The effects of diel vertical migration of Daphnia on zooplankton-phytoplankton interactions

Zooplankton populations which perform diel vertical migration (DVM) only spend the night in surface water layers but migrate downwards into the lower water layers during the day. The intention of this study was to investigate effects of DVM of Daphnia on phytoplankton dynamics and Daphnia life history parameters in a lake. I conducted field and laboratory experiments in which I compared ‘migration’ with ‘no-migration’ situations. It is generally assumed that phytoplankton communities in the epilimnion of stratified lakes profit from the presence of DVM. This might be caused by less grazing due to intermittent grazing and/or less grazing due to lower population densities of migrating Daphnia populations (as they migrate into the colder, lower hypolimnion during the day which leads to a lower temperature-dependent population growth). In a first laboratory experiment I showed that an enhanced phytoplankton biomass could develop under a migration regime solely due to intermittent grazing. I further showed that edible phytoplankton species with higher intrinsic growth rates benefited more from intermittent grazing than edible species with lower intrinsic growth rates. Field experiments also indicated that phytoplankton biomass in the epilimnion was higher when subject to a migrating zooplankton population and that additionally different phytoplankton community compositions arose from different migration regimes (‘migration’/’no-migration’). For example edible algae were at an advantage when zooplankton migrated, whereas large, inedible algae species had an advantage when zooplankton populations did not migrate. In an additional laboratory experiment I also showed that these changes in phytoplankton composition had strong feedback effects on life history parameters of daphnids and that food conditions experienced by migrating daphnids were advantageous. In a further laboratory experiment I exposed two Daphnia species to either constant or regularly changing temperature regimes to study whether a fluctuating temperature regime – as experienced by migrating daphnids – implies costs for daphnids. Somatic growth rates of juvenile Daphnia in the regularly changing temperature regime were almost as low as under constant low temperature conditions indicating that a regular change in temperature involves high costs. The results of my study indicate that DVM has a strong modulating effect on zooplankton-phytoplankton interactions in a lake.Zooplankter, die eine tagesperiodischen Vertikalwanderung (TPV) durchführen halten sich nur nachts in der Oberflächenschicht der Gewässer auf, den Tag dagegen verbringen sie in tieferen Schichten. Die vorliegende Arbeit beschäftigt sich mit den Auswirkungen der TPV von Daphnien auf Phytoplanktondynamiken und Populationsparameter der Daphnien. Dazu führte ich sowohl Freiland- als auch Laborversuche durch, in denen ich jeweils ‘Migrations-’ mit ‘Nicht-Migrations-’ Ansätzen verglich. Es wird generell angenommen, dass Phytoplanktongemeinschaften in den Epilimnien von geschichteten Seen der gemäßigten Zone von einer TPV profitieren können. Das kann durch zwei Mechanismen hervorgerufen werden: Erstens durch einen zeitlich gepulsten Fraßdruck (Grazing nur nachts), und zweitens durch einen geringeren Fraßdruck, hervorgerufen durch die meist geringere Dichte wandernder Zooplanktonpopulationen, da diese, bedingt durch die Wanderung tagsüber in die kalten, tiefergelegenen Wasserschichten, ein reduziertes Wachstum haben. In einem ersten Laborversuch konnte ich zeigen, dass allein durch einen zeitlich gepulsten, diskontinuierlichen Fraßdruck ein erhöhtes Algenwachstum entstehen konnte. Ich konnte weiterhin zeigen, dass fressbare Algenarten mit einer höheren intrinsischen Wachstumsrate einen größeren Vorteil von der TPV hatten als fressbare Arten mit einer niedrigeren Wachstumsrate. In Freilandexperimenten konnte ich zum einen ebenfalls zeigen, dass die Algenbiomasse bei Wanderung der Daphnien erhöht war, zum anderen konnte ich auch vom Wanderregime abhängige Veränderungen in der Phytoplanktongemeinschaft nachweisen. So hatten fressbare Phytoplanktonarten einen Vorteil von einer TPV, unfressbare Algenarten hatten dagegen einen Vorteil, wenn keine Wanderung stattfand. Ich konnte in einem weiteren Laborversuch zeigen, dass die oben genannten Veränderungen in der Phytoplanktondynamik starke rückwirkende Effekte auf Populationsparameter der Daphnien hatten. In einem weiteren Laborversuch setzte ich die Daphnien entweder konstanten, oder tageszeitenabhängig fluktuierenden Temperaturregimes aus. Dadurch untersuchte ich, ob eine sich zwei mal pro Tag ändernde Temperatur (wie Daphnien es bei einer Wanderung erleben) Kosten für Daphnien verursacht. Dabei war die somatische Wachstumsrate der juvenilen Daphnien bei einem fluktuierenden Temperaturregime fast genauso gering wie bei konstant tiefer Temperatur. Dies gibt Hinweise auf Kosten, die durch ein fluktuierendes Temperaturregime entstehen. Die Ergebnisse zeigen, dass eine TPV von Daphnien starke, modulierende Auswirkungen auf Zooplankton-Phytoplankton Interaktionen hat

