Ecological responses to ocean acidification and warming: scaling up from individuals to communities

Impacts of human CO2 emissions on ecosystems and their services are inherently difficult to predict, as ecosystem responses emerge from complex and dynamic networks of organisms and their interactions. Yet, our understanding of the ecological imprint of future climate remains largely based on tests of single species in the laboratory. Here I show how the responses of individual organisms to ocean acidification and ocean warming scale-up to species communities and reveal the underlying ecological dynamics. This was accomplished through the study of behaviour, bottom-up and top-down forcing, food web architecture, and functional composition in 1,800 L mesocosms that harboured a temperate near-shore community including various species of algae, invertebrates and fishes. The negative effects of ocean acidification were buffered effectively through stabilizing processes at both simple and complex levels of biological organisation. Consequently, acidification primarily acted as a resource (via CO2-enrichment) that increased productivity throughout the food web. In contrast, ocean warming shifted the balance in key ecological processes leading to a novel community structure that would likely undermine ecosystem services. Dynamics with the potential to compensate for the uneven sensitivities between functions failed to engage – given the fundamental influence of temperature on physiology – which allowed impacts to cascade through the community. This stress through warming also negated any positive effects of acidification. My findings bridge the gap between the simplicity of the laboratory and species communities in nature, by revealing how impacts of future climate can be countered or accelerated through ecological processes. A predictive understanding of stability or change in ecosystems is key to the management of natural resources in a future ocean.Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 201

Counteracting effects of nutrient composition (Si:N) on export flux under artificial upwelling

To keep global warming below 1.5°C, technologies that remove carbon from the atmosphere will be needed. Ocean artificial upwelling of nutrient-rich water stimulates primary productivity and could enhance the biological carbon pump for natural CO2 removal. Its potential may depend on the Si availability in the upwelled water, which regulates the abundance of diatoms that are key carbon exporters. In a mesocosm experiment, we tested the effect of nutrient composition (Si relative to N) in artificially upwelled waters on export quantity and quality in a subtropical oligotrophic environment. Upwelling led to a doubling of exported particulate matter and increased C:N ratios to well beyond Redfield (9.5 to 11.1). High Si availability stimulated this carbon over-consumption further, resulting in a temporary ~5-fold increase in POC export and ~30% increase in C:N ratios compared to Si-scarce upwelling. Whilst the biogenic Si ballast of the export flux increased more than 3.5-fold over the Si:N gradient, these heavier particles did not sink faster. On the contrary, sinking velocity decreased considerably under high Si:N, most likely due to reduced particle size. Respiration rates remained similar across all treatments indicating that biogenic Si did not protect particles against microbial degradation. Si availability thus influenced key processes of the biological carbon pump in counteracting ways by increasing the export magnitude and associated C:N ratios but decreasing the efficiency of carbon transfer to depth. These opposing effects need to be considered when evaluating the potential of artificial upwelling as negative emission technology

KOSMOS mesocosm experiment Gran Canaria 2019 on testing the effect of nutrient composition (Si:N) during artificial upwelling: mesozooplankton trophic level and fish biomass and feeding

Mesozooplankton trophic position (via copepod δ15N) to approximate food web length as well as fish production (via biomass increase) and feeding rate (via stomach content) during the mesocosm experiment in the Canary Islands in autumn 2019. Copepods >200µm were sampled depth-integrated (0-2.5m) on day 13. At this time, copepods represented the top of the food web as fish were not yet present. Individuals were picked in groups into tin capsules and δ15N measured in a mass spectrometer. Particulate organic matter(>0.7µm) δ15N was also measured to calculate an autotroph baseline. The difference between copepod and autotroph δ15N was used as trophic level proxy. A small pelagic fish (silverside, Atherina presbyter) was introduced to the mesocosms on day 15. On day 18, for a subset of fish, stomachs content was assessed to estimate feeding success. On day 21, all fish were sampled for biomass and abundance. The upwelling treatment started on day 6. Methodological details in Goldenberg et al. (doi:10.3389/fmars.2022.1015188) and Goldenberg et al. (under review)

KOSMOS mesocosm experiment Gran Canaria 2019 on testing the effect of nutrient composition (Si:N) during artificial upwelling: mesozooplankton carbon and nitrogen content and stable isotope δ15N

Mesozooplankton (mesoZP) per capita mass, elemental composition and stable N isotopes during the mesocosm experiment in the Canary Islands in autumn 2019. Depth-integrated (0-2.5m) water samples were taken over the course of 33 days. Metazoan zooplankton were split into three size fractions (55-200, 200-500 and >500 µm), picked into tin cups in groups and C, N and δ15N measured in an element analyser coupled to a mass spectrometer. Particulate organic matter (>0.7µm) was also measured for a comparison of C/N between the bottom of the food web and mesoZP grazers. The upwelling treatment started on day 6. Methodological details in Goldenberg et al. (doi:10.3389/fmars.2022.1015188) and Goldenberg et al. (under review)

KOSMOS mesocosm experiment Gran Canaria 2019 on testing the effect of nutrient composition (Si:N) during artificial upwelling: phytoplankton pigments and composition

