Low CO2 Sensitivity of Microzooplankton Communities in the Gullmar Fjord, Skagerrak: Evidence from a Long-Term Mesocosm Study

Abstract

Ocean acidification is considered as a crucial stressor for marine communities. In this study, we tested the effects of the IPCC RPC6.0 end-of-century acidification scenario on a natural plankton community in the Gullmar Fjord, Sweden, during a long-term mesocosm experiment from a spring bloom to a mid-summer situation. The focus of this study was on microzooplankton and its interactions with phytoplankton and mesozooplankton. The microzooplankton community was dominated by ciliates, especially small Strombidium sp., with the exception of the last days when heterotrophic dinoflagellates increased in abundance. We did not observe any effects of high CO2 on the community composition and diversity of microzooplankton. While ciliate abundance, biomass and growth rate were not affected by elevated CO2, we observed a positive effect of elevated CO2 on dinoflagellate abundances. Additionally, growth rates of dinoflagellates were significantly higher in the high CO2 treatments. Given the higher Chlorophyll a content measured under high CO2, our results point at mainly indirect effects of CO2 on microzooplankton caused by changes in phytoplankton standing stocks, in this case most likely an increase in small-sized phytoplankton of <8 μm. Overall, the results from the present study covering the most important part of the growing season indicate that coastal microzooplankton communities are rather robust towards realistic acidification scenarios.

Analyzing the Impacts of Elevated-CO2 Levels on the Development of a Subtropical Zooplankton Community During Oligotrophic Conditions and Simulated Upwelling

“…Overall, higher zooplankton abundances were observed at elevated pCO 2 conditions (medium-and high-) in the post-bloom phase. This result matches with a previous KOSMOS study in coastal mesotrophic conditions (Bach et al, 2016b) where certain groups of consumers capitalized on CO 2 -enhanced phytoplankton biomass, resulting in higher zooplankton abundances under moderate IPCC end-of-century pCO 2 scenarios (RCP6.0) (Horn et al, 2016;Algueró-Muñiz et al, 2017). However, unexpectedly, both microZP and mesoZP abundances (Figures 3G, 4G) increased much later in the experiment under high-pCO 2 than under medium-and low-pCO 2 .…”

“…For dinoflagellates, especially small-sized athecates, we expected a positive effect of high-pCO 2 levels due to findings from previous OA studies conducted in oligotrophic (Sala et al, 2015) and eutrophic regions (Horn et al, 2016). During the oligotrophic phase of the experiment, this expectation was confirmed since higher abundances of small athecate dinoflagellates at high-pCO 2 were also found in our study.…”

“…very subtle changes in the community (Suffrian et al, 2008;Aberle et al, 2013;Horn et al, 2016;Lischka et al, 2017) while other studies showed detrimental (Calbet et al, 2014) or even positive effects (Rose et al, 2009). In this study, an increase in aloricate ciliate abundances was observed in all treatments in response to the deep water-induced phytoplankton bloom, although the increase showed a considerable time-lag relative to the phytoplankton bloom, especially at high CO 2 conditions.…”

“…Plankton community OA studies to date have been mostly carried out in relatively eutrophic environments (but see Sala et al, 2015;Gazeau et al, 2017), and led to varying conclusions. Some studies showed the lack of major effects of elevated pCO 2 levels in micro- (Aberle et al, 2013;Horn et al, 2016) and mesozooplankton abundances (Niehoff et al, 2013), while others detected both changes in community size distributions and biomass (Lischka et al, 2017;Taucher et al, 2017b) as well as positive bottom-up pCO 2 responses on mesozooplankton abundances (Algueró-Muñiz et al, 2017). Inorganic nutrient availability can control these different responses to OA in planktonic communities, thereby the nutrient-deplete phases could determine the transfer of the pCO 2 effects on primary producers to primary consumers (Alvarez-Fernandez et al, 2018).…”

Simulated ocean acidification reveals winners and losers in coastal phytoplankton

“…They reached higher biomass in the high CO 2 treatment towards the end of the experiment ( Fig 4F ). Horn et al, investigated dinoflagellate abundance in the same mesocosm study by means of light microscopy and found the same CO 2 trend in phase IV [ 50 ]. In their analysis they focused on species which are traditionally considered as heterotrophic although still aware that many species are at least facultative mixotrophic [ 50 ].…”