The Baltic Sea spring phytoplankton bloom in a changing climate: an experimental approach

Abstract

The response of the Baltic Sea spring bloom was studied in mesocosm experiments, where temperatures were elevated up to 6°C above the present-day sea surface temperature of the spring bloom season. Four of the seven experiments were carried out at different light levels (32-202 Wh m -2 at the start of the experiments) in the different experimental years. In one further experiment, the factors light and temperature were crossed, and in one experiment, the factors density of overwintering zooplankton and temperature were crossed. Overall, there was a slight temporal acceleration of the phytoplankton spring bloom, a decline of peak biomass and a decline of mean cell size with warming. The temperature influence on phytoplankton bloom timing, biomass and size structure was qualitatively highly robust across experiments. The dependence of timing, biomass, and size structure on initial conditions was tested by multiple regression analysis of the y-temperature regressions with the candidate independent variables initial light, initial phytoplankton biomass, initial microzooplankton biomass, and initial mesozooplankton (=copepod) biomass. The bloom timing predicted for mean temperatures (5.28°C) depended on light. The peak biomass showed a strong positive dependence on light and a weaker negative dependence on initial copepod density. Mean phytoplankton cell size predicted for the mean temperature responded positively to light and negatively to copepod density. The anticipated mismatch between phytoplankton supply and food demand by newly hatched copepod nauplii occurred only under the combination of low light and warm temperatures. The analysis presented here confirms earlier conclusions about temperature responses that are based on subsets of our experimental series. However, only the comprehensive analysis across all experiments highlights the importance of the factor light.

The response of temperate aquatic ecosystems to global warming: novel insights from a multidisciplinary project

“…This prediction was supported by the mesocosm experiments by Müren et al (2005) and O'Connor et al (2009). Within AQUASHIFT, the Baltic Sea mesocosm experiments (Klauschies et al 2012;Sommer et al 2012;Winder et al 2012), the microcosm experiments of Burgmer and Hillebrand (2011), and modeling Lake Constance phytoplankton (Tirok and Gaedke 2007b) also showed a declining phytoplankton biomass, while the lake mesocosms did not show a decline of phytoplankton biomass under warmer conditions (Sebastian et al 2012;Winder et al 2012). However, both freshwater and marine mesocosm experiments agreed in demonstrating a dominant role of light for the determination of bloom biomass (Klauschies et al 2012).…”

“…Shifts toward smaller body size may be brought about by species replacements, increase in the share of juveniles, and size changes at defined developmental stages, for example, maturity. The AQUASHIFT projects found examples for shifts between differently sized species in phytoplankton (Klauschies et al 2012;Sommer et al 2012) and for a reduced size at maturity in zooplankton (Isla et al 2008;Sebastian et al 2012). Changes in age structure, however, were highly variable and depended on sampling time, because experimental populations were not in demographic equilibrium (Sebastian et al 2012).…”

Spring phenological responses of marine and freshwater plankton to changing temperature and light conditions

“…The natural seasonal temperature increase was simulated during the experiments based on the decadal (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) mean sea surface temperature in Kiel Fjord and a 6°C warming scenario (Table 1), the latter corresponding to the most drastic prediction of temperature increase by the end of this century (IPCC 2007). Light intensities were set to daily light curves and seasonal light patterns calculated from astronomic equations (Brock 1981) and February 4 (2005 Sommer et al (2012) suggests that variation in nutrient levels over these ranges does not affect phytoplankton peak magnitude.…”

Climate Change: Warming Impacts on Marine Biodiversity

“…Differential phenological shifts of prey and predator in response to temperature change might induce temporal mismatches in the occurrence (Fig. 18.3), leading to altered marine trophodynamics (Philippart et al 2003;Edwards and Richardson 2004;Durant et al 2005;Burthe et al 2012;Sommer et al 2012) and consequent declines in commercially important fish stocks such as cod (Beaugrand et al 2003). …”