Enhanced carbon overconsumption in response to increasing temperatures during a mesocosm experiment

T Dittmar,U Riebesell,K G Schulz,J Taucher,U Sommer,A Oschlies

doi:10.5194/bg-9-3531-2012

Abstract

Abstract. Increasing concentrations of atmospheric carbon dioxide are projected to lead to an increase in sea surface temperatures, potentially impacting marine ecosystems and biogeochemical cycling. Here we conducted an indoor mesocosm experiment with a natural plankton community taken from the Baltic Sea in summer. We induced a plankton bloom via nutrient addition and followed the dynamics of the different carbon and nitrogen pools for a period of one month at temperatures ranging from 9.5 °C to 17.5 °C, representing a range of ±4 °C relative to ambient temperature. The uptake of dissolved inorganic carbon (DIC) and the net build-up of both particulate (POC) and dissolved organic carbon (DOC) were all enhanced at higher temperatures and almost doubled over a temperature gradient of 8 °C. Furthermore, elemental ratios of carbon and nitrogen (C : N) in both particulate and dissolved organic matter increased in response to higher temperatures, both reaching very high C : N ratios of > 30 at +4 °C. Altogether, these observations suggest a pronounced increase in excess carbon fixation in response to elevated temperatures. Most of these findings are contrary to results from similar experiments conducted with plankton populations sampled in spring, revealing large uncertainties in our knowledge of temperature sensitivities of key processes in marine carbon cycling. Since a major difference to previous mesocosm experiments was the dominant phytoplankton species, we hypothesize that species composition might play an important role in the response of biogeochemical cycling to increasing temperatures.

Highlights

Climate change is expected to affect marine ecosystems and biogeochemical cycling in the oceans in a variety of ways (Riebesell et al, 2009; IPCC, 2007a)
The development of the phytoplankton bloom was characterized by a rapid decline in dissolved inorganic nutrients (Fig. 1), the drawdown of dissolved inorganic carbon and the build-up of particulate organic matter (Fig. 3)
Our study revealed a strong effect of temperature on the dynamics of particulate organic matter, which was not observed in previous experiments

Summary

Introduction

Climate change is expected to affect marine ecosystems and biogeochemical cycling in the oceans in a variety of ways (Riebesell et al, 2009; IPCC, 2007a). Recent climate projections suggest an increase in global surface air temperatures by about 1.1 to 6.4 ◦C by the end of this century (relative to 1980–1999), thereby leading to a further warming of the upper ocean (IPCC, 2007b). This will affect marine ecosystems indirectly as thermal stratification of the water column becomes stronger, leading to changes in the availability of nutrients and light. It is likely that sea surface warming will have pronounced direct effects on pelagic ecosystems and marine carbon cycling, as temperature is a major environmental factor controlling the rates of biological processes (Brown et al, 2004). Experimental evidence suggests a clear relationship between temperature and phytoplankton growth (Eppley, 1972)

Methods

Results

Discussion

Conclusion