Photoactive zone

Figure 2. Time variable exposure to UVR and PAR in a simulated mixed layer of a temperate lake. Data is from Lake Lucerne, September 15,1999 (adapted from [79]). (A) Profiles of UV-B (290-320 nm), UV-A (320-400 nm) and PAR (400-700 nm) as irradiance relative to the surface. A line shows the bottom of the photoactive zone as defined by 10% of surface UV-B (1.7 m in this case). (B) Temperature profiles showing an approximately 5 m thick upper mixed layer. Experimental mixing was conducted over the upper 4 m (circle). (C) Exposures for PAR (thick line) and (circles, UVR weighted for inhibition of photosynthesis) obtained when samples were mixed with a 8 min rotation time. Photosynthesis was estimated to be about 60% lower at the surface compared to the...

The previous section showed how mixing processes determine the way UML constituents (molecules or organisms) enter the photoactive zone where they may participate in a UVR-mediated process. Under strong stratification, such transport is very limited, so UVR effects will only involve those constituents already present in the active zone. Such extreme stratification can be episodically important in systems where diurnal thermoclines form, but more typically the UML extends below the photoactive zone so that constituents in waters below the photoactive zone will also participate in UVR photochemistry and photobiology. The mixing characteristics of the UML influence photoprocesses in at least two fundamental ways. First, the depth of the UML influences the average UVR exposure that occurs second, the rate of vertical transport, either due to... 

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 nearsurface photoactive zone are in continual motion and, over some time scale. 

This limitation was finally overcome by the concept of the bulk heterojunction, where the donor and acceptor materials are intimately blended throughout the bulk [28-30]. In this way, excitons do not need to travel long distances to reach the donor/acceptor interface, and charge separation can take place throughout the whole depth of the photoactive layer. Thus the active zone extends throughout the volume, as illustrated in Fig. 11. Conse-... 

The phenomenon was strong evidence that dothistromin was photoactive, as observed previously by others (10, 35) and also in accord with results obtained by us from antibiosis tests. In these, dothistromin gave rise to clear inhibition zones, but only in the... 

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