude and merely based on an increase in numbers in the control and lowest treated microcosms rather than a decrease in numbers in all other treatments. The standards based on algal toxicity data, as adopted by the Uniform Principles, consist of a safety factors of 0.1 to be multiplied with the EC50. The NOEC of coccales was lower than 0.1 × EC50. All other observed variables in this aquatic ecosystem were sufficiently protected against the mixture of herbicides by the safety factor as proposed in the Uniform Principles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Benthic macroinvertebrate community structure in relation to food and environmental variables

The relative contribution of sediment food (e.g. organic matter, carbohydrates, proteins, C, N, polyunsaturated fatty acids) and environmental variables (e.g. oxygen, pH, depth, sediment grain size, conductivity) in explaining the observed variation in benthic macroinvertebrates is investigated. Soft bottom sediments, water and benthic macroinvertebrates were sampled in several water systems across The Netherlands. The variance partitioning method is used to quantify the relative contributions of food and environmental variables in structuring the benthic macroinvertebrate community structure.It is assumed that detritivores show a significant relationship with sediment food variables and carnivores and herbivores do not. The results of the variance partitioning method with data sets containing only detritivores, herbivores or carnivores confirm this assumption. This indicates that the variance partitioning method is a useful tool for analyzing the impact of different groups of variables in complex situations. Approximately 45% of the total variation in the macroinvertebrate community structure could be explained by variables included in the analyses. The variance partitioning method shows that sediment food variables contributed significantly to the total variation in the macroinvertebrate dataset. The relative importance of food depends on the intensity of other environmental factors and is lower on broad spatial scales than on smaller scales.The results of the partitioning depend on the selected variables that are included in the analyses. The method becomes problematic in case variables from different groups of variables (e.g. one food variable and one environmental variable) have a high inflation factor and thus are collinear. The choice of the variable that is left out impacts the variance allocated to the different groups of variables.The variance partitioning method was able to detect the spatial scale dependent contribution of food variables in structuring macroinvertebrate communities. This spatial scale dependency can also be caused by the size, the composition, and the heterogeneity of the dataset. Performing extra analyses in which specific samples are removed from the original dataset can give insight in under- or overestimation of the impact of certain factors and offers the possibility to test the robustness of the obtained results.     

Effects of artificial ultraviolet-B radiation on experimental aquatic microcosms

1. The effects of prolonged ultraviolet-B (UVB) radiation on freshwater communities were studied in indoor microcosms (600 L) with artificial light sources, simulating a clear, shallow, mesotrophic aquatic ecosystem. A range of six intensities (in duplicate) of UVB radiation, ranging from 0 (control) to 9.56 kJ m⁻² day⁻¹ at the water surface, was applied for 8 weeks. The UVB radiation levels, attenuation, shading and scattering were comparable to those in Dutch shallow freshwater systems. Physical, chemical and biological variables were monitored weekly.
2. The UVB treatment did not affect the abundance, species composition or biovolume of the phytoplankton or zooplankton communities, nor did it affect the periphyton or the macroinvertebrate community. A few species showed a significant response on some of the sampling dates, but there was no negative UVB effect at the community level. Overall, the ecosystems in the microcosms were not affected by the UVB treatment.
3. In a bio-assay, a laboratory clone of Daphnia pulex , not subjected to UVB radiation, was fed with seston from the microcosms. Daphnia pulex feeding on seston from the control microcosms grew faster, had better survival and better reproduction than D. pulex feeding on seston from the UVB treated microcosms. The phytoplankton–zooplankton interaction may have been influenced by the UVB treatment.
4. The dissolved oxygen content (DOC) concentrations in the microcosms were around 5 mg L^−1. The DOC levels in Dutch systems rarely fall below 10 mg L^−1. This might provide sufficient protection against the detrimental effects of increased UVB radiation.