Abstract
Eutrophic coastal regions are highly productive and greatly influenced by human activities. Primary production supporting the coastal ecosystems is supposed to be affected by progressive ocean acidification driven by increasing CO2 emissions. In order to investigate the effects of high pCO2 (HC) on eutrophic plankton community structure and ecological functions, we employed 9 mesocosms and carried out an experiment under ambient (∼410 ppmv) and future high (1000 ppmv) atmospheric pCO2 conditions, using in situ plankton community in Wuyuan Bay, East China Sea. Our results showed that HC along with natural seawater temperature rise significantly boosted biomass of diatoms with decreased abundance of dinoflagellates in the late stage of the experiment, demonstrating that HC repressed the succession from diatoms to dinoflagellates, a phenomenon observed during algal blooms in the East China Sea. HC did not significantly influence the primary production or biogenic silica contents of the phytoplankton assemblages. However, the HC treatments increased the abundance of viruses and heterotrophic bacteria, reflecting a refueling of nutrients for phytoplankton growth from virus-mediated cell lysis and bacterial degradation of organic matters. Conclusively, our results suggest that increasing CO2 concentrations can modulate plankton structure including the succession of phytoplankton community and the abundance of viruses and bacteria in eutrophic coastal waters, which may lead to altered biogeochemical cycles of carbon and nutrients.
Highlights
The ocean CO2 sink has increased from 1.7 ± 0.4 Pg C yr−1 in 1980s to 2.5 ± 0.6 Pg C yr−1 in 2010s (Friedlingstein et al, 2020), leading to the pH drop of open ocean surface by 0.017–0.027 units per decade (IPCC, 2019), an environmental problem known as ocean acidification (OA)
Apart from monitoring environmental and biogenic changes, we focused on the phytoplankton community succession and the abundance of viruses and bacteria, and the results showed notable correlation between autotrophic assimilation and heterotrophic dissimilation
The results from our mesocosm experiment during the period of late spring and early summer indicate that elevated CO2 concentration to the level projected for the end of this century, along with natural rise of surface seawater temperature, altered phytoplankton community structure, with impeded succession from diatoms to dinoflagellates and enhanced abundance of freeliving viruses and heterotrophic bacteria
Summary
The ocean CO2 sink has increased from 1.7 ± 0.4 Pg C yr−1 in 1980s to 2.5 ± 0.6 Pg C yr−1 in 2010s (Friedlingstein et al, 2020), leading to the pH drop of open ocean surface by 0.017–0.027 units per decade (IPCC, 2019), an environmental problem known as ocean acidification (OA). Two mesocosm studies deployed in the Raunefjorden, western Norway, showed that Emiliania huxleyi, the most abundant coccolithophore species, reduced its growth rates under 710 μatm of pCO2 during May–June 2001 (Engel et al, 2005), and even lost blooming capacity under 1000–3000 μatm conditions during May–June 2011 (Riebesell et al, 2016) In another mesocosm experiment carried out in the Raunefjorden during May 2006, the dominant group of prasinophyte community changed from Bathycoccus-like phylotypes to Micromonas-like phylotypes, as pCO2 increased from current to predicted future levels (Meakin and Wyman, 2011). These results suggest that the OA effects on community structure can vary temporally in the same regions. By integrating and analyzing the results of mesocosm experiments, we noted that planktonic ecosystem level responses to elevated pCO2 can vary spatiotemporally, which may depend on the initial community structure and other abiotic environmental factors such as nutrient levels
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