Time Scales of Hypoxia-Induced Nutrient Recycling in Sediments: An Experimental Approach Using Engineered Aeration

Sediment-water exchanges of nutrients and oxygen are highly impacted by the overlying water oxygen conditions. Despite evidence that deoxygenation stimulates sediment recycling of nitrogen and phosphorus, extensive short-term measurements of these processes have not been made in coastal ecosystems. We leveraged an engineered aeration system in a small estuary to perform ecosystem-scale deoxygenation experiments and measure the responses of sediment-water fluxes to different time scales (days to weeks) of oxygen depletion. We found that sediment-water flux responses to deoxygenation were not immediate and that response times and types varied by solute. Deoxygenation was associated with elevated orthophosphorus fluxes and reduced sediment uptake of nitrate + nitrite, consistent with past studies. For each day the engineered aeration system was turned off, an additional 5.1 μmol P m^-2 h^-1 was released from sediments, leading to bottom water orthophosphate concentrations increasing by a factor of 3-10. In contrast, sediment-water ammonium fluxes decreased under deoxygenation, which is not consistent with previously described patterns. These results underscore solute-specific time scales of responses to deoxygenation and reflect the influence of factors beyond dissolved oxygen. We conclude that sediment responses to deoxygenation require sustained oxygen depletion and that the time scales of responses for nitrogen and phosphorus differ.