Hermann W. Bange

Tidal-induced mixing and diapycnal nutrient fluxes in the Mauritanian upwelling region

The Mauritanian coastal area is one of the most biologically productive upwelling regions in the world ocean. Shipboard observations carried out during maximum upwelling season and short-term moored observations are used to investigate diapycnal mixing processes and to quantify diapycnal fluxes of nutrients. The observations indicate strong tide-topography interactions that are favored by near-critical angles occurring on large parts of the continental slope. Moored velocity observations reveal the existence of highly nonlinear internal waves and bores and levels of internal wave spectra are strongly elevated near the buoyancy frequency. Dissipation rates of turbulent kinetic energy at the slope and shelf determined from microstructure measurements in the upper 200 m averages to ɛ = 5 × 10−8 W kg−1. Particularly elevated dissipation rates were found at the continental slope close to the shelf break, being enhanced by a factor of 100 to 1000 compared to dissipation rates farther offshore. Vertically integrated dissipation rates per unit volume are strongest at the upper continental slope reaching values of up to 30 mW m−2. A comparison of fine-scale parameterizations of turbulent dissipation rates for shelf regions and the open ocean to the measured dissipation rates indicates deficiencies in reproducing the observations. Diapycnal nitrate fluxes above the continental slope at the base of the mixed layer yielding a mean value of 12 × 10−2 μmol m−2 s−1 are amongst the largest published to date. However, they seem to only represent a minor contribution (10% to 25%) to the net community production in the upwelling region.

N-loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment

Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the oceans N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (e; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddys center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddys center (up to ~40 µmol L−1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable e for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall e for N-loss was calculated to be only ~13-14‰ (as compared to canonical values of ~20-30‰) assuming a closed system and only slightly higher assuming an open system (16-19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate e for NO3− reduction to NO2− and NO2− reduction to N2 of ~16-21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2-, confirm a role for NO2− oxidation in increasing the apparent e associated with NO3− reduction. The lower e for NO3− and NO2− reduction as well as N-loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N-loss.

Estimation of the Atmospheric Flux of Nutrients and Trace Metals to the Eastern Tropical North Atlantic Ocean

Atmospheric deposition contributes potentially significant amounts of the nutrients iron, nitrogen and phosphorus (via mineral dust and anthropogenic aerosols) to the oligotrophic tropical North Atlantic Ocean. Transport pathways, deposition processes and source strengths contributing to this atmospheric flux are all highly variable in space and time. Atmospheric sampling was conducted during 28 research cruises through the Eastern Tropical North Atlantic (ETNA) over a 12 year period and a substantial dataset of measured concentrations of nutrients and trace metals in aerosol and rainfall over the region was acquired. This database was used to quantify (on a spatial- and seasonal-basis) the atmospheric input of ammonium, nitrate, soluble phosphorus and soluble and total iron, aluminium and manganese to the ETNA. The magnitude of atmospheric input varies strongly across the region, with high rainfall rates associated with the Inter-tropical Convergence Zone contributing to high wet deposition fluxes in the south, particularly for soluble species. Dry deposition fluxes of species associated with mineral dust exhibited strong seasonality, with highest fluxes associated with winter-time low-level transport of Saharan dust. Overall (wet plus dry) atmospheric inputs of soluble and total trace metals were used to estimate their soluble fractions. These also varied with season and were generally lower in the dry north than in the wet south. The ratio of ammonium plus nitrate to soluble iron in deposition to the ETNA was lower than the N:Fe requirement for algal growth in all cases, indicating the importance of the atmosphere as a source of excess iron.

