Depth limits and minimum light requirements of freshwater macrophytes

1. Data for maximum colonization depth (Z(c)) of five groups of submerged macrophytes and light attenuation were collected for forty-five Danish lakes and 108 non-Danish lakes. The macrophyte groups were bryophytes, charophytes, caulescent angiosperms, rosette-type angiosperms and Isoetes spp. 2. The data showed systematic differences among the groups in the relationship of Z(c) to water transparency. In lakes with low transparency (Secchi disc transparency (Z(s)) less than 7 m) caulescent angiosperms and charophytes penetrated deepest followed by bryophytes and Isoetes spp. In more transparent lakes bryophytes grew deepest, followed by charophytes, caulescent angiosperms and Isoetes spp. Rosette-type angiosperms had the lowest Z(c) in all types of lakes. Charophytes and caulescent angiosperms had similar depth limits in lakes with Z(s) < 4 m but charophytes grew deeper in more transparent lakes. The depth limits of both groups were independent of light penetration in lakes with very low transparency (Z(s) < 1 m). The annual light exposure for the deepest growing macrophytes (bryophytes) was 20-95 mol photons m(-2). 3. The relationship between Z(c), macrophyte type and lake transparency could be explained by three distinct processes regulating Z(c). In lakes with low transparency (Z(s) < 1 m), tall macrophytes (caulescent angiosperms and charophytes) compensate for light limitation by shoot growth towards the water surface and Z(c) is therefore independent of transparency. In lakes with medium transparency (1 m < Z(s) < 4 m) Z(c) for angiosperms, charophytes and Isoetes spp. is constrained by light attenuation in the water column, corresponding to a linear relationship between Z(c) and Z(s). This pattern also applies to bryophytes, despite lake transparency. In transparent lakes, the minimum light requirement at Z(c) increased with increasing transparency for angiosperms, charophytes and Isoetes spp. 4. The minimum light requirements among submersed macrophytes (including marine macroalgae) depend on their plant-specific carbon value (plant biomass per unit of light-absorbing surface area) for the species/group, indicating that the light requirements of submersed plants are tightly coupled to the plants' possibility to harvest light and hence to the growth form. 5. The light requirements increased on average 0.04% surface irradiance per degree increase in latitude corresponding to an average decrease in Z(c) of 0.12 m per degree latitude.