Ultraviolet Radiation and Bio-optics in Crater Lake, Oregon, 2005
OPTICAL BACKGROUND
In studies of underwater light in aquatic ecosystems it is customary to characterize the transparency or attenuation of natural waters instead of underwater irradiance because transparency and attenuation are persistent properties from which underwater UVR irradiance can be calculated for any given time and depth. A useful measure of UVR transparency in natural waters is Kd,?, the spectral diffuse attenuation coefficient for downwelling irradiance Ed,? (Baker & Smith, 1979). Kd can be used to characterize water transparency and factors controlling transparency or to reconstruct underwater solar spectra as a function of depth. Kd is calculated either from discrete measurements of Ed made at several depths or from a set of Ed values recorded over a continuous range of depths. An average Kd is often calculated for a range of depths considered to be optically mixed (e.g. the upper mixed layer or epilimnion) but depth-specific Kdvalues can also be derived from underwater measurements to reveal how attenuation varies with depth.
Valid spectral applications of Kd values are difficult to obtain unless the wavebands are narrow enough to be “spectrally-neutral” (where Kd,? varies little across the waveband). An example of a broad yet spectrally-neutral waveband in clear water is 400–500 nm. In contrast, when underwater Kd is calculated in clear water for the PAR (400–700 nm, PAR = photosynthetically active radiation) waveband, attenuation of solar radiation varies strongly with depth even though the water is uniformly mixed (Kirk, 1994b). This is because spectral variation in Kd,? leads to shifts with depth in relative proportions of different wavelengths within the waveband. Kd,PAR in a uniformly-mixed body of clear water is much greater at the surface than it is deeper because at the surface the strongly attenuated red part of the solar spectrum contributes to the average Kd; at deeper depths only the weakly attenuated blue and violet wavelengths are still present and only these contribute to average Kd. Another example of a problematic broadband application is when the entire UV-B range of wavelengths is used to calculate a single Kd,UVB, yielding values that are difficult to compare or interpret (Hargreaves, 2003). Underwater spectral radiometers useful for Kddeterminations have moderate bandwidths in the range of 10 nm or less (Kirk et al., 1994), although significant spectral shifts can occur with moderate bandwidth sensors at the shorter UV-B wavelengths (Patterson et al., 1997).
Other sensor properties can influence spectral Kd measurements. For downwelling irradiance (Ed) a sensor with an accurate cosine response to the angle of incident photons is needed. Accurate determinations of Kd,? are possible when a sensor is not accurately calibrated as long as the same sensor is used in all measurements, travels in a vertical plane during displacement over precisely determined depths, and maintains stability of its wavelength sensitivity, calibration, and cosine response to the angular distribution of light. In practice a correction for a dark offset signal and response to changing temperature may need to be incorporated into the measurement protocol (Kirk et al., 1994). Internal radiation sources (fluorescence and Raman scattering, also called “inelastic scattering”) can interfere with attempts to relate Kd,? to other optical properties when these contribute a significant fraction of the detected irradiance (Haltrin et al., 1997; Gordon, 1999). Measurement of spectral reflectance ratios (either irradiance reflectance, Eu/Ed, or radiance reflectance, Lu/Ed) can suggest the wavelengths and depths where such interference is occurring (Haltrin et al., 1997) but must account for self shading of the upwelling signal when a large instrument package is deployed (Dierssen & Smith, 2000).
For depth-specific measurements the equation is
where Ed is downwelling cosine irradiance measured at two depths, Z1 and Z2 (Kirk, 1994a; Kirk, 1994b). When Ed is measured continuously with depth by a UV profiling instrument the equation becomes
The value of Kd in equation (2) is typically estimated for a specific wavelength (?) by regression analysis, solving for the slope of the straight line formed by plotting Loge(Ed,Z) versus depth, Z, after correcting Ed,Z for dark signal & other noise (Hargreaves, 2003). With either method an average value for Kd,? is attributed to a specific depth range. The value of Kd,? will be approximately constant throughout depths that are uniformly mixed but can increase or decrease somewhat with depth until an equilibrium extent of diffuseness develops (Gordon, 1989). Variations in Kd near the surface of well-mixed water are related to surface waves (Zaneveld et al., 2001) and to changes in diffuseness determined by sky conditions and sun angle (up to 20–25% for UV wavelengths, Hargreaves, 2003). It is because of the response of Kd to diffuseness and light angle that Kd has been called an apparent optical property (AOP) of the water body, in contrast to inherent optical properties discussed below.
