Extension of the growing season of phytoplankton in the western Baltic Sea in response to climate change

Abstract

Phenology of phytoplankton was investigated at a coastal station in the western Baltic Sea from 1988 to 2017 by means of microscopically determined biomass and chlorophyll a (chl a) data. The prolongation of the growing season in this marine area is much stronger than that known from terrestrial areas. The growing season, defined by biomass or chl a thresholds, increased by 125 or 129 d, respectively, and extends recently from February to December. The spring bloom started earlier at a rate of 1.4 d yr −1 and the end of the autumn bloom was delayed by 3.1 d yr −1. The duration of the growing season increased at a rate of 4.5 d yr −1. The earlier start of the growing season was correlated with a slight increase in sunshine duration during spring, whereas the later end of the growing season was correlated with a strong increase in water temperature in autumn. The period with sea surface temperature >10°C shifted towards the end of November. Correlations of the duration of the growing season with the phosphate and nitrate concentrations were probably not causative. The shifts in the spring and autumn blooms led to a prolongation of the summer biomass minimum. The earlier spring bloom was caused, among other factors, by a shift of the biomass maximum of the dominant diatom Skeletonema marinoi from May to February/March. The delay in the autumn bloom was induced by a retardation of dominant dinoflagellates and diatoms, such as Ceratium spp. and Dactyliosolen fragilissimus.

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

“…In the southwestern Baltic Sea, the spring bloom has advanced by 1.4 d yr −1 and the growing season increased by 127 days between 1988 and 2017 (Wasmund et al 2019). The earlier start of the spring bloom was related to increased sunshine duration and the prolongation in autumn due to warmer seawater temperature.…”

“…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).…”

Natural Hazards and Extreme Events in the Baltic Sea region

“…A phenomenon worth mentioning is the extension of the growing season of phytoplankton in the oceans (Gobler et al, 2017), but also in the Baltic Sea (Groetsch et al, 2016). The period with satelliteestimated chlorophyll a (chl a) concentrations of at least 3 mg m -3 has doubled from approximately 110 days in 1998 to 220 days in 2013 the central Baltic Sea (Kahru et al, 2016 Wasmund et al (2019) found an earlier start of the spring bloom with a rate of 1.4 days/year and a later end of the autumn bloom with 3.1 days/year and a corresponding extension of the growing season (Figure 10). The earlier start of the growing season was correlated with a slight increase in sunshine duration during spring whereas the later end of the growing season was correlated with a strong increase in water temperature in autumn.…”

Baltic Earth Assessment Report on the biogeochemistry of the Baltic Sea

“…The productive period is prolonged with increasing temperatures, already signified by the occurrence of earlier spring and later autumn phytoplankton blooms (Wasmund et al, 2019). This implies that more inorganic nutrients are intercepted, thereby enhancing the coastal filter efficiency.…”