A study of the optical characteristics of the suspended particles in the benthic nepheloid layer of the Scotian Rise

Self Cite

Measurements of inherent optical properties (IOP) were conducted over bottoms with different substrates by use of a sampling package mounted on and operated by a SCUBA diver. It was found that in areas of low ambient currents the distribution of IOP varies with bottom type in (1) its value relative to a nearby bottom of different type, (2) its vertical gradient, and (3) its variability. This implies that radiative transfer modeling in shallow environments may need to include, besides the bottom characteristics, the bottom effect on in-water IOP. In tidally flushed shallow banks, vertical and horizontal gradients over scales of O(1, 10 m), respectively, are as large as temporal gradients over scales of minutes and cannot be separated in our measurements. However, bottom-substraterelated processes over the banks result in gradients over large horizontal spatial scales and tidal timescales. The distribution of IOP is consistent with several biogeochemical processes that may be active at a given bottom substrate and suggest that optical measurements may provide a useful tool to infer and quantify bulk rates of biogeochemical processes.The classic forward problem in ocean optics is the calculation of the distribution of radiance in space given the optical properties of the medium (the inherent optical properties [IOP]; Preisendorfer 1976) and appropriate boundary conditions. To solve this ''forward'' problem, we need to know the details of the light source or sources (e.g., the solar spectrum and its direct and diffuse light just above the ocean), transmission through the boundaries of the domain (e.g., a wave-modulated air-ocean interface), and the optical properties of the medium. In the ocean, photons interact with the water molecules and with dissolved and particulate material within the water. These constituents determine the IOP of any given water mass. In shallow waters, an additional complexity comes from the existence of a bottom boundary, which interacts with light (through absorption and reflection).The forward problem is of importance for interpretation of remotely sensed ocean color in shallow areas (e.g., Lee et al. 1999;Carder et al. 2003) and for systems designed to image the bottom (e.g., for classification of bottom substrate and target recognition; McLean et al. 1995). In order to understand the distribution of IOP in space and time, we need to understand their relationship to processes occurring in the ocean as well as processes associated with the bottom.The ocean bottom affects light propagation not only through its own optical properties; the bottom substrate and AcknowledgmentsThis research was sponsored by the Coastal Benthic Optical Properties initiative of the Office of Naval Research. We thank R. Wheatcroft for providing us his current velocity measurements. Discussions with R. Zimmerman and D. Burdige initiated the porewater sampling. We thank P. Hill for an insightful review of an earlier version of this manuscript and J. Washburn and F. Baratange for assistance in sampling and i...

Section: Methodsmentioning

confidence: 99%

The effect of bottom substrate on inherent optical properties: Evidence of biogeochemical processes

Boss

Zaneveld

2003

Self Cite

“…As a result of elevated organic matter and nutrient content, they are often hotspots for microbial activity, characterized by elevated microbial cell numbers and metabolic turnover rates (Boetius et al 2000;Guo and Santschi 2000). Most studies to date have focused on BNLs in ocean basins (Hunkins et al 1969;Spinrad et al 1983;McCave 1986), and the few studies dealing with BNLs in lake systems were carried out in very large lakes (e.g., Lake Ontario, Lake Michigan, Lake Superior), approaching inland seas in size (Hawley and Murthy 1995;Hawley and Muzzi 2003;Hicks et al 2004). The exact controls on the formation of lacustrine BNLs remain enigmatic.…”

mentioning

confidence: 99%

Bacterial methanotrophs drive the formation of a seasonal anoxic benthic nepheloid layer in an alpine lake

Blees¹,

Niemann²,

Wenk³

et al. 2014

We investigated the formation and microbial composition of a seasonal benthic nepheloid layer (BNL) in the eutrophic, monomictic southern basin of Lake Lugano. During stratification, a BNL developed at the sedimentwater interface and progressively expanded 20-30 m into the water column, following the rising oxic-anoxic interface. The dominance of the fatty acids C 16:1v5 , C 16:1v6 , C 16:1v7 , and C 16:1v8 , with d 13 C values between 262% (v6) and 280% (v7), suggests that the BNL was composed primarily of Type I aerobic methane oxidizing bacteria (MOB). Indeed, MOB contributed . 75% to the fatty acid carbon pool in the fully developed BNL, with cell densities up to 8.5 3 10 5 cells mL 21 . In ex situ incubation experiments, CH 4 turnover rate coefficients were up to 2.1 d 21 , which translates into potential CH 4 oxidation rates as high as 20 mmol m 23 d 21 under in situ CH 4 concentrations. CH 4 oxidation was limited by the diffusive supply of O 2 , and O 2 consumption by aerobic CH 4 oxidation (up to 13.1 mmol m 22 d 21 ) appears to be the primary driver of the seasonal growth of the BNL and expansion of the hypolimnetic anoxic zone. Methanotrophic activity at the interface between oxic and anoxic water masses can actuate the formation of a BNL, which in turn functions as an effective microbial CH 4 filter in the water column, preventing CH 4 transport to surface waters and evasion to the atmosphere. In situ biomass production by methanotrophic bacteria may represent, in addition to sediment resuspension and detritus trapping, a novel BNL formation mechanism.

“…Values of c m p or b m p reported in the literature vary over one order of magnitude, from 0.05 m 2 g 21 to 1.5 m 2 g 21 (Baker and Lavelle 1984;Wells and Kim 1991;Gardner et al 2001, but see Hill et al 2011 for a comprehensive overview). Theoretical and experimental work focused on the effect of particle size on c m p and b m p (Pak et al 1970;Spinrad et al 1983;Baker and Lavelle 1984). However, despite large variability in particle size, in situ measurements of c m p vary much less (Bunt et al 1999;Mikkelsen and Pejrup 2000).…”

mentioning

confidence: 99%

In situ variability of mass‐specific beam attenuation and backscattering of marine particles with respect to particle size, density, and composition

Neukermans

Loisel

Mériaux

et al. 2011

174

179

This study analyzes relationships between concentration of suspended particles represented by dry mass, [SPM], or area, [AC], and optical properties including particulate beam attenuation (c p ), side scattering (b s ), and backscattering (b bp ), obtained from an intensive sampling program in coastal and offshore waters around Europe and French Guyana. First-order optical properties are driven by particle concentration with best predictions of [SPM] by b bp and b s , and of [AC] by c p . Second-order variability is investigated with respect to particle size, apparent density (dry weight-to-wet-volume ratio), and composition. Overall, the mass-specific particulate backscattering coefficient, b bp remains unexplained. Possible causes are the limitation of the measured size distributions to the 2-302-mm range and effects of particle shape and internal structure that affect b bp more than c p and were not accounted for.