Pigment concentration and pigment-based phytoplankton community composition data from the mesocosm experiment conducted in the Canary Islands in autumn 2019. Depth-integrated (0-2.5m) water samples were taken in 2-days intervals over the course of 33 days. One set of filters (one filter per sampling day and mesocosm) was analysed fluorometrically for Chl a. Another set of filters was analysed for a range of photosynthetic pigments using reverse-phase high-performance liquid chromatography (HPLC). Based on pigment concentrations, phytoplankton community composition was approximated using the CHEMTAX software with the original pigment ratios from Mackey et al (1996, doi:10.3354/meps144265). The input included Chl a, b, c2, and c3, peridinin, 19'-butanoyloxyfucoxanthin, fucoxanthin, neoxanthin, prasinoxanthin, violaxanthin, 19'-hexanoyloxyfucoxanthin, alloxanthin, and zeaxanthin. Divinyl Chl a was instead fully associated with Prochlorophyceae. The presence of the main phytoplankton groups is expressed in Chl a equivalents and their contribution to the phytoplankton community as percentage to total Chl a. The upwelling treatment started on day 6. Methodological details in Goldenberg et al. (doi:10.3389/fmars.2022.1015188)

KOSMOS 2018 Gran Canaria mesocosm study on artificial upwelling: suspended matter fatty acids, d15N stable isotopes and diatom dominance

Suspended matter fatty acid composition, nitrogen stable isotope ratios and diatom dominance of the phytoplankton community during the mesocosm experiment in the Canary Islands in autumn 2018. Depth-integrated (0-14 m) water samples were filtered (>0.7µm). Chlorophyll a is based on HPLC. Particulate organic matter C, N and δ15N were measured with an element analyser and mass spectrometer. Diatom biovolume was measured by flow imaging analysis. Fatty acids (FA) composition of particulate matter was determined by gas chromatography. The upwelling treatment started on day 4. Methodological details in Ortiz et al. (2022) (doi:10.3389/fmars.2021.7431059), Baumann et al. (2021) (doi:10.3389/fmars.2021.742142) and Goldenberg et al. (under review)

KOSMOS 2018 Gran Canaria mesocosm study on artificial upwelling: mesozooplankton carbon and nitrogen content and stable isotope δ15N

Mesozooplankton (mesoZP) per capita mass, elemental composition and stable N isotopes during the mesocosm experiment in the Canary Islands in autumn 2018. Depth-integrated (0-14 m) plankton nets were taken over the course of 38 days. Metazoan zooplankton were split into three size fractions (55-200, 200-500 and >500 µm), picked into tin cups in groups and C, N and δ15N measured in an element analyser coupled to a mass spectrometer. Particulate organic matter (>0.7µm) was also measured for a comparison of C/N between the bottom of the food web and mesoZP grazers. The upwelling treatment started on day 4. Methodological details in Ortiz et al. (2022), Baumann et al. (2021) and Goldenberg et al. (under review)

KOSMOS mesocosm experiment Gran Canaria 2019 on testing the effect of nutrient composition (Si:N) during artificial upwelling: phytoplankton microscopy

Abundance and biovolume data of the community of larger phytoplankton from the mesocosm experiment conducted in the Canary Islands in autumn 2019. Depth-integrated (0-2.5m) water samples were taken in 2-days intervals over the course of 33 days and autotrophic taxa assessed to the lowest taxonomic level possible using Utermöhl microscopy. Only taxa larger than approx. >5 µm could be considered with this method. Biovolume was calculated based on geometrical measurements (dominant taxa) or the literature (rare taxa). Carbon biomass estimates were purposefully not provided, as the standard literature conversion factors from biovolume to carbon biomass did not apply to many of our samples, likely due to low carbon density within cells. Predominantly mixotrophic or heterotrophic taxa are not provided in this dataset. The upwelling treatment started on day 6. Methodological details in Goldenberg et al. (doi:10.3389/fmars.2022.1015188)

KOSMOS 2018 Gran Canaria mesocosm study on artificial upwelling: mesozooplankton trophic position and diatom fatty acid markers

Mesozooplankton trophic markers to estimate food web length (δ15N) and dietary contribution of diatoms (fatty acids) during the mesocosm experiment in the Canary Islands in autumn 2018. Depth-integrated (0-14 m) plankton nets were taken and omnivorous copepods >200µm used for trophic marker analysis. For trophic level, copepods were picked in groups into tin capsules and δ15N measured in a mass spectrometer. Particulate organic matter (>0.7µm) δ15N was also measured to calculate an autotroph baseline. The difference between copepod and autotroph δ15N was used as trophic level proxy. Trophic level data represents averages across all samples from day 11 to 38. For fatty acids (FA), copepods were picked in groups and FA composition determined by gas chromatography. Particulate organic matter (>0.7µm) FA were also measured as baseline. The marker 20:5ω3/22:6ω3 was most suitable to track the propagation of diatom productivity up the food web. FA data represents averages across samples taken on day 30 and 36. The upwelling treatment started on day 4. Methodological details in Ortiz et al. (2022), Baumann et al. (2021) and Goldenberg et al. (under review)

KOSMOS 2021 Gran Canaria mesocosm study on ocean alkalinity enhancement: remineralization

The data presented herein originates from a mesocosm study conducted as part of the EU H2020 OceanNETs project, aimed at investigating the ecological ramifications of ocean alkalinity enhancement. Nine mesocosms were deployed in Taliarte Harbour, Gran Canaria, Spain, and systematically sampled using integrated water samplers over the period spanning from September 10th to October 25th, 2021. Alkalinity was employed in a gradient design, ranging from ambient (0 µeq kg-1 added alkalinity, OAE0) to elevated levels of 2400 µeq kg-1 additional alkalinity (OAE2400) in increments of 300 µeq kg-1. The dataset encompasses a spectrum of sediment trap particle flux data, water column biogeochemistry variables, including inorganic nutrients, carbonate chemistry parameters, and particulate matter, alongside chlorophyll a concentrations. The study and data set offer insights into impacts of alkalinity enhancement on marine ecosystems and their associated biogeochemistry