Norman B. Nelson

Chapter 11 – Chromophoric DOM in the Open Ocean

This chapter presents a discussion on chromophoric dissolved organic matter (DOM) in the open ocean. Recent research has shown that the optically active fraction of dissolved organic matter (chromophoric DOM or CDOM, also called “gelbstoff” or “gilvin”) plays a major role in determining underwater light availability in the open ocean. Open oceans are defined as the part of the global ocean where coastal runoff and riverine input are negligible on annual time scales. Furthermore, research suggested that significant variability in CDOM concentrations occur on the seasonal to interannual time scales in the upper water column. This “picture” of open ocean CDOM differs from the traditional view where light absorption by CDOM was thought to be minor and that CDOM variability was not significant on annual time scales. The change in the perception toward CDOM dynamics is important because CDOM can play a direct or indirect role in climate-related biogeochemical cycles—in particular, the carbon and sulfur cycles. Absorption of light by CDOM controls ultraviolet radiation penetration into the ocean, which has an impact upon phytoplankton and bacterial productivity. CDOM is also a primary reactant in the photoproduction of CO 2 , CO, H 2 O 2 , and Carbonyl sulfide and is a photosensitizer in the photolysis of dimethyl sulfide (DMS). Because of its impact upon the underwater light field, CDOM can influence the accuracy of global satellite-based measurements of ocean chlorophyll and primary productivity.

Maritime Aerosol Network as a component of Aerosol Robotic Network

The paper presents the current status of the Maritime Aerosol Network (MAN), which has been developed as a component of the Aerosol Robotic Network (AERONET). MAN deploys Microtops handheld Sun photometers and utilizes the calibration procedure and data processing (Version 2) traceable to AERONET. A web site dedicated to the MAN activity is described. A brief historical perspective is given to aerosol optical depth (AOD) measurements over the oceans. A short summary of the existing data, collected on board ships of opportunity during the NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project is presented. Globally averaged oceanic aerosol optical depth (derived from island-based AERONET measurements) at 500 nm is similar to 0.11 and Angstrom parameter (computed within spectral range 440-870 nm) is calculated to be similar to 0.6. First results from the cruises contributing to the Maritime Aerosol Network are shown. MAN ship-based aerosol optical depth compares well to simultaneous island and near-coastal AERONET site AOD.

The Global Distribution and Dynamics of Chromophoric Dissolved Organic Matter

Chromophoric dissolved organic matter (CDOM) is a ubiquitous component of the open ocean dissolved matter pool, and is important owing to its influence on the optical properties of the water column, its role in photochemistry and photobiology, and its utility as a tracer of deep ocean biogeochemical processes and circulation. In this review, we discuss the global distribution and dynamics of CDOM in the ocean, concentrating on developments in the past 10 years and restricting our discussion to open ocean and deep ocean (below the main thermocline) environments. CDOM has been demonstrated to exert primary control on ocean color by its absorption of light energy, which matches or exceeds that of phytoplankton pigments in most cases. This has important implications for assessing the ocean biosphere via ocean color-based remote sensing and the evaluation of ocean photochemical and photobiological processes. The general distribution of CDOM in the global ocean is controlled by a balance between production (primarily microbial remineralization of organic matter) and photolysis, with vertical ventilation circulation playing an important role in transporting CDOM to and from intermediate water masses. Significant decadal-scale fluctuations in the abundance of global surface ocean CDOM have been observed using remote sensing, indicating a potentially important role for CDOM in ocean-climate connections through its impact on photochemistry and photobiology.

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.

Physical and biogeochemical variability from hours to years at the Bermuda Testbed Mooring site : June 1994-March 1998

Abstract The Bermuda Testbed Mooring (BTM), initiated in 1994, provides important information concerning periodic and episodic processes. The BTM enables collection of virtually continuous data during periods of inclement weather when traditional sampling is not possible, and provides otherwise inaccessible data in the important spectral range of minutes to a month. New methodologies and analyses are evolving for interdisciplinary mooring measurements. For example, BTM and shipboard temperature and spectral downwelling irradiance data sets have been compared and are in good agreement. Several meteorological parameters determined using operational analyses and climatological estimates also compare favorably with BTM calculations. Kinetic energy (by frequency domain), de-correlation time scales, and spectra were computed using the BTM data sets. The ratio of kinetic energy attributed to periods less than 2 days to that in periods greater than 2 days is generally greater at 45 m than at 71 m (the winter season being the exception). The upper layer has more kinetic energy due to inertial motions, while the lower layer is more influenced by mesoscale eddies. It is inferred that important interactions occur between these layers. It is hypothesized that the specific timing of the onset of spring stratification and fall breakdown of stratification, as well as accompanying phytoplankton blooms, are controlled by the specific synoptic wind forcing and mesoscale eddy conditions. An example of this scenario is described using unique BTM observations of a fall bloom event. Specifically, (1) a mode eddy with a shallow (roughly 80 m) nutricline passed the site, (2) deepening of the mixed layer (from about 50–175 m) caused entrainment of nutrients into the euphotic layer, and (3) a three-fold increase in depth-integrated chlorophyll was observed. Finally, de-correlation time scales were found to lie between 5 and 15 days for the key physical and biological variables. These de-correlation scales set upper limits on sampling intervals necessary to resolve the important physical and biogeochemical processes.

