Supply of and demand for organic matter by sediment communities on two central North Pacific seamounts

Abstract

Concurrent measurements of sediment community oxygen consumption (SCOC) and particulate organic carbon flux (POCF) were made over two central North Pacific seamounts to test two hypotheses: (1) the benthic boundary layer community metabolism represented by SCOC (carbon demand) is inversely related to water depth; (2) the flux of particulate organic carbon entering the benthic boundary layer (carbon supply) is inversely related to water depth. To test these hypotheses, we chose two heavily sedimented (depositional) seamounts at different depths under surface waters with productivity of similar magnitude: Horizon Guyot ( ca 1490 m depth at summit cap) and Magellan Rise ( ca 3130 m depth at summit cap). A “control” station was established near each seamount. SCOC was measured with a free vehicle grab respirometer and Alvin-placed grab respirometers; POCF was measured with sediment traps moored at 100 m above the seamount summit cap and at a comparable depth from the surface at the control station. Significantly higher rates of SCOC (P < 0.01) were measured on the Horizon Guyot summit cap than on the Magellan Rise summit cap during the same month (March 1987). This evidence supports our first hypothesis of a depth-mediated gradient in SCOC. In contrast, POCF at Horizon Guyot was only half that measured at Magellan Rise in the same time period. This evidence does not support our second hypothesis. POCF over the seamounts was less than concurrently measured fluxes at the sane depth over the “control stations”. We attribute this difference to higher current speeds over the seamounts resulting in a lower trapping efficiency of sediment traps. The ratios of POCF: SCOC, which range from 0.12 to 0.71 over the seamounts, suggest indirect pelagic/benthic coupling, possibly through suspended POC and dissolved organic carbon.