TY - JOUR A1 - Tirok, Katrin A1 - Gaedke, Ursula T1 - Spring weather determines the relative importance of ciliates, rotifers and crustaceans for the initiation of the clear-water phase in a large, deep lake N2 - Clear-water phase (CWP) is an important event in seasonal plankton succession. We examined the influence of all herbivorous zooplankton on its initiation under different weather and climatic conditions using up to 19 years of observations from the large, deep Lake Constance (Europe) and estimates of relative clearance rates. A CWP occurred regularly, even if daphnid biomass was still very low. CWP was attributed to strong grazing either by a daphnid- dominated zooplankton community or by a diverse assemblage consisting of micro- and meso-zooplankton. Both types of zooplankton communities occurred with approximately the same frequency. The timing of the CWP was unrelated to the North Atlantic Oscillation (NAO) but correlated with the wind-dependent intensity of deep vertical mixing 3 months earlier, during early spring. Less mixing enabled early growth of phytoplankton, ciliates and rotifers despite low temperatures, which prevented daphnid development at this time. This resulted in enhanced grazing of ciliates and rotifers, which increased the importance of phytoplankton less edible for most ciliates, rotifers and daphnids. Ciliates clearly dominated the grazing pressure on phytoplankton throughout spring, maintaining high biomasses together with the phytoplankton for up to 2 months. A CWP was observed when herbivores grazing on larger phytoplankton developed in addition to ciliates Y1 - 2006 ER - TY - JOUR A1 - Tirok, Katrin A1 - Gaedke, Ursula T1 - Internally driven alternation of functional traits in a multispecies predator-prey system N2 - The individual functional traits of different species play a key role for ecosystem function in aquatic and terrestrial systems. We modeled a multispecies predator-prey system with functionally different predator and prey species based on observations of the community dynamics of ciliates and their algal prey in Lake Constance. The model accounted for differences in predator feeding preferences and prey susceptibility to predation, and for the respective trade-offs. A low food demand of the predator was connected to a high food selectivity, and a high growth rate of the prey was connected to a high vulnerability to grazing. The data and the model did not show standard uniform predator- prey cycles, but revealed both complex dynamics and a coexistence of predator and prey at high biomass levels. These dynamics resulted from internally driven alternations in species densities and involved compensatory dynamics between functionally different species. Functional diversity allowed for ongoing adaptation of the predator and prey communities to changing environmental conditions such as food composition and grazing pressure. The trade-offs determined whether compensatory or synchronous dynamics occurred which influence the variability at the community level. Compensatory dynamics were promoted by a joint carrying capacity linking the different prey species which is particularly relevant at high prey biomasses, i.e., when grazers are less efficient. In contrast, synchronization was enhanced by the coupling of the different predator and prey species via common feeding links, e.g., by a high grazing pressure of a nonselective predator. The communities had to be functionally diverse in terms of their trade-offs and their traits to yield compensatory dynamics. Rather similar predator species tended to cycle synchronously, whereas profoundly different species did not coexist. Compensatory dynamics at the community level thus required intermediately strong tradeoffs for functional traits in both predators and their prey. Y1 - 2010 UR - http://esapubs.org/esapubs/journals/ecology.htm U6 - https://doi.org/10.1890/09-1052.1 SN - 0012-9658 ER - TY - JOUR A1 - Tirok, Katrin A1 - Gaedke, Ursula T1 - The effect of irradiance, vertical mixing and temperature on spring phytoplankton dynamics under climate change : long-term observations and model analysis N2 - Spring algal development in deep temperate lakes is thought to be strongly influenced by surface irradiance, vertical mixing and temperature, all of which are expected to be altered by climate change. Based on long-term data from Lake Constance, we investigated the individual and combined effects of these variables on algal dynamics using descriptive statistics, multiple regression models and a processoriented dynamic simulation model. The latter considered edible and less-edible algae and was forced by observed or anticipated irradiance, temperature and vertical mixing intensity. Unexpectedly, irradiance often dominated algal net growth rather than vertical mixing for the following reason: algal dynamics depended on algal net losses from the euphotic layer to larger depth due to vertical mixing. These losses strongly