Seasonal variability in primary production and particle flux in the northwestern Sargasso Sea: U.S. JGOFS Bermuda Atlantic time-series study

Abstract

The relationship between primary production and sediment trap-derived downward flux of particulate organic matter was characterized over a 2 year period at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site to evaluate the importance of temporal variations in upper ocean biogeochemical processes. Water column-integrated primary production (∫PP), determined once each cruise using ¹⁴C incubations (in situ dawn-to-dusk), peaked in late winter/early spring of both 1989 and 1990. Smaller increases in ∫PP also occured in July 1989 and October–December 1990. Annual ∫PP was 9.2 mol C m⁻² y⁻¹ in 1989 and 12 mol C m⁻² y⁻¹ in 1990. This was higher than the 1959–1963 annual average (6.8 mol C m⁻² y⁻¹) determined at Station “S” located approximately 50 km northwest of the BATS site.

Fluxes associated with sinking of total particulate mass, particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured at 150, 200, 300 and 400 m using a free-floating sediment trap array generally deployed once each cruise for 72 h. Fluxes varied seasonally, and within our ability to resolve differences (i.e. monthly sampling), there was no distinguishable time offset between peaks in ∫PP and corresponding peaks in elemental flux. Fluxes generally decreased with increasing depth, and fluxes of POC and PON were positively correlated with particulate mass flux at all depths. POC/PON (C/N) ratios at 150 m during periods of high ∫PP were generally characteristics of live planktonic biomass. Higher C/N ratios in material collected by the deeper traps were consistent with more rapid losses of PON than POC from sinking particles. POC and PON fluxes at 150 m, nominally the base of the euphotic zone, were positively correlated with ∫PP. The fraction of ∫PP leaving the euphotic zone in the form of sinking particles (i.e. collected in traps) varied seasonally and was inversely proportionato ∫PP. Surface export of organic matter estimated by sediment traps at 150m was 0.78 mol C m⁻²y⁻¹ (0.10 mol N m⁻²y⁻¹) in 1989 and 0.77 mol C m⁻²y⁻¹ (0.11 mol N m⁻²y⁻¹) in 1990.