Extreme Levels of Ocean Acidification Restructure the Plankton Community and Biogeochemistry of a Temperate Coastal Ecosystem: A Mesocosm Study

Carsten Spisla,Mathias Haunost,Bettany D Jenkins,Ulf Riebesell,Jana Meyer,Fabrizio Minutolo,Kai T Lohbeck,Peter Kohnert,Alice Nauendorf,Andrew L King,Verena Kalter,Michael Sswat,Paul Stange,Tim Boxhammer,Andrea Ludwig,Michael Krudewig,Silke Lischka,Isabel Dörner,Anna K Lechtenbörger,Lennart T Bach,Jan Taucher,Nicole Aberle,Stefanie M H Ismar‐Rebitz ,Joselynn Wallace ,Jean‐Marie Bouquet ,Jaw Chuen Yong

doi:10.3389/fmars.2020.611157

Abstract

The oceans’ uptake of anthropogenic carbon dioxide (CO2) decreases seawater pH and alters the inorganic carbon speciation – summarized in the term ocean acidification (OA). Already today, coastal regions experience episodic pH events during which surface layer pH drops below values projected for the surface ocean at the end of the century. Future OA is expected to further enhance the intensity of these coastal extreme pH events. To evaluate the influence of such episodic OA events in coastal regions, we deployed eight pelagic mesocosms for 53 days in Raunefjord, Norway, and enclosed 56–61 m3 of local seawater containing a natural plankton community under nutrient limited post-bloom conditions. Four mesocosms were enriched with CO2 to simulate extreme pCO2 levels of 1978 – 2069 μatm while the other four served as untreated controls. Here, we present results from multivariate analyses on OA-induced changes in the phyto-, micro-, and mesozooplankton community structure. Pronounced differences in the plankton community emerged early in the experiment, and were amplified by enhanced top-down control throughout the study period. The plankton groups responding most profoundly to high CO2 conditions were cyanobacteria (negative), chlorophyceae (negative), auto- and heterotrophic microzooplankton (negative), and a variety of mesozooplanktonic taxa, including copepoda (mixed), appendicularia (positive), hydrozoa (positive), fish larvae (positive), and gastropoda (negative). The restructuring of the community coincided with significant changes in the concentration and elemental stoichiometry of particulate organic matter. Results imply that extreme CO2 events can lead to a substantial reorganization of the planktonic food web, affecting multiple trophic levels from phytoplankton to primary and secondary consumers.

Highlights

The world oceans currently absorb 2.5 ± 0.5 GtC y−1 of the total 11.5 ± 0.9 GtC y−1 anthropogenic CO2 emissions [2009 – 2018, Friedlingstein et al (2019)]
Our observations of ocean acidification (OA) effects on biogeochemical parameters in phases III and IV along with the consistent treatment differences already observed in chl a, lead to the confirmation of our second hypothesis
The reduced phytoplankton biomass in the high pressure of CO2 (pCO2) treatment was seen in POMWATER concentrations

Summary

Introduction

The world oceans currently absorb 2.5 ± 0.5 GtC y−1 of the total 11.5 ± 0.9 GtC y−1 anthropogenic CO2 emissions [2009 – 2018, Friedlingstein et al (2019)]. Realistic emission scenarios project that the pH of ocean surface waters will further decline by at least 0.2 units to about 7.9 by the end of the century [IPCC scenario RCP4.5, Pörtner et al (2014)] The effects of this alteration in the carbonate systems of the oceans on the inherent marine organisms has already been targeted by a variety of different experiments and approaches (Gattuso and Hansson, 2011). Recent studies have shown that consequences of an elevated partial pressure of CO2 (pCO2) vary strongly between different oceanic regions as well as between planktonic communities (Fabricius et al, 2011; Riebesell et al, 2013b; Paul et al, 2015; Gazeau et al, 2017; Taucher et al, 2017). What they have in common, is that the studies cover plankton communities in e.g., the Baltic Sea, the north western Mediterranean, the eastern subtropical North Atlantic or the Artic, they all discovered OA effects in similar trophic positions. Riebesell et al (2013b) and Paul et al (2015) both discovered predominantly positive effects of an OA simulation on pico- and nanophytoplankton organisms, along with corresponding changes in chl a or particulate organic matter (POM). Taucher et al (2017), observed a pronounced reorganization of the whole plankton community under elevated pCO2, still suspected to be driven by phytoplankton, but contrary to the other studies affecting micro- and mesozooplankton organisms (Algueró-Muñiz et al, 2019)

Methods

Results

Conclusion