James C. Kitchen

Scattering error correction of reflecting-tube absorption meters

In this paper we examine correction methods for the scattering error of reflecting tube absorption meters and spectrophotometers. We model the scattering error of reflecting tube absorption meters for different tube parameters and different inherent optical properties. We show that the only reasonable correction method for an absorption meter without attenuation measurements or a spectrophotometer is the method in which the measured absorption at a wavelength in the near infrared is subtracted. A better correction is obtained if attenuation is measured simultaneously and the absorption at the reference wavelength is multiplied by the ratio of the measured scattering at the measurement wavelength divided by the measured scattering coefficient at the reference wavelength. This is the proportional method. We showed that the important geometrical parameters of the reflecting tube can be obtained by a comparison of measurements and models of polystyrene beads. Finally, we examine the improvements that could be obtained if a direct scattering measurement were made simultaneously with the absorption and attenuation measurements.

The variation in the inherent optical properties of phytoplankton near an absorption peak as determined by various models of cell structure

Optical oceanography models of attenuation and scattering properties often contain simple spectral relationships. Electromagnetic theory, however, predicts fluctuations in the spectra of the attenuation coefficients and scattering properties of substances at wavelengths near an absorption peak. We have modeled these effects for phytoplankton using homogeneous, two-layered, and three-layered sphere models of cell structure and using a wide range of plausible particle size distributions. The magnitude of the scattering in backward directions is affected the most. The effect on the beam attenuation spectra is relatively small compared with the effect on the absorption and scattering coefficients. The backscattering coefficient shows large variability, varying by almost a factor of 3 for some models. The results suggest that beam attenuation at any wavelength in the red shorter than the wavelength of the chlorophyll absorption peak will be insensitive to the chlorophyll content of the particles. Increases in the pigment content per unit volume of phytoplankton will increase the index of refraction in the infrared and therefore increase the attenuation and scattering coefficients there.

Preliminary results from an in-situ spectral absorption meter

The in situ absorption meter, based on the reflective tube absorption meter principle in which both scattered and directly transmitted light are measured by a single receiver, was originally proposed as a alternative means to measuring in situ concentrations of chlorophyll a and phaeophytin. By measuring differential absorption between two wavelengths, 676 nm and 712 nm, a scattering correction mechanism was provided which provides accurate absorption measurements in natural waters. As the instrument design evolved six wavelengths were eventually installed to measure absorption throughout the visible and near IR spectrum. An operational overview of the instrument describes the primary optical and electrical components of the instrument and provides a basic understanding of how the absorption measurement is performed. After initial field tests, laboratory tests were performed to quantify the instruments operational characteristics. Precision, linearity, and performance in the presence of a scattering medium were tested to determine the instruments utility in performing in situ quantitative analysis of chlorophyll. The instrument demonstrated precision approaching 0.02 (mu) g/1 at a 7 Hz acquisition rate, excellent linearity over a 40 (mu) g/1 range, and less than two percent error in measurement accuracy under scatterer to absorber concentration ratios in excess of 1000:1.

Analysis of in-situ spectral absorption meter data

In situ absorption data collected with a reflective-tube absorption meter are presented. Various procedures for correcting the data for scattering error and for extracting chlorophyll absorption from the raw signal are explored. Based on our knowledge of the distribution of particle types and on measured backscattering, the scattering correction as a function of total scattering is found to vary significantly with depth. However, absorption in the near infrared is shown to be highly correlated with backscattering. We thus postulate that this signal is mainly due to the scattering error and possibly to absorption by dissolved substances and particles of a detrital nature. Thus, the infrared signal seems to provide a good correction for the measured a(676) to obtain chloropigment absorption. Indeed, a(676) - a(750) (corrected for water absorption and 750 temperature dependence) was found to correlate strongly with fluorescence. However, a(750) was found to be highly temperature dependent, so a(712) was chosen for this purpose in future measurements. An examination of optical microstructure in East Sound, Orcas Island, Washington showed numerous peaks with vertical dimensions of the order of tens of centimeters. These peaks may contain the majority of the biomass in the system. The relative magnitude of the a(676) - a(712) and the a(712) signals varied greatly from one peak to another and systematically with depth, presumably reflecting the nature and physiological states of the populations in the various peaks.

Reflective-tube absorption meter

The design and calibration of a proposed in situ spectral absorption meter is evaluated using a laboratory prototype. The design includes a silver coated (second-surface) glass tube, a tungsten light source (stabilized by means of optical feedback), a monochromator, and a solid state detector. The device measures the absorption coefficient plus a portion of the volume scattering function. Theoretical analyses and laboratory experiments which explore the magnitude and variation of the errors due to scattering and internal reflections are described. Similar analyses are performed on the Cary 1 18 Spectrophotometer to allow cross calibration. Algorithms to yield the abscrption coefficient and the zenith-sun diffuse attenuation coefficient are presented and evaluated. Simultaneous measurement of the beam attenuation or backscattering coefficient allows use of algoriThms with much narrower error bands. The various methods of obtaining absorption and diffuse attenuation values are compared. Procedures for using reverse osmosis filtration to produce a clean water calibration standard are described. An absorption spectrum for pure water is obtained. Development of the absorption meter is proceeding along two lines: 1) a two-wavelength side-by-side LED is being fabricated to allow an in situ chlorophyll a absorption meter to be constructed, and 2) scientific projects using a shipboard or laboratory flow.-through pumping system are being planned.

