Skip to main content

Southern Ocean Nanoplankton Response to CO2

Periodic Reporting for period 1 - SONAR-CO2 (Southern Ocean Nanoplankton Response to CO2)

Reporting period: 2018-02-01 to 2020-01-31

The impact of anthropogenic ocean acidification on calcifying organisms is expected to be imminent, particularly in high latitude ecosystems. The Southern Ocean is responsible for about 40% of the global oceanic uptake of anthropogenic CO2 with much of the drawdown occurring in the Sub-Antarctic Zone or SAZ. This large inventory of anthropogenic CO2 makes this region an ideal setting to assess the response of marine calcifying plankton to increasing anthropogenic CO2 levels in their natural habitat. Indeed, there is evidence that the ongoing ocean acidification in the SAZ is already affecting the calcification of key calcifying plankton, such as planktonic foraminifera and pteropods.
Coccolithophores, unicellular eukaryotic algae that secrete calcite plates (coccoliths; Figure 1), are the most abundant marine calcareous phytoplankton and play an important role in the marine carbon cycle by contributing to the oceanic pumps of organic matter and carbonate (Figure 2). The cosmopolitan coccolithophore Emiliania huxleyi is known to develop large-scale blooms in high latitudes systems where the production and shedding of coccoliths give the surface waters a milky-turquoise appearance allowing their detection from satellites.
The Marie Skłodowska-Curie funded project Southern Ocean Nanoplankton Response to CO2 (SONaR-CO2) aims to shed light on the ongoing debate whether or not ocean acidification will lead to a replacement of heavily-calcified coccolithophores by lightly-calcified ones in subpolar ecosystems. SONaR-CO2 aims to answer the call by the European Project on OCean Acidification (EPOCA) and Southern Ocean Observing System (SOOS) for urgent and increased effort in research initiatives that address the impacts of ocean acidification on marine and coastal ecosystems and resources.

Project Description
Australian and New Zealand sediment trap programs were launched in the late ‘90s along two latitudinal transects (140°E and 178°E meridians). These collections represent the longest deep Southern Ocean time-series and provide an exceptional opportunity to examine the response of marine calcifying organisms to ocean acidification.
In this project, cutting-edge and traditional microscopy techniques combined with biogeochemical analyses will be applied to the longest existing subantarctic time-series records and sediment samples from two sectors of the Southern Ocean in order to achieve the following objectives:
1. Detect changes in the calcification response of the dominant coccolithophore species Emiliania huxleyi in relation to changes in the CO2 concentration in the surface waters of the SAZ at different time scales.
2. Determine the diversity, abundance and temporal community changes of coccolithophores.
3. Estimate, for the first time, the partial contribution of coccolithophores to total carbonate export in the Subantarctic Zone.
4. Explore the potential impacts of environmental stressors on the biomarker signature of coccolithophores.
All the work packages of the project were successfully accomplished and summarized as follows. Samples from sediment traps, surface sediments and two sediment cores were shipped from NIWA and IMAS to USAL before the onset of the project. Samples were processed and analysed for characterization of coccolithophore assemblage composition and quantification, and measurement of coccolith morphological parameters. Chemical and biomarker training and analyses were conducted during secondment at IPMA. All datasets were statistically analysed and compared with existing physical, chemical and biological data from mooring platforms and satellites. A large part of the project results have been published in three open-access publications in top leading journals in the field of environmental and ocean sciences, while two more scientific articles are being prepared. The corresponding datasets are freely available through the Australian Antarctic Data Centre repository. Overall, the action has produced important results that reduce uncertainty as to the impacts of Climate Change on an important taxon in the Southern Ocean, and therefore, they are expected to be considered in future IPCC reports.
During the course of the project, Dr Rigual-Hernández gained expertise on coccolithophore taxonomy and ecology and learnt new microscopy and biogeochemical techniques. Moreover, he provided training to PhD, graduate, undergraduate and secondary school students on a variety of projects with SONaR-CO2 materials, while actively participating in the teaching of several units of Biology and Environmental Sciences degrees at USAL. He established new collaborations with research teams from Australia, New Zealand and India. As a result of his work the Southern Ocean community now has a taxonomic key for identification of key E. huxleyi morphotypes and extensive data available from the AADC to provide an understanding of the role of coccolithophore communities in the carbon cycle across two different physiochemical realms of the Southern Ocean. The project was successfully accomplished and resulted in raising the profile of Dr Rigual-Hernández as an international specialist on Southern Ocean phytoplankton.
The combination of taxonomic and morphometric analyses on Emiliania huxleyi coccoliths, together with in situ measurements of surface water properties allowed to monitor, with unprecedented detail, the seasonal cycle of this species in the Southern Ocean. The co-occurrence of maximum annual relative abundance of highly calcified morphotypes with peak annual TCO2 challenges the generally accepted notion that ocean acidification will necessarily lead to a replacement of heavily-calcified coccolithophores by lightly-calcified ones in subpolar ecosystems. Moreover, comparison of chemical particulate inorganic carbon measurements with coccolith flux estimates and species carbonate content allowed to determine that coccolithophore contribution to calcium carbonate export ranges approximately between 25 and 60% of the annual carbonate flux in the Subantarctic Zone while their contribution drops down to ~5% in the Antarctic Zone waters. Notably, we observed that not the most abundant coccolithophore species (Emiliania huxleyi) but rather less abundant and larger species (e.g. Calcidiscus leptoporus) make the greatest contribution to carbonate export to the deep sea. Since these larger species exhibit substantially different ecological traits from the opportunistic E. huxleyi, predictions of future response of Southern Ocean coccolithophore communities should not be based on the physiological results from experiments with E. huxleyi. Rather, new physiological response experiments of those less abundant, larger coccolithophore species are urgently needed to constrain responses of these important carbonate exporters to environmental change in the Southern Ocean. Unpublished comparison of sediment trap and sea-floor sediment E. huxleyi assemblages suggests that pre-industrial assemblages were more calcified than modern assemblages. Lastly, the combination of biomarker and taxonomic analyses provided important new insights into how the makeup of the coccolithophore community influences the biomarker imprint recorded in the sedimentary record and therefore are expected to improve calibrations of this widely used proxy.
Coccolithophore images
Ocean pumps