Development of Toxicological Risk Assessment Models for Acute and Chronic Exposure to Pollutants

Alert level frameworks advise agencies on a sequence of monitoring and management actions, and are implemented so as to reduce the risk of the public coming into contact with hazardous substances. Their effectiveness relies on the detection of the hazard, but with many systems not receiving any regular monitoring, pollution events often go undetected. We developed toxicological risk assessment models for acute and chronic exposure to pollutants that incorporate the probabilities that the public will come into contact with undetected pollution events, to identify the level of risk a system poses in regards to the pollutant. As a proof of concept, we successfully demonstrated that the models could be applied to determine probabilities of acute and chronic illness types related to recreational activities in waterbodies containing cyanotoxins. Using the acute model, we identified lakes that present a ‘high’ risk to develop Day Away From Work illness, and lakes that present a ‘low’ or ‘medium’ risk to develop First Aid Cases when used for swimming. The developed risk models succeeded in categorising lakes according to their risk level to the public in an objective way. Modelling by how much the probability of public exposure has to decrease to lower the risks to acceptable levels will enable authorities to identify suitable control measures and monitoring strategies. We suggest broadening the application of these models to other contaminants

The effects of diel vertical migration of Daphnia on zooplankton-phytoplankton interactions

Zooplankton populations which perform diel vertical migration (DVM) only spend the night in surface water layers but migrate downwards into the lower water layers during the day. The intention of this study was to investigate effects of DVM of Daphnia on phytoplankton dynamics and Daphnia life history parameters in a lake. I conducted field and laboratory experiments in which I compared ‘migration’ with ‘no-migration’ situations. It is generally assumed that phytoplankton communities in the epilimnion of stratified lakes profit from the presence of DVM. This might be caused by less grazing due to intermittent grazing and/or less grazing due to lower population densities of migrating Daphnia populations (as they migrate into the colder, lower hypolimnion during the day which leads to a lower temperature-dependent population growth). In a first laboratory experiment I showed that an enhanced phytoplankton biomass could develop under a migration regime solely due to intermittent grazing. I further showed that edible phytoplankton species with higher intrinsic growth rates benefited more from intermittent grazing than edible species with lower intrinsic growth rates. Field experiments also indicated that phytoplankton biomass in the epilimnion was higher when subject to a migrating zooplankton population and that additionally different phytoplankton community compositions arose from different migration regimes (‘migration’/’no-migration’). For example edible algae were at an advantage when zooplankton migrated, whereas large, inedible algae species had an advantage when zooplankton populations did not migrate. In an additional laboratory experiment I also showed that these changes in phytoplankton composition had strong feedback effects on life history parameters of daphnids