Effect of ocean acidification on the structure and fatty acid composition of a natural plankton community in the Baltic Sea

Rafael Bermúdez,Anna-Karin Almén,Annegret Stuhr,Ulf Riebesell,Jonna Engström-Öst,Monika Winder

doi:10.5194/bg-13-6625-2016

Abstract

Abstract. Increasing atmospheric carbon dioxide (CO2) is changing seawater chemistry towards reduced pH, which affects various properties of marine organisms. Coastal and brackish water communities are expected to be less affected by ocean acidification (OA) as these communities are typically adapted to high fluctuations in CO2 and pH. Here we investigate the response of a coastal brackish water plankton community to increasing CO2 levels as projected for the coming decades and the end of this century in terms of community and biochemical fatty acid (FA) composition. A Baltic Sea plankton community was enclosed in a set of offshore mesocosms and subjected to a CO2 gradient ranging from natural concentrations ( ∼ 347 µatm fCO2) up to values projected for the year 2100 ( ∼ 1333 µatm fCO2). We show that the phytoplankton community composition was resilient to CO2 and did not diverge between the treatments. Seston FA composition was influenced by community composition, which in turn was driven by silicate and phosphate limitation in the mesocosms and showed no difference between the CO2 treatments. These results suggest that CO2 effects are dampened in coastal communities that already experience high natural fluctuations in pCO2. Although this coastal plankton community was tolerant of high pCO2 levels, hypoxia and CO2 uptake by the sea can aggravate acidification and may lead to pH changes outside the currently experienced range for coastal organisms.

Highlights

The steady increase of carbon dioxide (CO2) due to anthropogenic emission since the beginning of the industrial era has increased the atmospheric concentration (Boyd et al, 2014)
A Baltic Sea plankton community was enclosed in a set of offshore mesocosms and subjected to a CO2 gradient ranging from natural concentrations (∼ 347 μatm f CO2) up to values projected for the year 2100 (∼ 1333 μatm f CO2)
Our results showed that the polyunsaturated fatty acids (PUFAs) concentration of the dominating copepod species, A. bifilosa and E. affinis did not vary between the different CO2 treatments

Summary

Introduction

The steady increase of carbon dioxide (CO2) due to anthropogenic emission since the beginning of the industrial era has increased the atmospheric concentration (Boyd et al, 2014). The ocean has a large carbon sink capacity, and increasing atmospheric CO2 absorbed by the ocean is changing the chemistry of the seawater, causing a decline in pH, termed “ocean acidification” (OA; Boyd et al, 2014). Coastal and brackish-water environments encounter wide and frequent fluctuations in pCO2 (Hinga, 2002; Rossoll et al, 2013) due to large fluxes of organic and inorganic carbon from river run-off (Hinga, 2002), seasonal processes (Melzner et al, 2013) and upwelling of CO2enriched water (Feely et al, 2008), all of which lead to wider pH variation in coastal systems compared to the open ocean (Hinga, 2002).

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