In areas of low productivity, such as the Bothnian Sea, a reduction in nutrients does not necessarily lead to improved oxygen status. Bacteria, which are the largest oxygen consumers in the sea, will continue to consume as much oxygen despite a decrease in nutrients. And the high costs for nutrient reducing measures risk being tossed into the sea.
Text: Kristina Viklund
Bacteria are the largest oxygen consumers in the seas. Through their respiration, they account for over half of the ocean's oxygen consumption. Thus, they play a key role in the problem of oxygen deficiency in different sea areas around the world. Despite this, bacterial respiration has been overlooked when discussing measures against hypoxic bottoms. Instead, the nutrient status has been at the center of the discussions, and expensive measures have been put in place to reduce the amount of nutrients supplied to the seas in areas where oxygen levels are low.
Nutrients are central to high-productivity areas with eutrophication problems. By reducing the amount of nitrogen and phosphorus there, you can improve the oxygen status, although it may take time to achieve the desired results. But in low-productivity areas, such as the Gulf of Bothnia, this type of measure may be futile. It can take years before effects are measurable and thus years before it is clear whether the efforts were fruitful or if the costs were literally thrown into the sea.
Respiration maintains life functions
The explanation as to why nutrient reduction may be ineffectual lies in the different types of bacterial respiration, and above all their maintenance respiration. Maintenance respiration is defined as the respiration that is in progress even though growth in the bacterial community is low. This maintains vital functions in the bacteria, just as how humans must breathe to live. More than half of the bacteria's respiration is maintenance respiration.
Unchanged oxygen consumption
Maintenance respiration appears to be relatively independent of nutrient status in the field, and is not affected significantly by limiting emissions of nitrogen and phosphorus. This is because bacteria switch to a higher degree of maintenance respiration during low-nutrient conditions. Bacterial respiration per cell even seems to be able to increase as nutrient levels reach starvation levels, perhaps as a way to increase the effort to find food.
What activities do the starving bacteria use the consumed oxygen for? This is one of the many bacterial respiration questions that remains unanswered, and more research is needed. However, bacteria maintenance respiration is a factor to take into account when determining measures to be taken, especially in low-productivity areas such as the Gulf of Bothnia. Otherwise, there is a risk that the measures will not have the desired effect on oxygen status that we strive for.
The article has earlier been published in the annual report EcoChange 2019. The text is based on the following scientific publication: