Vol.:(0123456789) 1 3 Marine Biology (2024) 171:9 https://doi.org/10.1007/s00227-023-04341-x ORIGINAL PAPER Giant kelp (Macrocystis pyrifera) morphological and reproductive strategies in two contrasting sub‑Antarctic forests Julieta Kaminsky 1  · María Bagur 1  · Irene Ruth Schloss 1,2,3  · Soledad Diodato 1,2  · Mariano Rodríguez 2  · Alejandro H. Buschmann 4  · María Liliana Quartino 3 Received: 13 April 2023 / Accepted: 24 October 2023 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023 Abstract Sub-Antarctic giant kelp (Macrocystis pyrifera) forests have remained relatively wild and far from anthropogenic pressures. However, they are increasingly exposed to multiple stressors that might afect kelp forest persistence. The urban impact of a coastal city, Ushuaia (54° S 68° W), in the Beagle Channel, Argentina, on giant kelp forest environments was evaluated in this study and implications on morphological variability were explored. Seasonal samplings were conducted from 2019 to 2021 in two contrasting sites: an urban site directly exposed to coastal discharges, and a non-urban site. Our results indi- cate that the combination of coastal discharges, kelp canopy and low wave exposure increases light attenuation inside kelp forests, promoting coastal darkening. Additionally, and based on the present results, we present the frst record of algal turf in the Beagle Channel. We further propose the Kelp Structure Index to describe the efect of environmental conditions on kelp structure, which highlights an increased blade surface in sites with urban impacts. These results suggest that giant kelp shows high plasticity in morphological traits under varying environmental conditions, particularly in environments exposed to coastal darkening efects. However, the presence of turf indicates that urban pressures might be severely limiting kelp forest health. Keywords Beagle Channel · Coastal darkening · Kelp forest · Algal turf · Urban impact Introduction Forests of giant kelp (Macrocystis pyrifera) are structur- ally complex and highly productive constituents of marine rocky coastal sub-Antarctic ecosystems (Steneck et al. 2002; Mora-Soto et al. 2020). Kelps are considered both founda- tion species (sensu Dayton 1972, 1985) and ecosystem engi- neers (sensu Jones et al. 1994; Jones and Gutiérrez 2007) as they promote the development and persistence of ecological communities by modifying the structure and physical condi- tions of a habitat (Teagle et al. 2017). Kelps can reduce light and wave force and increase sedimentation, creating suit- able conditions for other species (Miller et al. 2018). Kelp primary production supports food webs through diferent detrital pathways (Krumhansl and Scheibling 2012; Bruno et al. 2023). In addition, detached buoyant kelps facilitate long-distance dispersal of invertebrate species (Wichmann et al. 2012). Recently, kelps may be further considered in global blue carbon estimations, as they can assimilate and sequester CO 2 via primary production, transport and burial of kelp biomass in the deep ocean, thereby contributing to climate change mitigation eforts (Krause-Jensen and Duarte 2016; Bayley et al. 2021; Pessarrodona et al. 2023). Sub-Antarctic ecosystems have remained relatively iso- lated, wild, and far from anthropogenic pressures. How- ever, these ecosystems are increasingly exposed to multiple Responsible Regional Editor: Patrick Gagnon. Responsible Editor: Patrick Gagnon . * Julieta Kaminsky kaminsky.julieta@gmail.com 1 Centro Austral de Investigaciones Científcas, Consejo Nacional de Investigaciones Científcas y Técnicas, Ushuaia, Tierra del Fuego, Argentina 2 Instituto de Ciencias Polares, Ambiente y Recursos Naturales, Universidad Nacional de Tierra del Fuego, Ushuaia, Tierra del Fuego, Argentina 3 Instituto Antártico Argentino, San Martín, Provincia de Buenos Aires, Argentina 4 Centro i-mar, CeBiB and MASH, Universidad de Los Lagos, Puerto Montt, Región de Los Lagos, Chile