Description: Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs). There are numerous feedbacks between oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes. We investigated the pelagic biogeochemistry of OMZs in the eastern tropical North Atlantic and eastern tropical South Pacific during a series of cruise expeditions and mesocosm studies. The following summarizes the current state of research on the influence of low environmental oxygen conditions on marine biota, viruses, organic matter formation and remineralization with a particular focus on the nitrogen cycle in OMZ regions. The impact of sulfidic events on water column biogeochemistry, originating from a specific microbial community capable of highly efficient carbon fixation, nitrogen turnover and N2O production is further discussed. Based on our findings, an important role of sinking particulate organic matter in controlling the nutrient stochiometry of the water column is suggested. These particles can enhance degradation processes in OMZ waters by acting as microniches, with sharp gradients enabling different processes to happen in close vicinity, thus altering the interpretation of oxic and anoxic environments.

Description: Concerning their sensitivity to ocean acidification, coccolithophores, a group of calcifying single-celled phytoplankton, are one of the best-studied groups of marine organisms. However, in spite of the large number of studies investigating coccolithophore physiological responses to ocean acidification, uncertainties still remain due to variable and partly contradictory results. In the present study we have used all existing data in a meta-analysis to estimate the effect size of future pCO2 changes on the rates of calcification and photosynthesis and the ratio of particulate inorganic to organic carbon (PIC / POC) in different coccolithophore species. Our results indicate that ocean acidification has a negative effect on calcification and the cellular PIC / POC ratio in the two most abundant coccolithophore species: Emiliania huxleyi and Gephyrocapsa oceanica. In contrast, the more heavily calcified species Coccolithus braarudii did not show a distinct response when exposed to elevated pCO2/reduced pH. Photosynthesis in Gephyrocapsa oceanica was positively affected by high CO2, while no effect was observed for the other coccolithophore species. There was no indication that the method of carbonate chemistry manipulation was responsible for the inconsistent results regarding observed responses in calcification and the PIC / POC ratio. The perturbation method, however, appears to affect photosynthesis, as responses varied significantly between total alkalinity (TA) and dissolved inorganic carbon (DIC) manipulations. These results emphasize that coccolithophore species respond differently to ocean acidification, both in terms of calcification and photosynthesis. Where negative effects occur, they become evident at CO2 levels in the range projected for this century in the case of unabated CO2 emissions. As the data sets used in this meta-analysis do not account for adaptive responses, ecological fitness and ecosystem interactions, the question remains as to how these physiological responses play out in the natural environment.

Description: In open-ocean regions, as is the Eastern Tropical North Atlantic (ETNA), pelagic production is the main source of dissolved organic matter (DOM) and is affected by dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations. Changes in pelagic production under nutrient amendments were shown to also modify DOM quantity and quality. However, little information is available about the effects of nutrient variability on chromophoric (CDOM) and fluorescent (FDOM) DOM dynamics. Here we present results from two mesocosm experiments ("Varied P" and "Varied N") conducted with a natural plankton community from the ETNA, where the effects of DIP and DIN supply on DOM optical properties were studied. CDOM accumulated proportionally to phytoplankton biomass during the experiments. Spectral slope (S) decreased over time indicating accumulation of high molecular weight DOM. In Varied N, an additional CDOM portion, as a result of bacterial DOM reworking, was determined. It increased the CDOM fraction in DOC proportionally to the supplied DIN. The humic-like FDOM component (Comp.1) was produced by bacteria proportionally to DIN supply. The protein-like FDOM component (Comp.2) was released irrespectively to phytoplankton or bacterial biomass, but depended on DIP and DIN concentrations. Under high DIN supply, Comp.2 was removed by bacterial reworking, leading to an accumulation of humic-like Comp.1. No influence of nutrient availability on amino acid-like FDOM component in peptide form (Comp.3) was observed. Comp.3 potentially acted as an intermediate product during formation or degradation of Comp.2. Our findings suggest that changes in nutrient concentrations may lead to substantial responses in the quantity and quality of optically active DOM and, therefore, might bias results of the applied in situ optical techniques for an estimation of DOC concentrations in open-ocean regions.

