Minimum Light Requirements of Submerged Freshwater Macrophytes in Laboratory Growth Experiments

Abstract

SUMMARY (1) This study determined the minimum light requirements and the photo-adaptive behaviour of submerged freshwater macrophytes representing different taxonomic groups (charophytes, bryophytes and angiosperms), and their depth distributions in the field. Relative growth rates (,u) of macrophytes were measured in the laboratory at photon flux densities (PFD) between 0 and 105 molm-2 s-1 at 7?C. Dark respiration rates were measured subsequently on the adapted plants. (2) Mean rates of carbon loss were low in plants kept in the dark (5.9mmol C mol-1 cell C day-) and corresponded to experimental dark respiration rates. Mean dark respiration was only 7 Ommol C mol-h cell C day-' at the light compensation point (Ic), which was on average 69 9mol m-2s-1. Light compensation points and respiration rates were variable within species collected at different times and approached the variability among species. The growth efficiency in low light (the slope of ,u vs. PFD) varied less within species and showed differences among species which were positively related to the maximum growth rate at high light. Overall, macrophyte species with different depth distribution or taxonomy did not differ systematically in dark respiration and growth performance in low light in the experiments. (3) Experimental Ic levels of the two charophyte species, converted to annual means (78-165 mol m-2 year- '), corresponded to annual PFD levels of 40200 molM-2 year-l at the depth limit of charophyte species in the hypolimnion of oligotrophic temperate lakes. Experimental Ic levels of normally rooted angiosperms were much lower than PFD levels at their depth limits in epilimnetic waters. However, these plants formed no roots at low light in the laboratory and needed c. 24 molm-2 S- for proper root development. Considering that this light level (505 mol m-2 year-' or 7% of surface PFD) is needed for root development, correspondence with depth limits was much closer. These comparisons are complicated by daily and seasonal variability of light and temperature under field conditions. However, seasonal adaptations in the field, and the possible operation of loss processes (e.g. grazing and detachment), should cause greater light requirements for survival of field populations. (4) Comparisons between freshwater macrophytes and phytoplankton show larger chlorophyll:carbon and protein:carbon ratios of phytoplankters, which combined with their smaller size lead to higher growth-light efficiencies, higher respiration rates and overall lower Ic levels.