Removal of harmful nitrogen – another reason to ‘re-mussel the Gulf’

Nitrogen is an essential nutrient for plant life and small amounts are a natural component of healthy rivers and marine ecosystems.  However, in excess nitrogen becomes a pollutant, ultimately leading to algal blooms and depleted ‘dead zones’.  Scientists have recently discovered how restored mussels reefs can help.

70 percent of all of Aotearoa / New Zealand’s river length had at least one form of nitrogen exceeding the Australian & New Zealand Guidelines for fresh and marine water quality (ANZG 2018) for the period 2013–17. This means that median concentrations were above what’s ‘natural’ and could therefore pose a risk to aquatic species.  Agricultural and urban land use, and infrastructure such as wastewater treatment plants, adds more nitrogen to waterways. In excess, nitrogen becomes a pollutant, leading to ‘eutrophication’ – where excessive nutrients lead to run-away plant growth like algal blooms.  In the extreme this can lead to what is referred to as ‘dead zones’, where oxygen becomes so depleted that fish and other species can’t survive.

Compared to areas with native forest land cover, median total nitrogen concentrations were 10x higher in urban areas and 6x higher in pastoral areas.  Median nitrate-nitrogen concentrations were 22x times higher in urban areas and 11x times higher in pastoral areas (Stats NZ 2020).

Mussels to the rescue

In a world-first, scientists from the University of Auckland Institute of Marine Science have been investigating the role that mussels play in nitrogen removal, a process referred to as ‘denitrification’. The research conducted by Dr. Jenny Hillman and PhD student Mallory Sea looked at how restored mussel reefs influence nutrient recycling and remove nitrogen (Hillman 2021) and how this varies under different conditions within mussel beds (Sea 2021). The results were quite staggering.  By analysing the mussel beds and comparing them to areas of bare sediment they witnessed denitrification rates of 100 – 2,500 percent compared to neighbouring areas of plain sediment. “It’s exciting to see this invisible benefit”, explains Jenny Hillman, “To think these small animals, en masse, can quietly tackle a major anthropogenic (human-induced) stressor like nitrogen!”. 

Coastal ecosystems have a natural ability to remove nitrogen. Microbes in the sediment digest nitrates (NO₃⁻) and ammonium (NH₄⁺) and turn it into harmless nitrogen gas (N₂), a form of nitrogen that most algae cannot use. This denitrification process requires the ‘anoxic’ (low oxygen) conditions that you get in sediment and a supply of organic matter – and that’s where the mussels come in. Mussels act as the ‘conveyor belts’, moving organic material, along with the nitrogen they contain, down into the sediment. A single mussel can filter up to 150-300 litres of water per day, extracting the algae from the water column. Then, through the production of organic biodeposits; ‘pseudofaeces’ or mussel poop, this nitrogen is passed on to the sediment.  The packaging of the material in the biodeposits and the structural complexity of the reef itself helps trap the biodeposits on the seafloor where the microbes can get at them. When the conditions are right you get greatly enhanced denitrification. But, it’s not lock-tight. Converting the nitrogen in algae to N₂ gas involves a series of reactions and pathways, where there can be leaks, and some nitrogen may be recycled back to the water column before it is converted to N₂ gas. So the efficiency of this pipeline depends on a number of environmental conditions.

The follow-up study led by Mallory Sea looked at how nitrogen removal rates varied within individual reefs as a result of different mussel clumping patterns. The good news is that the sediments between the mussel clumps were removing as much nitrogen as sediments with mussels growing directly over them, meaning that the overall extent of the mussel reef seems to be more important than the density of the mussels within the reef itself. 

The implications for restoration

The ability of mussel reefs to greatly enhance denitrification is great news and adds to an already impressive kete / basket of ‘ecosystem services’ provided by kūtai / mussels (see our why mussels? page). Understanding that the habitat context of a restored mussel reef can significantly affect its delivery of ecosystem services and should be considered carefully in restoration and management plans will help with future decision making on where to restore, and how to get the most ‘bang for buck’. 

There are also financial implications that could lead to more funding for mussel reef restoration.  Just as there are carbon markets today, where carbon emitters get to offset their emissions by purchasing carbon credits which then go on to fund carbon sequestration or reduction projects, nitrogen markets are similarly emerging. Sometime in the not too distant future, funding for mussel reef restoration work could come from monetising the denitrification benefits the restored mussel reefs provide. 

Carbon sequestration by mussels is also under the microscope, although Jenny Hillman says that their current work, building a conceptual model which includes how the molluscs breathe, is very complicated because the carbon cycle interacts with the nitrogen cycle “and you’re looking at all these multiple cycles happening at the same time”. 

References

Hillman, J., O’Meara, T., Lohrer, A., & Thrush, S. (2021). Influence of restored mussel reefs on denitrification in marine sediments. Journal of Sea Research, 175(102099).

Sea, M., Thrush, S., & Hillman, J. (2021). Environmental predictors of sediment denitrification rates within restored green-lipped mussel Perna canaliculus beds. Marine Ecology Progress Series, 667(1-13).

Stats NZ. (2020). Home Indicators River water quality: nitrogen. Stats NZ. Retrieved December 7, 2021, from https://www.stats.govt.nz/indicators/river-water-quality-nitrogen