Decreased depth distribution of<b><i>Fucus vesiculosus</i></b>(Phaeophyceae) in the Western Baltic: effects of light deficiency and epibionts on growth and photosynthesis

For many coastal areas of the world, a decrease in abundance and depth penetration of perennial macroalgae and seagrasses has been documented and attributed to eutrophication. A surplus of nutrients impairs perennial seaweeds in at least two ways: increased phytoplankton densities reduce the depth penetration of light and in addition filamentous seaweeds and microalgae growing epiphytically shade their perennial hosts. A reduction of depth limit and total abundance has also been observed for the brown seaweed Fucus vesiculosus at many sites in the Baltic Sea. However, in most cases the mechanistic reason for the loss of Fucus has been deduced from observations rather than from experimental evidence. Here, we present results of a two-factorial (water depth/light supply and epibionts) experiment that was run in the Kiel Fjord, western Baltic, from August to October 2005. Performance of F. vesiculosus was recorded by growth and chlorophyll measurements, PI-curves and in situ measurements of the photosynthetic activity as the relative rate of electron transport (rETR). rETR and growth decreased with water depth. Chlorophyll a concentrations increased with reduced light intensities, but this apparently could not compensate for the light deficiency. Epibionts enhanced the negative effect of reduced light conditions on growth. According to these findings we estimated the physiological depth limit of F. vesiculosus in the Kiel Fjord to lie between 4 and 6 m water depth.