Continuous Hyperspectral Absorption Measurements of Colored Dissolved Organic Material in Aquatic Systems

Abstract

The majority of organic carbon in the oceans is present as dissolved organic matter (DO~f); therefore understanding the distribution and dynamics of DOM is central to understanding global carbon cycles. Describing the time-space variability in colored dissolved organic matter (eDOM) has been difficult, as standard spectrophotometric methods for eDOM determination are laborious and susceptible to meth· odological biases. Previously, measurements of eDOM absorption in discrete water samples by use of a liquid·waveguide capillary cell (LWCC) compared favorably with measurements made with a benchtop spectrophotometer. Given this, we focused on automating the LWCC technique to improve our spatial and temporal sampling capabilities for eDOM. We found strong correlations between CDOM absorp tion spectra collected from discrete water samples using standard methods and selected corresponding CDOM spectra collected by the automated LWCC system. The near-continuous measurements by the LWCC system made it possible to map the temporal, spatial, and spectral variability ofCDOM absorption along the ship track.

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“…In contrast, type II LCW measurements resulted in spectral slopes that increased with decreasing salinity (difference of 0.0012 nm -1 ), with the 25% seawater being significantly different from the 50% and 100% natural seawater samples. Although the cause for increasing S with dilution in the type II LCW is unclear, the data are consistent with previous reports (Kirkpatrick et al 2003). The high precision of LCW-mediated absorption coefficient scans renders these differences significant (with respect to data overlap within three standard deviations); however, the observed variations with dilution are within typical reported error values (Twardowski et al 2004).…”

“…A host of studies have found that CDOM concentrations and spectral signature vary in response to local production and destruction processes (both biotic and abiotic) ( These recent insights into marine CDOM dynamics have prompted the growing use of LCWs for CDOM measurements in coastal (D'Sa et al 2002;Miller et al 2002;Kirkpatrick et al 2003;Floge and Wells 2007) and open-ocean (Nelson et al 2007) regions. Indeed, Nelson et al (2007) used a type I cell (Ultrapath; WPI Inc.) to map CDOM distributions in the North Atlantic Ocean, whereas type II LCW cells are being incorporated into various autonomous in situ optical sensors (Kirkpatrick et al 2003). Unfortunately, concerns have been raised about wavelength-dependent biases and lack of compatibility in data collected with the two LCW cell types (D'Sa et al 1999).…”

“…Additionally, long-pathlength LCWs are being incorporated into various autonomous underwater vehicles and fixed oceanographic observation platforms for the purpose of developing comprehensive CDOM maps (Kirkpatrick et al 2003). However, there have been reports of discrepancies between absorption coefficient spectra obtained with type II capillary waveguides and conventional spectrophotometers (D'Sa et al 1999).…”

“…It has also been suggested that type I and type II LCWs differ significantly in their waveguiding characteristics (Byrne and Kaltenbacher 2001). At present, both cell types are used to generate CDOM absorption coefficient and slope data in diverse natural environments, but it is currently unclear whether data generated with each type are comparable (e.g., Kirkpatrick et al 2003, Miller et al 2002.…”

Observing the Oceans from the COOL Room: Our History, Experience, and Opinions

“…The strategy was based on scienceengineering partnerships where the scientist provided the motivation, the engineer developed the new tools, and the team collaborated on implementation. This model has been central to the growth of the New Jersey observation network, where science-engineering partnerships have focused on the development of satellite data-processing algorithms , surface current radars (Kohut et al, 1999;Kohut et al, in press), autonomous underwater gliders (Schofield et al, 2002, ship-towed systems (Creed et al, 1998;Creed and Glenn, 2000), and bio-optical instrumentation (Kirkpatrick et al, 2003). The partnerships were the key to ensuring success as all projects were funded through competitively awarded research grants.…”