HR: 10:20h
AN: B21G-01 INVITED     [PDF]
TI: Diatom Production of Opal and the Evolution of Nitrate to Silicic Acid Ratios in the Ocean
AU: * De La Rocha, C L
EM: christina00@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambrdige, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: Diatoms are highly successful phytoplankton and over the course of appearing and radiating are thought to have caused a drop in marine concentrations of silicic acid on the order of about 1000 $\mu$M (Siever 1991). Diatoms deposit opal in their cell wall and so require Si in what is a roughly equal proportion to N. Such a great need for Si should not hamper diatom growth in a Si-rich environment, such as the modern ocean with its average N/Si ratio of 0.6 (tabulated from the $>$17,000 WOCE stations), well below the 1:1 boundary between eventual N- and eventual Si-limitation. But the deeper pumping of Si than N prior to remineralization from biogenic particles produced in surface waters has pushed the average nitrate to silicic acid ratio (N/Si) of the upper kilometer to 1.4. Values peak around 300 m, and decline both above and below this depth. Average N/Si ratios do not drop below 1 until 800 m, suggesting that, in the absence of nitrate-utilization by non-siliceous phytoplankton, most waters upwelling into the euphotic zone of the modern day ocean contain N and Si in proportions that should be Si-limiting to diatom growth. The average ratio of utilization of N/Si appears to be 1.9, which could correspond to the uptake ratio of Fe-limited diatoms or to a significant proportion of the nitrate being taken up by non-siliceous phytoplankton. This high utilization ratio causes the drop in N/Si above 300 m, but the drop in N/Si below 300 m is due to the more efficient remineralization of N than Si from sinking particles. Despite the high N/Si ratio of surface waters, diatoms are not necessarily faced with Si-limitation as much of the N is taken up instead by non-siliceous phytoplankton. Marine N/Si ratios have been high probably only in the last third of the existence of the diatoms. Prior to the appearance and radiation of the diatoms surface ocean N/Si ratios were likely on the order of 0.02 and from that time have probably risen by a factor of 30 to 70. Now that the N/Si ratio of surface waters exceeds the value of 1, the total amount of Si input to the euphotic zone limits the proportion of available N that can be acquired by diatoms and diatoms may at times experience Si-limitation despite the competition with other phytoplankton for N stocks.
DE: 0330 Geochemical cycles
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1615 Biogeochemical processes (4805)
DE: 4845 Nutrients and nutrient cycling
SC: Biogeosciences [B]
MN: 2003 Fall Meeting