Bewertung der biogeochemischen Rolle und Kontrolle der Stickstofffixierung im östlichen tropischen Südpazifik

The Eastern Tropical South Pacific (ETSP), hosts an extensive Oxygen Minimum Zone (OMZ) in the water column which has a major imprint on local and global marine biogeochemistry. Due to the low oxygen conditions within the OMZ, microbial processes of nitrogen (N) loss, such as anammox and denitrification are sustained in the water column. These processes result in a pronounced N deficit which reduces bioavailable N for primary productivity and thus influences fisheries production in the region. To maintain a balanced marine N inventory regionally in ETSP, the N deficit would have to be compensated by N inputs via upwelling or N2 fixation. A classical assumption is that N2 fixation is favoured by iron (Fe) availability and a surplus of inorganic phosphate relative to inorganic nitrogen (this relativity is defined as P*), both conditions are present in the ETSP. Over the past decades, this assumption has been integrated into most coupled circulation and N-cycle biogeochemical models. These models indicate that there is a close spatial link between areas of high N loss, generally confined to OMZs and N2 fixation. On the contrary, other biogeochemical models have revealed that a close spatial link between N loss and N2 fixation in OMZ areas may give rise to run-away loss of fixed N in the ETSP, ultimately destabilizing the regional marine N inventory. While N loss processes are relatively well understood in the ETSP, the lack of a comprehensive dataset that resolves N2 fixation rates in both space and time constraints an accurate assessment of the regional marine N inventory, potential feedback mechanisms, and their impact on N turnover and productivity. Therefore, the main objective of this doctoral dissertation was to investigate the spatial distribution of N2 fixation relative to N loss in the ETSP, in order to understand potential feedbacks in the regional N cycle

Toward a Solution of the “Peruvian Puzzle”: Pelagic Food-Web Structure and Trophic Interactions in the Northern Humboldt Current Upwelling System Off Peru

The northern Humboldt Current upwelling system (HCS) belongs to the most productive marine ecosystems, providing five to eight times higher fisheries landings per unit area than other coastal upwelling systems. To solve this “Peruvian puzzle”, to elucidate the pelagic food-web structure and to better understand trophic interactions in the HCS, a combined stable isotope and fatty acid trophic biomarker approach was adopted for key zooplankton taxa and higher trophic positions with an extensive spatial coverage from 8.5 to 16°S and a vertical range down to 1,000 m depth. A pronounced regional shift by up to ∼5‰ in the δ15N baseline of the food web occurred from North to South. Besides regional shifts, δ15N ratios of particulate organic matter (POM) also tended to increase with depth, with differences of up to 3.8‰ between surface waters and the oxygen minimum zone. In consequence, suspension-feeding zooplankton permanently residing at depth had up to ∼6‰ higher δ15N signals than surface-living species or diel vertical migrants. The comprehensive data set covered over 20 zooplankton taxa and indicated that three crustacean species usually are key in the zooplankton community, i.e., the copepods Calanus chilensis at the surface and Eucalanus inermis in the pronounced OMZ and the krill Euphausia mucronata, resulting in an overall low number of major trophic pathways toward anchovies. In addition, the semi-pelagic squat lobster Pleuroncodes monodon appears to play a key role in the benthic-pelagic coupling, as indicated by highest δ13C’ ratios of −14.7‰. If feeding on benthic resources and by diel vertical migration, they provide a unique pathway for returning carbon and energy from the seafloor to the epipelagic layer, increasing the food supply for pelagic fish. Overall, these mechanisms result in a very efficient food chain, channeling energy toward higher trophic positions and partially explaining the “Peruvian puzzle” of enormous fish production in the HCS

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

Nitrogen and carbon fixation rates and organic matter concentrations from water column samples during RV Maria S. Merian cruise MSM80 off the northern Humboldt Upwelling System

This data is part of the BMBF projects CUSCO (Coastal Upwelling Systems in a Changing Ocean) and BioTip subproject Humboldt Tipping. Here we report water column nitrogen fixation, carbon fixation rates and particulate organic matter composition from the upper 300 m. Data was collected during cruise number MSM80 with research vessel Maria S. Merian from 23.12.2018 - 30.01.2019 (from Panama to Valparaiso) in the Humboldt Upwelling system off the Eastern Tropical south Pacific. Samples were taken by CTD- rosette sampler from different depths, injected with 15N labelled N2 gas based on the modified dissolution method (Großkopf et al., 2012 and Mohr et al., 2010) and additionally 13C labeled sodium bicarbonate. Samples were incubated for 24 hours at light intensities that resemble the in situ light conditions. After incubation a volume of the sample (20 - 1500 ml) was filtered onto a pre-combusted Whatman GF/75 filter. Filters were frozen, transported to the institute on dry ice and measured on a mass spectrometer for Delta15N and 13C (Delta V Advantage Isotope Ratio MS, ThermoFisher) with the ConFlo IV interface (ThermoFisher). Nitrogen fixation rates were calculated based on Montoya et al (1996) while carbon fixation rates were calculated based on the equation by Slawyk et al. (1977). Limits of detection (LOD) and minimum quantifiable rates (MQR) for Nitrogen fixation rates were calculated according to the criteria described by White et al. (2020) and standard error propagation methods described in Gradoville et al. (2017) respectively. POP and Biogenic silica concentrations were measured spectrophotometrically following Hansen and Koroleff (1999)

