Distribution of transparent exopolymer particles in the easter tropical south Pacific

Important for the carbon export to the Deep Sea, Transparent Exopolymer Particles (TEP) also serve as nutritional substrate and attachment surfaces for bacteria, supporting biological oxygen consumption. Spatial distribution of TEP in the Eastern South Pacific Ocean (ESP), an area influenced by a highly productive upwelling system and underlying extensive Oxygen Minimum Zone (OMZ), is largely unknown. In attempt to recognize how TEP stocks are affected by complex biogeochemistry of ESP and vice versa, we determined TEP distribution in the ESP (12°-14°S, 76°-79°W) during the Meteor 93 cruise (February - March 2013). Highest TEP concentrations (>1900µgXeq/l) were observed close to the coast, coinciding with upwelling of nutrient-rich waters. Generally TEP accumulated in the oxygenated photic layer and correlated significantly with chlorophyll-a-fluorescence (r=0.7, n=323, P<0.05). At the upper boundary the OMZ (70 m), TEP concentrations were moderate (<80µgXeq/l), slightly attenuating toward its lower border (<60µgXeq/l; 500m depth). The role of TEP for organic matter cycling in the ESP and their potential influence on oxygen and carbon fluxes in the OMZ are discussed

Effects of nitrate and phosphate supply on chromophoric and fluorescent dissolved organic matter in the Eastern Tropical North Atlantic: a mesocosm study

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

Effects of varied nitrate and phosphate supply on polysaccharidic and proteinaceous gel particles production during tropical phytoplankton bloom experiments

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

Chromophoric and fluorescent dissolved organic matter in and above the oxygen minimum zone off Peru

Key Points: - Two amino acid-like and three humic-like FDOM components were found in and above the oxygen minimum zone off the coast of Peru - The distribution of CDOM and amino acid-like FDOM covaried with chl a, suggesting phytoplankton as their major source - Presence of DOM microbial reworking and DOM release by anoxic sediment was illustrated by the distribution of humic-like FDOM As a result of nutrient upwelling, the Peruvian coastal system is one of the most productive regions in the ocean. Sluggish ventilation of intermediate waters, characteristic for the Eastern Tropical South Pacific (ETSP) and microbial degradation of a high organic matter load promotes deoxygenation at depth. Dissolved organic matter (DOM) plays a key role in microbial respiration and carbon cycling, but little is known on DOM distribution and cycling in the ETSP. DOM optical properties give important insights on DOM sources, structure and biogeochemical reactivity. Here, we present data and a conceptual view on distribution and cycling of chromophoric (CDOM) and fluorescent (FDOM) DOM in and above the oxygen minimum zone (OMZ) off Peru. Five fluorescent components were identified during PARAFAC analysis. Highest intensities of CDOM and of the amino acid-like fluorescent component (C3) occurred above the OMZ and coincided with maximum chl a concentrations, suggesting phytoplankton productivity as major source. High intensities of a marine humic-like fluorescent component (C1), observed in subsurface waters, indicated in situ microbial reworking of DOM. FDOM release from inner shelf sediment was determined by seawater analysis and continuous glider sensor measurement and included a humic-like component (C2) with a signature typical for terrestrially derived humic acids. Upwelling supplied humic-like substances to the euphotic zone. Photo-reactions were likely involved in the production of a humic-like fluorescent component (C5). Our data show that variable biological and physical processes need to be considered for understanding DOM cycling in a highly dynamic coastal upwelling system like the ETSP off Peru

Chromophoric and fluorescent dissolved organic matter in and above the oxygen minimum zone off Peru

