HR: 08:00h
AN: OS11B-01 INVITED     [Abstracts]
TI: River Influences on Shelf Ecosystems: Initial Impressions
AU: * Hickey, B M
EM: bhickey@u.washington.edu
AF: University of Washington, School of Oceanography Box 355351, Seattle, WA 98195 United States
AB: The overall goal of RISE (River Influences on Shelf Ecosystems) is to determine the extent to which regional productivity differences are a result of the presence of the river plume-e.g., it's turbidity, stratification, species composition and nutrient load, as well as its effect on mixing and advection. The first RISE cruise took place in July 2004, using the R/V Wecoma and the R/V Point Sur. Upwelling/downwelling was much more intermittent than is typical of this month; riverflow varied by almost a factor of two between the beginning and end of the cruise. Overall, a wide range of environmental conditions were sampled (although not persistent and strong upwelling). The often shallow (1-5 m) river plume environment combined with the intermittent wind and tidal forcing proved challenging for the observational team. Nevertheless sufficient data were collected to characterize those time and space scales and to present an initial look at sources and sinks of nutrients and plankton. Detailed nutrient/plankton measurements along repeated sections north and south of the plume and along its axis were made aboard the Wecoma. In addition, surface Argos-tracked drifters were deployed and a subset was followed to examine changes over time. Laboratory studies of growth, dilution and grazing were performed onboard. Profiles of turbulence as well as high frequency, high vertical and horizontal resolution measurements of fluorescence, optical backscatter and velocity were collected from the Point Sur towing instrumentation in proximity to the Wecoma. Detailed 3-D large scale and frontal water property/velocity/particle size surveys were performed from the Point Sur using a towed Triaxus. Tidal variability was much stronger than anticipated-even in modest riverflows surface drifters were transported all the way across the shelf to the slope in one tidal excursion: this result has important implications for plankton/larval/juvenile fish export, growth and survival. Variability on scales of minutes to days (i.e., internal waves) was important to fluorescence, nutrients and salinity within 10 km of the river mouth, prompting us to initiate multi-disciplinary anchor stations to examine higher frequencies in detail. Initial conclusions are that short spatial scales ($\sim$100 m) and time scales ($\sim$10-30 minutes) are likely critical to mixing and hence biological growth in the plume. Larger scales (shelf width) and longer periods (days) are also important; e.g., the baroclinic southward coastal jet over the outer shelf/slope over 100-200 km along the coast plays a critical role in plume direction and hence transport of plume material as well as providing nutrients to fuel plume productivity. We note that much of the water upwelled into the Columbia plume likely originates farther north where upwelling is enhanced by the estuarine/topographic processes near the Strait of Juan de Fuca.
UR: http://www.ocean.washington.edu/rise/index.htm
DE: 4805 Biogeochemical cycles (1615)
DE: 4815 Ecosystems, structure and dynamics
DE: 4845 Nutrients and nutrient cycling
DE: 4219 Continental shelf processes
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting