Description: Enhancing ocean productivity by artificial upwelling is evaluated as a nature-based solution for food security and climate change mitigation. Fish production is intended through diatom-based plankton food webs as these are assumed to be short and efficient. However, our findings from mesocosm experiments on artificial upwelling in the oligotrophic ocean disagree with this classical food web model. Here, diatoms did not reduce trophic length and instead impaired the transfer of primary production to crustacean grazers and small pelagic fish. The diatom-driven decrease in trophic efficiency was likely mediated by changes in nutritional value for the copepod grazers. Whilst diatoms benefitted the availability of essential fatty acids, they also caused unfavorable elemental compositions via high carbon-to-nitrogen ratios (i.e. low protein content) to which the grazers were unable to adapt. This nutritional imbalance for grazers was most pronounced in systems optimized for CO2 uptake through carbon-to-nitrogen ratios well beyond Redfield. A simultaneous enhancement of fisheries production and carbon sequestration via artificial upwelling may thus be difficult to achieve given their opposing stoichiometric constraints. Our study suggest that food quality can be more critical than quantity to maximize food web productivity during shorter-term fertilization of the oligotrophic ocean.

Description: As one of Earth's most productive marine ecosystems, the Peruvian Upwelling System transports large amounts of biogenic matter from the surface to the deep ocean. Whilst particle sinking velocity is a key factor controlling the biological pump, thereby affecting carbon sequestration and oxygen-depletion, it has not yet been measured in this system. During a 50-day mesocosm experiment in the surface waters off the coast of Peru, we regularly sampled sedimented material (sampling depth: 17 m) and analyzed the properties of sinking particles using an optical measurement approach. The presented dataset includes sinking velocity, particle size (ESD), compactness (porosity) and shape (aspect ratio) of 〉100.000 individually measured particles.

Description: As one of Earth's most productive marine ecosystems, the Peruvian Upwelling System transports large amounts of biogenic matter from the surface to the deep ocean. Whilst particle sinking velocity is a key factor controlling the biological pump, thereby affecting carbon sequestration and oxygen-depletion, it has not yet been measured in this system. During a 50-day mesocosm experiment in the surface waters off the coast of Peru, we regularly measured particle sinking velocities and their biogeochemical and physical drivers. We further characterized the general properties of sinking matter (sampling depth: 17 m) under different phytoplankton communities and nutritional states. This dataset contains mean velocities of sinking particles as well as their median size, compactness and shape. We further included the particulate organic carbon flux, the sinking matter nitrogen to phosphorus ratio and the relative contribution of opal and particulate inorganic carbon to the total flux. The particle flux characteristics are complemented by measurements of chlorophyll a concentration in the water column and the relative contribution of diatoms to total chlorophyll a.

Description: In this study we investigated the effect of artificial upwelling with different Si:N ratios on a natural oligotrophic plankton community. Data on primary production, flow cytometry, photophysiology, internal nutrient storage, diatom size, diatom silicification and diatom carbon density from the mesocosms experiment conducted on the Canary Islands in autumn 2019. Values are depth-integrated averages in (mostly) 2-days intervals over the course of 33 days. The upwelling treatment started on day 6. All samples were depth integrated (0-2.5m). Oxygen production and respiration rates were measured through incubations and Winkler titrations. Flow cytometry and photophysiology samples were measured fresh. Parameters used for diatom per capita size, silicification and carbon density measurements were taken from the following published datasets on the same experiment: https://doi.org/10.1594/PANGAEA.954852 and https://doi.org/10.1594/PANGAEA.951417

Description: Global warming and ocean acidification are forecast to exert significant impacts on marine ecosystems worldwide. However, most of these projections are based on ecological proxies or experiments on single species or simplified food webs. How energy fluxes are likely to change in marine food webs in response to future climates remains unclear, hampering forecasts of ecosystem functioning. Using a sophisticated mesocosm experiment, we model energy flows through a species-rich multilevel food web, with live habitats, natural abiotic variability, and the potential for intra- and intergenerational adaptation. We show experimentally that the combined stress of acidification and warming reduced energy flows from the first trophic level (primary producers and detritus) to the second (herbivores), and from the second to the third trophic level (carnivores). Warming in isolation also reduced the energy flow from herbivores to carnivores, the efficiency of energy transfer from primary producers and detritus to herbivores and detritivores, and the living biomass of detritivores, herbivores, and carnivores. Whilst warming and acidification jointly boosted primary producer biomass through an expansion of cyanobacteria, this biomass was converted to detritus rather than to biomass at higher trophic levels-i.e., production was constrained to the base of the food web. In contrast, ocean acidification affected the food web positively by enhancing trophic flow from detritus and primary producers to herbivores, and by increasing the biomass of carnivores. Our results show how future climate change can potentially weaken marine food webs through reduced energy flow to higher trophic levels and a shift towards a more detritus-based system, leading to food web simplification and altered producer–consumer dynamics, both of which have important implications for the structuring of benthic communities.

Description: Marine prey and predators will respond to future climate through physiological and behavioral adjustments. However, our understanding of how such direct effects may shift the outcome of predator–prey interactions is still limited. Here, we investigate the effects of ocean warming and acidification on foraging behavior and biomass of a common prey (shrimps, Palaemon spp.) tested in large mesocosms harboring natural resources and habitats. Acidification did not alter foraging behavior in prey. Under warming, however, prey showed riskier behavior by foraging more actively and for longer time periods, even in the presence of a live predator. No effects of longer-term exposure to climate stressors were detected on prey biomass. Our findings suggest that ocean warming may increase the availability of some prey to predators via a behavioral pathway (i.e., increased risk-taking by prey), likely by elevating metabolic demand of prey species.

Description: As human activities intensify, the structures of ecosystems and their food webs often reorganize. Through the study of mesocosms harboring a diverse benthic coastal community, we reveal that food web architecture can be inflexible under ocean warming and acidification and unable to compensate for the decline or proliferation of taxa. Key stabilizing processes, including functional redundancy, trophic compensation, and species substitution, were largely absent under future climate conditions. A trophic pyramid emerged in which biomass expanded at the base and top but contracted in the center. This structure may characterize a transitionary state before collapse into shortened, bottom-heavy food webs that characterize ecosystems subject to persistent abiotic stress. We show that where food web architecture lacks adjustability, the adaptive capacity of ecosystems to global change is weak and ecosystem degradation likely.

Description: 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.

Description: 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).

Description: 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) (doi:10.3389/fmars.2021.743105), Baumann et al. (2021) (doi:10.3389/fmars.2021.742142) and Goldenberg et al. (under review).