A Low Power, High Resolution, In Situ Fluorometer For Profiling And Moored Applications In Water

Sea Tech Inc. has developed an in situ fluorometer to measure chlorophyll a fluorescence in aquatic environments. The instrument has been developed within stringent constraints of size, weight and power consumption. The use of custom-designed components, including the flashlamp, excitation and emission filters, and energy storage capacitor for the light source has permitted optimal mechanical, optical and electrical design of the instrument. This new design results in efficient stimulation and detection of chlorophyll a fluorescence. The instrument is not sensitive to ambient light and has excellent stability over time and temperature. Chlorophyll a concentration is measurable on three selectable ranges of approximately 3, 10 or 30 µg/1 full scale with a minimum detectable signal of <0.02 gg/1. Time constants of 0.1, 1, 3 and 10 seconds are selectable to smooth the output data. Power requirements are nominally 12 VDC at 150 mA, and output signal is 0 to 5 VDC. These power requirements and signal levels make the fluorometer compatible with most oceanographic moored and profiling data acquisition systems. Operating depth for the instrument is rated at 500 meters with a plastic housing or 3000 meters with a stainless steel pressure housing.

The vertical structure and size distributions of suspended particles off Oregon during the upwelling season

Abstract A simple numerical model of the vertical distribution of two size classes of particles is developed for situations common during the coastal upwelling season off Oregon. The total particle concentration is assumed to be proportional to the phytoplankton population for the surface layer. The two size classes of particles are thus distinguished by their maximum specific growth rates, their half saturation constants for nitrate uptake, and their settling rates. The resulting vertical distributions and size distributions were similar in shape to the average profiles for regions inshore and offshore of a particle front during August 1974 but overestimated particle concentration by 40%. This was attributed to ignoring grazing by zooplankton. Sensitivity analyses showed the size preference was most responsive to the maximum specific growth rates and nutrient half-saturation constants. The vertical structure was highly dependent on the eddy diffusivity followed closely by the growth terms.

Backscattering efficiency of coccolithophorids: use of a three-layered sphere model

The spectral variations of the efficiency factors for absorption, total scattering and backscattering have been computed via Mie theory using a three-layered sphere model, with the size distribution function and the spectral values of the refractive index of each layer as input parameters. When compared with the results of a model for homogeneous spherical cells (with an equivalent bulk refractive index), these theoretical predictions allow the modifications of the efficiency factors due to heterogeneities within algal cells to be assessed. Such a comparison has been performed for a coccolithophorid suspension (Emiliania huxleyi), for which the spectral values of the refractive index have been derived from the experimental absorption and scattering coefficients. While the internal structures induce insignificant modifications in absorption and only weak modifications in total scattering, they appear to be able to increase the backscattering efficiency by a factor as high as 50, depending mainly on the calcite shell thickness. The internal structures also induce spectral changes in backscattering.

Some observations on the effect of ozone treatment on suspended particulate matter in seawater

Effects of ozone treatment on suspended particles in seawater have been observed. The observations consist of measuring the particle size distribution using an electronic particle counter before and after treating the sample with ozone. Three kinds of response have been observed in the laboratory and in the field: (i) the total elimination of a peak in the size spectra; (ii) a shift of a peak to a smaller size with correspondingly less volume; and (iii) a mild decrease in the total particle concentration. Our interpretation of these responses as observed in the field are respectively: (i) phytoplankton with a very small amount of refractory structural materials (e.g. naked flagellates); (ii) particles that individually contain both refractory and non-refractory components (e.g. diatoms); and (iii) primarily non-organic and refractory organic materials.

Observations of natural fluorescence with an underwater radiometer

Measurements of subsurface irradiance spectra in the ocean consistently indicate intensities of long-wavelength visible light (λ≧589 nm) greater than could be derived from the penetration of sunlight. This can be attributed to natural fluorescence and/or to spectral crosstalk due to light leakage through the blocking filters of each discrete detector. A comparison of observed profiles with modelled contributions from these two possible sources indicates that both factors are contributing. At 671 and 694 nm wavelength, the excess signal appears to be predominantly fluorescence while that at 589 nm is mostly crosstalk. Both effects appear to be important at 625 nm although the amount of excess light is small compared to the other wavelengths.The above observations and interpretations are consistent with the optical design of the instrument used and the shape of natural irradiance spectra. Fluorescence efficiencies derived from the irradiance measurements correlate well with measuredin situ fluorescence.