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Optical effect on the nitracline in a coastal upwelling area Dag L. Aksnes1

Summary: Optical effect on the nitracline in a coastal upwelling area
Dag L. Aksnes1
Department of Biology, University of Bergen, N-5020 Bergen, Norway
Mark D. Ohman
Scripps Institution of Oceanography, University of California­San Diego, La Jolla, California 92093-0218
Pascal Rivie`re
LEMAR, Institut Universitaire EuropeŽen de la Mer, 29280 PlouzaneŽ, France
The transport of nitrate into the euphotic zone is an important regulator of primary production. This transport
is facilitated by physical processes that involve the depth and the steepness of the nitracline, but transport is
complicated by the dynamical nature of the euphotic zone. Here we derive an analytical model that predicts two
optical effects of the euphotic zone on the nitracline: the nitracline depth should vary inversely with light
attenuation for downwelling irradiance, and the nitracline steepness should be directly proportional to light
attenuation. We show that observations of nitrate and Secchi depth, which have been obtained over 21 yr in the
coastal upwelling region off Southern California (CalCOFI area), are consistent with these predictions.
Chlorophyll a measurements also indicate an optical signature in the nitracline: while the amount of chlorophyll
correlated poorly with the nitracline depth, the nitracline depth correlated strongly with the optical effect of
chlorophyll, and the nonlinear nature of this relationship was consistent with the model prediction. These optical
effects on the nitracline may involve positive feedback mechanisms with phytoplankton production that have
implications for interpretation and modeling of primary production.


Source: Aksnes, Dag L. - Institutt for biologi, Universitetet i Bergen


Collections: Environmental Sciences and Ecology; Biology and Medicine