Abstract
ABSTRACT Global warming is causing changes in the food web structure and seasonal plankton dynamics. The Baltic Sea is one of the fastest-warming sea areas and warming consequently affects the timing and magnitude of phytoplankton blooms. Based on available Chlorophyll a data from nine years between 1979 and 2018, from the entrance to the Gulf of Finland, we studied the timing of the phytoplankton spring bloom in relation to spring seawater temperature. We found the peak of the bloom to occur earlier in years with higher spring seawater temperature. In warmer years, there was also a shorter time lag between phytoplankton and zooplankton biomass peaks. In addition, it seems as the spring bloom total biomass has decreased with time, with more extensive summer blooms during later years, in line with the general trend observed in the Baltic Sea. The spring bloom has traditionally been considered the most important part of the season, but we argue that the whole growth season should be investigated since summer blooms appear to increase with warming.
Highlights
In temperate areas, the onset of the phytoplankton spring bloom is determined by irradiance (Sommer and Lengfellner 2008) and is affected by water column stability and in polar and high-latitude areas to variable extent by sea ice (Sommer et al 2012)
Based on available Chlorophyll a data from nine years between 1979 and 2018, from the entrance to the Gulf of Finland, we studied the timing of the phytoplankton spring bloom in relation to spring seawater temperature
It seems as the spring bloom total biomass has decreased with time, with more extensive summer blooms during later years, in line with the general trend observed in the Baltic Sea
Summary
The onset of the phytoplankton spring bloom is determined by irradiance (Sommer and Lengfellner 2008) and is affected by water column stability and in polar and high-latitude areas to variable extent by sea ice (Sommer et al 2012). Ocean temperature is increasing globally, and the Baltic Sea is one of the fastest-warming sea areas (Belkin 2009) with a projected change of 2–3°C before the end of the century (Meier 2015). Warming is causing changes to the pelagic food web both directly by regulating light-saturated growth rates of primary producers and indirectly through stratification of the water column, affecting nutrient availability (Winder and Sommer 2012; Spilling et al 2018). 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). 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)
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