Behavioral effects of low dissolved oxygen on the bivalve Macoma balthica

Hypoxia, a dissolved oxygen concentration (DO) below 2 mg l – 1 , is a significant stressor in many estuarine ecosystems. Many sedentary organisms, unable to move to avoid hypoxic areas, have metabolic and behavioral adaptations to hypoxic stress. We tested the effects of hypoxia on the behavior and mortality of the clam Macoma balthica, using four levels of dissolved oxygen in flow-through tanks. We used five replicates of each of four treatments: (1) Hypoxic (DO mean ± SE = 1.1 ± 0.06 mg O 2 l – 1 ), (2) Moderately hypoxic (DO 2.6 ± 0.05 mg O 2 l – 1 ), (3) Nearly normoxic (DO 3.2 ± 0.04 mg O 2 l – 1 ), (4) Normoxic (DO = 4.9 ± 0.13 mg O 2 l – 1 ). We lowered the dissolved oxygen with a novel fluidized mud-bed, designed to mimic field conditions more closely than the common practice of solely bubbling nitrogen or other gasses. This method for lowering the DO concentrations for a laboratory setup was effective, producing 1.4 l min –1 of water with a DO of 0.8 mg O 2 l – 1 throughout the experiment. The setup greatly reduced the use of compressed nitrogen and could easily be scaled up to produce more low-DO water if necessary. The lethal concentration for 50% of the M. balthica population (LC 50) was 1.7 mg O 2 l – 1 for the 28-day experimental period. M. balthica decreased its burial depth under hypoxic and moderately hypoxic (~2.5 mg O 2 l – 1 ) conditions within 72 hours of the onset of hypoxia. By the sixth day of hypoxia the burial depth had been reduced by 26 mm in the hypoxic tanks and 10 mm in the moderately hypoxic tanks. Because reduced burial depth makes the clams more vulnerable to predators, these results indicate that the sub-lethal effects of hypoxia could change the rate of predation on M. balthica in the field.