Abstract

Kelps are foundation species in upwelling coastal ecosystems, sustaining benthic food webs and providing habitat. They also support fisheries and subsidize adjacent ecosystems through the export of particulate organic matter (POM). Stable isotope ratios of carbon (S13C) and nitrogen (S15N) in kelp tissues are widely used to assess how upwelling affects productivity and to estimate the contribution of kelp-derived POM to coastal food webs. However, studies have reported inconsistent relationships between kelp isotopic values and upwelling dynamics. Here, we examined the isotopic composition of bulk tissues of three common kelp species in central Chile: the intertidal Lessonia spicata and Durvillaea incurvata, and the subtidal L. trabeculata. Samples were collected at five sites along an upwelling gradient during the springs of 2021 and 2022. We compared S13C and S15N values with upwelling intensity and thermal variability indicators over weekly, monthly, and three-month windows preceding sampling. We found pronounced isotopic differences between the subtidal L. trabeculata and the two intertidal species, with the latter exhibiting more depleted S15N (10.20%o) and S13C (-20.03%o) values than intertidal species (average S15N = 11.97%o, S13C =-14.55%o). This pattern suggests that depth exerts a stronger influence on isotopic fractionation than taxonomic relatedness. The magnitude of interspecific differences varied across sites and years, but these changes were not associated with the regional upwelling gradient, nor with the thermal conditions during the preceding weeks to months. Critically, most of the variation occurred among individuals separated by only a few meters (average 65% of variance in S15N, 81% of S13C), rather than among sites kilometers apart. These results emphasize the need for caution when using kelp isotopic composition as a simple reference for environmental factors in food web studies. They highlight the need to identify the temporal scales at which kelp isotopic composition responds to environmental variability, as well as the determinants of fine-scale variability among individuals.