Kd and other optical measurements respond to the concentration of particulate and dissolved matter and can be used to investigate factors controlling transparency of natural waters. In addition to Kd, (an AOP) these include inherent optical properties of the water, or IOPs (Tyler & Presiendorfer 1962), properties controlled by the composition of the water and not influenced by the light field. IOPs include the beam absorption coefficient, a, the beam scattering coefficient, b, and their sum, the beam attenuation coefficient, c. Direct measurement of c in the water column has been common for years using the beam transmissometer, typically with a red (e.g. 660 nm) light source. When measured at a long wavelength where CDOM absorption is negligible, variations in c are correlated with the concentration of particles because of their impact on scattering. Particulate organic carbon (POC), microbial biomass, or phytoplankton cells are the dominant particles in many aquatic systems (Boss et al., this issue). At the typical wavelength of 660 nm used in transmissometers to measure beam c, the relatively constant absorption of water (cw660 = 0.411 m-1, varying slightly with temperature, Pope & Fry 1997; Morel 1974; Pegau etal., 1997) can be subtracted to yield the particulate beam attenuation coefficient, cp660.
Field measurements of a and b are relatively rare in visible wavelengths and extremely scarce in UV wavelengths (but see Boss et al., this issue). The value of a is the optical sum of absorption by dissolved and particulate constituents of natural waters in combination with aw, absorption by H20. The primary contributor to absorption by dissolved constituents in natural waters is colored dissolved organic matter (CDOM);acdom is typically measured using a laboratory spectrophotometer after particles are removed from the water sample by filtration. Values for aCDOM can also be estimated from measurements of DOC concentration if DOC-specific absorption can be estimated as well. Although suspended mineral particles can sometimes make a large contribution to attenuation, especially in shallow water or near inflow from glaciers or rivers, the optically-important particles in lakes are typically phytoplankton. Spectral absorption by phytoplankton can be measured in a spectrophotometer by concentrating a water sample onto a glass fiber filter. While primarily used for visible wavelengths (Yentsch & Phinney, 1989; Mitchell 1990; Lohrenz 2000), the technique has also been used for UV wavelengths (Ayoub et al., 1996; Sosik, 1999; Helbling, et al., 1994; Belzile et al., 2002; Hargreaves 2003; Laurion et al., 2003).
Indirect measures of optical properties can be predictive of phytoplankton abundance and Kd,UV in low-CDOM systems. The concentration of the primary photosynthetic pigment in phytoplankton (chlorophyll a) can be detected in vivo by its red absorption peak (676 nm) or by fluorescence measurements (emission peak at 683 nm) in the water column. Solar-stimulated fluorescence from phytoplankton pigments can also be detected by spectral reflectance meters after correction for Raman scattering. In natural waters the cp660 signal described above primarily responds to particle concentration because of scattering at 660 nm but when the particles are predominantly biotic, cp660 is expected to covary also with absorption and also attenuation at other wavelengths. None of these indirect measures is likely to be useful alone in predicting UV attenuation over a range of depths because of photoacclimation: the deeper phytoplankton adjust to dim light by increasing the efficiency of light utilization, the concentration of chlorophyll per cell, and the absorption per unit of chlorophyll, and decreasing the proportion of UV-screening pigments (MacIntyre et al 2002). Summing Fchl and cp660, with proper adjustment of their relative contribution, might provide a useful index of changing UV attenuation with depth when direct measures of UV attenuation are unavailable.
Another optical measurement that should be related to Kd,UV in UV-transparent systems is Secchi depth (ZSD). Measurement of ZSD has been used for many years as a simple transparency index of water quality (Larson et al., 1996a). The depth at which a 20 cm white disk is barely visible under ideal conditions (flat surface, no reflections from the surface, and adequate solar radiation) depends on a combination of scattering that obscures the image of the underwater disc and absorption that diminishes the light reaching the disk from the surface. The inverse of Secchi depth (1/ZSD, unit m-1) has been shown to correlate with [Kd + c] where Kd and c are measured for the appropriate range of wavelengths dependent on the combination of human vision and peak transmission wavelengths (Tyler,1968; Preisendorfer, 1986, ) and depth-averaged from the surface to ZSD. Human visibility of black objects underwater has been shown to vary inversely with beam attenuation in green wavebands (530 nm, Davies-Colley, 1988; Zaneveld & Pegau, 2003) but the blue waveband is likely to be more important in the case of very clear water such as Crater Lake. Because phytoplankton contribute to both scattering and absorption in blue wavelengths and typically have UV-absorbing protective pigments in a high UVR environment, blue attenuation and 1/ZSD should be correlated with UV attenuation when the latter is affected by phytoplankton. In other studies where phytoplankton and suspended mineral sediments control transparency, Secchi depth has been correlated with Kd measurements for the PAR waveband and with the concentration of suspended sediments (e.g. Jassby et al., 1999). In systems where the relative contributions to optical attenuation by phytoplankton and suspended mineral particles are variable, the relationships among cp660, 1/ZSD, Kd, and phytoplankton concentration would be expected to vary somewhat, with Kd less responsive to increases in scattering than the other two measurements.
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