Hi all,
Assuming the flow is shallow, and relatively slow, the UV lamp will potentially kill all cells that flow through it. UV lamps provide a radiation at a wavelength of about 254 nm which damages DNA and RNA, and they can be used for sterilising drinking water etc. <
http://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation>.
In ponds the lamps kill the green water algae like
Chlorella and
Volvox, as well as any planktonic micro-organisms (
Euglaena &
Daphnia etc.). Because photosynthetic micro-organisms tend to be well up in the water column they tend to be preferentially killed. As has been said they only effect planktonic organisms, their effect on benthic organisms, attached algae etc is non-direct.
The real problem is that this is a cosmetic effect, the "Green water" algae is there because of the combination of nutrients and light, you haven't removed either of these, you have just removed their visible result. If you can reduce the nutrient input, encourage organisms that eat phyto-plankton, or grow a greater biomass of other photosynthetic organisms, the green water will go away. If you keep fish like Koi carps or Gold-fish the problem is that they will tend to eat macrophytes (higher plants) and zooplankton, so that the end result of using the UV filter will be a greater growth of filamentous "blanket weed" algae. All the time the potential for plant growth is still there, plants will grow.
A better approach to water quality is to use a planted fish free basin as a phyto-filter, If this is planted with fast growing plants (like Reeds
Phragmites or Eichornia (Water Hyacinth) etc.) this will convert the nutrients into reeds/Water Hyacinths, that can be easily removed, exporting your nutrients and these will also provide a refugium for zoo-plankton like
Daphnia which will eat the green water phytoplankton. If you don't mind frequently cleaning the filter, an over-sized pond filter with a fine sponge will also catch the algal cells, allowing their removal.
cheers Darrel