Consequences of consumer-resource stoichiometric imbalance in planktonic food webs

Abstract: Resource imbalance between consumers and their resources can come from inadequate resource quantity or quality. The ecological stoichiometry theory focuses on understanding the consequences of imbalance in elemental composition.  In this thesis, I have used both resource quality (e.g., inorganic vs organic forms of nutrients) and resource quantity (e.g., terrestrial and freshwater nutrient loading to natural coastal systems) to address the consequences of consumer-resource imbalance in planktonic food webs. First, I provided a framework that summarizes how the stoichiometric imbalance is transferred from one biological level to another. The framework highlights the importance of the distribution of elements among different chemical forms and the distribution of elements among connected ecosystems. The framework then served as a guideline for the empirical work of my thesis.  Second, I studied the response of bacterial community mineralization to the relative availability of different forms of nitrogen (inorganic vs. organic form) in a batch culture experiment. The study shows that different forms of nitrogen can significantly influence the growth of bacteria. More importantly, my results show that it is crucial to measure the actual bacterial carbon to nitrogen consumption ratio, rather than use classical theoretical models, to be able to make an accurate prediction of bacterial ammonium regeneration. Third, I tested the effect of different forms of nitrogen on microplankton food web dynamics in a microcosm experiment. I found that differences between nitrogen forms have a strong impact on food web dynamics that is channeled by the bacteria-phytoplankton interaction at the base of the food web. The whole microplankton food web benefits from organic forms of nitrogen as a result of increased mutualistic interactions between bacteria and phytoplankton. Hence, the form of nitrogen is an important factor to be considered in microplanktonic food web dynamics, at least on the short-term. In the final part of this thesis, I explored resource quality and quantity effects on the stoichiometric response of a natural coastal ecosystem in a field study. I expected that the relative availability of inorganic or organic forms of carbon, nitrogen and phosphorus in our sampling bays may affect organismal elemental composition both temporally and spatially. The results indicate that the stoichiometry among seston size fractions and zooplankton varied more through time than in space. However, zooplankton stoichiometry was relatively stable among species within specific months. Overall, the concentration of dissolved organic carbon and dissolved organic nitrogen in the water column were the major explanatory variables for the seston stoichiometry. In summary, this thesis uses multiple systems to elucidate how the form and input of nutrients shape the plankton food web dynamics and its stoichiometric responses.