Climatic and Tidal Forcing of Hydrography and Chlorophyll Concentrations in the Columbia River Estuary |
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Authors: | G Curtis Roegner Charles Seaton António M Baptista |
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Institution: | (1) NOAA Fisheries, Northwest Fisheries Science Center, Point Adams Research Station, Box 155, Hammond, OR 97013, USA;(2) Science and Technology Center for Coastal Margin Observation and Prediction, Oregon Health and Science University, Beaverton, OR 97006, USA |
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Abstract: | Hydrographic patterns and chlorophyll concentrations in the Columbia River estuary were compared for spring and summer periods
during 2004 through 2006. Riverine and oceanic sources of chlorophyll were evaluated at stations along a 27-km along-estuary
transect in relation to time series of wind stress, river flow, and tidal stage. Patterns of chlorophyll concentration varied
between seasons and years. In spring, the chlorophyll distribution was dominated by high concentrations from freshwater sources.
Periods of increased stream flow limited riverine chlorophyll production. In summer, conversely, upwelling winds induced input
of high-salinity water from the ocean to the estuary, and this water was often associated with relatively high chlorophyll
concentrations. The frequency, duration, and intensity of upwelling events varied both seasonally and interannually, and this
variation affected the timing and magnitude of coastally derived material imported to the estuary. The main source of chlorophyll
thus varied from riverine in spring to coastal in summer. In both spring and summer seasons and among years, modulation of
the spring/neap tidal cycle determined stratification, patterns of mixing, and the fate of (especially freshwater) phytoplankton.
Spring tides had higher mixing and neap tides greater stratification, which affected the vertical distribution of chlorophyll.
The Columbia River differs from the more tidally dominated coastal estuaries in the Pacific Northwest by its large riverine
phytoplankton production and transfer of this biogenic material to the estuary and coastal ocean. However, all Pacific Northwest
coastal estuaries investigated to date have exhibited advection of coastally derived chlorophyll during the upwelling season.
This constitutes a fundamental difference between Pacific Northwest estuaries and systems not bounded by a coastal upwelling
zone. |
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