Nitrogen processes in coastal and marine ecosystems

Nature of the problem • Nitrogen (N) inputs from human activities have led to ecological deteriorations in large parts of the coastal oceans along European coastlines, including harmful algae blooms and anoxia. • Riverine N-loads are the most pronounced nitrogen sources to coasts and estuaries. Other signifi cant sources are nitrogen in atmospheric deposition and fi xation. Approaches • Th is chapter describes all major N-turnover processes which are important for the understanding of the complexity of marine nitrogen cycling, including information on biodiversity. • Linkages to other major elemental cycles like carbon, oxygen, phosphorus and silica are briefl y described in this chapter. • A tentative budget of all major sources and sinks of nitrogen integrated for global coasts is presented, indicating uncertainties where present, especially the N-loss capacity of ocean shelf sediments. • Finally, specifi c nitrogen problems in the European Regional Seas, including the Baltic Sea, Black Sea, North Sea, and Mediterranean Sea are described. Key fi ndings/state of knowledge • Today, human activity delivers several times more nitrogen to the coasts compared to the natural background of nitrogen delivery. Th e source of this is the land drained by the rivers. Th erefore, the major European estuaries (e.g. Rhine, Scheldt, Danube and the coastlines receiving the outfl ow), North Sea, Baltic Sea, and Black Sea as well as some parts of the Mediterranean coastlines are aff ected by excess nutrient inputs. • Biodiversity is reduced under high nutrient loadings and oxygen defi ciency. Th is process has led to changes in the nutrient recycling in sediments, because mature communities of benthic animals are lacking in disturbed coastal sediments. Th e recovery of communities may not be possible if high productivity and anoxia persist for longer time periods. Major uncertainties/challenges • Th e magnitude of nitrogen sources are not yet well constrained. Likewise the role of nutrient ratios (N:P:Si ratios) may be a critical variable in the understanding of the development of harmful algae blooms. • Whether only inorganic forms of nitrogen are important for productivity, or whether organic nitrogen is also important is not well understood and needs future attention. Recommendations • For the future it will be necessary to develop an adaptive transboundary management strategy for nitrogen reduction. Th e starting point for such regulation is located in the catchments of rivers and along their way to the coastal seas. • An overall reduction of nitrogen inputs into the environment is urgently necessary, especially in the case of diff use nitrogen inputs from agricultural activities.

Air-Sea Interactions of Natural Long-Lived Greenhouse Gases (CO2, N2O, CH4)in a Changing Climate

Understanding and quantifying ocean–atmosphere exchanges of the long-lived greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) are important for understanding the global biogeochemical cycles of carbon and nitrogen in the context of ongoing global climate change. In this chapter we summarise our current state of knowledge regarding the oceanic distributions, formation and consumption pathways, and oceanic uptake and emissions of CO2, N2O and CH4, with a particular emphasis on the upper ocean. We specifically consider the role of the ocean in regulating the tropospheric content of these important radiative gases in a world in which their tropospheric content is rapidly increasing and estimate the impact of global change on their present and future oceanic uptake and/or emission. Finally, we evaluate the various uncertainties associated with the most commonly used methods for estimating uptake and emission and identify future research needs.

Introduction to special section: Results of the Meteor 55: Tropical SOLAS Expedition

This special section presents results from an interdisciplinary research cruise to the northern tropical Atlantic Ocean, which took place in October–November 2002. The cruise was planned as a pilot study for the international Surface Ocean – Lower Atmosphere Study (SOLAS) project. This introduction summarizes the goals as well as the hydrographic and atmospheric setting of the expedition. We also present a brief review of the findings published in this section and elsewhere concerning controls on trace gas fluxes and the biogeochemical significance of dust composition and deposition.