Key Points: - Two amino acid-like and three humic-like FDOM components were found in and above the oxygen minimum zone off the coast of Peru - The distribution of CDOM and amino acid-like FDOM covaried with chl a, suggesting phytoplankton as their major source - Presence of DOM microbial reworking and DOM release by anoxic sediment was illustrated by the distribution of humic-like FDOM As a result of nutrient upwelling, the Peruvian coastal system is one of the most productive regions in the ocean. Sluggish ventilation of intermediate waters, characteristic for the Eastern Tropical South Pacific (ETSP) and microbial degradation of a high organic matter load promotes deoxygenation at depth. Dissolved organic matter (DOM) plays a key role in microbial respiration and carbon cycling, but little is known on DOM distribution and cycling in the ETSP. DOM optical properties give important insights on DOM sources, structure and biogeochemical reactivity. Here, we present data and a conceptual view on distribution and cycling of chromophoric (CDOM) and fluorescent (FDOM) DOM in and above the oxygen minimum zone (OMZ) off Peru. Five fluorescent components were identified during PARAFAC analysis. Highest intensities of CDOM and of the amino acid-like fluorescent component (C3) occurred above the OMZ and coincided with maximum chl a concentrations, suggesting phytoplankton productivity as major source. High intensities of a marine humic-like fluorescent component (C1), observed in subsurface waters, indicated in situ microbial reworking of DOM. FDOM release from inner shelf sediment was determined by seawater analysis and continuous glider sensor measurement and included a humic-like component (C2) with a signature typical for terrestrially derived humic acids. Upwelling supplied humic-like substances to the euphotic zone. Photo-reactions were likely involved in the production of a humic-like fluorescent component (C5). Our data show that variable biological and physical processes need to be considered for understanding DOM cycling in a highly dynamic coastal upwelling system like the ETSP off Peru

Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterized by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilized by microbial communities. The degradation of POM is associated with dissolved organic matter (DOM) production and reworking. The release of recalcitrant DOM to the overlying waters may represent an important organic matter escape mechanism from remineralization within sediments but received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion–driven fluxes and net in situ fluxes of DOC and DON in order to investigate processes affecting DOM cycling at the sediment–water interface along a transect 12° S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore-water DOC and DON accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralization within sediments. High DON accumulation resulted in very low pore water DOC / DON ratios (> 1) which could be caused by either an "imbalance" in DOC and DON remineralization, or to the presence of an additional nitrogen source. Diffusion driven fluxes of DOC and DON exhibited high spatial variability. They varied from 0.2–0.1 mmol m−2 d−1 to 2.52–1.3 mmol m−2 d−1 and from −0.042–0.02 mmol m−2 d−1 to 3.32–1.7 mmol m−2 d−1, respectively. Generally low net in situ DOC and DON fluxes as well as steepening of spectral slope (S) of CDOM and accumulation of humic-like FDOM at the near-bottom waters over time indicated active microbial DOM utilization at the sediment–water interface, potentially stimulated by nitrate (NO3−) and nitrite (NO2−). The microbial DOC utilization rates, estimated in our study, may be sufficient to support denitrification rates of 0.2–1.4 mmol m−2 d−1, suggesting that sediment release of DOM contributes substantially to nitrogen loss processes in the ETSP off Peru

Diapycnal dissolved organic matter supply into the upper Peruvian oxycline

The eastern tropical South Pacific (ETSP) hosts the Peruvian upwelling system, which represents one of the most productive areas in the world ocean. High primary production followed by rapid heterotrophic utilization of organic matter supports the formation of one of the most intense oxygen minimum zones (OMZs) in the world ocean, where dissolved oxygen (O2) concentrations reach less than 1 µmol kg−1. The high productivity leads to an accumulation of dissolved organic matter (DOM) in the surface layers that may serve as a substrate for heterotrophic respiration. However, the importance of DOM utilization for O2 respiration in the Peruvian upwelling system in general and for shaping the upper oxycline in particular remains unclear so far. This study reports the first estimates of diapycnal fluxes and supply of O2, dissolved organic carbon (DOC), dissolved organic nitrogen, dissolved hydrolysable amino acids (DHAA) and dissolved combined carbohydrates (DCCHO) for the ETSP off Peru. Diapycnal flux and supply estimates were obtained by combining measured vertical diffusivities and solute concentration gradients. They were analysed together with the molecular composition of DCCHO and DHAA to infer the transport of labile DOM into the upper OMZ and the potential role of DOM utilization for the attenuation of the diapycnal O2 flux that ventilates the OMZ. The observed diapycnal O2 flux (50 mmol O2 m−2 d−1 at maximum) was limited to the upper 80 m of the water column; the O2 supply of ∼1 µmol kg−1 d−1 was comparable to previously published O2 consumption rates for the North and South Pacific OMZs. The diapycnal DOM flux (31 mmol C m−2 d−1 at maximum) was limited to ∼30 m water depth, suggesting that the labile DOM is extensively consumed within the upper part of the shallow oxycline off Peru. The analyses of DCCHO and DHAA composition support this finding, suggesting that DOM undergoes comprehensive remineralization within the upper part of the oxycline, as the DOM within the core of the OMZ was found to be largely altered. Estimated by a simple equation for carbon combustion, aerobic respiration of DCCHO and DHAA, supplied by diapycnal mixing (0.46 µmol kg−1 d−1 at maximum), could account for up to 38 % of the diapycnal O2 supply in the upper oxycline, which suggests that DOM utilization plays a significant role for shaping the upper oxycline in the ETSP

