Sub-lethal and lethal effects of chronic ammonia exposure and hypoxia on a New Zealand bivalve

Eutrophication-induced hypoxia is among the most widespread anthropogenically deleterious environmental issues occurring globally in coastal marine environments, contributing to a suite of major stressors on marine organisms. Changes to biogeochemical processes and loss of biodiversity and ecosystem function are symptoms of stressors on aerobic organisms, particularly in estuaries. The response of a species to a single stressor is often very different when compared to exposure in a multiple stressor environment. Using an experimental approach, we tested the sub-lethal and lethal effects of hypoxia and chronic nutrient enrichment on the critical estuarine bivalve Austrovenus stutchburyi. Findings of the present study highlight the cumulative mortality of A. stutchburyi nearly doubled when exposed to elevated ammonia when it was combined with elevated hypoxic stress. A positive feedback loop is hypothesized to explain the result, whereby A. stutchburyi stressed by low oxygen produce and excrete more ammonia waste, increasing their risk of mortality and subsequent lethal ammonia concentrations. Water samples taken across the 45-day experiment support the hypothesis, with ammonia concentrations differing between treatments with bivalves in hypoxic treatments producing more ammonia than normoxic treatments. Siphon activity and respiration rate responses to hypoxic stress also demonstrated the importance of hypoxia as a primary driver of stress responses by A. stutchburyi. These findings highlight the threats to estuarine ecosystems with bivalve populations, whereby, extensive degradation of algal mats results in increased oxygen consumption from microbial respiration, which leads to hypoxia and shallowing of the oxygenated layer. Consequently, chemical reduction in sediments can be associated with an efflux of ammonium into the water column. The experimental results presented here on interactions between stressors highlights the substantial detrimental implications for bivalve populations and subsequent ecosystem functions given increased prevalence of warming and eutrophication in estuaries and coastal environments.