Seasonal variability of the particulate organic matter vertical flux in the central region of the Gulf of California

Abstract

Seasonal variability of particulate organic matter (POM) fluxes and the amount exported from the upper 200 m of the water column was studied during four oceanographic cruises in Guaymas Basin during autumn 1994 and spring, summer, and autumn 1995. We used an array of drifting sediment traps to measure the particulate fluxes at 50, 100, and 200 m depths in the upper water column. The mean depths of the mixed layer varied from 20.8 m in summer 1995 to 29.6 m in spring 1995, and of the euphotic zone from 33.2 m in spring 1995 to 51.6 m in summer 1995. Mean phytoplankton biomass ranged from 36.9 mg Chla m^–2 in autumn 1995 to 85.9 mg Chla m^–2 in spring 1995. Higher POM fluxes occurred during autumn 1994 and spring 1995, while lower fluxes occurred during summer and autumn 1995. The percentage of daily primary productivity exported from the upper 200 m of the water column varied from 3.6% to 8.9% and is within the range reported for other regions. The C:N ratios were homogeneous, regardless of depth, and varied from 9.0 to 13.8. The particulate organic carbon residence time varied from 3.7 to 11.2 d, and was higher in summer and lower in autumn. Consequently, benthonic heterotrophic organisms get their food from seasonal pulses. The total exported carbon for the area was estimated to be about 300 × 10³ t yr^–1, which suggests that during summer, this region acts as a carbon sink; however, more data are needed to understand the yearly balance.