depended on the vertical algal gradient which, in turn, was determined by the mixing intensity during the previous days, thereby introducing a memory effect. This observation implied that during intense mixing that had already reduced the vertical algal gradient, net losses due to mixing were small. Consequently, even in deep Lake Constance, the reduction in primary production due to low light was often more influential than the net losses due to mixing. In the regression model, the dynamics of small, fast-growing algae was best explained by vertical mixing intensity and global irradiance, whereas those of larger algae were best explained by their biomass 1 week earlier. The simulation model additionally revealed that even in late winter grazing may represent an important loss factor during calm periods when losses due to mixing are small. The importance of losses by mixing and grazing changed rapidly as it depended on the variable mixing intensity. Higher temperature, lower global irradiance and enhanced mixing generated lower algal biomass and primary production in the dynamic simulation model. This suggests that potential consequences of climate change may partly counteract each other. Y1 - 2007 UR - http://www.springerlink.com/content/p450177134590p61/ U6 - https://doi.org/10.1007/s00442-006-0547-4 ER - TY - JOUR A1 - Tirok, Katrin A1 - Gaedke, Ursula T1 - Regulation of planktonic ciliate dynamics and functional composition during spring in Lake Constance N2 - Protozoans are among the most important grazers of phytoplankton and remineralizers of nutrients in marine and freshwater ecosystems, but less is known about the regulation of their population dynamics. We analyzed a 12 yr data set of ciliate biomass and species composition in large, deep Lake Constance to understand the factors influencing ciliate spring development. The start of ciliate net growth in spring was closely linked to that of edible algae, chlorophyll a and the vertical mixing intensity, but independent of water temperature. During ciliate spring growth, the relative contribution of ciliated interception feeders was positively related to that of cryptomonads, whereas the relative contribution of filter feeders correlated positively with that of non-cryptomonads. The duration of ciliate dominance in spring was largely controlled by the highly variable onset of the phytoplankton bloom, as the termination of the ciliate bloom was less variable. During years with an extended spring bloom of algae and ciliates, internally forced species shifts were observed in both communities. Interception feeders alternated with filter feeders in their relative importance as did cryptomonads and non-cryptomonads. Extended spring blooms were observed when vertical mixing intensity was low at low temperatures during early spring, which will become less likely under the anticipated climate change scenarios. The termination of the ciliate spring bloom occurred prior to a reduction in food concentration and mostly also prior to the mass development of daphnids alone, but coincided with increased grazing by various predators together, such as rotifers, copepods and daphnids in late May/early June. Y1 - 2007 UR - http://www.int-res.com/articles/ame2007/49/a049p087.pdf U6 - https://doi.org/10.3354/Ame01127 ER - TY - JOUR A1 - Tirok, Katrin A1 - Bauer, Barbara A1 - Wirtz, Kai A1 - Gaedke, Ursula T1 - Predator-Prey Dynamics Driven by Feedback between Functionally Diverse Trophic Levels JF - PLoS one N2 - Neglecting the naturally existing functional diversity of communities and the resulting potential to respond to altered conditions may strongly reduce the realism and predictive power of ecological models. We therefore propose and study a predator-prey model that describes mutual feedback via species shifts in both predator and prey, using a dynamic trait approach. Species compositions of the two trophic levels were described by mean functional traits-prey edibility and predator food-selectivity- and functional diversities by the variances. Altered edibility triggered shifts in food-selectivity so that consumers continuously respond to the present prey composition, and vice versa. This trait-mediated feedback mechanism resulted in a complex dynamic behavior with ongoing oscillations in the mean trait values, reflecting continuous reorganization of the trophic levels. The feedback was only possible if sufficient functional diversity was present in both trophic levels. Functional diversity was internally maintained on the prey level as no niche existed in our system, which was ideal under any composition of the predator level due to the trade-offs between edibility, growth and carrying capacity. The predators were only subject to one trade-off between food-selectivity and grazing ability and in the absence of immigration, one predator type became abundant, i.e., functional diversity declined to zero. In the lack of functional diversity the system showed