Sediment release of dissolved organic matter in the oxygen minimum zone off Peru

In combination to sluggish ventilation by ocean currents, the nutrient upwelling and high surface productivity, followed by organic matter remineralization, leads to a pronounced oxygen minimum zone (OMZ) in the eastern tropical South Pacific (ETSP). There, oxygen concentrations drop below 1 �mol/kg at a water depth <80 m. The high productivity results in the supply of organic matter (OM) to the anoxic sediments and its utilization by heterotrophic communities. The microbial utilization of OM under anoxia leads to nitrogen loss processes, and an accumulation of sulphide and methane. The proximity of the OMZ to the ocean surface in the ETSP may lead to an active outgassing of climate relevant products of the anoxic OM remineralization. The degradation of OM in sediments is associated with production of dissolved organic matter (DOM) from organic particles (POM) that is further remineralized into inorganic nutrients and dissolved inorganic carbon, which then can be released back to the water column, fuelling productivity. Part of the DOM pool may be released to the overlying water column and serve as ligands for micronutrients, such as iron, or provide an additional substrate for microbial communities to respire, affecting overlying water column biogeochemistry. Despite the potential relevance for biogeochemical processes, the quality of the DOM in the pore waters that may be released to the overlying water column has been barely studied in the ETSP off Peru. High spatial resolution measurements of DOM fluorescence (FDOM) during the research cruise M93 (Feb-March 2013) indicated elevated intensities near the sediments in the ETSP off Peru. Those intensities were interpreted as a sediment release of DOM, the quantification of dissolved organic carbon (DOC) flux, however, was not possible at the time. To estimate DOM fluxes and DOM quality, DOC and DOM samples were collected from the sediment pore waters and from benthic incubation chambers from six stations along the 12°S transect in the Peruvian upwelling in 2017 (cruises M136, M137). Samples were collected using a multiple-corer and by Biogeochemical Observatories, respectively. Here, we evaluate DOC fluxes from the sediments and relate them to the measurements of FDOM. We evaluate the quality of DOM by Excitation Emission spectroscopy, followed by parallel factor analysis. The possible implications of the DOM release for water column biogeochemistry are discussed

Changing nutrient stoichiometry affects phytoplankton production, DOP accumulation and dinitrogen fixation – a mesocosm experiment in the eastern tropical North Atlantic

Ocean deoxygenation due to climate change may alter redox-sensitive nutrient cycles in the marine environment. The productive eastern tropical North Atlantic (ETNA) upwelling region may be particularly affected when the relatively moderate oxygen minimum zone (OMZ) deoxygenates further and microbially driven nitrogen (N) loss processes are promoted. Consequently, water masses with a low nitrogen to phosphorus (N : P) ratio could reach the euphotic layer, possibly influencing primary production in those waters. Previous mesocosm studies in the oligotrophic Atlantic Ocean identified nitrate availability as a control of primary production, while a possible co-limitation of nitrate and phosphate could not be ruled out. To better understand the impact of changing N : P ratios on primary production and N2 fixation in the ETNA surface ocean, we conducted land-based mesocosm experiments with natural plankton communities and applied a broad range of N : P ratios (2.67–48). Silicic acid was supplied at 15 µmol L−1 in all mesocosms. We monitored nutrient drawdown, biomass accumulation and nitrogen fixation in response to variable nutrient stoichiometry. Our results confirmed nitrate to be the key factor determining primary production. We found that excess phosphate was channeled through particulate organic matter (POP) into the dissolved organic matter (DOP) pool. In mesocosms with low inorganic phosphate availability, DOP was utilized while N2 fixation increased, suggesting a link between those two processes. Interestingly this observation was most pronounced in mesocosms where nitrate was still available, indicating that bioavailable N does not necessarily suppress N2 fixation. We observed a shift from a mixed cyanobacteria–proteobacteria dominated active diazotrophic community towards a diatom-diazotrophic association of the Richelia-Rhizosolenia symbiosis. We hypothesize that a potential change in nutrient stoichiometry in the ETNA might lead to a general shift within the diazotrophic community, potentially influencing primary productivity and carbon export