Dissolved inorganic and organic nutrients from water column samples during RV Maria S. Merian cruise MSM80 off the northern Humboldt Upwelling System

This data is part of the BMBF projects CUSCO (Coastal Upwelling Systems in a Changing Ocean) and BioTip subproject Humboldt Tipping. Data was collected during cruise number MSM80 with research vessel Maria S. Merian from 23.12.2018 - 30.01.2019 (from Panama to Valparaiso) in the Humboldt Upwelling system off the Eastern Tropical south Pacific. Samples were taken by CTD- rosette sampler from different depths and analysed onboard for dissolved inorganic nutrients and total dissolved nutrients. Triplicate nutrient samples were analysed for concentrations with an autosampler (XY-2 autosampler, SEAL Analytical) and a continuous flow analyzer (QUAAtro Autoanalyzer, SEAL Analytical) using standard colorimetric and flourometric methods by Kastriot Qelaj. Dissolved organic nutrients were calculated as the difference of the two for respective nitrogen and phosphorous nutrients. Phosphate was measured according to Murphy and Riley (1962). Ammonium was measured fluorometrically based on Holmes et al. (1999). An empty cell means that corresponding nutrient samples were not taken for the respective depth

KOSMOS 2020 Peru mesocosm study on ecosystem responses to different light and upwelling intensities: particulate organic matter concentrations of water samples

This data is part of the BMBF project CUSCO (Coastal Upwelling Systems in a Changing Ocean). Here we report particulate organic matter concentrations collected during a 35-day experiment where we enclosed natural plankton communities in in-situ mesocosms off Peru. The experiment investigated the interactive effects of light and upwelling on the Humboldt upwelling ecosystem by mimicking a gradient of upwelling intensities (0%, 15%, 30%, 45% and 60%) under summer-time high light and winter-time low light. Integrated seawater samples from a depth between 0 and 10m were collected using a 5L Integrating Water sampler (IWS; Hydro-Bios, Kiel). Samples (150-1000 ml) were filtered onto a 0.7 µm pre-combusted glass-fiber filters (GFF, Whatman). The filters were acidified and then dried in the oven at 60 °C for 24 hours and total particulate carbon (TPC), particulate organic carbon (POC) and nitrogen (PON) content were measured using a CN analyzer (Euro EA3000, HEKAtech GmbH, Wegberg, Germany). POP filters were autoclaved for 30 min in 100 mL Schott Duran glass bottles using an oxidizing decomposition solution (Merck, catalogue no. 112936) to convert organic phosphate to orthophosphate. Phosphate concentrations were determined spectrophotometrically following Hansen and Koroleff (1999; doi:10.1002/9783527613984.ch10)

Coastal N 2 fixation rates coincide spatially with N loss in the Humboldt Upwelling System off Peru

Marine nitrogen (N2) fixation supports significant primary productivity in the global ocean. However, in one of the most productive regions of the world ocean, the northern Humboldt Upwelling System (HUS), the magnitude and spatial distribution of this process remains poorly characterized. This study presents a spatially resolved dataset of N2 fixation rates across six coastal transects of the northern HUS off Peru (8°S – 16°S) during austral summer. N2 fixation rates were detected throughout the waters column including within the OMZ between 12°S and 16°S. N2 fixation rates were highest where the subsurface Oxygen Minimum Zone (OMZ, O2 <20 µmol L-1) was most intense and estimated nitrogen (N) loss was highest. There, rates were measured throughout the water column. Hence the vertical and spatial distribution of rates indicates colocation of N2 fixation with N loss in the coastal productive waters of the northern HUS. Despite high phosphate and total dissolvable iron (TdFe) concentrations throughout the study area, N2 fixation was still generally low (1.19 ± 3.81 nmol L-1 d-1) and its distribution could not be directly explained by these two factors. Our results suggest that the distribution was likely influenced by a complex interplay of environmental factors including phytoplankton biomass and organic matter availability, and potentially iron, or other trace metal (co)-limitation of both N2 fixation and primary production. In general, our results support previous conclusions that N2 fixation in the northern HUS plays a minor role as a source of new N and to replenish the regional N loss. Key Points: A north-to-south pattern in N2 fixation rates was observed implying increased N turnover between 12°S and 16°S where N loss was pronounced Highest N2 fixation rates were measured in coastal productive waters above and within the OMZ, showing no clear relationship with Fe or P The magnitude of N2 fixation was low compared to predictions, estimated to account for ∼0.3% of primary production and <2% of local N los

