Marine Biology

Oxygen

The regular influx of freshwater and salt water into the estuary, coupled with the shallowness, turbulence, and wind mixing, usually means an ample supply of oxygen in the water column. Since the solubility of oxygen in water decreases with increased temperature and salinity, the precise amount of oxygen in the water will vary . In slat-wedge estuaries, or any deep estuary during the summer when a thermocline develops and where there is a vertical salinity stratification, there is often little interchange between the oxygen rich surface waters and the deeper layers. This isolation of the deep waters from oxygen interchange, coupled with high biological activity and slow removal or flushing rate, may deplete the oxygen of the bottom waters.

Oxygen is severely depleted in the substrate. The high organic content and high bacterial population of the sediment exert a large oxygen demand on the interstitial water. Since the fine particle size of the sediments restricts the interchange of the interstitial water with the water column above, oxygen is quickly depleted. Estuarine sediments are, therefore, anoxic below the first few centimeters – unless they have large particle size or large numbers of burrowing animals, such as the ghost shrimp Calliamasa and the hemichordate worm Balamoghossus, which by their activities oxygenate lower sediment layers.