and that food conditions experienced by migrating daphnids were advantageous. In a further laboratory experiment I exposed two Daphnia species to either constant or regularly changing temperature regimes to study whether a fluctuating temperature regime – as experienced by migrating daphnids – implies costs for daphnids. Somatic growth rates of juvenile Daphnia in the regularly changing temperature regime were almost as low as under constant low temperature conditions indicating that a regular change in temperature involves high costs. The results of my study indicate that DVM has a strong modulating effect on zooplankton-phytoplankton interactions in a lake.Zooplankter, die eine tagesperiodischen Vertikalwanderung (TPV) durchführen halten sich nur nachts in der Oberflächenschicht der Gewässer auf, den Tag dagegen verbringen sie in tieferen Schichten. Die vorliegende Arbeit beschäftigt sich mit den Auswirkungen der TPV von Daphnien auf Phytoplanktondynamiken und Populationsparameter der Daphnien. Dazu führte ich sowohl Freiland- als auch Laborversuche durch, in denen ich jeweils ‘Migrations-’ mit ‘Nicht-Migrations-’ Ansätzen verglich. Es wird generell angenommen, dass Phytoplanktongemeinschaften in den Epilimnien von geschichteten Seen der gemäßigten Zone von einer TPV profitieren können. Das kann durch zwei Mechanismen hervorgerufen werden: Erstens durch einen zeitlich gepulsten Fraßdruck (Grazing nur nachts), und zweitens durch einen geringeren Fraßdruck, hervorgerufen durch die meist geringere Dichte wandernder Zooplanktonpopulationen, da diese, bedingt durch die Wanderung tagsüber in die kalten, tiefergelegenen Wasserschichten, ein reduziertes Wachstum haben. In einem ersten Laborversuch konnte ich zeigen, dass allein durch einen zeitlich gepulsten, diskontinuierlichen Fraßdruck ein erhöhtes Algenwachstum entstehen konnte. Ich konnte weiterhin zeigen, dass fressbare Algenarten mit einer höheren intrinsischen Wachstumsrate einen größeren Vorteil von der TPV hatten als fressbare Arten mit einer niedrigeren Wachstumsrate. In Freilandexperimenten konnte ich zum einen ebenfalls zeigen, dass die Algenbiomasse bei Wanderung der Daphnien erhöht war, zum anderen konnte ich auch vom Wanderregime abhängige Veränderungen in der Phytoplanktongemeinschaft nachweisen. So hatten fressbare Phytoplanktonarten einen Vorteil von einer TPV, unfressbare Algenarten hatten dagegen einen Vorteil, wenn keine Wanderung stattfand. Ich konnte in einem weiteren Laborversuch zeigen, dass die oben genannten Veränderungen in der Phytoplanktondynamik starke rückwirkende Effekte auf Populationsparameter der Daphnien hatten. In einem weiteren Laborversuch setzte ich die Daphnien entweder konstanten, oder tageszeitenabhängig fluktuierenden Temperaturregimes aus. Dadurch untersuchte ich, ob eine sich zwei mal pro Tag ändernde Temperatur (wie Daphnien es bei einer Wanderung erleben) Kosten für Daphnien verursacht. Dabei war die somatische Wachstumsrate der juvenilen Daphnien bei einem fluktuierenden Temperaturregime fast genauso gering wie bei konstant tiefer Temperatur. Dies gibt Hinweise auf Kosten, die durch ein fluktuierendes Temperaturregime entstehen. Die Ergebnisse zeigen, dass eine TPV von Daphnien starke, modulierende Auswirkungen auf Zooplankton-Phytoplankton Interaktionen hat

Population genomics of resource exploitation: insights from gene expression profiles of two Daphnia ecotypes fed alternate resources

