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.
Life cycle and spring phenology of Temora longicornis in the Baltic Sea