During summer 2009, we conducted a field experiment and a field survey at 30uS in the coastal Humboldt Current to determine how floating time affects the physiological performance of kelp rafts. For the field experiment kelp rafts were tethered in coastal waters and the field survey was specifically designed to collect freefloating Macrocystis pyrifera across a latitudinal temperature gradient that reflects natural floating time. Experimental kelps were kept under photosynthetic active radiation (PAR) and PAR + ultraviolet (UV; PAR + UV) using filter foils, and tethered at the sea surface in their natural habitat. Ultraviolet radiation (UVR) did not affect kelp physiology, but caused a decrease in kelp biomass. The field survey confirmed that sea-surface temperature increased with distance from upstream source populations of M. pyrifera. Rafts sampled at increasing distance from sources showed high epibiont cover and reduced blade lengths. Physiological performance declined with increasing size of algal epibionts, which are indicators of floating time. Rafts that were farthest from the southern source populations had lost their sporophylls, suggesting that dispersal potential decreases with increasing floating time. The combined effects of abiotic (UVR and temperature) and biotic factors accelerate degradation of M. pyrifera and, thus, can impede successful dispersal in the Humboldt Current at 30uS. This suggests that floating macroalgae can be important dispersal vectors in areas with moderate environmental stress (i.e., in temperate oceans).