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Arctic coastal ecosystems are rapidly changing due to climate warming. This makes modeling their productivity crucially important to better understand future changes. System primary production in these systems is highest during the pronounced spring bloom, typically dominated by diatoms. Eventually the spring blooms terminate due to silicon or nitrogen limitation. Bacteria can play an important role for extending bloom duration and total CO2 fixation through ammonium regeneration. Current ecosystem models often simplify the effects of nutrient co-limitations on algal physiology and cellular ratios and simplify nutrient regeneration. These simplifications may lead to underestimations of primary production. Detailed biochemistry- and cell-based models can represent these dynamics but are difficult to tune in the environment. We performed a cultivation experiment that showed typical spring bloom dynamics, such as extended algal growth via bacterial ammonium remineralization, reduced algal growth and inhibited chlorophyll synthesis under silicate limitation, and gradually reduced nitrogen assimilation and chlorophyll synthesis under nitrogen limitation. We developed a simplified dynamic model to represent these processes. Overall, model complexity in terms of the number of parameters is comparable to the phytoplankton growth and nutrient biogeochemistry formulations in common ecosystem models used in the Arctic while improving the representation of nutrient-co-limitation-related processes. Such model enhancements that now incorporate increased nutrient inputs and higher mineralization rates in a warmer climate will improve future predictions in this vulnerable system.

Natural ventilation is a basic quantity to reach comfort in passively acting buildings. It delivers not only fresh air to breath but can also be used to temper the room if the indoor temperature is above the outdoor one. Driving forces are temperature differences (buoyancy) and wind. However, both may be weak in hot and especially humid locations. A solar chimney uses solar radiation to heat up the exhausted air and to increase buoyancy, thus could help to improve that situation at least during the daytime. Nevertheless, the implementation of solar chimneys is quite rare. It may be that the idea to use heat to cool and ventilate a building seems strange. The literature reports about the potential of solar chimneys, characteristics like volume flow and temperatures are measured or simulated. Though, the findings of these publications are based on a special geometry and provide not enough detailed information about the optimized shape (width, height, length, etc.) of solar chimneys. To overcome that situation, this paper presents general design rules for the geometry of solar chimney systems that could be adapted to existing or newly erected buildings. A solar chimney system is assumed as composed by three components: An absorber as the main part to reach a higher temperature, a stack extension on top for further acceleration of the exhausted air, and a stack at the bottom connected to the (lower) storeys that should be ventilated. A full set of equations of fluid mechanics and thermodynamics is presented and describes the physical behaviour of the air in the system. These equations are coupled with each other and can be solved in iterations, also with a simple spreadsheet. As a compromise between accuracy in results and complexity of the simulation method, the physical model is based on some simplifying assumptions like turbulent flow, friction on walls, air as an incompressible medium, and immediate heat transfer from the absorber to the air. Nevertheless, the outco