Light Requirements of Water Lobelia (Lobelia dortmanna L.)

Maximum depth of colonization (z_C) and total area covered by a population of Lobelia dortmanna, as well as underwater light regime were studied in 25 soft water lobelia lakes in north-western Poland. Variations in underwater light conditions among the lakes were described by Secchi disc depths (z_SD), and by attenuation coefficients of irradiance within photosynthetically active radiation range (K_d,PAR), and euphotic zone depths (z_EU) derived from photometric measurements conducted twice a year (in midspring and midsummer) during the period 2014–2015. Maximum depth of colonization of water lobelia ranged from 0.1 to 2.2 m (median z_C = 0.8 m; mean z_C = 1.0 m). Nine lakes showed the relative coverage of the littoral zone (RCLZ) by L. dortmanna to be greater than the mean value, which was 4.8%. Studies showed that light requirements of water lobelia increase when the maximum depth of colonization also increases. This pattern could be partially related to the greater energy needs of deeper growing individuals due to enlarged seed production and their incubation, and for the creation of much heavier inflorescences. Assessment of the light requirements of L. dortmanna along the depth gradient indicates that relative irradiance (percentage of subsurface irradiance of PAR) should be at the level of: (i) 47–50% (annual total of quantum irradiance 3083–3280 mol m⁻² yr⁻²) for plants growing within a depth range of 2.0–2.5 m; (ii) 44–47% (2886–3083 mol m⁻²yr⁻¹) for plants growing within a depth range of 1.5–2.0 m; (iii) 41–44% (2690–2886 mol m⁻²yr⁻²) for plants growing within a depth range of 1.0–1.5 m; and (iv) 34–41% (2230–2690 mol m⁻¹ yr⁻¹) for those growing in the littoral zone at a depth of between 0.5 and 1.0 m. In average conditions in the Pomeranian lakes, the maximum depth of colonization by L. dortmanna accounts for approximately a third of the Secchi disc depth and a fifth of the depth of the euphotic zone with irradiance of PAR at z_C equal to about 43% of subsurface irradiance. It has also been demonstrated that the light factor is a crucial one that limits the absolute maximum depth of lobelia population occurrence in Pomeranian lakes. The cleanest and most transparent lakes of this region have light attenuation coefficients (K_d,PAR) within the range of 0.35–0.42 m⁻¹, which corresponds to the maximum colonization depths of 1.8–2.2 m.