Seasonal dynamics of the biogenic silica cycle in surface sediments of the Helgoland Mud Area (southern North Sea)

摘要

In coastal waters and the ocean silicic acid (Si(OH)ud4ud) is a key nutrient for primary producers (ude.gud. diatoms)udand other siliceous organisms, because it is required for the formation of frustules and other hard partsudmade of biogenic silica (bSi). Especially in shallow waters like the southern North Sea, dissolution of bSiudin surface sediments and the reudfludux of silicic acid from sediments into the water column is an importantudfeedback mechanism for sustaining primary production. We investigated the temporal variability ofudbenthic silicic acidudfluduxes and the recycling efudfiudciency of bSi in surface sediments of the Helgoland MududArea (southern North Sea). For this purpose we used different methods including a benthic chamberudlander system forudin situudfludux studies of Si(OH)ud4ud,udex situudsediment incubations, pore water studiesudand sediment analysis. Ourudin situudmeasurements revealed considerable temporal variations with lowudsilicic acidudfluduxes in winter (0.3ud–ud1.0 mmol mud�ud2uddud�ud1udin March 2013 and 2014), increasedudfluduxes ofud2.0ud–ud4.0 mmol mud�ud2uddud�ud1udin November 2013, and highudfluduxes in June and August 2012ud(3.6ud–ud8.3 mmol mud�ud2uddud�ud1ud). The relevance of biological mediated transport for the recycling of Si(OH)ud4udwasudunderlined by comparingudin situudandudex situudsediment incubations, pore water studies, as well as depthudproudfiudles of benthic macrofauna. Mass budget calculations indicate that about 1.7ud–ud2.2 mol bSi mud�ud2udsettleudannually at the seaudfludoor, off which about 60ud–ud81% are recycled within surface sediments and transportedudback into the water column