Trace metal data from water column samples during RV Maria S. Merian cruise MSM80 off the northern Humboldt Upwelling System

This data is part of the BMBF projects CUSCO (Coastal Upwelling Systems in a Changing Ocean) and BioTip subproject Humboldt Tipping. The file contains total dissolvable trace metal concentrations (Fe, Cd, Ni, Cu, Zn and Co) from various depths of sampled stations. Trace metal concentrations were determined via Inductively coupled plasma mass spectrometry (ICP-MS, Element XR, ThermoFisher Scientific) after pre-concentration as per Rapp et al. (2017)

KOSMOS 2020 Peru mesocosm study on ecosystem responses to different light and upwelling intensities: carbon and nitrogen uptake rates from water samples

This data is part of the BMBF project CUSCO (Coastal Upwelling Systems in a Changing Ocean). Here we report carbon and nitrogen uptake rates by phytoplankton collected during a 35-day experiment where we enclosed natural plankton communities in in-situ mesocosms off Peru. The experiment investigated the interactive effects of light and upwelling on the Humboldt upwelling ecosystem by mimicking a gradient of upwelling intensities (0%, 15%, 30%, 45% and 60%) under summer-time high light and winter-time low light. Integrated seawater samples from a depth between 0 and 10m were collected using a 5L Integrating Water sampler (IWS; Hydro-Bios, Kiel). Samples were spiked with 13C labelled sodium bicarbonate and 15N labelled potassium nitrate. Samples were incubated for 24 hours at light intensities that resemble the light conditions in the mesocosms. After incubation a subsample (150-1000 ml) was filtered onto a 0.7 µm pre-combusted glass-fiber filters (GFF, Whatmann). The filters were acidified, then dried in the oven at 60 °C and later measured on a mass spectrometer for Delta15N and 13C (Delta V Advantage Isotope Ratio MS, ThermoFisher) with the ConFlo IV interface (ThermoFisher). Carbon and nitrogen uptake rates were calculated based on Slawyk et al. (1977, doi:10.4319/lo.1977.22.5.0925) and Shiozaki et al. (2009, doi:10.3354/meps07837) respectively

KOSMOS 2020 Peru mesocosm study on ecosystem responses to different light and upwelling intensities: HPLC pigment concentrations and phytoplankton community composition based on CHEMTAX analysis of water samples

This data is part of the BMBF project CUSCO (Coastal Upwelling Systems in a Changing Ocean). Here we report pigment concentrations and the phytoplankton community composition based on CHEMTAX analysis during a 35-day experiment, where we enclosed natural plankton communities in in-situ mesocosms off Peru. The experiment investigated the interactive effects of light and upwelling on the Humboldt upwelling ecosystem by mimicking a gradient of upwelling intensities (0%, 15%, 30%, 45% and 60%) under summer-time high light and winter-time low light. Integrated seawater samples from a depth between 0 and 10m were collected using a 5L Integrating Water sampler (IWS; Hydro-Bios, Kiel). To obtain pigment concentrations 0.15 – 1L of water sampled from the water column were filtered onto glass-fiber filters (precombusted at 450°C for 6h, GFF, 0.7 µm nominal pore size, Whatman) using a low vacuum of 200 mbar. Afterwards the filters were stored in cryovials at -80°C until analysis of photosynthetic pigments using reverse-phase high-performance liquid chromatography (HPLC) following (Barlow et al., 1997; doi:10.3354/meps161303) as described by (Paul et al., 2015; doi:10.5194/bg-19-5911-2022). The phytoplankton community composition was calculated with CHEMTAX, which classifies phytoplankton taxa based upon taxon-specific pigment ratios (Mackey et al., 1996; doi:10.3354/meps144265). The values for the Peruvian upwelling system determined by (DiTullio et al., 2005; doi:10.4319/LO.2005.50.6.1887) as described by (Meyer et al., 2017; doi:10.3389/fmars.2017.00001) were used as pigment ratios. As phytoplankton microscopy revealed high abundances of the raphydophite Fibrocapsa japonica, the initial pigment matrix was complemented by ratios for F. japonica from (Laza-Martinez et al., 2007; doi:10.1093/plankt/fbm069) from the Atlantic