Ocean deoxygenation: Past, present, and future

To a first order, the oxygen content of the ocean interior is determined by the influx of the gas across the air-sea surface (i.e., ventilation) and consumption due primarily to microbial respiration. As these two competing processes vary in space and time, so does the concentration of oxygen in the ocean interior. Although oxygen concentrations on continental margins are declining in many regions due to increased anthropogenic nutrient loadings [e.g., Rabalais et al., 2002], oxygen also appears to be declining in both the central North Pacific Ocean and the tropical oceans worldwide [Emerson et al., 2004; Whitney et al., 2007; Keeling et al., 2010] (see Figure 1). It is unclear whether the loss throughout the basins in the open ocean is a long-term, nonperiodic (secular) trend related to climate change, the result of natural cyclical processes, or a combination of both (Figure 2). If related to climate change, a number of important factors may be involved, including decreased solubility of oxygen as waters warm, decreased ventilation at high latitudes associated with increased ocean stratification, and changes in respiration in the ocean interior.

Calibration of an integrating sphere for determining the absorption coefficient of scattering suspensions

Measuring the absolute absorption of suspensions of absorbing particles with unknown scattering characteristics is not possible in conventional spectrophotometers or in integrating spheres that have the sample located outside the sphere. A method for the calibration and use of an integrating sphere with a centrally located sample to measure absolute absorption coefficients of scattering suspensions is presented. Under the tested conditions the integrating sphere used in this study was insensitive to changes in the scattering coefficient of the sample but had a nonlinear response to increasing absorption of the sample, which could be corrected with an empirically derived function. This response was analyzed by using a Monte Carlo simulation, and results indicated that amplification of the absorption signal was primarily due to photons reflected from the sphere surface and the baffle reentering the cuvette. The calibration procedure described here may be generally applicable to spheres of different configurati n. An example of the use of the sphere for determining the absorption and scattering coefficients of marine phytoplankton samples is presented.

The Optical Properties of DOM in the Ocean

Abstract In this chapter, a summary of the absorption and fluorescence (UV-Visible spectroscopic) properties of colored dissolved organic matter (CDOM) in the marine environment is given. Although the chemical compounds responsible for these properties to date remain elusive, much can be gained from studying the distribution and dynamics of CDOM. The results from global surveys and experimental incubations indicate that CDOM measurements have the potential to be optical markers which trace the turnover of a DOM fraction that is not otherwise apparent from elemental analyses. A remaining key challenge is to evaluate how much of the carbon bound in DOM is associated with CDOM.

Decoupling of net community and export production on submesoscales in the Sargasso Sea

Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux-regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed-layer biogeochemical parameters and proxies for NCP and EP during 10, high-spatial resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common but rarely did the same parameters covary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in >5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However, when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect-averaged EP determinations were slightly smaller than similar NCP values (Type-II regression slope of 0.93, standard deviation = 0.32) but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations.

Optical Signatures of Dissolved Organic Matter Transformation in the Global Ocean

Characterization of dissolved organic matter in terms of its composition and optical properties, with an eye toward ultimately understanding its deep ocean dynamics, is the currently active frontier in DOM research. We used UV-visible absorption spectroscopy and fluorescence excitation-emission matrix (EEM) spectroscopy to characterize dissolved organic matter in the open ocean along sections of the U.S. CO2/CLIVAR Repeat Hydrography Project located in all the major ocean basins outside the Arctic. Despite large differences in fluorescence intensity between ocean basins, some variability patterns were similar throughout the global ocean, suggesting similar processes controlling the composition of the DOM. We find that commercially available single channel CDOM sensors are sensitive to the fluorescence of humic materials in the deep ocean and thermocline but not to the UVA-fluorescing and absorbing materials that characterize freshly produced CDOM in surface waters, revealing fundamental diversity in the DOM profile. In surface waters, UVA fluorescence and absorption signatures indicate the presence of freshly produced material and the process of bleaching removal, but in the upper mesopelagic and in the main thermocline these optical signatures are replaced by those of humic materials, with distribution patterns correlated to apparent oxygen utilization and other signatures of remineralization. Empirical orthogonal analysis (EOF) of the EEM data suggests the presence of two (unidentified) processes which convert “fresh” DOM to humic materials: one located in the surface ocean (shallower than 500m) and one located in the main thermocline. These hypothetical humification processes represent less than 5% of the overall variability in oceanic humic DOM fluorescence, which appears to be dominated by terrestrial input and solar bleaching of humic materials.