Future directions

Fundamental to progress in understanding UV attenuation by natural waters is an improved understanding of absorption by pure water. Crater Lake and others like it offer the possibility to achieve optical closure for IOPs and AOPs with appropriate instrumentation. Advanced spectral absorption metres (e.g., the ICAM, [101,107]) should be extended into UV-B wavelengths and applied to testing ultra-pure water in the laboratory as well as the absorption of phyto-plankton and other suspended particles at low concentrations. Temperature and salinity effects should be established for absorption by water in UV wavelengths.

Optical methods can be improved in large part by extending existing methods developed for visible wavelengths. A multispectral scalar UV sensor combined with existing uplooking and downlooking cosine sensors would improve our ability to measure /1 and thus to relate IOP's with AOP's. Lacking this, published algorithms for estimating /Z from multispectral reflectance meters should be extended into the UV wavelengths. Future underwater measurements should be more consistently combined with determination of sun angle (requires geographic coordinates and time of day) and the diffuse fraction for solar UV

irradiance in order that attenuation calculations can be standardized for sun and sky effects. CDOM measurements should be combined more often with measurement of [DOC] and with CDOM spectral fluorescence using the new method of McKnight et al. [80] to help characterize DOM optical qualities. At very low levels of CDOM the use of a proxy such as fluorescence will be needed for field measurements; even better would be an instrument like the Wetlabs AC-9 that operates in UV wavebands. CDOM calculations should consistently include corrections for scattering (where needed) and baseline offset, and regression techniques should avoid the noise region of the measuring instrument (where negative absorption coefficients can create a bias in calculated spectral slope). Spectral slope should be calculated for UV-B and UY-A wavebands. CDOM values should be checked against matched diffuse attenuation measurements to detect (and correct) errors where Kd < (aC(jom + aphyto + «water)- Measurement of UV absorption and attenuation by particles should be improved, including better methods for calibration of QFT using field samples. Phytoplank-ton pigment should be measured in combination with field optical measurements to develop trends in chlorophyll-specific absorption in the water column.

More field measurements of UV absorption by phytoplankton are needed and these should be scaled to chlorophyll concentration and evaluated for spatial and temporal patterns across gradients (trophic, UV, nutrient, grazing). The effect of depth and mixing should be evaluated for effects on UV exposure and on the response of cells to adjust UV-absorbing and photosynthetic pigments.

Sources and sinks of CDOM should be evaluated in lakes and ocean regions to achieve closure for carbon and optical budgets. Spatial correlation between microbial communities and CDOM is now possible with field absorption instruments but more work is also needed on microbial processing of DOC, especially in low-DOC systems. Effects of climate change on DOC and UV attenuation should be explored for different geographic regions, and should take into account changes in DOC-specific absorption as well as DOC concentration. Patterns of DOC-specific CDOM absorption and other DOC characteristics should be determined for a larger range of saline environments, including saline lakes, coastal and the open ocean, with comparisons between surface and deep water. More work is needed on the roles for watersheds and hydrology (including concentration of [DOC] by regulating its influx rate or by evaporative loss of water) in determining DOC optical qualities. The counterpart for oceans is to better determine basin-scale changes in CDOM quality as a function of water column biotic processes, photochemical processes, mixing, and other sources and sinks [74].

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