Elevated pCO2 affects behavioural patterns and mechano-sensation in predatory phantom midge larvae Chaoborus obscuripes

Effect of an antidepressant on aquatic ecosystems in the presence of microplastics: A mesocosm study

Emerging pollutants, such as pharmaceuticals and microplastics have become a pressing concern due to their widespread presence and potential impacts on ecological systems. To assess the ecosystem-level effects of these pollutants within a multi-stressor context, we simulated real-world conditions by exposing a near-natural multi-trophic aquatic food web to a gradient of environmentally relevant concentrations of fluoxetine and microplastics in large mesocosms over a period of more than three months. We measured the biomass and abundance of different trophic groups, as well as ecological functions such as nutrient availability and decomposition rate. To explore the mechanisms underlying potential community and ecosystem-level effects, we also performed behavioral assays focusing on locomotion parameters as a response variable in three species: Daphnia magna (zooplankton prey), Chaoborus flavicans larvae (invertebrate pelagic predator of zooplankton) and Asellus aquaticus (benthic macroinvertebrate), using water from the mesocosms. Our mesocosm results demonstrate that presence of microplastics governs the response in phytoplankton biomass, with a weak non-monotonic dose-response relationship due to the interaction between microplastics and fluoxetine. However, exposure to fluoxetine evoked a strong non-monotonic dose-response in zooplankton abundance and microbial decomposition rate of plant material. In the behavioral assays, the locomotion of zooplankton prey D. magna showed a similar non-monotonic response primarily induced by fluoxetine. Its predator C. flavicans, however, showed a significant non-monotonic response governed by both microplastics and fluoxetine. The behavior of the decomposer A. aquaticus significantly decreased at higher fluoxetine concentrations, potentially leading to reduced decomposition rates near the sediment. Our study demonstrates that effects observed upon short-term exposure result in more pronounced ecosystem-level effects following chronic exposure.

Long-term effects of elevated pCO2 levels on the expression of Chaoborus-induced defences in Daphnia pulex

Increased carbon dioxide from fossil fuel combustion results in an enrichment of CO₂ in the global carbon cycle. Recent evidence indicates that rising atmospheric CO₂ impacts the partial pressure of carbon dioxide (pCO₂) in freshwaters. This affects freshwater biota by disrupting chemical communication between predator and prey. One such well-described predator-prey interaction is the phantom midge larva Chaoborus preying on the freshwater crustacean Daphnia pulex. To counter Chaoborus predation, D. pulex develops defensive neckteeth in response to chemical cues. The strength of neckteeth expression is reduced when D. pulex experience elevated pCO₂ levels. This is discussed to directly impair predator perception and results in reduced defence expression. However, it is not known whether there are also long-term effects associated with continuous elevated pCO₂. Here, we investigated the effect of long-term exposure of D. pulex to elevated pCO₂ levels in a life-table experiment over three generations. Using a flow-through system, we continuously exposed D. pulex to cues released by the predatory larva Chaoborus and control or elevated pCO₂ levels. We determined morphological defence expression in the 2^nd juvenile instar and the number of neonates as a measure for life-history traits over three successive generations. We detected that elevated pCO₂ significantly reduces the expression of predator-induced morphological defences (i.e. neckteeth) and life-history parameters (i.e. number of neonates) in successive generations. Our data clearly show that at least three generations become more vulnerable to predation without indications of transgenerational acclimation. As Daphnia is a keystone grazer of freshwater ecosystems, this may destabilise population growth rates. In conclusion, long-term effects of pCO₂-induced reduction of predator-induced plasticity may significantly affect trophic interactions.