HR: 08:30h
AN: OS11B-03    [Abstracts]
TI: Importance of Vertical Mixing for Additional Sources of Nitrate and Iron to Surface Waters of the Columbia River Plume: Implications for Biology.
AU: * Lohan, M C
EM: mlohan@es.ucsc.edu
AF: Institute of Marine Sciences, Univeristy of California Santa Cruz, Santa Cruz, CA 95064 United States
AU: Bruland, K W
EM: bruland@ucsc.edu
AF: Institute of Marine Sciences, Univeristy of California Santa Cruz, Santa Cruz, CA 95064 United States
AB: In contrast to other large estuaries, the Columbia River is a unique study area as it supplies very little nitrate (5 $\mu$M at salinities of 1-2) to coastal waters. Elevated dissolved iron and nitrate concentrations (20 nM and 20 $\mu$M) were observed in the near field Columbia River plume. In order to characterize the chemical signatures of the near-field plume and underlying water masses, vertical profiles of dissolved iron and nutrients were collected by lowering a trace metal towed `fish' sampler to depths of 1 to 20m at one station close to the mouth of the estuary over a 35 hour period. The plume was identified as a shallow surface lens (less than 4m) of low salinity water with salinities as low as 12. During the ebb flows, surface concentrations of dissolved iron and nitrate were as high as 6 nM and 14 $\mu$M, respectively. Surface nitrate concentrations were higher than observed in the Columbia River itself and therefore must be added by entrainment of the higher nitrate concentrations in the subsurface waters. Over this tidally influenced region, during flood flows the advection of higher salinity (32) water, decreased surface concentrations of iron and nitrate to 1.2 nM and 0.3 $\mu$M, respectively. Vertical mixing was identified to provide an additional source (to that supplied during ebb flow) of both nitrate and iron to surface waters causing a concomitant increase in chlorophyll concentrations. Vertical profiles of dissolved iron and nitrate were also determined when a surface drifter deployed during an ebb tide was utilized to follow a parcel of high iron and nitrate plume water over a three day as it advected offshore. Initial concentrations of nitrate and dissolved iron in surface waters were 8 $\mu$M and 4 nM respectively and had decreased to 0.31 $\mu$M and 1.2 nM within a few hours. The plume drifted directly westward, over a subsurface nitrate depleted water mass. Iron concentrations in the surface waters are sufficient to meet the biological demand. However, due to the extremely low nitrate in the Columbia River itself, nitrate in the plume is primarily dependent on mixing with nitrate rich subsurface waters. Without such an additional source the plume rapidly becomes nitrate limited.
DE: 4805 Biogeochemical cycles (1615)
DE: 4808 Chemical tracers
DE: 4815 Ecosystems, structure and dynamics
DE: 4235 Estuarine processes
DE: 4271 Physical and chemical properties of seawater
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting