Trophic interactions and abiotic forcing in the aquatic ecosystems : a modeling approach

Abstract

Planktonic food webs are easy to monitor and manipulate, thus offer a wide array of opportunities for deriving general ecological and evolutionary principles. Not least, for predicting and managing the consequences of the increasing stresses on aquatic ecosystems due to anthropogenic activity, we need a better understanding of their ecological dynamics more than ever, in the face of growing threats such as perturbation of biogeochemical cycles and global warming. In this study, I focused on various aspects of plankton dynamics and the physical environment influencing them. In chapter 1, anomalous limnological conditions that occurred after an extremely warm winter were elaborated for gaining insights into the shape of things to come in a warmer future. In chapter 2, the regulation of phytoplankton growth by top-down and bottom-up factors and their variation over inter-decadal, inter-annual and seasonal scales were inspected. In chapter 3, the focus was on ciliates, where the performance of different model formulations regarding their loss terms were evaluated, based on extensive phytoplankton and ciliate data sets collected from Lake Constance. In chapter 4, the influence of cell size on algal competitive abilities in an incompletely mixed water column where nutrients and light are found in opposing gradients was analyzed.The simulations with the hydro-dynamical model forced by artificially sequenced meteorological conditions revealed that the occurrence of relatively shallow mixed layer depth during the 2006/07 winter was brought about by the collective work of two winters: anomalously high air temperatures starting from 2006 November resulted in a lack of cooling, hence, insufficient densification of surface waters to replace the denser water at the deep layers, which was a result of cold air temperatures of 2005/06 winter. Therefore the limnological parameters that primarily depend on mixing dynamics such as phosphorus concentrations at surface and the timing of phytoplankton bloom are likely to overestimate the future conditions in which such warm winters are anticipated to be the norm. Contrastingly, the temperature dependent parameters such as timing of the emergence of Daphnia are shown to be potentially underestimating the future conditions, as the simulations forced with the meteorology of warm 2006/07 revealed that, it takes almost a decade in Lake Constance for the temperature profiles to reach an equilibrium annual cycle.In order to improve the understanding on the regulation of the phytoplankton and ciliate populations, a vertically resolved phytoplankton-ciliate model was used, which was driven by hydro-dynamically calculated temperature and turbulent diffusivity profiles and field measurements of phosphorus and various other zooplankton groups Across a gradient of trophic states encompassing meso\eutrophic to oligotrophic conditions reflecting three decades of reduced phosphorus loading in Lake Constance, the relative importance of bottom-up regulation of spring phytoplankton growth gradually and non-linearly exceeded that of top-down regulation. While the increase in the absolute magnitude of bottom-up limitation played a major role in this alteration of the predominance of regulation mechanisms, the decrease in the absolute magnitude of top-down regulation as a result of decreasing ciliate biomasses also had a contribution. An understanding of the seasonal scale variations in the strength of top-down and bottom-up factors were shown to be essential for understanding the variations on a long-term scale. While the decline in ciliate abundances with decreasing phytoplankton concentrations suggest that the regulation of ciliate population is predominantly bottom-up, the detailed investigation of the performance of alternative model formulations for simulating the ciliate dynamics indicates that some self-limitation process keep their population in certain bounds and prevent them from fully exploiting their algal resources. With a density dependent mortality rate, it was possible to realistically simulate the ciliate biomasses in a wide range of environmental settings, suggesting that this formulation offers a simple solution for inclusion of a microzooplankton compartment to the ecosystem models.Extensive simulations with the vertically resolved Droop-model of phytoplankton growth revealed a novel mechanism of algal competition. When the competing species differ with respect to their nutrient storage abilities, for instance, due to differences in their typical cell sizes, storage advantages can override other disadvantages such as lower growth rates or lower nutrient affinities, if the upward transport rate of nutrient-rich cells to the photic zone is sufficiently high. The supply rate of nutrient-rich cells to the surface do not only depend on the mixing intensities throughout the water column but also on other system parameters, such as background turbidity and nutrient concentrations at the bottom of the water column, which can effect the vertical distribution of light and nutrient availabilities. Moreover, the spatio-temporal heterogeneities in the mixing intensities were shown to introduce further complexities, which need to be investigated in future.