D. A. Siegel

Colored dissolved organic matter and its influence on the satellite‐based characterization of the ocean biosphere

[1] Satellite ocean color data enable the global assessment of the ocean biosphere through determinations of chlorophyll concentrations. However, ocean color is not a function of chlorophyll alone. We assess differences between two ocean color models with nearly identical validation statistics. The resulting chlorophyll retrievals show systematic differences which are consistent with each model’s ability to account for the absorption of light by colored dissolved organic materials. These differences are often large and approach 100% poleward of 40� latitude. We conclude that the discrepancies are due to fundamental differences in model assumptions and their empirical tuning using geographically limited, in situ data. This source of uncertainty is important as the choice of ocean color model alters modeled rates of global net primary production by more than 30%. The ultimate resolution of this issue requires continued improvements in remote sensing algorithms and validation data as well as satellite technology. Citation: Siegel, D. A., S. Maritorena, N. B. Nelson, M. J. Behrenfeld, and C. R. McClain (2005), Colored dissolved organic matter and its influence on the satellite-based characterization of the ocean biosphere, Geophys. Res. Lett., 32, L20605, doi:10.1029/2005GL024310.

An improved bio‐optical model for the remote sensing of Trichodesmium spp. blooms

L � 1 . Using the in situ data set, this model is trained to successfully predict Trichodesmium blooms (� 92%) while minimizing false positive retrievals (� 16% of nonbloom observations). A validation of the approach applied to Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery shows that the model correctly predicts 76% of the bloom occurrences of an independent validation data set of in situ Trichodesmium observations. Ultimately, maps of Trichodesmium bloom occurrence will provide a means of addressing the ecology of Trichodesmium and its contribution to new production of the world oceans.

Estimating suspended sediment concentrations in turbid coastal waters of the Santa Barbara Channel with SeaWiFS

A technique is presented for estimating suspended sediment concentrations of turbid coastal waters with remotely sensed multi-spectral data. The method improves upon many standard techniques, since it incorporates analyses of multiple wavelength bands (four for Sea-viewing Wide Field of view Sensor (SeaWiFS)) and a nonlinear calibration, which produce highly accurate results (expected errors are approximately ±10%). Further, potential errors produced by erroneous atmospheric calibration in excessively turbid waters and influences of dissolved organic materials, chlorophyll pigments and atmospheric aerosols are limited by a dark pixel subtraction and removal of the violet to blue wavelength bands. Results are presented for the Santa Barbara Channel, California where suspended sediment concentrations ranged from 0–200+ mg l−1 (±20 mg l−1) immediately after large river runoff events. The largest plumes were observed 10–30 km off the coast and occurred immediately following large El Niño winter floods.

Ocean color observations and modeling for an optically complex site: Santa Barbara Channel, California, USA

[1] An extensive bio-optical data set from the Santa Barbara Channel (SBC), California, is used to assess the in-water constituents responsible for ocean color variability in this optically complex coastal site and to develop locally optimized ocean color models. The Santa Barbara Channel is a productive region of the California Current System. Chlorophyll concentrations are highly variable (range from 0.06 to 19.8 mg m � 3 ) and are higher than the typical open ocean conditions (mean of 2.2 mg m � 3 and standard deviation of 2.4 mg m � 3 ). Thus chlorophyll pigments are a prime driver of reflectance variability in the SBC. Reflectance is also influenced by colored dissolved organic matter and suspended sediments which only weakly covary with chlorophyll. Detrital particulates are responsible for only a small (ca. 10%) portion of nonwater absorption, whereas phytoplankton and colored dissolved organic matter contribute approximately equally to the total nonwater absorption coefficient at 440 nm. Remote-sensing reflectance spectra show the expected relationships with in situ determinations of absorption and backscattering where reflectance variability is absorption-driven in the blue and backscattering-driven in the green. The performance of existing empirical and semianalytical ocean color algorithms is shown to be satisfactory, although clearly not excellent. The optical complexity of the SBC suggests that regionally specific semianalytical models should be developed to best account for the independently varying optical constituents. Importantly, an optimization of a semianalytical model using the present data set does not result in substantial improvements in performance. Analyses of the causes and the implications of this conundrum are discussed.