Abstract
Abstract. The co-variation of key variables with simulated phytoplankton biomass in the Baltic proper has been examined using wavelet analysis and results of a long-term simulation for 1850–2008 with a high-resolution coupled physical–biogeochemical circulation model for the Baltic Sea. By focusing on inter-annual variations, it is possible to track effects acting on decadal timescales such as temperature increase due to climate change as well as changes in nutrient input. The strongest inter-annual coherence indicates that variations in phytoplankton biomass are determined by changes in concentrations of the limiting nutrient. However, after 1950 high nutrient concentrations created a less-nutrient-limited regime, and the coherence was reduced. Furthermore, the inter-annual coherence of mixed-layer nitrate with riverine input of nitrate is much larger than the coherence between mixed-layer phosphate and phosphate loads. This indicates a greater relative importance of the vertical flux of phosphate from the deep layer into the mixed layer. In addition, shifts in nutrient patterns give rise to changes in phytoplankton nutrient limitation. The modelled pattern shifts from purely phosphate limited to a seasonally varying regime. The results further indicate some effect of inter-annual temperature increase on cyanobacteria and flagellates. Changes in mixed-layer depth affect mainly diatoms due to their high sinking velocity, while inter-annual coherence between irradiance and phytoplankton biomass is not found.
Highlights
The Baltic Sea is a semi-enclosed brackish water body separated from the North Sea and Kattegat through the Danish straits
The Baltic Sea is naturally prone to eutrophication, and organic matter degradation leads to low deepwater oxygen concentrations in between deep-water renewal events
With a focus on simulated inter-annual variations, the wavelet coherence of the mixed-layer phytoplankton biomass with key variables affecting the primary production has been examined for the Baltic proper
Summary
The Baltic Sea is a semi-enclosed brackish water body separated from the North Sea and Kattegat through the Danish straits. It stretches from about 54 to 66◦ N, and the limited water exchange with the ocean in the south gives rise to a large meridional salinity gradient. The Baltic Sea is naturally prone to eutrophication, and organic matter degradation leads to low deepwater oxygen concentrations in between deep-water renewal events. This leads to complex nutrient cycling with different processes acting in oxygenized vs low-oxygen environments
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
