Seaweed structure shapes trophic interactions: A case study using a mid-trophic level fish species

Biogenic habitat structure, such as that created by foundation macrophytes, shapes the interactions of higher trophic level organisms by creating three-dimensional refuge spaces. In recent decades, many kelp habitats have transformed into turf-dominated communities. This represents a fundamental change in the overall habitat structure in these communities, with an unknown impact on upper trophic level organisms. We investigated how macroalgae morphology affects a common residential mid-trophic level wrasse, Tautogolabrus adspersus (cunner), which utilizes macroalgae for both refuge and foraging. Three studies were conducted: in situ behavioral video observations, a refuge choice experiment, and a foraging efficiency experiment. Video observations revealed that in kelp-dominated and mixed habitat types cunner use macroalgae more often for refuge than for foraging, but in turf-dominated habitats refuge and foraging events were equal. In these habitats, refuge-seeking was more often associated with a tall, morphologically simple kelp. The refuge choice experiment supported our video observations with cunner preferentially seeking refuge beneath taller but less morphologically complex algae instead of shorter filamentous forms. In predation trials, macroalgae complexity did not significantly affect the number of prey the fish captured. Our results provide evidence that the refuge-seeking behavior of this residential mid-trophic level fish may be impacted by the ongoing shifts in macroalgae dominance in the Gulf of Maine. Loss of its preferred refuge (tall, canopy-forming kelps) may force it to use the less-preferred introduced turf algae instead. However, whether turf provides sufficient protection for this species remains unclear. With the ongoing loss of kelp in temperate coastal ecosystems worldwide, it is important to understand the potential indirect effects that changes in habitat structure will have on the trophic interactions of upper level organisms.