OS11B-01 INVITED 08:00h
River Influences on Shelf Ecosystems: Initial Impressions
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.
http://www.ocean.washington.edu/rise/index.htm
OS11B-02 08:15h
Fission of Internal Solitary Waves from a Decelerating Density Front
Solitary internal waves are typically thought to be generated through topographic interactions. However, the ubiquitous presence of solitary waves in regions of mild topography (such as the northern Oregon Coast) has remained a mystery. We present an alternative generation mechanism: that from a decelerating plume front. In July 2004, we obtained a detailed set of near-surface turbulence and acoustics observations of a front of the Columbia River Plume that illustrate the generation process. Nine transects over a seven hour period document: (1) the growth of a plume front, (2) the fission of nonlinear waves that inherit the physical characteristics of the front, and (3) the free propagation of a nonlinear internal wave packet. Linear wave theory is used to interpret the results and describe the timing of wave release.
http://kai.oce.orst.edu
OS11B-03 08:30h
Importance of Vertical Mixing for Additional Sources of Nitrate and Iron to Surface Waters of the Columbia River Plume: Implications for Biology.
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.
OS11B-04 08:45h
Does the Columbia River Plume Influence Copepod and Euphausiid Biomass and Growth Rates in Continental Shelf Waters off Washington and Oregon?
We have been making hydrographic measurements and sampling the zooplankton and juvenile salmonids along a grid of stations off Washington (WA) and Oregon (OR) in June and September from 1998 to present. We have found that upwelling is weaker off WA state as compared to OR, but that chlorophyll concentration is on average three times higher than off WA than OR. Given the higher biomass of phytoplankton, are zooplankton biomass and production rates higher off WA? With funding from the National Science Foundation (the RISE program) we have been evaluating this question through measurement of copepod and euphausiid biomass, egg production and molting rates. We have not found a consistent difference in copepod biomass off WA as compared to OR, but have found that the concentration of euphausiid eggs is nearly an order of magnitude higher off WA than OR. From one cruise in June 2003, we did not find a significant difference in copepod {\it(Calanus marshallae)} egg production rates between stations off WA as compared to OR, probably because phytoplankton concentrations were very high in both regions thus did not limit copepod growth. We found a significant correlation between copepod egg production and water depth, regardless of latitude with egg production rates highest inshore and lowest offshore. Thus although there were large differences in chl-a between WA and OR, the onshore-offshore differences are more important in controlling egg production rates than alongshore differences.
OS11B-05 INVITED 09:00h
Mississippi River Plume and Adjacent Continental Shelf
The Mississippi River system dominates the flux of fresh water and associated dissolved and particulate materials on the central northern Gulf of Mexico shelf. The waters of the continental shelf adjacent to the Mississippi River are affected on varying spatial and temporal scales by a discharge that integrates runoff from 41 percent of the lower 48 United States. The Mississippi and Atchafalaya rivers combined contribute 580 km$^{3}$ per yr of fresh water to the Gulf of Mexico along with sediment yields of 210\times10$^{6}$ t per yr, 1.6\times10$^{6}$ t per yr nitrogen, of which 0.95\times10$^{6}$ t is nitrate and 0.58\times10$^{6}$ t is organic nitrogen, 0.1\times10$^{6}$ t per yr phosphorus and 2.1\times10$^{6}$ t per yr silica. The ecosystem adjacent to the discharges of the Mississippi River system, unlike many estuaries and confined seas, is an open continental shelf system. There is daily, weekly and seasonal variability in currents and stratification on the shelf and, therefore, no simple description of the couplings between nutrient delivery, carbon production in surface waters and delivery to and cycling in bottom waters. There are, however, multiple lines of evidence to implicate changes in riverine nutrient loads with overall primary and secondary production, carbon accumulation at the seabed, and low oxygen conditions on the shelf. The seasonally severe hypoxic zone that develops at the terminus of the Mississippi River system is the largest in the world's coastal ocean, reaching up to 20,000 km$^{2}$ of bottom water in mid-summer.
OS11B-06 09:30h
Numerical Simulations of Hypoxia on the Texas-Louisiana Continental Shelf
Numerical simulations of the mechanisms controlling hypoxia on the Louisiana/Texas Continental Shelf are presented using two different biogeochemical models of benthic oxygen demand. The first model defines benthic oxygen demand as a simple function of local temperature and oxygen concentration based on measurements of benthic respiration. This simple model reproduces many of the salient features such as vertical structure, approximate spatial extent, and timing of hypoxia on the Louisiana/Texas shelf. A more complex model that includes nutrient inputs from the local rivers, water column production, and sinking detritus is used to examine the effects on carbon supply to the benthos.
OS11B-07 INVITED 09:45h
Phosphorus Limitation of Phytoplankton Growth in the Mississippi River Plume: A Case for Dual Nutrient Control?
The Action Plan agreed to in 2001 by federal, state and tribal agencies calls for reducing the area of the Mississippi plume hypoxic zone to less than 5,000 square kilometers by 2015, to be achieved by a 30 percent reduction in the dissolved inorganic nitrogen entering the Gulf of Mexico. Evidence collected over that last fifteen years suggests that seasonal phosphorus limitation of phytoplankton growth on the Louisiana shelf coincides with the periods of both highest river flow and maximum primary productivity in the spring. The phosphorus limitation documented here almost certainly results from increased nitrogen input from the Mississippi River over the past fifty years. This phosphorus-limited productivity is most likely responsible for the summer hypoxia, though the detailed mechanism of hypoxia generation is unclear. If the Mississippi River is the major source of phosphorus as well as nitrogen to this system, then it may be time to recommend reductions in phosphorus as well as nitrogen. This would be consistent with recent recommendations for control of both nitrogen and phosphorus inputs to estuarine systems, as well as with the partial recovery of parts of the Black Sea since the major declines in nitrogen and phosphorus inputs began in the early 1990s. Though unanswered questions about the sources and supply rate of phosphorus to the Mississippi plume system remain, the need for phosphorus reductions should be thoroughly evaluated.