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

Ocean acidification (OA) is affecting marine ecosystems through changes in carbonate chemistry that may influence consumers of phytoplankton, often via trophic pathways. Using a mesocosm approach, we investigated OA effects on a subtropical zooplankton community during oligotrophic, bloom, and post-bloom phases under a range of different pCO2 levels (from ∼400 to ∼1480 μatm). Furthermore, we simulated an upwelling event by adding 650 m-depth nutrient-rich water to the mesocosms, which initiated a phytoplankton bloom. No effects of pCO2 on the zooplankton community were visible in the oligotrophic conditions before the bloom. The zooplankton community responded to phytoplankton bloom by increased abundances in all treatments, although the response was delayed under high-pCO2 conditions. Microzooplankton was dominated by small dinoflagellates and aloricate ciliates, which were more abundant under medium- to high-pCO2 conditions. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO2 treatments during the oligotrophic phase of the experiment but were found in higher abundance under medium- and high-pCO2 conditions toward the end of the experiment, most likely as a response to increased phyto- and microzooplankton standing stocks. The second most abundant mesozooplankton taxon were appendicularians, which did not show a response to the different pCO2 treatments. Overall, CO2 effects on zooplankton seemed to be primarily transmitted through significant CO2 effects on phytoplankton and therefore indirect pathways. We conclude that elevated pCO2 can change trophic cascades with significant effects on zooplankton, what might ultimately affect higher trophic levels in the future

Effects of Elevated CO2 on a Natural Diatom Community in the Subtropical NE Atlantic

Diatoms are silicifying phytoplankton contributing about one quarter to primary production on Earth. Ocean acidification (OA) could alter the competitiveness of diatoms relative to other taxa and/or lead to shifts among diatom species. In spring 2016, we set up a plankton community experiment at the coast of Gran Canaria (Canary Islands, Spain) to investigate the response of subtropical diatom assemblages to elevated seawater pCO2. Therefore, natural plankton communities were enclosed for 32 days in in situ mesocosms (∼8 m3 volume) with a pCO2 gradient ranging from 380 to 1140 μatm. Halfway through the study we added nutrients to all mesocosms (N, P, Si) to simulate injections through eddy-induced upwelling which frequently occurs in the region. We found that the total diatom biomass remained unaffected during oligotrophic conditions but was significantly positively affected by high CO2 after nutrient enrichment. The average cell volume and carbon content of the diatom community increased with CO2. CO2 effects on diatom biomass and species composition were weak during oligotrophic conditions but became quite strong above ∼620 μatm after the nutrient enrichment. We hypothesize that the proliferation of diatoms under high CO2 may have been caused by a fertilization effect on photosynthesis in combination with reduced grazing pressure. Our results suggest that OA in the subtropics may strengthen the competitiveness of (large) diatoms and cause changes in diatom community composition, mostly under conditions when nutrients are injected into oligotrophic systems

Factors controlling plankton community production, export flux, and particulate matter stoichiometry in the coastal upwelling system off Peru

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can modify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predictions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes controlling organic matter cycling in the coastal Peruvian upwelling system. Eight mesocosms, each with a volume of ∼55 m3, were deployed for 50 d ∼6 km off Callao (12∘ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected subsurface waters at two different locations in the regional oxygen minimum zone (OMZ) and injected these into four mesocosms (mixing ratio ≈1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the individual mesocosms. The mesocosm phytoplankton communities were initially dominated by diatoms but shifted towards a pronounced dominance of the mixotrophic dinoflagellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. sanguinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decoupled from surface production and sustained by the remaining plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self-shading by phytoplankton and by inorganic nitrogen limitation which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few opportunities for blooms, which represents an event where the system becomes unbalanced. Overall, our mesocosm study revealed some key links between ecological and biogeochemical processes for one of the most economically important regions in the oceans

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: 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 2014 mesocosm study: mesozooplankton abundances

Using a mesocosm approach, we investigated ocean acidification effects on a subtropical zooplankton community during oligotrophic, bloom, and post-bloom phases under a range of different pCO2 levels (from ~400 to ~1480 µatm). To do that, we simulated an upwelling event by adding 650 m-depth nutrient-rich water to the mesocosms, which initiated a phytoplankton bloom. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO2 treatments during the oligotrophic phase of the experiment but were found in higher abundance under medium- and high-pCO2 conditions towards the end of the experiment, most likely as a response to increased phyto- and microzooplankton standing stocks. The second most abundant mesozooplankton taxon were appendicularians, which did not show a response to the different pCO2 treatments. Overall, CO2 effects on zooplankton seem to be primarily transmitted through significant CO2 effects on phytoplankton and therefore indirect pathways

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 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)

Responses of a subtropical zooplankton community to ocean acidification during oligotrophic conditions and simulated upwelling (KOSMOS 2014)

Ocean acidification (OA) is affecting marine ecosystems through changes in carbonate chemistry that may influence consumers of phytoplankton, often via trophic pathways. Using a mesocosm approach, we investigated OA effects on a subtropical zooplankton community during oligotrophic, bloom, and post-bloom phases under a range of different pCO2 levels (from ∼400 to ∼1480 μatm)