TY - JOUR A1 - Jäger, Christoph G. A1 - Diehl, S. A1 - Matauschek, C. A1 - Klausmeier, C. A. A1 - Stibor, H. T1 - Transient dynamics of pelagic producer-grazer systems in a gradient of nutrients and mixing depths JF - Ecology N2 - Phytoplankton–grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton–grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon : nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light–nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the “paradox of energy enrichment” (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment. KW - algae-grazer interaction KW - C:P ratio KW - Daphnia KW - ecological stoichiometry KW - food quality KW - light enrichment KW - nutrient enrichment KW - paradox of enrichment KW - population model KW - population oscillations KW - seasonal succession Y1 - 2008 UR - https://doi.org/10.1890/07-0347.1 VL - 89 IS - 5 SP - 1272 EP - 1286 ER - TY - JOUR A1 - Jäger, Christoph G. A1 - Vrede, T. A1 - Persson, L. A1 - Jansson, M. T1 - Interactions between metazoans, autotrophs, mixotrophs and bacterioplankton in nutrient‐depleted high DOC environments: a long‐term experiment JF - Freshwater Biology N2 - 1. Humic lakes with a high external supply of DOC and low input of nutrients can often support a high biomass of metazoan zooplankton. In such lakes, autotrophic algae compete with bacteria for inorganic nutrients, but bacteria support mixotrophic growth. Consequently, planktonic communities are often dominated by mixotrophic flagellates, while obligate autotrophic phytoplankton occurs in low numbers for extended periods. 2. To test the importance of autotrophic phytoplankton and mixotrophic flagellates as food resources for metazoan grazers and, in turn, the feedback effects of grazers on basal food‐web interactions, we conducted a long‐term experiment where we simulated abiotic resource relationships of humic lakes (high DOC [glucose] and low P input). We examined the population dynamics of Daphnia galeata when inoculated in systems with autotrophic algae only, mixotrophic algae only and a mixture of autotrophic and mixotrophic algae, and how the systems changed after the inoculation of Daphnia. All combinations were run at high‐ and low‐light conditions to analyse the effects of light on food quantity and quality. 3. Daphnia grew to high densities only when mixotrophs were present at high‐light conditions and showed no or only weak growth at low‐light conditions or with autotrophs as the only food source. 4. Autotrophic algae and bacteria showed a strong competition for nutrients. Autotrophic algae were released from competition for nutrients after Daphnia grazed on bacteria, which led to a probable change of the bacteria community to less edible but less competitive taxa. As a consequence, there was a mutualistic interaction between autotrophs and mixotrophs before Daphnia were introduced which turned into competition after Daphnia inoculation. 5. We suggest that mixotrophic flagellates can be a critical resource for cladocerans and thereby also have a cascading effect on higher trophic levels, and cladocerans, in turn, have important indirect effects on basal planktonic food webs; hence, both might affect whole lake ecosystems. KW - Daphnia KW - Dinobryon KW - Food quality KW - Indirect effects KW - Scenedesmus Y1 - 2014 UR - https://doi.org/10.1111/fwb.12366 VL - 59 IS - 8 SP - 1596 EP - 1607 ER -