The conspicuous retreat of the key species Fucus vesiculosus from the deeper parts of its former distribution area in the Baltic Sea has triggered extensive research on the factors that control its growth. Based on recently obtained knowledge on a large number of potential drivers, we developed a numerical model incorporating effects of abiotic factors on the physiological functions of photosynthesis, respiration, and reproduction and the ecological processes of competition, grazing, and epibiosis. For all input combinations, the model delivers the monthly net growth rate near the bladder wrack's depth limit and the maximum depth of its vertical distribution. The use of data corresponding to conditions presently observed in the western Baltic Sea sets the year's maximum algal net growth rate in late spring and 2 minima in early spring and autumn. The depth limit of the wrack's distribution is set at ~9 m. Light and its absorption by phytoplankton represent by far the most important factors controlling the modeled net growth rate and depth penetration, with the role of epibiosis requiring further investigation. Lacking findings on population dynamics and biotic interactions restrict the generated model to an exploratory rather than a predictive tool.
KEY WORDS: Depth limit · Benthic ecology · Eutrophication · Ecological modeling · Bladder wrack · Biotic interactions · Fucus physiology
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 463: [59][60][61][62][63][64][65][66][67][68][69][70][71][72] 2012 of its distribution during the second half of the last century (e.g. Kautsky et al. 1986, Vogt & Schramm 1991, Eriksson et al. 1998. Although partial recovery has been observed at some sites in recent years (Nilsson et al. 2004), the general pattern of restricted depth penetration of F. vesiculosus in the Baltic Sea seems to persist (Torn et al. 2006). Among other factors, such as direct habitat destruction or increased grazing due to overfishing of the mesograzers' predators, eutrophication is considered to play the most prominent role in this change (Kautsky et al. 1986, Eriksson et al. 1998, Torn et al. 2006. The ways in which high nutrient levels can hinder the propagation and growth of F. vesiculosus are manifold, the most commonly reported being increased water turbidity (Kautsky et al. 1986), increased sedimentation rates (Eriksson & Johansson 2003), increased grazing pressure (Kangas et al. 1982), direct inhibition of spore settlement and development (Bergström et al. 2003), increased epibiotic load (Wahl 1989), and increased competition with ephemeral algae and the blue mussel Mytilus edulis (Vogt & Schramm 1991).Considering all these possible effects of eutrophication on Fucus vesiculosus, one might be tempted to consider the question of its reduced depth penetration as answered. However, the fact that so many mechanisms and functional groups of organisms are involved, some of them, like epibionts, even having been credited with possibly positive eff...