Abstract
Abstract. Dense blooms of filamentous diazotrophic cyanobacteria are formed every summer in the Baltic Sea. These autotrophic organisms may bypass nitrogen limitation by performing nitrogen fixation, which also governs surrounding organisms by increasing bioavailable nitrogen. The magnitude of the nitrogen fixation is important to estimate from a management perspective since this might counteract eutrophication reduction measures. Here, a cyanobacteria life cycle model has been implemented for the first time in a high-resolution 3D coupled physical and biogeochemical model of the Baltic Sea, spanning the years 1850–2008. The explicit consideration of life cycle dynamics and transitions significantly improves the representation of the cyanobacterial phenological patterns compared to earlier 3D modeling efforts. Now, the rapid increase and decrease in cyanobacteria in the Baltic Sea are well captured, and the seasonal timing is in concert with observations. The current improvement also had a large effect on the nitrogen fixation load and is now in agreement with estimates based on in situ measurements. By performing four phosphorus sensitivity runs, we demonstrate the importance of both organic and inorganic phosphorus availability for historical cyanobacterial biomass estimates. The model combination can be used to continuously predict internal nitrogen loads via nitrogen fixation in Baltic Sea ecosystem management, which is of extra importance in a future ocean with changed conditions for the filamentous cyanobacteria.
Highlights
Bioavailable nitrogen is globally limiting primary production in the ocean (Moore et al, 2013)
In Hense and Burchard (2010) and Hense et al (2013), the Cyanobacteria Life Cycle (CLC) model is implemented in a one-dimensional water column model representing the Eastern Gotland Basin, and the results show a clear improvement in the timing and duration of blooms compared to conventional one compartment models
Through a series of sensitivity experiments, we have shown that the inclusion of phosphorus dependence in cyanobacteria is essential for the CLC model in the Baltic Proper, but only a weak limitation is necessary
Summary
Bioavailable nitrogen is globally limiting primary production in the ocean (Moore et al, 2013). Diazotrophic cyanobacteria can bypass this limitation by performing nitrogen fixation. They may release up to 50 % of their newly fixed nitrogen, which stimulates surrounding organisms (Wannicke et al, 2009; Ploug et al, 2010, 2011). As nitrogen-fixing cyanobacteria are suggested to be enhanced by elevated temperatures (Paerl and Huisman, 2008; Wannicke et al, 2018), there is an increasing need to further understand their bloom dynamics and ecosystem impact. The Baltic Sea is a semi-enclosed brackish water body exposed to significant impacts from eutrophication because of the combination of a large increase in nutrient supplies since World War II (Gustafsson et al, 2012), permanent stratification (e.g., Leppäranta and Myrberg, 2009), and long water residence times (Meier, 2007), which reduce the deep water ventilation and enhance the widespread oxygen deficiency. The early history of multistressors and long-term data series in the Baltic Sea provides an opportunity to study consequences and possible mitigation strategies for future management of aquatic systems (Reusch et al, 2018)
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