Rhizosolenia mats: An overlooked source of silica production in the open sea

The contribution of Rhizosolenia mats to silica cycling in the central Nort h Pacific and the coupling of mat silicon metabolism and their vertical mig ration was examined in areas to the west of the Hawaiian Islands (23-28 deg rees N and 159-175 degrees W) in 1995 and to the east of Hawaii along 31 de grees N (160-127 degrees W) in 1996. The biogenic silica content of Rhizoso lenia mats sampled in 1995 averaged 1.82 +/- 1.87 (SD) mu mol Si mat(-1). L arger mats that averaged 4.56 +/- 5.54 (SD)(-) mu mol Si mat(-1) were obser ved in 1996. Kinetic experiments indicated that substrate limitation of mat silica production was widespread across the study legion, with ambient [Si (OH)(4)] restricting silica production to 33% of maximum potential rates. T hree lines of evidence indicate that silicon metabolism is not tightly coup led to the migration of mats to and from the nutricline. In 1996, mats in s urface waters could double their Si content in 0.55 d on average without mi grating to the nutricline to obtain Si. However, average doubling times (9. 8 d) in 1996 were of the same order as a migration cycle, necessitating sig nificant Si uptake at depth. Si uptake rates did not differ significantly b etween ascending and descending mats,suggesting that mats ascending from th e nutricline had not fulfilled their Si uptake requirements. Finally, small internal peals of Si in ascending mats indicated that if significant amoun ts of Si were taken up at depth, they were not stored for use in the surfac e waters. The biomass and silica production rates of mats collected using S CUBA in the upper 20 m were extrapolated to 150 m by using abundances deter mined using a video plankton recorder (VPR). The results suggest that mats account for about 3% of the standing stock of biogenic silica and 26% of si lica production in the upper 150 m. The daily silica production by Rhizosol enia mats (317 mu mol Si m(-2) d(-1)) is 50-76% of the total silica product ion in the Sargasso Sea. This high rate of silica production combined with the wide geographic distribution of mats throughout several mid-ocean gyres suggests that mats may contribute significantly to global silica productio n.