Zooplankton community responses to Ocean Acidification

Abstract

Ocean acidification is affecting marine ecosystems directly through changes in pH, as well as indirectly, via trophic pathways. Thus, to evaluate impacts of ocean acidification on marine communities it is necessary to consider the potential pCO2 effects on population dynamics as well as community trophic interactions. Within the framework of the BIOACID II project (Biological Impacts of Ocean ACIDification), the overarching goal of this thesis was to study the effects of ocean acidification on zooplankton, focusing on copepods and jellyfish. The main results are described in four chapters (CHAPTER I to IV), each of which corresponds to a manuscript. The first part of this thesis evaluated pCO2 effects on natural mesozooplankton communities from a boreal fjord (CHAPTER I) and the subtropical Northeast Atlantic (CHAPTER II). Large-scale pelagic mesocosm units (a Kiel Off-Shore Mesocosms for Future Ocean Simulations : KOSMOS) were artificially enriched in CO2 to simulate future ocean conditions. In both experiments, we detected species-specific sensitivities to ocean acidification in copepods, as well as positive pCO2 effect on total mesozooplankton abundances under high-CO2 bloom conditions, caused by a bottom-up effect. During the Gullmar Fjord KOSMOS2013 experiment (CHAPTER I) species-specific sensitivities to CO2 were detected in copepods, as well as in hydromedusae. However, these effects on single species were not translated into the structure or the diversity of the community, likely due to the overwhelmingly dominance of Pseudocalanus acuspes, which resulted to be more abundant under acidic conditions, especially the younger (copepodite) life stage. In the Gran Canaria KOSMOS2014 study (CHAPTER II) a significant effect of pCO2 on phytoplankton succession was detected, ultimately affecting the development of the plankton community only after a simulated bloom event. The zooplankton community responded to the phytoplankton bloom in all mesocosms, although the response was delayed under high pCO2 conditions. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO2 treatments during the pre-bloom phase of the experiment. However calanoids were more abundant under elevated pCO2 conditions than in low- pCO2 levels in the post-bloom phase. Bottom-up effects of CO2-driven increases in phyto- and microzooplankton standing stocks would explain the increase in copepod abundance during both experiments. These results suggest that, under realistic end-of-century scenarios, the above-mentioned ocean acidification effects detected on copepods could potentially affect biomass transfer to higher trophic levels. As in community experiments it is not possible to separate out the pCO2 direct and indirect effects, mesocosms studies were combined with laboratory experiments in the second part of this thesis work. The aim was to evaluate direct and indirect effects of global change conditions on the two main groups of interest for this thesis: copepods and jellyfish. Apart from direct acidification effects, the increasing carbon availability in the marine environment will likely change primary production and the quality of phytoplankton as food for higher trophic levels, showing higher C:nutrient ratios as CO2 availability increases. Hence, a change in biochemical composition when culturing algae (Rhodomonas salina) in elevated pCO2 levels caused a change in food quality, affecting zooplankton by decreased growth and development. Indirect negative pCO2 effects were observed on the dinoflagellate Oxyrrhis marina and nauplii and copepodite stages of the copepod Acartia tonsa. Direct pH effects on these consumers seem to be of lesser importance than the indirect effects caused by a CO2-associated decrease in algal quality when having only a food source (CHAPTER III), unlike the positive CO2-effect observed in copepods when feeding on natural plankton communities. Direct pH effects on zooplankton, however, must be placed in a global change context, considering that ocean acidification in future oceans will not act alone but in combination with other climate factors such as warming and deoxygenation. The direct effects of these three stressors in conjunction were thus studied on 1-day-old ephyrae of the moon jellyfish (Aurelia aurita) from a North Sea subpopulation off Helgoland Island (Germany). The results obtained during this experiment point that end-of-century pCO2 scenarios will not affect these ephyrae in a substantial way. However, A. aurita may not be robust to larger changes in ocean pH, warming and deoxygenation, especially if simultaneous increases in atmospheric pCO2 levels and seawater temperature occur (CHAPTER IV). A. aurita is an ecologically and economically relevant species due to its interactions with commercially important fish species, hence the tolerance or resilience of this jellyfish to climate change might be detrimental for future fisheries.