the same dynamics as conventional models of predator-prey interactions ignoring the potential for shifts in species composition. This way, our study identified the crucial role of trade-offs and their shape in physiological and ecological traits for preserving diversity. Y1 - 2011 U6 - https://doi.org/10.1371/journal.pone.0027357 SN - 1932-6203 VL - 6 IS - 11 PB - PLoS CY - San Fransisco ER - TY - JOUR A1 - Gaedke, Ursula A1 - Ruhenstroth-Bauer, Miriam A1 - Wiegand, Ina A1 - Tirok, Katrin A1 - Aberle-Malzahn, Nicole A1 - Breithaupt, Petra A1 - Lengfellner, Kathrin A1 - Wohlers, Julia A1 - Sommer, Ulrich T1 - Biotic interactions may overrule direct climate effects on spring phytoplankton dynamics N2 - To improve our mechanistic understanding and predictive capacities with respect to climate change effects on the spring phytoplankton bloom in temperate marine systems, we used a process-driven dynamical model to disentangle the impact of potentially relevant factors which are often correlated in the field. The model was based on comprehensive indoor mesocosm experiments run at four temperature and three light regimes. It was driven by time-series of water temperature and irradiance, considered edible and less edible phytoplankton separately, and accounted for density- dependent grazing losses. It successfully reproduced the observed dynamics of well edible phytoplankton in the different temperature and light treatments. Four major factors influenced spring phytoplankton dynamics: temperature, light (cloudiness), grazing, and the success of overwintering phyto- and zooplankton providing the starting biomasses for spring growth. Our study predicts that increasing cloudiness as anticipated for warmer winters for the Baltic Sea region will retard phytoplankton net growth and reduce peak heights. Light had a strong direct effect in contrast to temperature. However, edible phytoplankton was indirectly strongly temperature-sensitive via grazing which was already important in early spring at moderately high algal biomasses and counter-intuitively provoked lower and later algal peaks at higher temperatures. Initial phyto- and zooplankton composition and biomass also had a strong effect on spring algal dynamics indicating a memory effect via the broadly under-sampled overwintering plankton community. Unexpectedly, increased initial phytoplankton biomass did not necessarily lead to earlier or higher spring blooms since the effect was counteracted by subsequently enhanced grazing. Increasing temperature will likely exhibit complex indirect effects via changes in overwintering phytoplankton and grazer biomasses and current grazing pressure. Additionally, effects on the phytoplankton composition due to the species-specific susceptibility to grazing are expected. Hence, we need to consider not only direct but also indirect effects, e.g. biotic interactions, when addressing climate change impacts. Y1 - 2010 UR - http://www3.interscience.wiley.com/cgi-bin/issn?DESCRIPTOR=PRINTISSN&VALUE=1354-1013 U6 - https://doi.org/10.1111/j.1365-2486.2009.02009.x SN - 1354-1013 ER - TY - JOUR A1 - Ebenhoeh, Oliver A1 - Houwaart, Torsten A1 - Lokstein, Heiko A1 - Schlede, Stephanie A1 - Tirok, Katrin T1 - A minimal mathematical model of nonphotochemical quenching of chlorophyll fluorescence JF - Biosystems : journal of biological and information processing sciences N2 - Under natural conditions, plants are exposed to rapidly changing light intensities. To acclimate to such fluctuations, plants have evolved adaptive mechanisms that optimally exploit available light energy and simultaneously minimise damage of the photosynthetic apparatus through excess light. An important mechanism is the dissipation of excess excitation energy as heat which can be measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). In this paper, we present a highly simplified mathematical model that captures essential experimentally observed features of the short term adaptive quenching dynamics. We investigate the stationary and dynamic behaviour of the model and systematically analyse the dependence of characteristic system properties on key parameters such as rate constants and pool sizes. Comparing simulations with experimental data allows to derive conclusions about the validity of the simplifying assumptions and we further propose hypotheses regarding the role of the xanthophyll cycle in NPQ. We envisage that the presented theoretical description of the light reactions in conjunction with short term adaptive processes serves as a basis for the development of more detailed mechanistic models by which the molecular mechanisms of NPQ can be theoretically studied. KW - Photosynthesis KW - Light reactions KW - Nonphotochemical quenching of chlorophyll fluorescence KW - Chlorophyll fluorescence KW - Mathematical model Y1 - 2011 U6 - https://doi.org/10.1016/j.biosystems.2010.10.011 SN - 0303-2647 VL - 103 IS - 2 SP - 196 EP - 204 PB - Elsevier CY - Oxford ER -