Description: Upwelling of nutrient loaded water masses with low inorganic nitrogen (N) to phosphorus (P) ratios is thought to favor non-Redfield primary production by phytoplankton species adapted to exponential growth. Additionally, an excess of P (P*) in OMZ-influenced waters is also supposed to provide a niche for nitrogen fixing organisms. In order to assess the influence of low inorganic nutrient ratios on the stoichiometry and composition of primary producers, biogeochemical measurements were carried out in the eastern tropical South Pacific during R/V Meteor cruise M93. A succession of different functional types of phytoplankton was observed along onshore – offshore transects with diatoms dominating the productive upwelling region, while haptophytes, cryptophytes and crysophytes prevailed in the more oligotrophic open ocean. Simultaneously, particulate organic nitrogen to phosphorus ratios increased with increasing distance from shore. The stoichiometry of organic matter, however, always exceeded ratios of 16:1, although nutrient supply ratios were below Redfield proportions in the whole sampling area. A considerable amount of P* was detected in the surface ocean layer above the shelf, which decreased as water masses were advected beyond the shelf slope. Phytoplankton pigment analyses with HPLC revealed the existence of diazotrophic marker pigments in the study area, hinting towards a local replenishment of the N-deficit via nitrogen fixation.

Description: Gel particles such as the polysaccharidic transparent exopolymer particles (TEP) and the proteinaceous Coomassie stainable particles (CSP) play an important role in marine biogeochemical and ecological processes like particle aggregation and export, or microbial nutrition and growth. So far, effects of nutrient availability or of changes in nutrient ratios on gel particle production and fate are not well understood. The tropical ocean includes large oxygen minimum zones, where nitrogen losses due to anaerobic microbial activity result in a lower supply of nitrate relative to phosphate to the euphotic zone. Here, we report of two series of mesocosm experiments that were conducted with natural plankton communities collected from the eastern tropical North Atlantic (ETNA) close to Cape Verde in October 2012. The experiments were performed to investigate how different phosphate (experiment 1, Varied P: 0.15–1.58 μmol L−1) or nitrate (experiment 2, Varied N: 1.9–21.9 μmol L−1) concentrations affect the abundance and size distribution of TEP and CSP. In the days until the bloom peak was reached, a positive correlation between gel particle abundance and Chl a concentration was determined, linking the release of dissolved gel precursors and the subsequent formation of gel particles to autotrophic production. After the bloom peak, gel particle abundance remained stable or even increased, implying a continued partitioning of dissolved into particulate organic matter after biomass production itself ceased. During both experiments, differences between TEP and CSP dynamics were observed; TEP were generally more abundant than CSP. Changes in size distribution indicated aggregation of TEP after the bloom, while newly formed CSP decomposed. Abundance of gel particles clearly increased with nitrate concentration during the second experiment, suggesting that changes in [DIN] : [DIP] ratios can affect gel particle formation with potential consequences for carbon and nitrogen cycling as well as food web dynamics in tropical ecosystems.

Description: The COVID-19 pandemic necessitates a change in conference formats for 2020. This shift offers a unique opportunity to address long-standing inequities in access and issues of sustainability associated with traditional conference formats, through testing online platforms. However, moving online is not a panacea for all of these concerns, particularly those arising from uneven distribution of access to the Internet and other technology. With conferences and events being forced to move online, this is a critical juncture to examine how online formats can be used to best effect and to reduce the inequities of in-person meetings. In this article, we highlight that a thoughtful and equitable move to online formats could vastly strengthen the global socio-ecological research community and foster cohesive and effective collaborations, with ecology and society being the ultimate beneficiaries.

