River and Freshwater Influences

A coupled circulation-and-ecosystem model, developed as part of the RISE (River Influences on Shelf Ecosystems) program, is used to examine the role of the Columbia River plume in shaping broad patterns of nutrient flux and primary production on the Washington and Oregon coasts. Realistic simulations of summer 2004 and 2005 in ROMS (Regional Ocean Modeling System) are forced by winds from the MM5 atmospheric model, Columbia riverflow, and climatological boundary conditions. The ecosystem model adds a budget for nitrate, ammonium, phytoplankton, zooplankton, and detritus to each grid cell. Model predictions are validated against surface chlorophyll distributions from SeaWiFS, the nitratesalinity relationship observed during RISE cruises, and phytoplankton growth and grazing rates from dilution experiments in and out of the Columbia plume. Preliminary results show vertical entrainment of nitrate into surface waters in the Columbia estuary and early plume, and also lateral entrainment of phytoplankton into the plume immediately north of the bulge region at the Columbia mouth. The fate of these upwelling-derived blooms once they encounter the plume—the fraction that pass through or beneath the plume onto the Oregon shelf, the fraction exported across the shelf in the plume itself, and the fraction retained on the Washington shelf—is examined using three-dimensional, diffusive particle tracking. Ed Dever, Oregon State University Observed Nitrate Variability off the Columbia River: June-August 2004 Authors: E. Dever (COAS, Oregon State University) Abstract: As part of the RISE (NSF CoOP) program, 3 moorings were placed over the Oregon and Washington shelves in the Columbia River plume from June to September 2004. The moorings were deployed north, south and immediately offshore of the Columbia River mouth near the 70 m isobath. The moored array was designed to sample the plume under various synoptic As part of the RISE (NSF CoOP) program, 3 moorings were placed over the Oregon and Washington shelves in the Columbia River plume from June to September 2004. The moorings were deployed north, south and immediately offshore of the Columbia River mouth near the 70 m isobath. The moored array was designed to sample the plume under various synoptic conditions. The distances between the northern and central moorings was 31 km and that between the central and southern mooring was 14 km. All moorings included anemometers, temperature loggers, acoustic Doppler current profilers (ADCPs) and near-surface salinity, fluorescence, and light transmission. The moored instruments were concentrated near the surface in order to sample the plume, which can be trapped very closely (5 m or less) to the surface. Salinity, fluorescence, and light transmission were measured 1, 5, and 20 m beneath the surface. Temperature data was collected about every 5 m throughout the upper water column. Each mooring included 2 ADCPs, a downward looking 300 kHz unit in the surface buoy, and an upward looking 1200 kHz ADCP moored 15 m beneath the surface. All moorings also had dissolved inorganic nitrogen (DIN, essentially nitrate) autoanalyzers at 1 m beneath the surface, however only the sensor on the central mooring returned useable data. In contrast to measurements of DIN over upwelling shelves, the DIN in the Columbia River plume shows lower overall values. The mean DIN from June 20 to Aug 23, 2004 was 2.5 uM, near the lower detection limit of the sensor. The peak DIN was 13.9 uM, and the standard deviation was 3.1 uM. The surface DIN in the Columbia river plume is correlated with several other measured parameters. The strongest correlation is with temperature. For hourly interpolated measurements, the correlation with temperature is -0.51 (i.e., colder water is associated with higher DIN values). DIN is also negatively correlated with near surface fluorescence. If upwelling were the dominant source of nitrate, one might expect that DIN is positively correlated with salinity. Somewhat surprisingly, there is a weak negative correlation with salinity (-0.33). One possible mechanism for nitrate delivery to the surface is baroclinic tides. Velocity and subsurface temperature observations suggest a strong mode one internal tide. Between June and August, the strength of the baroclinic tide varies on time scales of weeks. Periods of high internal tide activity seem to be associated with elevated near-surface DIN. Richard Dugdale, Romberg Tiburon Center Nutrient and new production effects of the San Francisco Bay coastal plume Authors: R. Dugdale, F. Wilkerson, A. Marchi, V. Hogue, and A. Paker (Romberg Tiburon Center, San Francisco State University, San Francisco, CA) Abstract: San Francisco Bay is a river-driven, turbid estuary characterized by high ambient nutrient concentrations and low primary productivity. Although seasonal and strong San Francisco Bay is a river-driven, turbid estuary characterized by high ambient nutrient concentrations and low primary productivity. Although seasonal and strong EPOC 2006 Abstracts 2 interannual changes in river flow modulate the exchanges of nutrients between estuary and coastal ocean, most primary nutrients of riverine source are exported to the coastal ocean. In heavy precipitation years, the salinity signal can be detected for more than 100 km from the Golden Gate. The nutrient plume is shortened by biological processes, primarily by phytoplankton uptake. The nutrient plume is dominated by Si(OH)4 with concentrations at the freshwater end of the estuary in excess of 200 μM, ensuring a good supply of that essential nutrient for nearshore diatoms. NO3 and NH4 are also exported through the Golden Gate, PO4 to a lesser extent. Enhancement of new production by the export of these nutrients will be especially important during non-upwelling conditions and may serve to promote diatom production year-round and aid in avoiding harmful algal blooms. Nutrient and new production data from the Gulf of the Farallones will be presented. Modification of NO3 uptake (new production) by NH4 is common in the estuary and has been seen during upwelling off Bodega Bay as a result of NH4 advected from the south near the coast. Mark Halverson, University of British Columbia The Fraser River plume's impact on the magnitude and spatial distribution of phytoplankton biomass Authors: Mark Halverson and Rich Pawlowicz (University of British Columbia, Vancouver, BC) Abstract: The Fraser River discharge forms a large, fresh, and often cloudy, buoyant plume upon entering the lower Strait of Georgia. The salinity, stratification, surface area, and optical clarity of the plume change dramatically because the Fraser River discharge varies by a factor of 10 throughout a