Shift towards larger diatoms in a natural phytoplankton assemblage under combined high-CO2 and warming conditions.

Scarlett Sett,Kai G Schulz,Ulf Riebesell,Lennart T Bach

doi:10.1093/plankt/fby018

Abstract

An indoor mesocosm experiment was carried out to investigate the combined effects of ocean acidification and warming on the species composition and biogeochemical element cycling during a winter/spring bloom with a natural phytoplankton assemblage from the Kiel fjord, Germany. The experimental setup consisted of a “Control” (ambient temperature of ~4.8 °C and ~535 ± 25 μatm pCO2), a “High-CO2” treatment (ambient temperature and initially 1020 ± 45 μatm pCO2) and a “Greenhouse” treatment (~8.5 °C and initially 990 ± 60 μatm pCO2). Nutrient replete conditions prevailed at the beginning of the experiment and light was provided at in situ levels upon reaching pCO2 target levels. A diatom-dominated bloom developed in all treatments with Skeletonema costatum as the dominant species but with an increased abundance and biomass contribution of larger diatom species in the Greenhouse treatment. Conditions in the Greenhouse treatment accelerated bloom development with faster utilization of inorganic nutrients and an earlier peak in phytoplankton biomass compared to the Control and High CO2 but no difference in maximum concentration of particulate organic matter (POM) between treatments. Loss of POM in the Greenhouse treatment, however, was twice as high as in the Control and High CO2 treatment at the end of the experiment, most likely due to an increased proportion of larger diatom species in that treatment. We hypothesize that the combination of warming and acidification can induce shifts in diatom species composition with potential feedbacks on biogeochemical element cycling.

Highlights

Climate change, driven primarily by anthropogenic utilization of fossil fuels over the past 250 years, is changing sea surface temperatures and seawater carbonate chemistry
Optimum temperatures for maximum metabolic rates (Boyd et al, 2013), one of the direct effects of warming is the enhancement of metabolic activities (Eppley, 1972) which is considered to be more pronounced in heterotrophs than autotrophs (Pomeroy and Wiebe, 2001)
Numerous laboratory and mesocosm-based experiments have been carried out to examine the effects of warming on natural plankton communities and some of the most common and recurrent patterns observed were increased primary production (PP) (Lewandowska et al, 2012), shifts in species composition (Sommer et al, 2007; Lassen et al, 2010), shifts in the partitioning of organic matter into dissolved and particulate pools (Wohlers et al, 2009) and decreased biomass build-up (Keller et al, 1999; Sommer and Lengfellner, 2008a; O’Connor et al, 2009; Lassen et al, 2010; Wohlers-Zöllner et al, 2012; Biermann et al, 2014) with one exception (Taucher et al, 2012)

Summary

Introduction

Driven primarily by anthropogenic utilization of fossil fuels over the past 250 years, is changing sea surface temperatures (i.e. ocean warming) and seawater carbonate chemistry (i.e. ocean acidification). The IPCC 2014 projects an additional increase in sea surface temperature of 1–6 °C and a decrease in surface pH of ~0.3 units by the end of this century (Field et al, 2014). Both of these environmental drivers are expected to affect marine primary producers (Riebesell and Tortell, 2011; Boyd and Brown, 2015) with potential consequences for marine biogeochemical element cycling in the future ocean (Riebesell et al, 2009; Gehlen et al, 2011; Rees, 2012). Numerous laboratory and mesocosm-based experiments have been carried out to examine the effects of warming on natural plankton communities and some of the most common and recurrent patterns observed were increased primary production (PP) (Lewandowska et al, 2012), shifts in species composition (Sommer et al, 2007; Lassen et al, 2010), shifts in the partitioning of organic matter into dissolved and particulate pools (Wohlers et al, 2009) and decreased biomass build-up (Keller et al, 1999; Sommer and Lengfellner, 2008a; O’Connor et al, 2009; Lassen et al, 2010; Wohlers-Zöllner et al, 2012; Biermann et al, 2014) with one exception (Taucher et al, 2012). Taucher et al (2012) suggested that the enhanced biomass build-up with warming during their experiments, contradictory to previous results, was most likely due to differences in the phytoplankton community composition

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Discussion

Conclusion