On land we take oxygen for granted, everywhere we go we have access to a near unlimited supply of oxygen to fill our lungs with. The ocean is a much different world where oxygen can be a scarce resource that can disappear in a matter of minutes and take decades to return. On a per volume basis water has much less oxygen than air. A high oxygen concentration in water would be 12 mg of oxygen per liter. Air on the other hand has approximately 250 mg of oxygen per liter. What this means is a fish needs to either be more efficient at removing oxygen from water or it needs to “breathe” in a much higher volume. Fortunately for fish they do not breathe like us, moving air into our lungs and then back out, instead water continuously flows across their gills either by swimming or by pumping it across. This allows fish to survive and thrive in their low oxygen concentration world.
In the ocean the oxygen content of water can vary considerably and it is primarily a function of temperature. Counter intuitively lower water temperature leads to higher gas saturation concentrations. Typically we think of hot water having higher saturation concentrations, think about dissolving sugar into a hot cup of tea versus a cold cup of tea. However, the chemical properties that control the dissolution of gases in water are different then solids, and the cold waters around Antarctica are typically much richer in oxygen than the warm waters near the equator. The oxygen saturation concentration difference between 0°C seawater and 30°C seawater is about 54%. Saturation concentration though only indicates the amount of dissolved oxygen that the water is capable of holding under ideal conditions, and it does not indicate the actual, measured concentration. The actual oxygen concentration is controlled by the production and consumption of oxygen by plankton and other animals, as well as the diffusion of oxygen from the air into the water.
Typically oxygen concentrations are higher near the surface where the water mixes with air. In areas where the conditions are very calm and there is little water exchange the oxygen concentrations near the bottom can be at or near 0 mg/L. This is a result of bacteria consuming all of the oxygen and the water at the bottom not being able to mix with the water near the surface. In other extreme cases oxygen concentrations can be very low, this can happen when there is an algae bloom as we discussed in our last blog post, or it can happen inside of a fish cage with a high density of fish and not much water motion.
In contrast to water, oxygen levels in air are relatively homogenous and do not vary considerably as a result of temperature. They do vary with changes in altitude, at 3000 meters elevation the oxygen concentration of air is approximately 30% less than at sea level. Just as humans have evolved to be able to adapt to changes in oxygen levels at higher altitudes, although it takes a few days, fish also have the ability to adapt to the changes in the oxygen concentrations of their environment.
Fish typically need a minimum oxygen concentration of 5mg/L to live a healthy life, some species can survive below these concentrations, but higher concentrations are generally better. In nature if a fish encounters a body of water with low oxygen concentration they will swim away, in a pen they do not have this freedom and it is the responsibility of the farmer to insure their fish have enough oxygen. Shellfish also require oxygen to survive and many of them cannot move around. Mussels, clams, and oysters can survive temporarily in low oxygen conditions by closing their shells, but they will die eventually if new oxygen rich water does not flow past them. Even seaweed which produces oxygen by photosynthesis also uses oxygen in respiration especially at night when there is no sunlight. All farmers need to keep a close eye on oxygen levels near their farm and they need to understand how it will affect their fish.
Try Pulse for free and check the dissolved oxygen levels near your farm by visiting www.pulse.umitron.com.
Also check out our previous blog posts about temperature and chlorophyll.
