Predator exclusion and riverine input: Effects on sub-Arctic intertidal soft-bottom macrobenthos

Consumption significantly structures marine benthic communities, and understanding predator-prey interactions is crucial for determining the dynamics of these systems. Yet consumer effects in sub-Arctic regions, particularly at sites with different environmental conditions, remain underexplored. This study investigated the impact of potential predators, such as crabs and birds, on macrobenthos abundance, biomass, and biodiversity in the mid-intertidal of one site with low and another site with high freshwater influence in Malangenfjord, Northern Norway. Moreover, microphytobenthos biomass and δ 13 C and δ 15 N stable isotope values of macrobenthos and different organic matter endmembers were measured. A three-month predator-exclusion experiment, run in two consecutive years, revealed weak to moderate predator effects on macrobenthos abundance and biomass, while biodiversity remained unaffected by predator-exclusion and riverine input. Predator-exclusion effects were generally stronger at the more river-influenced site. Microphytobenthos biomass increased slightly with predator exclusion, indicating complex trophic interactions. Stable isotopes revealed limited assimilation of terrestrial organic matter by macrobenthos, suggesting that the benthos is dependent on marine-derived carbon sources. These results suggest limited predation pressure on macrobenthos and that riverine inputs potentially mitigate predator-mediated reductions in prey biomass through nutrient enrichment, as the latter leads to higher primary production and food availability. This study underscores the importance of local environmental conditions in shaping predator-prey dynamics and highlights complex interplay between riverine inputs and the ecology of coastal ecosystems. The results contribute to understanding the interactions within sub-Arctic intertidal systems and indicate the need for future research to explore mechanisms and long-term effects of riverine-coastal interactions.