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

Abstract

The contribution of Rhizosolenia mats to silica cycling in the central North Pacific and the coupling of mat silicon metabolism and their vertical migration was examined in areas to the west of the Hawaiian Islands (23-28°N and 159-175°W) in 1995 and to the east of Hawaii along 31°N (160-127°W) in 1996. The biogenic silica content of Rhizosolenia mats sampled in 1995 averaged 1.82 ± 1.87 (SD) μmol Si mat-1. Larger mats that averaged 4.56 ± 3.54 (SD) μmol Si mat-1 were observed in 1996. Kinetic experiments indicated that substrate limitation of mat silica production was widespread across the study region, with ambient [Si(OH)4] restricting silica production to 33% of maximum potential rates. Three lines of evidence indicate that silicon metabolism is not tightly coupled to the migration of mats to and from the nutricline. In 1996, mats in surface waters could double their Si content in 0.55 d on average without migrating 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 significant Si uptake at depth. Si uptake rates did not differ significantly between ascending and descending mats, suggesting that mats ascending from the nutricline had not fulfilled their Si uptake requirements. Finally, small internal pools of Si in ascending mats indicated that if significant amounts of Si were taken up at depth, they were not stored for use in the surface waters. The biomass and silica production rates of mats collected using SCUBA in the upper 20 m were extrapolated to 150 m by using abundances determined 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 silica production in the upper 150 m. The daily silica production by Rhizosolenia mats (317 μmol Si m-2 d-1) is 50-76% of the total silica production 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 production.