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

Area of Science:

• Environmental Science
• Marine Chemistry
• Ecosystem Dynamics

Background:

• Sediment-water nutrient and oxygen exchanges are critical in coastal ecosystems.
• Deoxygenation is known to enhance sediment nutrient recycling, particularly for nitrogen and phosphorus.
• Limited short-term data exist on these processes during deoxygenation events in coastal environments.

Purpose of the Study:

• To investigate ecosystem-scale sediment-water flux responses to experimental deoxygenation.
• To measure the impact of varying oxygen depletion timescales (days to weeks) on nutrient fluxes.
• To understand solute-specific responses and temporal dynamics of sediment recycling.

Main Methods:

• Utilized an engineered aeration system in a small estuary to control oxygen levels.
• Conducted ecosystem-scale deoxygenation experiments.
• Measured sediment-water fluxes of key nutrients (nitrogen and phosphorus species) under different oxygen conditions and durations.

Main Results:

• Sediment-water flux responses to deoxygenation were not immediate, with varied response times and types among solutes.
• Deoxygenation led to increased orthophosphorus fluxes (5.1 μmol P m⁻² h⁻¹ per day of aeration shutdown) and reduced nitrate + nitrite uptake.
• Orthophosphate concentrations in bottom waters increased 3-10 fold.
• Unexpectedly, ammonium fluxes decreased under deoxygenation, diverging from previous findings.

Conclusions:

• Sediment responses to deoxygenation necessitate sustained oxygen depletion for significant effects.
• The timescales for nitrogen and phosphorus responses to deoxygenation are distinct.
• Factors beyond dissolved oxygen influence sediment-water nutrient exchange dynamics.