Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession: First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

Abstract

Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes–summarized by the term ocean acidification (OA)–could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (~380 μatm pCO2), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (~760 μatm pCO2). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a “long-term mesocosm” approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.

Ocean Acidification Experiments in Large-Scale Mesocosms Reveal Similar Dynamics of Dissolved Organic Matter Production and Biotransformation

“…In a previous mesocosm experiment in a Swedish Fjord with elevated pCO 2 levels of 760 µatm, we observed no effects of pCO 2 on DOM compounds being consumed or produced over time (Zark et al, 2015a;Bach et al, 2016). The same was true for this study in the subtropical North Atlantic Ocean.…”

Influence of Ocean Acidification and Deep Water Upwelling on Oligotrophic Plankton Communities in the Subtropical North Atlantic: Insights from an In situ Mesocosm Study

“…They allow the incubation of entire plankton communities from bacteria to fish larvae, and can be sustained on time scales sufficiently long to study the seasonal succession of organisms under close-tonatural conditions (Gamble and Davies, 1982;Riebesell et al, 2013a). Although, only few such "whole community" studies have been conducted so far, it already becomes apparent that the response to elevated CO 2 is highly variable among different ocean regions and plankton communities and often differs from effects on single species observed in the laboratory Riebesell et al, 2013b;Paul et al, 2015;Bach et al, 2016;Gazeau et al, 2016). The few reported community-level studies mostly focused on rather eutrophic environments at higher latitudes, such as the Arctic Ocean or temperate waters, since these regions are commonly assumed to be most vulnerable to ongoing changes in carbonate chemistry (Orr et al, 2005;Yamamoto-Kawai et al, 2009).…”

“…The few reported community-level studies mostly focused on rather eutrophic environments at higher latitudes, such as the Arctic Ocean or temperate waters, since these regions are commonly assumed to be most vulnerable to ongoing changes in carbonate chemistry (Orr et al, 2005;Yamamoto-Kawai et al, 2009). However, recent evidence from the Baltic Sea, North Sea, and Mediterranean Sea indicated that OA effects might be most pronounced when inorganic nutrient concentrations are low (Paul et al, 2015;Sala et al, 2015;Bach et al, 2016;Hornick et al, 2016). How plankton communities in the vast oligotrophic regions of the subtropical gyres might respond to OA is presently unknown.…”

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

“…While these studies helped in developing a principal understanding of how OA affects the physiology of single species, we are still largely lacking the understanding of how OA effects may manifest on the community level (Riebesell and Gattuso, 2015). Existing "whole community" studies show that both direct effects of OA on single species and indirect effects through changes in plankton community structure (Hall-Spencer et al, 2008;Fabricius et al, 2011;Schulz et al, 2013;Paul et al, 2015;Bach et al, 2016;Gazeau et al, 2016) can significantly influence biogeochemical cycles in the ocean.…”

“…Recent mesocosm experiments showed that OA effects can be particularly pronounced during extended periods of low inorganic nutrient concentrations (Paul et al, 2015;Sala et al, 2015;Bach et al, 2016;Thomson et al, 2016). We therefore conducted an in situ mesocosm experiment in the eastern subtropical Atlantic Ocean off the coast of Gran Canaria (Canary Islands)-a region characterized by low concentrations of inorganic nutrients in particular during fall when the study took place (Arístegui et al, 2001).…”