Floating seaweeds serve as dispersal agents for various organisms, but their survival at the sea surface may be compromised by physiological stress and epibiont overgrowth. Most previous experiments have been conducted in laboratory mesocosms where epibiont colonization is limited, but in their natural environment floating seaweeds are frequently overgrown by epibionts, which might negatively affect seaweeds or even cause their sinking. To test this hypothesis, we conducted field experiments in northern-central Chile (30°S) with floating giant kelps Macrocystis pyrifera to determine the time until sinking, epibiotic bryozoan load, and their physiological status across different seasons. Floating sporophytes persisted for at least 4 weeks at the sea surface and sank in all seasons after bryozoan loads exceeded 40 % of the raft biomass. At the time of sinking, the kelp rafts were physiologically viable and biomass losses were relatively minor. In autumn, kelp rafts stayed afloat for an average of 41 days (maximum: 52 days) during moderate environmental conditions (cool water temperature and moderate solar radiation) and slow growth of epibionts. However, higher water temperatures in summer seemed to enhance the growth of epibiotic bryozoans but not the growth of M. pyrifera, causing earlier sinking. The results indicate that the high growth rates of encrusting bryozoans provoke sinking of the kelp rafts, representing the first demonstrated case of epibiont-induced sinking of otherwise healthy floating seaweeds. Increasing global temperatures may enhance epibiont growth and thereby suppress the dispersal potential of floating seaweeds, even of species known for their high acclimation potential to the conditions at the sea surface.
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