Distribution of the invasive calanoid copepod <i>Pseudodiaptomus marinus</i> (Sato, 1913) in the Belgian part of the North Sea

The population structure of the non-indigenous calanoid copepod Pseudodiaptomus marinus (Sato, 1913) in the Belgian part of the North Sea (BPNS) is reported for the first time. Detailed P. marinus abundance data including sex and age class of the individuals was gathered on a monthly basis from February 2015 to February 2016 at six sites within the BPNS and Belgian harbors. Relevant environmental variables were analysed to identify potential drivers explaining the population structure of P. marinus within the BPNS. The abundances found were unexpectedly high, with peak densities of up to 560 ± 163 ind.m-3. Even though P. marinus was found in all stations sampled, large spatial and temporal differences were found in the abundance of this species. P. marinus population structure was best explained by water temperature and chlorophyll a concentrations, while salinity and concentrations of dissolved inorganic nitrogen did not influence the distribution. The reported high abundances of the species, especially in the harbor of Zeebrugge, together with the high relative abundances of copepodites indicate that the species is able to reproduce within the BPNS and Belgian harbors, possibly leading to an established, permanent population. It is crucial to study the distribution of this species for a longer period in order to determine the possible establishment of this species in the BPNS and consequences for local planktonic populations

Mechanisms of chronic waterborne Zn toxicity in <i>Daphnia magna</i>

In order to gain better insights in the integrated response of Daphnia magna following chronic zinc exposure, several physiological parameters were measured in a time-dependent manner. D. magna juveniles were exposed for 21 days to dissolved Zn concentrations up to 340 µg/L. Next to standard endpoints such as mortality, growth and reproduction the following sub-lethal endpoints were measured: filtration and ingestion rate, respiration rate, energy reserves, internal Zn and total Ca concentrations in the organisms. Organisms exposed to 80 µg/L generally performed better than the Zn deprived control organisms. The former were used to elucidate the effects of higher Zn concentrations on the endpoints mentioned above. After 1 week, only 7% of the organisms exposed to 340 µg/L survived. Body Zn contents of these organisms were 281 ± 76 µg g dry weight and a 37% decrease of the Ca contents was observed. This suggests a competitive effect of Zn on Ca uptake. Filtration rate (-51%), individual weight (-58%) and energy reserves (-35%) also exhibited a decreasing trend as a function of increasing Zn exposure concentrations. During the second and third exposure week an overall repair process was observed. In the surviving organisms mortality and reproduction were only slightly affected. This can be explained by (over)compensation reactions at lower levels of biological organisation: Ca contents (+24%) and filtration rate (+90%) increased as a function of the exposure concentration while respiration rate decreased (-29%) resulting in energy reserves remaining constant as a function of Zn exposure. It is hypothesized that a disturbed Ca balance is probably the first cause for zinc toxicity effects in D. magna

The effects of chronic dietary nickel exposure on growth and reproduction of <i>Daphnia magna</i>

Although there is growing evidence that dietborne metals can be toxic to various aquatic species, there is still insufficient knowledge to integrate this information in environmental risk assessment procedures. In this study, we investigated the effects of a 21-day exposure of Daphnia magna to a control diet (i.e. the green alga Pseudokirchneriella subcapitata containing D. magna, i.e. between 49.6 and 72.5 µg Ni/g dry wt, was observed when they were fed with diets containing between 85.6 and 837 µg Ni/g dry wt. This was paralleled by a significant reduction of reproduction (by 33.1%), measured as the total number of juvenile offspring per female and growth (by 9.1%), measured as the carapax length of 21-day-old females. Life-history analysis showed that the time to first brood of Ni exposed organisms was between 7.8 and 8.2 days, and occurred 0.7–1.1 days earlier than for the control organisms (time to first brood = 8.9 days). The number of offspring in the first brood was significantly reduced (by 21–33% compared to the control) in all dietary treatments. Longer exposure (=8.9 days, i.e. from the second brood onwards) led to a reduction of brood size only when given diets containing 85.6 and 837 µg Ni/g dry wt. The results suggest that a variety of mechanisms may be involved in the effects of dietary Ni exposure, including altered resource allocation or targeted reproductive inhibition. While Ni exposure clearly altered the quality of the diet (measured as essential ?3 polyunsaturated fatty acid content and C:P ratio), we found no conclusive evidence that these diet quality shifts could have affected growth or total reproductive output. More research is required to fully understand the mechanisms of Ni toxicity associated with the dietary exposure route

Non-simultaneous ecotoxicity testing of single chemicals and their mixture results in erroneous conclusions about the joint action of the mixture

The ecotoxicity of binary chemical mixtures with a common mode of action is often predicted with the concentration addition model. The assumption of concentration addition is commonly tested statistically based on results of toxicity experiments with the two single chemicals and their binary mixture. The present simulation study shows that if not all these experiments are performed simultaneously, one has a 20–80% chance of concluding synergism or antagonism while the mixture is actually additive (false positive rate). Truly synergistic or antagonistic mixtures have a 10–50% chance of being falsely categorized as additive (false negative rate). Additionally, false positive rates decrease with increasing experimental error, while false negative rates increase with increasing experimental error. Based on these results, we put forward a number of recommendations for future mixture ecotoxicity evaluation.

Comparison of different toxic effect sub-models in ecosystem modelling used for ecological effect assessments and water quality standard setting

Ecosystem models, combining a food web model with a toxic effect sub-model, have been proposed to incorporate ecological interactions in ecological effect assessments. Toxic effect sub-models in different studies tend to differ in (1) the used single-species toxicity data, (2) the effects they consider, (3) the concentration-effect function used. In this paper, we constructed four ecosystem models, each with a different toxic effect sub-model, and tested their capacity to predict biomass changes, and no observed effect concentrations (NOECs) established in an experimental microcosm. For most populations, these predictions depended heavily on the type of ecosystem model. The ecosystem model with a toxic effect sub-model incorporating mortality effects using a logistic concentration-effect function made accurate predictions for most populations. Additional incorporation of sub-lethal effects did not result in better predictions. Ecosystem models using linear concentration-effect functions predict biomass decreases at concentrations that are four times lower than the observed NOECs

Reduction of growth and haemolymph Ca levels in the freshwater snail Lymnaea stagnalis chronically exposed to cobalt.

The ecological risk assessment and the development of water quality criteria for Co are currently still hampered by insufficient knowledge about the toxicity of Co to freshwater organisms. A relevant group of organisms, for which no toxicity data with Co are available, is the class of the herbivorous pulmonate freshwater snails, which fulfil a pivotal role in the consumption and decomposition of aquatic plants and epihyton. We measured the growth rate of the pond snail Lymnaea stagnalis chronically exposed for 28 days to a series of Co concentrations. The no observed effect concentration (NOEC) and the lowest observed effect concentration (LOEC) for growth rate were 26 µg Co/L and 79 µg Co/L, respectively. Growth rate of snails exposed to 79 µg Co/L and higher concentrations was more impaired in the final two weeks of exposure than in the first two weeks of exposure. The reduced growth rate at 79 µg Co/L was accompanied by a reduced concentration of Ca in the haemolymph at the end of the exposure. Possible mechanisms of toxicity of Co to snail growth were suggested to be an impairment of Ca uptake and homeostasis and/or feeding inhibition. Although additional research is needed to investigate the relative importance of these mechanisms, as well as the interrelatedness between them, the toxicity data currently presented can assist in risk assessment and water quality criteria development