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Biological stoichiometry is an approach that focuses on the balance of elements in biological interactions. It is a theory that has the potential to causally link material processes at all biological levels—from molecules to the biosphere. But the lack of a coherent operational framework has so far restricted progress in this direction. Here, we provide a framework to help infer how a stoichiometric imbalance observed at one level impacts all other biological levels. Our framework enables us to highlight the areas of the theory in need of completion, development and integration at all biological levels. Our hope is that this framework will contribute to the building of a more predictive theory of elemental transfers within the biosphere, and thus, to a better understanding of human-induced perturbations to the global biogeochemical cycles.

The mineralization of nitrogen (N) and especially the regeneration of ammonium are critical processes performed by bacteria in aquatic ecosystems. Quantifying these processes is complicated because bacteria simultaneously consume and produce ammonium. Here we use experimental data on the effects of the molecular composition of the supplied substrates, combined with a classical stoichiometric model of ammonium regeneration, to demonstrate how the quantification of these processes can be improved. We manipulated a batch culture experiment with an isolated bacterial community by adding three different types of N substrates: dissolved inorganic nitrogen (DIN, nitrate), dissolved organic nitrogen (DON, amino acid) and a mixture of DIN and DON. With such experiment set-up, the ammonium regeneration per se could be easily tracked without using complicated methods (e.g. isotope dilution). We compared the experimental data with the predictions of Goldman et al’ model (1987) as well as with a revised version, using the measured consumption carbon:nitrogen ratio (C:N ratio), rather than an estimated consumption ratio. We found that, for all substrates, and in particular, mixed substrates where C and N are partially dissociated between different molecules, estimates of ammonium regeneration rates can be improved by measuring the actual consumption C: N ratio.

In aquatic microplankton food webs, the relative availability of dissolved inorganic nitrogen (DIN) and organic nitrogen (DON) can shape trophic interactions, food web structure and the stoichiometry of the organisms. To evaluate the importance of nitrogen forms, i.e., whether microplankton have access to organic or inorganic forms of nitrogen, we performed a short-term microcosm study of a coastal microplankton food web (organism size < 50 µm). In this experiment, the microplankton community was exposed to two different carbon (C) and nitrogen (N) sources: C and N were either associated in a single organic molecule or dissociated in two different molecules. The results showed that the different nitrogen forms had a strong impact on the food web composition, which resulted in different nitrogen food web use efficiency. The entire microplankton food web benefited from the association of C and N in a single DON molecule. The association or dissociation of C and N input had marked effects on all trophic levels, most probably through its effect on bacteria-phytoplankton interaction, which switched from mutualism to competition. Hence, the degree of association between N and C is an important factor to be considered in microplanktonic food web dynamics, at least for the short-term food web response.

Climate-driven changes, e.g. increasing precipitation, will increase terrestrial runoff and freshwater inputs to coastal ecosystems in the future. Consequently, we expect that altered availability of inorganic and organic forms of carbon (C), nitrogen (N) and phosphorus (P) will lead to spatial and temporal variation in the elemental composition of coastal organisms. To identify the major spatiotemporal patterns in seston and zooplankton elemental (C, N and P) composition, we sampled four bays of northern Baltic Sea from May to September 2018. This sampling design covers spatial and temporal variation in freshwater input, as it includes the spring flood driven by snowmelt in May, and bays receiving varying catchment areas. We analyzed CNP composition for different size fractions of seston, and dominant zooplankton taxa. Our results showed that the stoichiometric composition of individual seston size fractions varied more through time than in space. The concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in water were the major explanatory variables for seston stoichiometry. We also found that zooplankton stoichiometry was relatively similar between taxa within specific months, although the dominant zooplankton taxa differed over time and among sites. This could partly be explained by the similar stoichiometric requirements and homeostasis among zooplankton taxa. Our findings imply that the spatiotemporal variation in physicochemical characteristics of coastal ecosystems will alter the quality of seston and zooplankton. Yet, further investigations on the role of zooplankton in stoichiometry related nutrient cycling in the Baltic Sea coastal ecosystems are needed.