Article: Petersen, J. K.; Hasler, B.; Timmermann, K.; Nielsen, P.; Bruunshoj Torring, D.; Mork Larsen, M.; Holmer, M. Mussels as a tool for mitigation of nutrients in the marine environment. Mar. Poll. Bull. 2014, 82, 137-143. DOI: 10.1016/j.marpolbul.2014.03.006
Without modern fertilizers, the world would not be able to produce enough food for about half of our current population. Plants need nitrogen and phosphorous to grow, and humans need plants to survive. In our quest to produce more and more food for more and more people, we have released immense amounts of nutrients into aquatic environments. As we intentionally fertilize our fields, rivers and coastal areas are also unintentionally fertilized – nutrients from croplands, farms, and wastewater treatment plants all end up in marine environments. What happens next? The desired effect of fertilizer (stimulated growth) becomes undesirable, and aquatic systems become eutrophic.
Eutrophication: Too Much of a Good Thing
Eutrophication can have all sorts of nasty effects. Excess nutrients can stimulate growth of unwanted algal species that may be toxic, or at least unsightly. Even more worrisome, however, is what happens after the algae die and sink to the sea floor. While the bloom may disappear from the water’s surface, problems for the marine ecosystem are just beginning. When large amounts of nutrients and biomass sink to the sea floor, the influx of organic matter stimulates microbial communities, which use up oxygen as they consume and degrade the dead algae. This can lead to hypoxic (low oxygen) conditions that can be fatal for fish and organisms that live on the sea floor. In coastal areas that suffer from chronic inputs of nutrient pollution, eutrophication can drastically alter entire marine ecosystems.
In this study, researchers investigated a simple way to reduce the amounts of nutrients entering our waters. By farming mussels in nutrient-rich waters, excess nitrogen and phosphorous can be effectively “soaked up” by mussels as they feed on passing particulates in the water. The nutrients can be removed, along with the mussels, at harvest time. The researchers operated a mussel farm for a year, measuring the amount of nutrients taken up by their mussels and the costs of maintaining the farm, to determine whether increased mussel farming is a feasible way to fight eutrophication on a large scale.
Methods & Results
When cultivating mussels for mitigation rather than for human consumption, the main goal is to optimize total “mussel mass” – the faster the mussels grow, the more nutrients they are removing from the environment and transforming into body mass. This is different from traditional mussel farming for human consumption, where more care is taken to optimize the size and appearance of individual mussels.
To test how effectively mussels remove nutrients from the water, the research team rented a commercial mussel farm in Skive Fjord, Denmark, an area known to experience problems with nutrient enrichment and hypoxia. They ran the farm for one year and calculated how much it cost to establish and maintain the farm. When the mussels were ready to be harvested, the researchers removed portions of the total harvest for counting and weighing, and then extrapolated their results to the area of the whole farm to determine the total mass of mussels grown. The researchers were able to produce a total biomass of 1100 tons – that’s a lot of mussels!
The researchers measured nutrient content of the shells, meat, and byssus (filaments secreted by the mussel to adhere to a surface) for 30 individual mussels. They used this data to estimate the total amount of nitrogen and phosphorous removed from the water by the entire mussel farm and estimated that they removed 10 – 16 tons of nitrogen and 0.5 – 7 tons of phosphorous.
The results show that mussels were taking up nutrients from water, but is this method an economically viable approach to clean up the water column? The authors computed the cost of nitrogen removal by this method and found that removal costs per unit of nitrogen were similar to many other common mitigation techniques. Additionally, they noted that other methods, such as using coastal plants to soak up metals before they reach the water, were limited by how much space they took up on land.
Costs of running a “mitigation mussel” farm could be further reduced if the mussels could be sold as food, but first the researchers had to find out whether mussels grown in this polluted area were safe to eat. They measured concentrations of toxic heavy metals like arsenic and mercury in the mussels and compared them with safety guidelines. They found that the mussels were clean enough for use as feed in fish farms, or even for human consumption. This makes the farms more economically viable and allows nutrients that were lost to the marine system to be effectively recycled back into food for human populations. However, the metal content of mussels would vary depending on where the mussel farm is located, so these kinds of tests would need to be redone at any new location used for mitigation farming before mussels could be sold as food.