Nitrogen is quite literally all around us: 78 percent of Earth’s atmosphere is made up of nitrogen gas, and all living things include some form of nitrogen. It’s a necessary part of life, but as with most nutrients, too much of a good thing can quickly become a bad thing.
In Lake Erie, excess nitrogen can affect harmful algal blooms (HABs), from increasing the overall size of the bloom to making it produce more of the algal toxins that can affect human and wildlife health. Ohio Sea Grant researchers at Wright State University have been looking into these impacts for several years, and their results suggest that controlling HABs is about more than phosphorus runoff.
Nitrifying bacteria and archaea are the only microorganisms that can recycle nitrogen from its fertilizer forms – such as urea, ammonia or the organic nitrogen contained in manure – into nitrate. That nitrate is the main compound that can be removed from the ecosystem naturally by another microbial process called denitrification. Dr. Silvia Newell and Dr. Mark McCarthy have found that the cyanobacteria that cause harmful algal blooms disrupt denitrification and outcompete nitrifying bacteria, exacerbating bloom problems because more nitrogen is available to fuel its growth instead of being released back into the atmosphere as nitrogen gas.
“The trouble is that these nitrifiers have to compete with the harmful algal bloom for the kind of nitrogen they can use,” said Newell. “And they’re really not very good at competing with the harmful algal bloom, it turns out. Most of the data from our doctoral student Daniel Hoffman are showing that when the bloom is smaller or not around, nitrifiers’ rates of processing nitrogen are much higher than when the bloom is around.”
Research by their Master’s student Ashlynn Boedecker confirmed that denitrification, the natural pathway for removing nitrogen, is seriously reduced during high bloom years. Focusing on sediments, where most of the nitrogen-containing organic compounds are decomposed, she determined that the bacteria contained in western Lake Erie sediments removed about 35 percent of the basin’s annual nitrogen load in 2016. However, denitrification rates were inhibited during the larger 2017 bloom.
Of additional concern when it comes to nitrogen are the toxins many harmful algal blooms can produce. Not all blooms become toxic, and large blooms aren’t necessarily more toxic than small blooms, but a growing body of research suggests that the amount of excess nitrogen in the ecosystem is closely connected to toxin production.
“In this particular case, what happens if you’ve got extra nitrogen around is that not only do you get a much bigger bloom, but if you reduce phosphorus inputs to the lake and not nitrogen, you’re likely to produce a more toxic bloom,” Newell said.
Scientists aren’t sure why cyanobacteria produce toxins, but they do know that each molecule of microcystin, the most common algal toxin in Lake Erie, contains ten nitrogen atoms and no phosphorus, making nitrogen availability a major factor in the bloom’s ability to become toxic.
With Ohio’s continued attention on Lake Erie, harmful algal blooms and their impacts on residents, Newell emphasized that research into the effects of nitrogen on these problems is mostly in its beginning stages.
“As a whole, the research community is starting to shift to looking at the role of nitrogen, and that’s the first step before we can make policy recommendations,” Newell said. “Less nitrogen is probably a good idea, and there are things that farmers are already doing to control phosphorus that are also going to help control nitrogen.”
With researchers addressing similar problems across the world – harmful algal blooms also happen in lakes throughout Europe and China – the hope is that reducing external nutrient loads beyond phosphorus will help address modern blooms and their impacts from a range of angles, making it easier to get a handle on a global problem with direct impacts on local communities.