Ocean Acidification-Induced Restructuring of the Plankton Food Web Can Influence the Degradation of Sinking Particles

Paul Stange,Alice K Nauendorf,Lennart T Bach,Henriette G Horn,María Algueró-Muñiz,Tim Boxhammer,Jan Taucher,Luana Krebs,Ulf Riebesell

doi:10.3389/fmars.2018.00140

Abstract

Ocean acidification (OA) is expected to alter plankton community structure in the future ocean. This, in turn, could change the composition of sinking organic matter and the efficiency of the biological carbon pump. So far, most OA experiments involving entire plankton communities have been conducted in meso- to eutrophic environments. However, recent studies suggest that OA effects may be more pronounced during prolonged periods of nutrient limitation. In this study, we investigated how OA-induced changes in low-nutrient adapted plankton communities of the subtropical North Atlantic Ocean may affect particulate organic matter (POM) standing stocks, POM fluxes, and POM stoichiometry. More specifically, we compared the elemental composition of POM suspended in the water column to the corresponding sinking material collected in sediment traps. Three weeks into the experiment, we simulated a natural upwelling event by adding nutrient-rich deep-water to all mesocosms, which induced a diatom-dominated phytoplankton bloom. Our results show that POM was more efficiently retained in the water column in the highest CO2 treatment levels (>800 μatm pCO2) subsequent to this bloom. We further observed significantly lower C:N and C:P ratios in post-bloom sedimented POM in the highest CO2 treatments, suggesting that degradation processes were less pronounced. This trend is most likely explained by differences in micro- and mesozooplankton abundance during the bloom and post-bloom phase. Overall, this study shows that OA can indirectly alter POM fluxes and stoichiometry in subtropical environments through changes in plankton community structure.

Highlights

The increase in anthropogenic carbon dioxide (CO2) emissions during the last century has led to atmospheric CO2 concentrations that are unprecedented in the recent geological history (IPCC, 2014)
We show that ocean acidification (OA) induced changes in plankton community structure can significantly influence the degradation of sinking particulate organic matter (POM)
We found that C:N ratios in suspended POM were slightly elevated after a diatom-dominated phytoplankton bloom in the two mesocosms with highest pCO2, most likely due to enhanced carbon overconsumption

Summary

Introduction

The increase in anthropogenic carbon dioxide (CO2) emissions during the last century has led to atmospheric CO2 concentrations that are unprecedented in the recent geological history (IPCC, 2014). About 25% of these emissions is taken up by the oceans each year (Le Quéré et al, 2016). While this dampens the atmospheric CO2 increase, it results in shifts in carbonate chemistry. Food-Webs Affect Sinking Particle Degradation and a reduction of seawater pH, commonly referred to as ocean acidification (OA) (Wolf-Gladrow et al, 1999; Caldeira and Wickett, 2003). Estimates predict a 0.3–0.4 reduction in surface pH until the end of this century (Orr et al, 2005), which is expected to have significant impacts on physiological processes of marine biota (Doney et al, 2009)

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Discussion

Conclusion