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第7回極域科学シンポジウム:[OA] 南極隕石/はやぶさ11月30日（水）国立国語研究所 2階講堂

On a global scale the ratio of fixed nitrogen (N) and phosphate (P) is characterized by a deficit of N with regard to the classical Redfield ratio of N : P = 16 : 1 reflecting the impact of N loss occurring in the oceanic oxygen minimum zones. The northern Benguela upwelling system (NBUS) is known for losses of N and the accumulation of P in sub- and anoxic bottom waters and sediments of the Namibian shelf resulting in low N : P ratios in the water column. To study the impact of the N : P anomalies on the regional carbon cycle and their consequences for the export of nutrients from the NBUS into the oligotrophic subtropical gyre of the South Atlantic, we measured dissolved inorganic carbon (CT), total alkalinity (AT), oxygen (O2) and nutrient concentrations in February 2011. The results indicate increased P concentrations over the Namibian shelf due to P efflux from sediments resulting in a C : N : P : -O2 ratio of 106 : 16 : 1.6 : 138. N reduction further increase C : N and reduce N : P ratios in those regions where O2 concentrations in bottom waters are < 20 μmol kg−1. However, off the shelf along the continental margin, the mean C : N : P : -O2 ratio is again close to the Redfield stoichiometry. Additional nutrient data measured during two cruises in 2008 and 2009 imply that the amount of excess P, which is created in the bottom waters on the shelf, and its export into the subtropical gyre after upwelling varies through time. The results further reveal an inter-annual variability of excess N within the South Atlantic Central Water (SACW) that flows from the north into the NBUS, with highest N values observed in 2008. It is postulated that the N excess in SACW occurred due to the impact of remineralized organic matter produced by N2 fixation and that the magnitude of excess P formation and its export is governed by inputs of excess N along with SACW flowing into the NBUS. Factors controlling N2 fixation north of the BUS need to be addressed in future studies to better understand the role of the NBUS as a P source and N sink in the coupled C : N : P cycles.

The northern Benguela upwelling system is a nutrient-replete region with high plankton biomass production and a seasonally changing oxygen minimum zone. Nitrate:phosphate ratios in fresh upwelling water are low due to denitrification in the near-seafloor oxygen minimum zone and phosphate efflux from sediments. This makes the region a candidate for substantial dinitrogen fixation, for which evidence is scarce. Nutrient and oxygen data, N isotope data of nitrate, nitrogen isotope ratios of particulate matter, particulate organic carbon content, and suspended matter concentrations on a transect across the shelf and upper slope at 23°S illustrate N-cycling processes and are the basis for estimating the contribution of N-sources and N-sinks to the reactive nitrogen pool. It appears that N-removal due to denitrification exceeds N gain by N2 fixation and physical mixing processes by a factor of >6, although inorganic N:P ratios again increase as surface water is advected offshore. Nitrate and ammonium regeneration, nutrient assimilation with N:P < 16, shelf break mixing, atmospheric input, and N2 fixation all contribute to the restoration of inorganic N:P ratios back to Redfield conditions, but in seasonally changing proportions. The Benguela upwelling system thus is a nutrient source for the oceanic-mixed layer where N-sources and N-sinks are not in balance and Redfield conditions can only re-adjust by advection and mixing processes integrated over time.

This study presents, for the first time, micro-structure observations in tidally energetic, weakly stratified regimes obtained in the Wadden Sea, a tidal shallow coastal area in the South-Eastern North Sea characterised by barrier islands separated by tidal gulleys. The tidal currents are typically overlaid by a weak horizontal density gradient due to freshwater run-off from land. The observations in an energetic tidal channel clearly show the expected effects of tidal straining: destratification during flood and increased stratification during ebb. Microstructure observations are consistent with the tidal straining dynamics: during the flood relatively high values of viscous dissipation are observed whilst during ebb the values are substantially smaller. It is also shown that the tidal cycle of stratification and destratification depends on the position in the tidal channel. In parts of the channel, increased stratification occurs during full flood, a phenomenon which can only be explained by advection of stratified water masses formed outside the tidal channel. The observations presented here show the general significance of the tidal straining process for tidally energetic weakly stratified regimes.

It is thought that the penetration of Angolan waters through the Angola–Benguela Frontal Zone (ABFZ) into the much cooler Benguela regime may come about by a poleward slope undercurrent as well as by cross-frontal filaments. To test this hypothesis, two zonal transects off the northern Namibian coast were surveyed by CTD casts, current measurements and ichthyoplankton samples during April 1999. Simultaneous sea-surface temperature (SST), chlorophyll-a concentration, and wind data were obtained from satellite. The multidisciplinary results are described.An intense intrusion of the Angola Current into northern Namibian waters occurred with distinct signals of high temperatures and salinities, low chlorophyll-a concentration, and Angolan fish larvae of both the neritic and oceanic communities. This intrusion was temporarily displaced offshore by strong southeasterly winds, which also caused coastal upwelling and enhanced productivity. The expected slope undercurrent was not found. The Benguela Upwelling Front coincided with a sharp boundary between equatorward flow inshore, and generally southward flow offshore, whilst the offshore component of the Angola– Benguela Frontal Zone was located much farther south than anticipated, and showed only weak temperature gradients. Inshore of the Benguela Upwelling Front temperate Benguela fauna had characteristics of anomalously warm conditions. Tropical fish larvae offshore were clearly related to advection in Angola Current water, but not to recent spawning of their parents in it. Inconsistencies were observed in some cross slope boundaries between oceanic versus neritic fish larvae that can only partly be explained by Ekman drift of the surface layer, indicating that both the hydrographic and faunistic structures resulted from opposing meridional flows over time scales as different as 6 days to 4 weeks, intense mixing in the friction zone between them, and the westward displacement of a mesoscale gyre contributing its own anticyclonic flowfield. A conceptual transport model is presented. Includes references Published Benguela Current Inshore Northern Namibia Angola Current Intrusion Benguela Upwelling front Zonal transects