A particle conveyor belt process in the Columbia River estuary: Evidence from chlorophyll<Emphasis Type="Italic">a</Emphasis> and particulate organic carbon |
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Authors: | Email author" target="_blank">Lawrence?F?SmallEmail author Fredrick?G?Prahl |
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Institution: | (1) Environmental and Biomolecular Systems, Oregon Health and Science University, Beaverton, OR 97006, USA;(2) Department of Civil and Environmental Engineering, Portland State University, Portland, 97207, USA;(3) Science and Technology Center for Coastal Margin Observation and Prediction, Oregon Health and Science University, Beaverton, OR 97006, USA |
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Abstract: | Using both the photosynthetically active chlorophylla (chla) content of the organic carbon fraction of suspended particulate matter (chla/POC) and the percentage of photosynthetically, active chla in fluorometrically measured chla plus pheophytina (% chla), we determined that under specified hydrodynamic conditions, neap-spring tidal differentiation in particle dynamics could
be observed in the Columbia River estuary. During summer time neap tides, when river discharge was moderate, bottom chla/POC remained relatively unchanged from riverine chla/POC over the full 0–30 psu salinity range, suggesting a benign trapping environment. During summertime spring tides, bottom
chla/POC decreased at mid range salinities indicating resuspension of chla-poor POC during flood-ebb transitions. Bottom % chla during neap tides tended to average higher than that during spring tides, suggesting that neap particles were more recently
hydrodynamically trapped than those on the spring tides. Such differentiation supported the possibility of operation of a
particle conveyor belt process, a process in which low-amplitude neap tides favor selective particle trapping in estuarine
turbidity maxima (ETM)., while high-amplitude spring tides favor particle resuspension from the ETM. Untrapped river-derived
particles at the surface would continue through the estuary to the coastal ocean on the neap tide; during spring tide some
particles eroded from the ETM would combine with unsettled riverine particles in transit toward the ocean. Because in tensified
biogeochemical activity is associated with ETM, these neap-spring differences may be critical to maintenance and renewal of
populations and processes in the estuary. Very high river discharge (15, 000 m3 s−1) tended to overwhelm neap-spring differences, and significant oceanic input during very low river discharge (5,000 m3 s−1) tended to do the same in the estuarine channel most exposed to ocean input. During heavy springtime phytoplankton blooms,
development of a thick bottom fluff layer rich in chla also appeared to negate neapspring differentiation because spring tides apparently acted to resuspend the same rich bottom
material that was laid down during neap tides. When photosynthetic assimilation numbers μgC (μgchl,a)−1h−1] were measured across, the full salinity range, no neap-spring differences and no river discharge effects occurred, indicating
that within our suite of measurements the compositional distinction of suspended particulate material was mainly a function
of chla/POC, and to a lesser extent % chla. Even though these measurements suggest the existence of a conveyor belt process, proof of actual operation of this phenomenon
requires scalar flux measurements of chla properties in and out of the ETM on both neap and spring tides. |
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