Diurnal variation in light intensity underneath floating macrophyte leaves and implications for the behaviour of plant-attached zooplankton

Abstract

Light regulates vertical and horizontal movements of free swimming zooplankton due to its effects on the feeding efficiency of visual predators. The role of light in regulating the behaviour of zooplankton periodically attached to floating plant leaves is unknown. The behaviour of such zooplankters cannot be predicted from studies on free-swimming pelagic species, because the floating leaves modify the light environment. Therefore, we explored the daily variations in the factors that govern the intensity of light underneath floating objects, i.e. light intensity in the air, the proportion of light transmitted to the water and refraction of light at the air–water boundary. We show that due to the combined effects of these factors the intensity of light underneath macrophyte leaves may decline temporarily at noon when the solar altitude is highest, allowing attached zooplankton to switch into the swimming mode. The noon minimum does not take place under circumstances where the maximum solar elevation angle remains so low that the low proportion of light transmitted to the water (high proportion of reflected light) negates the effects of increasing the nadir angle of refracted light. The density of zooplankters attached to the lower surfaces of floating macrophyte leaves should thus often show two maxima, one between sunrise and noon and another between noon and sunset.

Keywords:

• plant-attached zooplankton
• migrations
• light
• macrophytes

Acknowledgements

This work was supported by the Academy of Finland (projects 50320 and 211156) and by the Alfred Kordelin Foundation. Jyrki Lappalainen and Kaarina Weckström gave valuable comments on the manuscript.