The chemistry, biology and vertical flux of particulate matter from the upper 400 m of the Cape Basin in the southeast Atlantic Ocean

Abstract

Particulate matter, divided into <1-, 1- to 53- and >53-μm size fractions, was obtained using the Large Volume in situ Filtration System (LVFS), Southlant Expedition, R.V. Chain 115, and was analysed for chemical (dry weight, Na, K, Mg, Ca, carbonate, opal, Sr, C and N), organismal (species assemblage and population densities), and morphological distributions. Profiles from LVFS Stas. 4 to 8 covering regions of low to high productivity and coastal upwelling in the southeast Atlantic are compared with that from LVFS Sta. 2, equatorial Atlantic.

Maxima in organism abundances and particulate mass were generally coincident and occurred at the base of the mixed layer or near-surface when the mixed layer was poorly developed or absent. A consistent distributional pattern of organisms was observed.

Features of the particulate matter distributions attributed to the feeding activities of zooplankton are: strong vertical concentration gradients of mass, organic matter, and organisms; 10-fold enrichment with depth of the > 53-μm fraction with coccolith carbonate; decrease in organic content from 100% at the surface to 50 to 60% at 400 m; fragmentation of most test material below 100 m; and the production of fecal pellets and fecal matter. Coccolithophorids and diatoms were the dominant sources of particulate carbonate and opal.

One- to 53-μm organic C/N ratios were 7.3±0.5 (σ) in productive waters; the <1-μm ratios were lower. Particulate organic carbon was distributed uniformly below 200 m with concentrations reflecting surface productivity. The calcium to carbonate ratios exceeded unity with values as high as 2.5 being typical of surface waters near Capetown where diatoms dominated; the cycling of excess calcium is 1 to 2 × 10¹³ mol yr⁻¹ or approximately 20% of the annual carbonate precipitation by organisms.

Vertical mass fluxes through 400m of Foraminifera, fecal pellets, and fecal matter were calculated for three stations using settling models and particle size distributions. Corresponding chemical fluxes of organic carbon, carbonate, and opal are given. Comparisons are made with recalculated fluxes for LVFS Sta. 2.

Over 90% of the organic matter produced in the euphotic zone is recycled in the upper 400 m. The recycling efficiency is nearly 99% in areas of low productivity; the organic to carbonate carbon and the silicate to carbonate ratios are highest at locations where the mass flux is greatest.