Interactive effects of body size and environmental gradient on the trophic ecology of sea stars in an Antarctic fjord

ABSTRACT: Antarctic sea stars can occupy different trophic niches and display different trophic levels, but, while the impacts of their body size and environmental features on their trophic niches are potentially important, they are presently understudied. Here we assessed the trophic ecology in relation to the size and habitat of sea stars in a fjord on King George Island (South Shetland Islands) using stable isotope values of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulphur (δ³⁴S). The disc radius influenced δ¹³C and δ¹⁵N values, whereas more limited changes in δ¹³C or δ³⁴S values were related to arm length. Specifically, δ¹³C and δ¹⁵N values were linked to disc radius in generalist species (<i>Diplasterias brandti</i> and <i>Odontaster validus</i>), which could indicate ontogenetic diet shifts, while this relationship occurred less frequently in more specialised species (<i>Bathybiaster loripes</i>, <i>Notasterias bongraini</i>, and <i>Perknaster sladeni</i>). <i>O. validus</i> had a smaller isotopic niche size in the inner than the outer fjord. The niche overlap between <i>D. brandti</i> and <i>O. validus</i> was low in the inner fjord. Low resource availability within the fjord, linked to higher turbidity, could induce trophic niche constriction and interspecific resource segregation. This could represent a mechanism for competition avoidance in a resource-limited system. Conversely, higher resource availability could allow <i>O. validus</i> to expand and share its isotopic niche with <i>D. brandti</i> in the outer fjord with a limited risk of competition. This trophic plasticity will likely influence how <i>O. validus</i> copes with the present and future modification of environmental conditions induced by climate change.