The Ocean's Vital Skin: Toward an Integrated Understanding of the Sea Surface Microlayer

Despite the huge extent of the oceans surface, until now relatively little attention has been paid to the sea surface microlayer (SML) as the ultimate interface where heat, momentum and mass exchange between the ocean and the atmosphere takes place. Via the SML, large-scale environmental changes in the ocean such as warming, acidification, deoxygenation, and eutrophication potentially influence cloud formation, precipitation, and the global radiation balance. Due to the deep connectivity between biological, chemical, and physical processes, studies of the SML may reveal multiple sensitivities to global and regional changes. Understanding the processes at the oceans surface, in particular involving the SML as an important and determinant interface, could therefore provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. This review identifies gaps in our current knowledge of the SML and highlights a need to develop a holistic and mechanistic understanding of the diverse biological, chemical, and physical processes occurring at the ocean-atmosphere interface. We advocate the development of strong interdisciplinary expertise and collaboration in order to bridge between ocean and atmospheric sciences. Although this will pose significant methodological challenges, such an initiative would represent a new role model for interdisciplinary research in Earth System sciences.

Soluble trace metals in aerosols over the tropical south-east Pacific offshore of Peru

Bulk aerosol samples collected during cruise M91 of FS Meteor off the coast of Peru in December 2012 were analysed for their soluble trace metal (Fe, Al, Mn, Ti, Zn, V, Ni, Cu, Co, Cd, Pb, Th) and major ion (including NO−3 and NH+4 ) content. These data are among the first recorded for trace metals in this relatively poorly studied region of the global marine atmosphere. To the north of ∼ 13 S, the concentrations of several elements (Fe, Ti, Zn, V, Ni, Pb) appear to be related to distance from the coast. At the south of the transect (∼ 15–16 S), elevated concentrations of Fe, Cu, Co, and Ni were observed, and we calculated dry deposition fluxes of soluble Cu approximately an order of magnitude higher than a recent model-based estimate of total Cu deposition to the region. The model did not take account of emissions from the large smelting facilities in the south of Peru and northern Chile, and our results may indicate that these facilities constitute an important source of trace metals to the region. Calculated dry deposition fluxes (3370–17800 and 16–107 nmol m d for inorganic nitrogen and soluble Fe respectively) indicated that atmospheric input to the waters of the Peru upwelling system contains an excess of Fe over N, with respect to phytoplankton requirements. This may be significant as primary production in these waters has been reported to be limited by Fe availability, but atmospheric deposition is unlikely to be the dominant source of Fe to the system.

Nitrous oxide emissions from the upwelling area off Mauritania (NW Africa)

Nitrous oxide (N2O) flux densities across the ocean/atmosphere interface from the Mauritanian upwelling (16°–18.5°W, 16°–21°N) were computed with a simple upwelling model using N2O measurements from four cruises between 2006 and 2008 as well as wind data from the QuikSCAT satellite. The resulting N2O flux densities show a strong seasonality reflecting the wind-driven seasonality of the upwelling: N2O flux densities are highest in the northern part (19.5°–21°N) and show a decreasing trend towards the south. The summer periods with no upwelling (and thus associated with no or negligible N2O flux densities) are most pronounced in the southern part (16°–17°N). The mean seasonally and regionally weighted annual N2O emissions from the Mauritanian upwelling were estimated to 1.0 Gg N. This is low compared to other major upwelling areas (Arabian Sea, off Chile) indicating that N2O emissions from the Mauritanian upwelling are a minor source of atmospheric N2O.

Nitrite removal improves hydroxylamine analysis in aqueous solution by conversion with iron(III)

Dissolved hydroxylamine (NH2OH) is a hort-lived compound produced in the oceanic environment during nitrification and dissimilatory eduction of nitrate to ammonium (DNRA). The ferric ammonium sulfate (FAS) conversion method is the only method available so far to determine dissolved NH2OH in nanomolar concentrations in seawater. We show that side reactions of dissolved nitrite (NO2-) can result in a significant bias in the NH2OH concentration measurements when applying the FAS conversion method. We propose to scavenge dissolved NO2- by addition of sulfanilamide to suppress effectively the undesired side reactions by NO2-. This modification of the FAS conversion method will allow a NH2OH determination even in oceanic regions with high NO2- concentrations. A reliable detection of NH2OH in seawater samples can give us a clue about the occurrence of active nitrification or DNRA in the ocean and,therefore, will provide further insights about the oceanic nitrogen cycle.