Vertical flux of particulate matter in Alfonso Basin, La Paz Bay during 2002

Every three months during 2002, a 1/8-m² Technicap PPS-3 time-series sediment trap was moored 50 m above the bottom of Alfonso Basin and provided 7-8 day resolution. Under the dissecting microscope, marine snow was abundant, while fecal pellets varied in number, size (0,25-10 mm) and form. Centric diatoms were common year-round, while chain-forming diatoms were clearly abundant only during a two-week period in June. The bulk samples were sieved (1 mm) to remove swimmers, the most frequent being polychaetes and pteropods, followed by gelatinous organisms and, rarely, crustaceans, The material was separated into ten equal parts using a rotary splitter. Four of the splits were rinsed of seawater, dried and weighed to determine the total mass flux. This varied between 0. 17 and 2.48 g m^-2 d^-1. The highest fluxes occurred in February and the lowest in mid-April and late June. The average mass flux (300 g m^-2 yr^-1) is equivalent to a sediment accumulation rate of 0.4 mm yr^-1, similar to that obtained from the dating of sediment cores from the basin. The carbonate fraction, determined by coulometry of the carbon content before and after acidification, was considerable, accounting for 8-26% of the total. The mean (14.6%) is similar to the concentration reported for surficial sediments. Peak concentrations occurred during periods in January, March, May, October and November. The CaCO₃ flux averaged 0.128 g m^-2 d^-1, ranged between 0.036 and 0.304 g m^-2 d^-1 and was most intense in autumn and winter. The total mass an carbonate fluxes in La Paz Bay are considerably greater than those observed in Guaymas Basin, in the open Gulf of California.