Climate Driven Changes in Timing, Composition and Magnitude of the Baltic Sea Phytoplankton Spring Bloom

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 ciliate Mesodinium 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 and M. rubrum at 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.

Reconstructing marine plankton food web interactions using DNA metabarcoding

“…Given high phytoplankton biovolume but low copepod abundance, competition for the resource seems relaxed. The dominance of dinoflagellates during the spring bloom found in this study (Figure 1) is becoming frequent in the Baltic Sea, with a consequent decrease in diatom abundance (Hjerne, Hajdu, Larsson, Downing, & Winder, 2019; Klais, Tamminen, Kremp, Spilling, & Olli, 2011), and this will have consequences in the food web structure (Kremp, Tamminen, & Spilling, 2008).…”

Decoupling salinity and carbonate chemistry: Low calcium ion concentration rather than salinity limits calcification in Baltic Sea mussels

“…Environmental variability is high on both spatial and temporal scales in the Baltic Sea with variations in carbonate chemistry and salinity resulting from occasional inflow events of highly saline North Sea water, wind driven upwelling of hypoxic and hypercapnic deep water, lateral transport of water masses and pronounced regional salinity gradients (Thomas and Schneider, 1999;Melzner, et al, 2013;Saderne, et al, 2013;Mohrholz, et al, 2015). Eutrophication promotes phytoplankton blooms, which vary in intensity between different Baltic basins providing vital sources of energy for secondary consumers (HELCOM, 2014, Gustafsson, et a., 2019Hjerne, et al, 2019). High phytoplankton food availability correlates with improved reproductive condition in mussels and has been shown to counteract the negative effects of sub-optimal carbonate chemistry on calcification in Baltic Mytilus in Kiel Fjord (Wołowicz, et al, 2006;Melzner, et al, 2011;Thomsen, et al, 2013).…”

Temperature-related timing of the spring bloom and match between phytoplankton and zooplankton

“…Chl a levels in spring increased in the western Gulf of Finland between 1970s and the 1980s but decreased in the 1990s (Raateoja et al 2005). Similarly, high spring bloom average biomass was observed between 1977 and 1989, while spring biomass was lower than the summer biomass after 1989, outside Askö, on the Swedish East coast (Hjerne et al 2019). Surprisingly, Raateoja et al (2018) did not find a relationship between the intensity of the spring bloom and the wintertime DIN pool or any significant temporal trends related to nutrients in the western Gulf of Finland.…”

“…The earlier start of the spring bloom was related to increased sunshine duration and the prolongation in autumn due to warmer seawater temperature. In the western Baltic Sea, off the Swedish coast, the advancement of the spring bloom was also likely caused by more sunshine and less windy conditions (Hjerne et al 2019), rather than sea temperature (Hjerne et al 2019). However, in the Gulf of Finland, the temperature is strongly connected to the prevailing climate forcing (Almén et al 2017) which may increase the inter-annual variability and make temporal trends in plankton dynamics difficult to detect.…”

“…The number of days with sea surface temperature above 17°C almost doubled between 1983 and 2014 and between 1998 and 2013 both light attenuation and near-surface Chlorophyll a (Chl a) increased in the central Baltic Sea (Kahru et al 2016). Earlier, more prolonged spring blooms with lower average biomass during the past 20 years have been observed (Raateoja et al 2005;Groetsch et al 2016;Kahru et al 2016;Hjerne et al 2019;Wasmund et al 2019).…”