Changing seasonality of the Baltic Sea

Abstract

Abstract. Changes in the phenology of physical and ecological variables associated with climate change are likely to have significant effect on many aspects of the Baltic ecosystem. We apply a set of phenological indicators to multiple environmental variables measured by satellite sensors for 17-36 years to detect possible changes in the seasonality in the Baltic Sea environment. We detect significant temporal changes, such as earlier start of the summer season and prolongation of the productive season, in several variables ranging from basic physical drivers to ecological status indicators. While increasing trends in the absolute values of variables like sea-surface temperature (SST), diffuse attenuation of light (Ked490) and satellite-detected chlorophyll concentration (CHL) are detectable, the corresponding changes in their seasonal cycles are more dramatic. For example, the cumulative sum of 30 000 W m −2 of surface incoming shortwave irradiance (SIS) was reached 23 days earlier in 2014 compared to the beginning of the time series in 1983. The period of the year with SST of at least 17 • C has almost doubled (from 29 days in 1982 to 56 days in 2014), and the period with Ked490 over 0.4 m −1 has increased from about 60 days in 1998 to 240 days in 2013 -i.e., quadrupled. The period with satellite-estimated CHL of at least 3 mg m −3 has doubled from approximately 110 days in 1998 to 220 days in 2013. While the timing of both the phytoplankton spring and summer blooms have advanced, the annual CHL maximum that in the 1980s corresponded to the spring diatom bloom in May has now shifted to the summer cyanobacteria bloom in July.

Large-Scale Nutrient Dynamics in the Baltic Sea, 1970–2016

“…After the two assessments were released, changes in the duration of vegetation season and the intensity of phytoplankton development related to warming have been detected in satellite measurements (Kahru et al, 2016). Similar changes in seasonal dynamics of the surface water temperature (SST) are reconstructed from the field observations and presented in this study for the first time.…”

“…For the Bothnian Sea similar time series were obtained from three-dimensional fields reconstructed with DAS as decadal averages for consecutive half-months. Such a basin-wide approach also made it possible to demonstrate changing seasonality in the field measurements (Figure 4), in addition to that revealed by remote sensing and numerical simulation (Kahru et al, 2016). The general warming of the Baltic Sea surface waters (Figures 4A,D,G), from the Kattegat to the Bothnian Bay (not shown), could also imply an earlier start to the productive season.…”

“…Figure 4). In the surface layer, T JA have been increasing since 1979 on average by 0.08 • C yr −1 (R = 0.65) that may cause a prolongation of the blooming period (Kahru et al, 2016), while inter-annual variations may play a role in fluctuations of TCA (cf. Figure 9B).…”

Environmental determinants of larval herring (Clupea harengus) abundance and distribution in the western Baltic Sea

“…Both global and regional climate models generally agree that the Baltic region is likely to warm up by about 2–4°C by the end of the 21st century (Elmgren et al ). More importantly, changes in the phenology of a variety of physical and ecological variables measured by satellite sensors for the past 17–36 yr have been quantified (Kahru et al ). Of particular importance in the present context, the cumulative sum of a phenological indicator of incoming shortwave irradiance, a key variable affecting sea surface temperature, was reached 23 d earlier in 2014 compared to the beginning of the time series in 1983.…”

“…This is reflected in springtime warming that has become significantly earlier. In combination with later cooling during the autumn, the period with sea surface temperature above 16°C has increased at a mean rate of 0.98 d yr −1 (Kahru et al ).…”