Rafts of Macrocystis pyrifera (L.) C. Agardh can act as an important dispersal vehicle for a multitude of organisms, but this mechanism requires prolonged persistence of floating kelps at the sea surface. When detached, kelps become transferred into higher temperature and irradiance regimes at the sea surface, which may negatively affect kelp physiology and thus their ability to persist for long periods after detachment. To examine the effect of water temperature and herbivory on the photosynthetic performance, pigment composition, carbonic anhydrase (CA) activity, and the nitrogen (N) and carbon (C) content of floating M. pyrifera, experiments were conducted at three sites (20 degrees S, 30 degrees S, 40 degrees S) along the Chilean Pacific coast. Sporophytes of M. pyrifera were maintained at three different temperatures (ambient, ambient - 4 degrees C, ambient + 4 degrees C) and in presence or absence of the amphipod Peramphithoe femorata for 14 d. CA activity decreased at 20 degrees S and 30 degrees S, where water temperatures and irradiances were highest. At both sites, pigment contents were substantially lower in the experimental algae than in the initial algae, an effect that was enhanced by grazers. Floating kelps at 20 degrees S could not withstand water temperatures > 24 degrees C and sank at day 5 of experimentation. Maximal quantum yield decreased at 20 degrees S and 30 degrees S but remained high at 40 degrees S. It is concluded that environmental stress is low for kelps floating under moderate temperature and irradiance conditions (i.e., at 40 degrees S), ensuring their physiological integrity at the sea surface and, consequently, a high dispersal potential for associated biota.