The effects of shading on Chara zeylanica KLEIN ex WILD. and associated epiphytes

Abstract

The effect of shading on the growth of Chara zeylanica Klein ex Wild. and associated epiphytes, mainly Rhizoclonium africanum KUTZ., was investigated in a large outdoor tank using water, sediment, and plants from Lake Okeechobee, Florida. Plants were grown in peat sediment and lake water, under ambient temperature (27.1-30.3 °C) and photoperiod (13L:11D). Treatments were established by differentially shading plants with varying numbers of layers of fiberglas window screen. Photon flux density (PFD) ranged from 8 to 153 μmole photons m -2 s -1 , or 1.1 to 21.6% of average incident photosynthetically active solar radiation (PAR), based on percent transmittance in the tank and averaged continuous daytime measurements from a mid-lake PAR sensor. Parameters examined included the ash-free dry mass (AFDM) and growth rate of C. zeylanica and its associated epiphytic community. Both C. zeylanica and epiphytic AFDM, but not their AFDM ratio, decreased linearly with decreasing PAR, were strongly correlated, and had statistically significant treatment effects. The apparent photosynthetic PFD for no net growth (NNG) of heavily epiphytized C. zeylanica, measured approximately a quarter meter above the sediment surface, was estimated to average 16 μmole photons m -2 s -1 , or 2.3 % of mean incident PAR, with an upper 95 % confidence limit of 33 μmole photons m -2 s -1 , or 4.7% of mean incident PAR. The apparent NNG photosynthetic PFD estimate mean of the epiphytic community, however, was negative, with an upper 95 % confidence limit of 10 μmole photons m -2 s -1 , or 1.4% of mean incident PAR. These results suggest that C. zeylanica, and particularly its associated epiphytes, can grow in very low light, which may be an important adaptation given the poor light climate typical of this and many other culturally eutrophic waterbodies.