The Baltic Sea is especially vulnerable to both climatically and anthropogenically driven changes. Climate change and eutrophication are predicted to have a significant effect on the Baltic Sea ecosystem and certain changes are already obvious.

The so-called spring bloom of phytoplankton is the most productive season and about half of the annual aquatic oxygen production occurs in only one month. The most abundant plankton species at the onset of the bloom dominates at least parts of this event. The dominance patterns throughout the bloom determine its “outcome” in terms of primary and heterotrophic production as well as the nutrient pools available for the summer community including, for example, cyanobacteria (blue-green algae).

“This thesis was motivated by the need to understand the consequences of the recently observed shift in the phytoplankton community composition during the Baltic Sea spring bloom. In the last decades, the contribution of dinoflagellates to the total biomass increased at the expense of diatoms in several sub-basins, which was expected to have ecosystem-wide consequences. The main objectives were to investigate the effects of these changes at the base of the food web during the Baltic Sea spring bloom”, says researcher Tobias Lipsewers.

Assessing a changing environment requires a constant advancement of monitoring protocols

The change from one species to another in the course of the spring bloom has been noted before but the detailed study was lacking until the present. Studies are based on four subsequent research cruises during the years 2013–2016 on Research Vessel Aranda (SYKE) covering the main phases of the bloom and sub-basins of the Baltic Sea (127 stations) and two experiments at Tvärminne Zoological Station (University of Helsinki).

Four species/groups of microplankton dominated the biomass considering all cruises: the phototrophic dinoflagellates Peridiniella catenata and the so-called DinoComplex (2-3 species) as well as ciliates (heterotrophic and M. rubrum).

The community composition of nano- and microplankton was clearly driven by the dynamics of temperature and inorganic nutrient levels. After the peak the proportion of diatoms declined and heterotrophic ciliates thrived, whereas the biomass of M. rubrum and the relevant dinoflagellates remained rather constant throughout the spring bloom.

The resulting decrease in the diatom-dinoflagellate-proportion after the peak might have far-reaching consequences for the ecosystem. Furthermore, the cruise data have shown that the plankton community composition explains 19% (average) in seston C:N:P ratios in spring, showing similar trends and differences along with the bloom.

“In the future, it will be important to follow these changes at the base of the food web to study the consequences for the aquatic environment and the atmosphere in the long-term. The marine monitoring programs around the Baltic Sea do a great job assessing the development of many relevant plankton species and groups. On the other hand, monitoring a changing environment requires changes to existing protocols sometimes”, says Tobias Lipsewers.

Further information

Tobias Lipsewers, M.Sc. (Tech, University of Rostock, Germany)

Researcher, Finnish Environment Institute SYKE.
tel. +358 295 251 787,

Tobias Lipsewers will defend his dissertation at Helsinki University in the Faculty of Biological and Environmental Sciences on Thursday, 20 August 2020 at 10:00 a.m.. The defence of the dissertation will be in English.

Address: Helsinki University, Metsätalo, room 6, Unioninkatu 40, City center campus, Helsinki.

Due to the current situation, 38 people from the public can join in person. Event will be streamed live
Meeting ID: 66 059 985 869
Password: 114976

Dissertation: Spring bloom dynamics in the Baltic Sea: From the environment to macroelements and microbial interactions

Link to the original press release (Finnish Environment Institute SYKE).