Summary

This chapter has covered some aspects of the physics of the upper mixed layer that modulate how molecules and organisms in aquatic ecosystems are exposed to UVR. The basic implications of a vertically mixed environment have been realized by aquatic UVR researchers for some time. The contents of the near-surface "photoactive" zone are in continual motion and, over some time scale, exposed constituents at the surface will exchange with relatively unexposed constituents at depth. The modulation of UVR exposure is greater than for visible (PAR) part of the spectrum because of the stronger vertical gradients in the UV. Moving beyond this qualitative understanding to quantitative descriptions of mixing effects, however, has been slow.

To some extent, progress has been limited by the availability of measurements on exchange processes. Until recently, temperature microstructure measurements were the primary approach to quantify near-surface turbulence. The instruments needed to do this were expensive and difficult to operate. The situation is now considerably improved. Microstructure sensors more suited to field use are commercially available, and are more "user-friendly". Alternative methods to observe or infer mixing processes have also been perfected, including free-fall CTDs, acoustic doppler sensors and acoustically monitored floats. As these techniques are refined and deployment, operation, and analysis become more routine, it will become increasingly practical to incorporate a "mixing" component into field studies of UVR effects.

Even with good measurements, another technical problem in understanding how mixing modifies UVR effects is translating a given mixing regime into an exposure regime. Up to the present, experiments on mixing and UVR have used simplified schemes to simulate mixing. In field experiments, the exposure usually consists of deterministic cycles as would occur in a circular, vertically rotating eddy. Numerical models, on the other hand, have mainly used a random walk approach which emphasizes turbulent diffusion. Actual mixing is more complicated in that it is a combination of deterministic and random motions, with the contribution of each type of motion varying with circumstances. At present, we don't know if simplified versions of vertical mixing used in experiments or models result in the losing some important aspect of the exposure regime which affects overall response. We can attempt to use more realistic schemes in future studies. For example, with microprocessor controls it is now easier to structure complex sequences of light exposures (e.g. [91]). Another approach that has been developed (but not yet applied in UVR studies) is to measure photosynthesis in a sample attached to a Lagrangian float [92]. There are now sophisticated numerical models of mixing which capture the full range of motions [93] and increases in computing power make it increasingly possible to run such models on a desktop computer. Implementation of full numerical mixing models with accurate estimates of the spectral and temporal dependence of UVR responses will be a significant step towards accurate assessment of UVR effects in the aquatic environment.

The few studies that have been done of mixing and UVR effects on molecules and non-motile or slow moving organisms have confirmed the expectation that mixing strongly affects the vertical distribution of such effects as photobleaching, photoinhibition and DNA damage. The deeper the mixed layer and the more vigorous the mixing, the more products of UVR photochemistry or UVR affected plankton will be found at depths below the photoactive zone. However, the extent to which vertical mixing just changes the distribution of UVR effects or, in addition, affects the integral damage (and/or repair) over the water column is a more open question. In the examples considered here, including UVR effects on CDOM, phytoplankton photosynthesis, bacterial DNA damage and zooplankton mortality, there are some cases in which mixing effects on the integral (or average) response were significant and other cases in which it was not. The primary requirement for defining when mixing will be important (or not) is a good understanding of the kinetics of UVR damage and repair. A recommendation to those in the UVR research community who would like to see progress in understanding interactions with mixing processes would be to include a kinetics component in any study of UVR exposure and response. With increased attention to kinetics and more widespread measurements of mixing processes, we expect that the next few years will see rapid progress in our understanding of this topic.

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