Combined effects of high environmental ammonia and low oxygen conditions on the bioturbation performance and bioenergetics of the ragworm Hediste diversicolor

Nutrient inputs from agriculture, wastewater, and coastal management practices like rewetting, can lead to high environmental ammonia (HEA) and low dissolved oxygen (DO) conditions in coastal habitats. Coastal benthic infauna can be negatively impacted by these stressors, experiencing physiological and behavioral disruptions that may compromise their performance and, consequently, the essential ecosystem functions they support. We investigated the impacts of a realistic eutrophication scenario consisting of combined HEA (0, 5, and 50 mg L^-1 total ammonia) and low DO (2.4 mg L^-1) levels on the bioturbation activities and physiological status of the ragworm Hediste diversicolor. Survival was synergistically lowered at very high HEA under low DO conditions. However, there was no strong evidence for an effect of HEA alone or in combination with low DO conditions on burrowing or particle reworking activity but bioirrigation activity increased significantly under low DO conditions. Oxygen consumption rate decreased under low DO conditions but increased with increasing HEA, while the energy reserves and cellular respiration showed no systematic response to either stressor. We conclude that ragworms exhibit a degree of tolerance to nutrient-rich, low DO conditions, maintaining energetic homeostasis through behavioral adaptations. This highlights the crucial role of tolerant marine bioturbators in colonizing and transforming degraded or restored habitats. However, combined stressors and further environmental deterioration may compromise their fitness.