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

• Published in: Biogeosciences Vol. 15; no. 16; pp. 5113 - 5129
• Main Authors: Hieronymus, Jenny, Eilola, Kari, Hieronymus, Magnus, Meier, H. E. Markus, Saraiva, Sofia, Karlson, Bengt
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 27-08-2018; Copernicus Publications
• Subjects: Annual temperatures; Annual variations; Biogeochemical cycles; Biogeochemistry; Biomass; Climate change; Coherence; Computer simulation; Cyanobacteria; Deep layer; Diatoms; Ecological research; Environmental aspects; Flagellates; Irradiance; Long-term changes; Marine phytoplankton; Mineral nutrients; Mixed layer; Mixed layer depth; Nutrient concentrations; Phosphates; Phytoplankton; Plankton; Temperature; Temperature effects; Temperature rise; Vertical flux; Wavelet analysis; Baltic Sea
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-15-5113-2018

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.