Benthic microalgal colonization in streams of differing riparian cover and light availability

Benthic microalgal colonization, light climate, and algal photosynthesis-irradiance relationships were analyzed in two vegetated creeks in southeast Australia, differing in their degree of riparian cover and hence light availability. Diatoms were the main component of the benthic microalgal communities in both creeks. The light received was three times higher in the partly shaded than in the completely shaded creek throughout the 45-day study period. The algal community in the most shaded stream (Dandenong Creek [DG]) was almost permanently under subsaturating light, with a range of 0.06-0.25 light h a day above saturation irradiance (I-k), whereas in the partly shaded stream (Deep Creek [DC]) a higher period of saturating light, 1.2 light h a day above I-k, was seen. The initial slopes (alpha) of photosynthesis versus irradiance relationships were similar in the two creeks during the initial stages of colonization, indicating that early algal colonizers could behave as generalists with respect to light. In later stages of colonization, however, alpha and I-k diverged in the two streams. In DG the alpha values remained high, up to the end of the colonization, whereas in DC alpha substantially decreased. In DG there was a higher chl concentration per surface area, and algae had a higher content of chl per cell than in the partly shaded creek. In DC the algal community became adapted to increasing light availability during the course of the colonization, but changes in the photosynthesis-irradiance parameters were accompanied by a shift in the composition of the diatom community. We concluded that the algal community is highly influenced by adaptation of the algal species to governing light conditions throughout colonization.