year. Seasonal changes in the plume properties have a noticeable impact on the distribution and magnitude of phytoplankton biomass in the lower Strait of Georgia. The Fraser River discharge forms a large, fresh, and often cloudy, buoyant plume upon entering the lower Strait of Georgia. The salinity, stratification, surface area, and optical clarity of the plume change dramatically because the Fraser River discharge varies by a factor of 10 throughout a year. Seasonal changes in the plume properties have a noticeable impact on the distribution and magnitude of phytoplankton biomass in the lower Strait of Georgia. Utilizing a 3+ year high resolution surface water dataset based on an instrumented ferry and intermittent CTD casts, the seasonal changes in phytoplankton biomass are examined with respect to the Fraser plume. In three of (nearly) four years of ferry data, the average biomass in the plume exceeded that found out of the plume. Though the annual mean biomass is greater in the plume than out, the short time-scale behaviour shows that the non-plume biomass may exceed the plume biomass by up to a factor of ten. Inferences of integrated biomass taken from the ferry surface readings can be misleading. Vertical profiles of chlorophyll show that the depth-integrated biomass is typically lower inside the plume. Lower integrated values within the plume are partially due to the tendency for the chlorophyll profiles to be narrower than the profiles outside the plume, which is likely caused by differences in light penetration and stratification. Diane Masson, Institute of Ocean Sciences Spatial and temporal chlorophyll distribution in the Straits of Georgia and Juan de Fuca Authors: Diane Masson and Angelica Pena (Institute of Ocean Sciences, Sidney BC) Abstract: We investigate the spatial and temporal distribution of planktonic biomass along with some factors influencing its distribution within the Straits of Georgia and Juan de Fuca. These two straits are the main constituents of a large coastal estuary system on the southern coast of We investigate the spatial and temporal distribution of planktonic biomass along with some factors influencing its distribution within the Straits of Georgia and Juan de Fuca. These two straits are the main constituents of a large coastal estuary system on the southern coast of British Columbia. A large amount of freshwater enters the coastal basin, with the main source being the Fraser River, and drives a two-way estuarine circulation. Complex biological dynamics (responses to light, nutrients, grazing, etc) dictate complex biomass distribution. However, a unique data set from a relatively long duration (over 5 years) and systematic (every season, at the same 70 stations) sampling program allows us to accurately describe the chlorophyll distribution in both time and space within the coastal basin. Over the study area, the main features of the spatial distribution of chlorophyll are shown to be closely related to variations in upper water column stratification. In addition, it is shown that, during the summer, plankton growth is nutrient limited within the central Strait of Georgia. Finally, the possible role of light limitation of the plankton production within Juan de Fuca Strait is discussed. Jonathan Nash, Oregon State University Effect of discharge rate on the structure and mixing of Columbia River plume Authors: Levi Kilcher, Jonathan Nash and Jim Moum (Oregon State University, Corvallis, OR) Abstract: Observations of turbulence within the Columbia River estuary and plume were made during periods of low and high river discharge (August 2005 and May 2006). Dramatic differences in structure during these periods Observations of turbulence within the Columbia River estuary and plume were made during periods of low and high river discharge (August 2005 and May 2006). Dramatic differences in structure during these periods signify a regime change. Within the estuary, turbulent energy dissipation rates (and vertical fluxes of freshwater) are similar during both low and high flow. As a result, plume waters are significantly fresher during high flows, approximately scaling with river discharge rate. In the plume nearfield, the freshness alters the dynamics, since the highly-stratified plume quickly detaches from the bottom, whereas the weakly-stratified plume exhibits top-to-bottom turbulence and high bottom stress. These differences extend well into the coastal ocean and have important implications for sediment transport, turbulent exchanges across plume boundaries, and cross-shelf transport. Nikolva Nezlin, Southern California Coastal Water Research Project Ocean-color satellite observations of stormwater plumes in Southern California Authors: Nikolay P. Nezlin and Paul M. DiGiacomo (Southern California Coastal Water Research Project, Westminster, CA) Abstract: Ocean-color observations from the satellite sensors SeaWiFS and MODIS provide synoptic information about spatio-temporal dynamics of stormwater plumes, including their direction of propagation, size, and persistence over time. In southern California, river discharge is a major source of pollutants and pathogens to coastal waters, and as such the knowledge of plume dynamics is important for management of coastal water quality. Plume size is a function of the total volume of precipitated water. This relationship is quantitatively different in different regions, and can be explained by the differences in watershed size, terrain, and land-use characteristics. The direction of plume propagation results from the near-shore circulation, which is modulated by local wind stress. Ocean surface geophysical and optical properties (e.g., salinity, transmissivity, and suspended particulate matter) can be used for direct or indirect assessment of water quality Ocean-color observations from the satellite sensors SeaWiFS and MODIS provide synoptic information about spatio-temporal dynamics of stormwater plumes, including their direction of propagation, size, and persistence over time. In southern California, river discharge is a major source of pollutants and pathogens to coastal waters, and as such the knowledge of plume dynamics is important for management of coastal water quality. Plume size is a function of the total volume of precipitated water. This relationship is quantitatively different in different regions, and can be explained by the differences in watershed size, terrain, and land-use characteristics. The direction of plume propagation results from the near-shore circulation, which is modulated by local wind stress. Ocean surface geophysical and optical properties (e.g., salinity, transmissivity, and suspended particulate matter) can be used for direct or indirect assessment of water quality