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

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

Influence of food quality on depth selection of Daphnia pulicaria

We studied the habitat choice of juvenile and adult Daphnia pulicaria in thermally stratified water columns (plankton towers) with a deep water algal maximum (DCM). The DCM consisted of either filamentous cyanobacteria (Planktothrix agardhii), non-filamentous Chlorophyceae (Scenedesmus obliquus) or a mixture of both. Adult D. pulicaria spent more time at colder temperatures in the presence of P. agardhii than in the presence of S. obliquus, either as the sole food source or when mixed with P. agardhii. Juvenile D. pulicaria did not show a different habitat choice in the three food treatments. In a fourth treatment, we also determined Daphnia distribution in the absence of food. Comparing the habitat choice of juveniles and adults in each of the four treatments, the latter spent more time at colder temperatures when food was absent or when in the sole presence of P. agardhii. Additional grazing and stable isotopic marker experiments showed that D. pulicaria ingested and assimilated Planktothrix filaments. The results suggest that the differences in habitat choice between adult D. pulicaria in the presence of different food types were influenced by food quality effects: adult Daphnia which move to colder waters in the presence of low quality P. agardhii decrease their metabolic rate and might thus be able to invest more resources into reproduction when environmental conditions improve

Effects of the Distribution of a Toxic Microcystis Bloom on the Small Scale Patchiness of Zooplankton.

Toxic cyanobacterial blooms can strongly affect freshwater food web structures. However, little is known about how the patchy occurrence of blooms within systems affects the spatial distribution of zooplankton communities. We studied this by analysing zooplankton community structures in comparison with the spatially distinct distribution of a toxic Microcystis bloom in a small, shallow, eutrophic lake. While toxic Microcystis was present at all sites, there were large spatial differences in the level of cyanobacterial biomass and in the zooplankton communities; sites with persistently low cyanobacterial biomass displayed a higher biomass of adult Daphnia and higher zooplankton diversity than sites with persistently high cyanobacterial biomass. While wind was the most likely reason for the spatially distinct occurrence of the bloom, our data indicate that it was the differences in cyanobacterial biomass that caused spatial differences in the zooplankton community structures. Overall, our study suggests that even in small systems with extensive blooms 'refuge sites' exist that allow large grazers to persist, which can be an important mechanism for a successful re-establishment of the biodiversity in an ecosystem after periods of cyanobacterial blooms

Biomass of cyanobacteria and other phytoplankton at each site in August to December 2010.

<p>Other phytoplankton is comprised of chlorophyta, diatoms, cryptophyta. Numbers represent cyanobacterial fraction, which is % of cyanobacteria of total phytoplankton. Please note that y-axes are at different scales.</p

Phytoplankton, cyanobacteria and microcystin parameters.

<p>Median and range (minimum – maximum) for phytoplankton, cyanobacteria, microcystin, and zooplankton parameters measured between August and December 2010 for each site at Lake Yangebup. CB fraction = cyanobacterial biomass as % of total phytoplankton biomass, H’_Phyto = Shannon-Wiener Index based on phytoplankton biomass, H’_Zoo = Shannon-Wiener Index based on zooplankton biomass, <i>Daphnia</i>/cal. cop. = ratio of <i>Daphnia</i> to calanoid copepoda, Intracell. MC = intracellular microcystin concentration (µg L⁻¹), Bioaccum. MC = bioaccumulated microcystin concentration in zooplankton (µg MC g⁻¹dry mass). N = 5 unless stated otherwise in superscript brackets. Superscript letters indicate results of post-hoc tests for differences between sites for each parameter with sites having identical letters being not significantly different.</p

The Importance of Lake Sediments as a Pathway for Microcystin Dynamics in Shallow Eutrophic Lakes

Microcystins are toxins produced by cyanobacteria. They occur in aquatic systems across the world and their occurrence is expected to increase in frequency and magnitude. As microcystins are hazardous to humans and animals, it is essential to understand their fate in aquatic systems in order to control health risks. While the occurrence of microcystins in sediments has been widely reported, the factors influencing their occurrence, variability, and spatial distribution are not yet well understood. Especially in shallow lakes, which often develop large cyanobacterial blooms, the spatial variability of toxins in the sediments is a complex interplay between the spatial distribution of toxin producing cyanobacteria, local biological, physical and chemical processes, and the re-distribution of toxins in sediments through wind mixing. In this study, microcystin occurrence in lake sediment, and their relationship with biological and physicochemical variables were investigated in a shallow, eutrophic lake over five months. We found no significant difference in cyanobacterial biomass, temperature, pH, and salinity between the surface water and the water directly overlying the sediment (hereafter ‘overlying water’), indicating that the water column was well mixed. Microcystins were detected in all sediment samples, with concentrations ranging from 0.06 to 0.78 µg equivalent microcystin-LR/g sediments (dry mass). Microcystin concentration and cyanobacterial biomass in the sediment was different between sites in three out of five months, indicating that the spatial distribution was a complex interaction between local and mixing processes. A combination of total microcystins in the water, depth integrated cyanobacterial biomass in the water, cyanobacterial biomass in the sediment, and pH explained only 21.1% of the spatial variability of microcystins in the sediments. A more in-depth analysis that included variables representative of processes on smaller vertical or local scales, such as cyanobacterial biomass in the different layers and the two fractions of microcystins, increased the explained variability to 51.7%. This highlights that even in a well-mixed lake, local processes are important drivers of toxin variability. The present study emphasises the role of the interaction between water and sediments in the distribution of microcystins in aquatic systems as an important pathway which deserves further consideration

Determining trophic niche width: an experimental test of the stable isotope approach

Determining the trophic niche width of an animal population and the relative degree to which a generalist population consists of dietary specialists are long-standing problems of ecology. It has been proposed that the variance of stable isotope values in consumer tissues could be used to quantify trophic niche width of consumer populations. However, this promising idea has not yet been rigorously tested. By conducting controlled laboratory experiments using model consumer populations (Daphnia sp., Crustacea) with controlled diets, we investigated the effect of individual- and population-level specialisation and generalism on consumer d13C mean and variance values. While our experimental data follow general expectations, we extend current qualitative models to quantitative predictions of the dependence of isotopic variance on dietary correlation time, a measure for the typical time over which a consumer changes its diet. This quantitative approach allows us to pinpoint possible procedural pitfalls and critical sources of measurement uncertainty. Our results show that the stable isotope approach represents a powerful method for estimating trophic niche widths, especially when taking the quantitative concept of dietary correlation time into account

Two-dimensional scatter plot of sampling sites on the two principal components (PC).

<p>Sites are clustered along the x-axis (Factor 1) in site 5 and all other sites and along the y-axis (Factor 2) in sites 6, 7 and 1–5. See text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066674#pone-0066674-t005" target="_blank">Table 5</a> for which parameters represent best each of the factors.</p