Description: Deoxygenation of shelf waters is known to enhance the sedimentary source of nutrients, including PO4 and Fe, into the water column. Yet substantial uncertainty remains with respect to what fraction of these nutrients are transported into the euphotic layer and ultimately advected offshore and thus how an increased benthic supply of nutrients affects offshore ecosystem productivity. In October 2015 a conservative tracer (CF3SF5) was released within the bottom boundary layer at 3 sites along the Peruvian coastline within the Peruvian Oxygen Minimum Zone (OMZ). During March/April 2017 the dispersion of this tracer along the shelf and laterally into the ETSP was tracked alongside a comprehensive suite of nutrient measurements including macronutrients (NO3, NO2, PO4, Si), dissolved organic nitrogen/phosphorus (DON/DOP) and dissolved trace metals (including Fe and Co). The tracer distribution demonstrated transport northwards along the shelf, in addition to lateral advection offshore, which was most pronounced along a transect at 17° S. Here we compare and contrast the distribution of our inert tracer with a broad range of nutrients along the 17° S transect in order to understand how internal cycling affects the lateral advection of bio-essential nutrients from anoxic shelf benthic boundary waters to offshore ecosystems. Constraining the relative strength of laterally advected Fe and bioavailable nitrogen/phosphorus sources will be critical to understanding how primary productivity and microbial community structure within the Peruvian Oxygen Minimum Zone will respond to intensified deoxygenation over the Peruvian shelf.

Description: In the coastal upwelling system of the eastern tropical North Atlantic (ETNA), dissolved organic phosphorus (DOP) production and its release by phytoplankton is increasingly recognized as an important supply mechanism for phosphorus (P) to the oligotrophic open ocean. Photoautotrophs and dinitrogen (N2) fixing organisms (diazotrophs) are thought to be able to consume DOP, thus alleviating P stress in areas with extremely low dissolved inorganic phosphate (DIP) concentrations. In this study, nutrient addition bioassay experiments were conducted to investigate the bioavailability of different organic and inorganic phosphorus components to the phytoplankton community in the ETNA. We specifically examined how DIP and DOP amendments affected N2 fixation in that area. Our observations showed that phytoplankton growth was primarily limited by nitrogen. Moreover, DIP addition resulted in a significant increase in N2 fixation rates in almost all experiments relative to control treatments, suggesting that diazotrophs were limited by P availability in our study. At very oligotrophic sampling stations, all P compounds stimulated N2 fixation rates compared to the control. This demonstrates the capability of the diazotrophic community to utilize various DOP compounds, especially under P limiting conditions. Our findings provide a mechanism explaining how high N2 fixation rates can be sustained under extremely low DIP concentrations in the oligotrophic North Atlantic.

Description: Coastal upwelling systems associated to the eastern continental margins of the Atlantic and Pacific Oceans are among the most productive realms of the marine ecosystems. Although they only occupy a small area, they play a globally important role in the cycling of nitrogen (N), phosphorus (P) and other biologically relevant elements. In subsurface waters of upwelling systems, oxygen minimum zones (OMZs) persist as a result of biological degradation and sluggish ventilation. Reduced oxygen concentrations influence redox sensitive nutrient inventories by promoting N loss processes and P release from the sediment. Hence, water masses upwelled to the surface feature low N:P ratios that deviate from canonical Redfield proportions of 16:1. Due to the excess P over N, upwelling systems are thought to favor the growth of dinitrogen (N2) fixing organism (diazotrophs) that could potentially restore inorganic nutrient ratios back to Redfield proportions and replenish the N deficit in those waters. Contrary to this assumption, the presence of nondiazotrophic phytoplankton utilizing nutrients in lower than Redfield proportions has been suggested to eliminate the niche for diazotrophs. Thus, the dominance of either Redfield or non-Redfield primary production is thought to determine the amount of N fixed in upwelling systems. In light of expanding OMZs and the predicted modification of nutrient inventories, this doctoral dissertation aimed to investigate the impact of changing N:P supply ratios on phytoplankton and organic matter composition. Moreover, the potential of primary producers to modify nutrient supply anomalies and their role in coupling or decoupling sources and sinks of fixed N was assessed. To accomplish this, nutrient manipulation experiments and a field study were conducted in the eastern tropical North Atlantic (ETNA) and eastern tropical South Pacific (ETSP).

Description: This deliverable explains how the OceanNETs project ensures that it is compliant with data protection requirements. It outlines the methodology chosen to ensure compliance, as well as providing an overview of relevant tasks, and the measures employed to ensure compliance.