Seasonality modulates coral trophic plasticity in an extreme, multi-stressor environment

Corals with high trophic plasticity, i.e., the ability to change the relative contribution of heterotrophic and autotrophic nutrition to their mixotrophic diet, can have increased tolerance to individual stressors, but it is poorly understood how trophic strategies shift in response to combined global (e.g., warming, acidification) and local stressors (e.g., nutrient input). Furthermore, it remains unclear how season-associated changes in physicochemical conditions modulate trophic strategies and which coral species generally have trophic plasticity. We measured the tissue stable isotopes (δ¹³C and δ¹⁵N) of three coral species (Siderastrea siderea, Siderastrea radians, and branching Porites sp.) from two distinct habitats: extreme, multi-stressor inland bay habitats and nearby fringing reefs with more benign environmental conditions. We further captured trophic plasticity between dry and wet seasons, as well as the effects of in situ heat stress on trophic strategies. Bay corals tended to be more autotrophic than fringing reef corals, which may be driven by higher nutrient input in the bays. All three coral species shifted their trophic strategy between the cool dry and warm wet seasons; however, the direction of trophic shifts varied between δ¹³C and δ¹⁵N. Bay S. siderea had the highest trophic plasticity across seasons, which likely facilitates their success in these multi-stressor habitats. Interestingly, not all species relied equally on heterotrophy, as bay Porites had a primarily autotrophic diet, even during the wet season when conditions were more extreme. This highlights that coral tolerance to more extreme conditions is promoted through dynamic shifts in diet, rather than only increasing heterotrophy.