Detrital aggregates on the sea floor: chemical composition and aerobic decomposition rates at a time-series station in the abyssal NE Pacific

Seasonal pulses of particulate matter sink to the sea floor and form visible detrital aggregates between July and December at an abyssal station (Sta. M, 4100 m depth) in the NE Pacific. We hypothesized that these detrital aggregates would create localized regions of intense biological activity on the sea floor. Detrital aggregates were collected and incubated on the sea floor using the submersible Alvin in August and three weeks later in September 1994. As a control, samples of the sediments were collected and incubated in situ in April 1995, when no aggregates were present. Detrital aggregates representing a range of sizes, colors and textures were sampled individually with Alvin using tube corers; others were incubated on the sea floor in tube core respirometers. In addition, a number of aggregate locations were documented photographically then sampled 23 and 221–223 d later, after the aggregates were no longer visible. A total of 55 tube cores with aggregates were collected in August 1994, and another 88 were collected in September 1994. Concentrations of total and organic carbon, total nitrogen, chlorophyll a and phaeopigments in the aggregates were similar to those in the sinking particulate matter collected concurrently in sediment traps at 50 m above bottom, but higher than those in the surface sediments. Oxygen consumption of diffuse floc was similar to that of the background sediment, whereas discrete aggregates and radiolarian-patch samples had rates significantly higher than background sediment. Detrital aggregates had no measurable influence on the chemical composition of the underlying sediment on time scales from 23 to 223 d or on sediment oxygen consumption after 222 d. Distinct detrital aggregates monitored visually with a time-lapse camera covered up to 4.9% of the sea floor and contributed substantially to the supply of organic carbon to the sediment community in fall 1994 while producing a minimal impact on sediment community oxygen consumption (SCOC). The annual oxygen consumption associated with detrital aggregates represented a minor contribution (0.34%) to the annual SCOC estimated at Sta. M in 1994–1995.