Consumer–resource interactions are a central issue in evolutionary and community ecology because they play important roles in selection and population regulation. Most consumers encounter resource variation at multiple scales, and respond through phenotypic plasticity in the short term or evolutionary divergence in the long term. The key traits for these responses may influence resource acquisition, assimilation, and/or allocation. To identify relevant candidate genes, we experimentally assayed genome-wide gene expression in pond and lake Daphnia ecotypes exposed to alternate resource environments. One was a simple, high-quality laboratory diet, Ankistrodesmus falcatus. The other was the complex natural seston from a large lake. In temporary ponds, Daphnia generally experience high-quality, abundant resources, whereas lakes provide low-quality, seasonally shifting resources that are chronically limiting. For both ecotypes, we used replicate clones drawn from a number of separate populations. Fourteen genes were differentially regulated with respect to resources, including genes involved in gut processes, resource allocation, and activities with no obvious connection to resource exploitation. Three genes were differentially regulated in both ecotypes; the others may play a role in ecological divergence. Genes clearly linked to gut processes include two peritrophic matrix proteins, a Niemann–Pick type C2 gene, and a chymotrypsin. A pancreatic lipase, an epoxide hydrolase, a neuroparsin, and an UDP-dependent glucuronyltransferase are potentially involved in resource allocation through effects on energy processing and storage or hormone pathways. We performed quantitative rt-PCR for eight genes in independent samples of three clones of each of the two ecotypes. Though these largely confirmed observed differential regulation, some genes’ expression was highly variable among clones. Our results demonstrate the value of matching the level of biological replication in genome-wide assays to the question, as it gave us insight into ecotype-level responses at ecological and evolutionary scales despite substantial variation within ecotypes

Combatting cyanobacteria with hydrogen peroxide: a laboratory study on the consequences for phytoplankton community and diversity

Experiments with different phytoplankton densities in lake samples showed that a high biomass increases the rate of hydrogen peroxide (HP) degradation and decreases the effectiveness of HP in the selective suppression of dominant cyanobacteria. However, selective application of HP requires usage of low doses only, accordingly this defines the limits for use in lake mitigation. To acquire insight into the impact of HP on other phytoplankton species, we have followed the succession of three phytoplankton groups in lake samples that were treated with different concentrations of HP using a taxa-specific fluorescence emission test. This fast assay reports relatively well on coarse changes in the phytoplankton community; the measured data and the counts from microscopical analysis of the phytoplankton matched quite well. The test was used to pursue HP application in a Planktothrix agardhii-dominated lake sample and displayed a promising shift in the phytoplankton community in only a few weeks. From a low-diversity community, a change to a status with a significantly higher diversity and increased abundance of eukaryotic phytoplankton species was established. Experiments in which treated samples were re-inoculated with original P. agardhii-rich lake water demonstrated prolonged suppression of cyanobacteria, and displayed a remarkable stability of the newly developed post-HP treatment state of the phytoplankton community

Effects of multiple stressors on cyanobacteria abundance vary with lake type

Blooms of cyanobacteria are a current threat to global water security that is expected to increase in the future because of increasing nutrient enrichment, increasing temperature and extreme precipitation in combination with prolonged drought. However, the responses to multiple stressors, such as those above, are often complex and there is contradictory evidence as to how they may interact. Here we used broad scale data from 494 lakes in central and northern Europe, to assess how cyanobacteria respond to nutrients (phosphorus), temperature and water retention time in different types of lakes. Eight lake types were examined based on factorial combinations of major factors that determine phytoplankton composition and sensitivity to nutrients: alkalinity (low and medium‐high), colour (clear and humic) and mixing intensity (polymictic and stratified). In line with expectations, cyanobacteria increased with temperature and retention time in five of the eight lake types. Temperature effects were greatest in lake types situated at higher latitudes, suggesting that lakes currently not at risk could be affected by warming in the future. However, the sensitivity of cyanobacteria to temperature, retention time and phosphorus varied among lake types highlighting the complex responses of lakes to multiple stressors. For example, in polymictic, medium‐high alkalinity, humic lakes cyanobacteria biovolume was positively explained by retention time and a synergy between TP and temperature while in polymictic, medium‐high alkalinity, clear lakes only retention time was identified as an explanatory variable. These results show that, although climate change will need to be accounted for when managing the risk of cyanobacteria in lakes, a ‘one‐size fits‐all’ approach is not appropriate. When forecasting the response of cyanobacteria to future environmental change, including changes caused by climate and local management, it will be important to take this differential sensitivity of lakes into account