Ecological Recovery Potential of Freshwater Organisms: Consequences for Environmental Risk Assessment of Chemicals

Understanding Ecological Complexity in a Chemical Stress Context: A Reflection on Recolonization, Recovery, and Adaptation of Aquatic Populations and Communities

Recovery, recolonization, and adaptation in a chemical stress context are processes that regenerate local populations and communities as well as the functions these communities perform. Recolonization, either by species previously present or by new species able to occupy the niches left empty, refers to a metacommunity process with stressed ecosystems benefiting from the dispersal of organisms from other areas. A potential consequence of recolonization is a limited capacity of local populations to adapt to potentially repeating events of chemical stress exposure when their niches have been effectively occupied by the new colonizers or by new genetic lineages of the taxa previously present. Recovery, instead, is an internal process occurring within stressed ecosystems. More specifically, the impact of a stressor on a community benefits less sensitive individuals of a local population as well as less sensitive taxa within a community. Finally, adaptation refers to phenotypic and, sometimes, genetic changes at the individual and population levels, allowing the permanence of individuals of previously existing taxa without necessarily changing the community taxonomic composition (i.e., not replacing sensitive species). Because these processes are usually operating in parallel in nature, though at different degrees, it seems relevant to try to understand their relative importance for the regeneration of community structure and ecosystem functioning after chemical exposure. In the present critical perspective, we employed case studies supporting our understanding of the underlying processes with the hope to provide a theoretical framework to disentangle the relevance of the three processes for the regeneration of a biological community after chemical exposure. Finally, we provide some recommendations to experimentally compare their relative importance so that the net effects of these processes can be used to parameterize risk-assessment models and inform ecosystem management. Environ Toxicol Chem 2023;42:1857-1866. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Characterization of patterns and variability in the dynamics of outdoor aquatic mesocosms: exploring the capabilities and challenges in data supporting aquatic system models

Aquatic mesocosms are complex test systems used within regulatory risk assessment of plant protection products. These model ecosystems allow researchers to capture interactions of multiple species under realistic environmental conditions. They enable assessment of direct and indirect effects of stressors at all trophic levels (i.e., from primary producers to secondary consumers) and impacts on ecosystem functions. Due to the limited ability to test the multitude of potential exposure scenarios, cross-linking aquatic mesocosm studies with virtual mesocosms, i.e., aquatic system models (ASMs), can serve to meet the demand for more environmental realism and ecological relevance in risk assessment. In this study, full control data sets from seven aquatic mesocosm studies conducted at a single test facility under GLP were analysed graphically and using descriptive statistics. Thereby, not only a comprehensive data base but also an insight into the species present, their dynamics over time, and variability in unchallenged mesocosms was observed. While consistency in dynamics could be discerned for physical and chemical parameters, variability was evident for several biological endpoints. This variability points to amplification of small differences over time as well as to stochastic processes. The outline of existing gaps and uncertainties in data leads to the estimation of what can be expected to be captured and predicted by ASMs.

Ecological effects of imidacloprid on a tropical freshwater ecosystem and subsequent recovery dynamics

This study aimed to investigate the effect of imidacloprid on structural (invertebrates and primary producers) and functional (organic matter decomposition and physicochemical parameters) characteristics of tropical freshwaters using acute single species and mesocosm studies performed in Ethiopia. The recovery of affected endpoints was also studied by using a mesocosm study period of 21 weeks. Our acute toxicity test showed that Cloeon dipterum (96-h EC50 = 1.5 μg/L) and Caenis horaria (96-h EC50 = 1.9 μg/L) are relatively sensitive arthropods to imidacloprid. The mesocosm experiment evaluated the effects of four applications of imidacloprid with a weekly interval and the results showed that the macroinvertebrate and zooplankton community structure changed significantly due to imidacloprid contamination in mesocosms repeatedly dosed with ≥0.1 and ≥ 0.01 μg/L, respectively (time weighted average concentrations of 112 days (TWA_112d) of ≥0.124 and ≥ ≈0.02 μg/L, respectively). The largest responses were found for C. dipterum, C. horaria, Brachionus sp. and Filinia sp. Chlorophyll-a concentrations of periphyton and phytoplankton significantly increased in the ≥0.1 μg/L treatments levels which are indirect effects as a result of the release of grazing pressure. A significant, but quantitatively small, decrease of organic matter decomposition rate was observed in mesocosms treated with repeated doses of 1 μg/L (TWA_112d of 2.09 μg/L). No recovery was observed for the macroinvertebrates community during the study period of 21 weeks, but zooplankton recovered after 9 weeks. We observed spatio-temporal related toxicity differences between tropical and temperate aquatic taxa, with tropical taxa generally being more sensitive. This suggests that use of temperate toxicity data for the risk assessment of imidacloprid in tropical region is not recommended.

Resting egg banks can facilitate recovery of zooplankton communities after short exposure to glyphosate

We assessed the short-term viability and recovery of zooplankton communities after exposure to glyphosate (active ingredient-a.i.). We conducted a hatching experiment in two steps: Step 1-natural lake sediments containing resting egg banks were placed into individual trays and exposed to a solution medium of glyphosate at three different treatments (LD = Values below the detection limits, LD < 0.05, 0.44, and 0.89 mg a.i./L) for 14 days; and Step 2-we replaced the exposure solution of glyphosate with distilled freshwater, keeping them all trays under freshwater conditions for another 14 day. The results from Step 1 showed significant effects of glyphosate on the emergence patterns of resting eggs, with a reduction in hatching of rotifers, mainly at concentrations of 0.44 and 0.89 mg a.i./L. On the other hand, the results from Step 2 showed an increase in the emergence of viable eggs for rotifers after restoration of freshwater conditions in all treatments; there was no effect for total zooplankton and microcrustaceans. These findings suggest that (i) glyphosate may, effectively, impair zooplankton hatching from resting egg banks; (ii) the magnitude of the negative effects depends on the the zooplanktonic group considered; and (iii) the restoration of freshwater conditions may, in some way, allow the recovery of the zooplankton community from viable egg banks. Our results can be useful in predicting the influence of glyphosate on the distribution patterns of freshwater zooplankton, which can represent vital information for environmental managers.

Mechanistic Effect Modeling Approach for the Extrapolation of Species Sensitivity

In the higher-tier environmental risk assessment of chemicals, species sensitivity distributions (SSDs) are used to statistically describe differences in sensitivity between species and derive community level endpoints. SSDs are usually based on the results from short-term laboratory experiments performed under constant environmental conditions. However, different species may be kept at different "optimal" temperatures, which influence their apparent sensitivity and thus the derivation of endpoints. Also, the extrapolation capacity of SSDs is largely limited to the tested species and conditions. Time-variable exposures and effects at higher levels of biological organization, including biological interactions, are not considered. The quantitative effect prediction at higher tiers would ultimately require the extrapolation of toxicokinetics and toxicodynamics to untested species and the involvement of population and community modeling. In this regard, we tested a toxicokinetic-toxicodynamic modeling approach to mechanistically consider and correct endpoints for ambient temperature and demonstrate the significance for SSDs. We explored correlations in toxicokinetic-toxicodynamic model parameters which would allow for the extrapolation of sensitivities to untested species. Finally, we illustrate the applicability of the approach for higher level effect predictions using an individual-based model. Our results suggest that mechanistic effect modeling approaches can reduce the uncertainties in higher tier effect assessments related to knowledge gaps.