At least one previous report of photosynthetic activity under conditions of artificial glitter lines seems to have indicated an enhancement in the rate of photosynthesis (Walsh and Legendre, 1983). Although an explanation of this apparent contraction is beyond the scope of this article, it should be noted that natural glitter in shallow waters differs considerably in frequency, intensity and prismatic quality from that of artificial glitter.
Figure 4: Natural glitter lines dance across the surface of a Porites coral in a tide pool in Kahalu'u (Kona), Hawai'i. Note how these glitter lines differ from artificially-generated glitter lines in Figure 2.
Even when various parameters were manipulated in this experiment (such as water depth, surface agitation and distance between the lamp and water surface), it was not possible to replicate glitter lines as seen in shallow tide pools here in Kona, Hawai'i. Natural glitter lines can produce light pulses about three times that of surface intensity (which is immediately followed by 'de-focused' light resulting in pulses of light intensity much lower than average). See Figure 5 for measurements made with a Spectrum Technologies (Plainfield, Illinois, USA) Watch Dog™ data logger and two PAR sensors (one above water, one below). In no case in this experiment did artificial glitter lines produce the high intensities seen with natural glitter lines (data not shown).
Figure 5: Light intensities both above and below water in a Hawai'ian tide pool. Various factors (such as wind and tidal water motion, among others) affect the focusing effect and hence light intensity of the glitter lines. Underwater light intensity at about 7 cm depth can be briefly almost 3X that of surface light intensity due to glitter lines. By the same token, light intensity can be dramatically lower due to de-focusing of light by wave action.
The lack of differences in photosynthetic yield under artificial glitter in the present experiment is fairly easy to explain. Since yield at each light intensity was initially measured with 'calm' (i.e., no artificial agitation, hence 'no glitter') conditions and moments later 'with agitation', it was easy to determine the high and low photosynthetically active radiation amplitudes. The average PAR value of glitter lines measured over short periods (a minute or two) always very closely equaled the PAR measured when a condition of 'no glitter' prevailed. In other words, the average intensity of the light field produced by glitter lines over a few seconds' time cannot possibly exceed the average light intensity in a 'no glitter' condition under the same set of circumstances. One may wonder why photosynthetic yield, instead of electron transport rate (ETR), was used to compare effects. The answer is quite simple – computation of ETR requires a known PAR value be used in the equation, and this was not possible in the conditions of this experiment. The PAM 210 does not include a PAR meter (more expensive models do have an internal PAR meter) and it was not possible to synchronize instantaneous recordings of Yield and PAR (a measured with my Li-Cor quantum meter) in a highly variable light yield such as that seen 'with' glitter lines. In addition, one should not confuse the drop in yield in Figure 3 as Photoinhibition. It is natural for photosynthetic yield to drop with increasing light intensity.
The real question is whether the high light amplitudes produced by artificial glitter lines would encourage a 'ramping up' of photosynthesis able to maintain a higher rate of photosynthesis during the brief (and inevitable) lower light intensities sure to follow the bright 'flicker'. This report suggests it does not. The story may be different with 'real' glitter lines in nature, or in aquaria capable of generating larger waves with high frequency (which could approximate the waves seen in the observed tide pools). For now, it seems that glitter lines are more 'show' than 'go' (photosynthetically speaking) in most aquaria situations.