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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 16 | Copyright
Biogeosciences, 15, 5113-5129, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Aug 2018

Research article | 27 Aug 2018

Causes of simulated long-term changes in phytoplankton biomass in the Baltic proper: a wavelet analysis

Jenny Hieronymus1, Kari Eilola1, Magnus Hieronymus1, H. E. Markus Meier2,1, Sofia Saraiva3, and Bengt Karlson1 Jenny Hieronymus et al.
  • 1Research and Development Department, Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
  • 2Department of Physical Oceanography and Instrumentation, Leibniz Institute for Baltic Sea Research Warnemünde, Rostock, Germany
  • 3University of Lisbon, Instituto Superior Técnico, Environment and Energy Section, Lisbon, Portugal

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.

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This paper investigates how phytoplankton concentrations in the Baltic Sea co-vary with nutrient concentrations and other key variables on inter-annual timescales in a model integration over the years 1850–2008. The study area is not only affected by climate change; it has also been subjected to greatly increased nutrient loads due to extensive use of agricultural fertilizers. The results indicate the largest inter-annual coherence of phytoplankton with the limiting nutrient.
This paper investigates how phytoplankton concentrations in the Baltic Sea co-vary with nutrient...