Abstract
Spring phytoplankton blooms contribute a substantial part to annual production, support pelagic and benthic secondary production and influence biogeochemical cycles in many temperate aquatic systems. Understanding environmental effects on spring bloom dynamics is important for predicting future climate responses and for managing aquatic systems. We analyzed long-term phytoplankton data from one coastal and one offshore station in the Baltic Sea to uncover trends in timing, composition and size of the spring bloom and its correlations to environmental variables. There was a general trend of earlier phytoplankton blooms by 1–2 weeks over the last 20 years, associated with more sunshine and less windy conditions. High water temperatures were associated with earlier blooms of diatoms and dinoflagellates that dominate the spring bloom, and decreased diatom bloom magnitude. Overall bloom timing, however, was buffered by a temperature and ice related shift in composition from early blooming diatoms to later blooming dinoflagellates and the autotrophic ciliateMesodinium rubrum.Such counteracting responses to climate change highlight the importance of both general and taxon-specific investigations. We hypothesize that the predicted earlier blooms of diatoms and dinoflagellates as a response to the expected temperature increase in the Baltic Sea might also be counteracted by more clouds and stronger winds. A shift from early blooming and fast sedimenting diatoms to later blooming groups of dinoflagellates andM. rubrumat higher temperatures during the spring period is expected to increase energy transfers to pelagic secondary production and decrease spring bloom inputs to the benthic system, resulting in lower benthic production and reduced oxygen consumption.
Highlights
The spring phytoplankton bloom accounts for a substantial part of the annual production in many temperate marine and freshwater systems, supports pelagic and benthic secondary production and influences biogeochemical cycles (Fulweiler and Nixon, 2009; Nixon et al, 2009; Behrenfeld and Boss, 2014; Griffiths et al, 2017)
At the coastal station B1, the average of the spring bloom was about twice as large than the summer biomass during 1977–1989 (Figure 2A) and slightly higher than the summer bloom during 1990– 2011 (Figure 2B), which corresponds to the sampling period of BY31. Offshore (BY31)
The spring bloom at both stations generally started with a diatom bloom, followed by dinoflagellates
Summary
The spring phytoplankton bloom accounts for a substantial part of the annual production in many temperate marine and freshwater systems, supports pelagic and benthic secondary production and influences biogeochemical cycles (Fulweiler and Nixon, 2009; Nixon et al, 2009; Behrenfeld and Boss, 2014; Griffiths et al, 2017). Climate Effects on Baltic Sea Spring Blooms (Winder and Sommer, 2012). In shallow lakes and coastal areas, light is the most important factor influencing the timing of the spring bloom (Townsend et al, 1994; Sommer and Lengfellner, 2008). Temperature can indirectly influence the spring bloom composition toward smaller individuals and a lower proportion of diatoms by increased selective grazing pressure at higher temperatures (Sommer and Lengfellner, 2008; Winder et al, 2012, but see Peter and Sommer, 2012). A meta-analysis (Winder et al, 2012) of mesocosm experiments showed that high light levels and low water temperatures can result in larger spring bloom magnitude. The results are discussed in relation to the existing mechanistic understanding of spring bloom dynamics and future climate change
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