Upward Nitrate Flux Research Articles

Spatial and temporal variability of particle flux at the N.W. European continental margin

A synopsis of results from two sediment trap moorings deployed at the mid- and outer slope (water depths 1450 and 3660 m, respectively) of the Goban Spur (N.E. Atlantic Margin) is presented. Fluxes increase with trap deployment depth; below 1000 m resuspended and advected material contributes increasingly to bulk flux. Fluxes of dry weight, POC and diatoms in the traps 400 m above bottom (mab) are smaller than those recorded at the sediment surface due to lateral fluxes in the benthic nepheloid layer. These near-bottom fluxes are larger at shallower water depths. Pa-231/Th-230 ratios in sedimenting material suggest that boundary scavenging is not significant at the Goban Spur. Fluxes of Pb-210 in the intermediate and deep traps are comparable to the Pb-210 supply rate at this site. At the outer slope, sediment Pb-210 fluxes are similar to those measured in the traps 400 mab; at the mid- slope they are a factor of 2 higher, once again indicating large near-bottom lateral particle input. Based on POC- normalised biomarkers in sedimenting material, we followed changes in the quality of sedimenting material with differing trap depth and on seasonal and event-related time scales. In spring fresh, diatom-dominated sedimentation occurs, with progressive degradation of POC with time (to winter) and depth (from 600 to 3220 m). Deeper traps are distinguished on the basis of opal and aluminium fluxes that are dominant in lateral input. A storm event during late September 1993 was clearly reflected in the delta N-15 isotope ratio of sedimenting material, with a time lag of 2-3 weeks. Diatom and opal fluxes were elevated in this storm-related signal, and its biomarker composition in the 600-m trap was similar to that during spring. An estimate made of upward nitrate flux (new production) at the shelf break and at the outer slope indicated a 2-fold higher new (export) production at the shelf break. Particulate organic carbon export from the shelf break to below the depth of maximal seasonal mixing ranges between 3 and 9% of primary production. [KEYWORDS: Isotopic composition; north-atlantic; goban spur; organic-matter; open-ocean; sediments; nitrogen; carbon; phytoplankton; 20-degrees-w]

Annual and interannual variations of phytoplankton pigment concentration and upwelling along the Pacific equator

The following variables along the Pacific equator from 145°E to 95°W were employed: surface layer phytoplankton pigment concentrations derived from Nimbus 7 coastal zone color scanner (CZCS) measurements of ocean color radiances; vertical velocities simulated at the 90‐m bottom of the euphotic layer from a wind‐driven ocean general circulation model; and nitrate concentrations estimated from model‐simulated temperature. The upward flux of nitrate into the euphotic layer was calculated from the simulated vertical motion and nitrate concentration. The CZCS‐derived phytoplankton pigment concentration was uniform from 175° to 95°W. Longitudinal profiles of upwelling, phytoplankton biomass, and 90‐m nitrate flux were of different shapes. The small annual cycles of the phytoplankton pigment concentration and nitrate flux were in phase: increased phytoplankton biomass was associated with increased upward nitrate flux, but the phase was not consistent with the annual cycles of the easterly wind or of the upwelling intensity. Variation of phytoplankton pigment concentration was greater during El Niño than during the annual cycle. The substantially reduced phytoplankton pigment concentration observed during El Niño was associated with smaller upward nitrate flux. Phytoplankton biomass during non‐El Niño conditions was not related to nitrate flux into the euphotic layer.

Nitrate flux into the euphotic zone near Bermuda

Throughout most of the year, there exists within the North Atlantic subtropical ocean mixed layer a3He excess which can only be attributed to a flux of tritiugenic3He from below. The upward flux required to support this excess can be computed as the convolution of the time series of3He excess and gas-exchange rate. This flux is quantitatively consistent with the long-term evolution of the main thermocline inventories of tritium and 3He. Using the observed correlation of 3He with nitrate within the upper main thermocline, I estimate the annually averaged upward flux of nitrate into the euphotic zone to be 0.6 ±0.2 moles m−2 per yr. This flux is sufficient to support a new, non-regenerative primary productivity in excess of 3 moles carbon m−2per yr.