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Every Drop Counts: What’s behind the Gallatin’s nuisance algal blooms?




If you’ve fished, floated, driven or walked along the Gallatin River recently, you’ve undoubtedly noticed the recent reappearance of algae growth. We’ve all become familiar with these late summer blooms and associate the presence of this bright green algae with sufficient nutrients, a detriment to our fish habitat and to our river recreation experience. 

There is more to the bloom than meets the eye, however. It is important to understand that the algae of concern in the Gallatin watershed is naturally occurring. Its natural growth is driven by several factors: nitrogen and phosphorus concentrations, water clarity, water and air temperature, available sunlight, pH, water velocity and water hardness. 

What we need to understand is why these growth drivers—both natural and man-made—are leading to more wide-spread recurring algal blooms, how much these blooms are being caused by human land use and what we need to do as a community to solve the problem.  

A variety of conditions exacerbate algae growth. Understanding how these factors work together to cause algal blooms is like fitting together the pieces of a jigsaw puzzle. Is this natural? If not, what’s causing it? Is it bad for river health and our wild trout? Does it have consequences to river recreation? How can it be prevented?

To answer some of these questions, the Gallatin River Task Force relies on our own research, and that of the Montana Department of Environmental Quality, which is doing extensive work studying the primary drivers of the algal blooms and what can be done to stop them. Based on the research, and our own contributions to data derived from water monitoring, here’s what we know.

A recent algal bloom as seen near the Upper Deer Creek section of the Gallatin River. PHOTO COURTESY OF THE GALLATIN RIVER TASK FORCE

On the Gallatin, algae growth is natural, but the scale of the growth in recent years is exceptional. Data so far shows that nutrient levels, even from human activity, are not high enough alone to cause algal blooms. Combined with a variety of changing environmental factors, however, we are seeing conditions that create the perfect storm for excessive algae.   

Climate change has resulted in drier conditions, more sunny days, and warmer air temperatures; all factors that increase the likelihood of a bloom. These factors, along with the man-made variety are critical to include when looking at the propensity for algal blooms. The blooms are occurring from a variety of sources, and while nutrients are a key element, it is in conjunction with these other environmental factors that the appropriate circumstances for algal blooms to occur are created.  

What is clear is that algal blooms do not occur without nutrient loading. Put another way, but for the presence of available nutrients in a river, the river cannot experience severe algal blooms. Unnatural algal blooms indicate an imbalance exists within a freshwater river system. In Montana the science is well-established that our headwater freestone streams, like the Gallatin, do not in their natural condition experience severe, nuisance algal blooms.  

Rather, decades of scientific research indicates that widespread algal blooms are nearly always linked to sufficient available nutrients, and additional contributing elemental and environmental factors. For the Gallatin, recurrent nuisance algal blooms mean that the river is reaching a tipping point where it cannot assimilate all the different changes happening to the system, both within our community and larger climate issues.

The job of the Gallatin River Task Force is to continue doing the work to figure out the leading causes of these blooms, both the ones we can control, and those we cannot. Our ongoing data collection helps give the community, along with our state and federal partners in conservation, a better understanding of the algal blooms and the reasons behind why they occur in some years, and not in others.  

More importantly, with the recent DEQ determination of the middle segment of the Gallatin as impaired by these algal blooms, we have the right tools, and the critically needed next step to lead us to a plan, and a system by which we can mitigate the algae growth in the future. A clean and healthy river is paramount, for the habitat, for the community. Addressing the changes and the causes that impair the Gallatin is critical as we see changes both in our community, and in our environment.  

Marne Hayes is the communications manager for the Gallatin River Task Force.

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