Phoebe works to investigate how the endangered fish can thrive.
The Little Colorado River has a brilliant turquoise-blue color due to the calcium carbonate minerals suspended in the water. Travertine, a chalky limestone that settles out of the water and coats the riverbed with a white hue, adds to the river’s amazing color.
The Little Colorado River can be divided into the upper and lower reaches, with the boundary between the two marked by a series of travertine waterfalls. The river is one of the last remaining places where you can find the endangered humpback chub. Science Moab talked with Phoebe Brown, a river guide and researcher who as an undergraduate was part of a larger study looking at the growth rates of the humpback chub.
Science Moab: What inspired this study of growth rates for humpback chub in the Little Colorado River?
Brown: Humpback chub populations declined during the 1990s, so in 2003 Arizona Fish and Wildlife researchers started taking humpback chub from the mouth or the lower reach of the Little Colorado and put them in the upper reach. They’ve been doing that study and have observed that the fish grow faster up there. So that’s the incentive for the work that I was part of: to figure out why.
Most of the humpback chub growth models on the main stem [of the Colorado River] find that the two predominant factors that seem to strongly describe humpback chub growth rates are temperature and food availability. People have researched temperature on the Little Colorado River, because right now most of the perennial water in the Little Colorado River is coming from a spring. There are times of year when the water coming out of the water in the upper reach is warmer than the water in the lower reach. That’s likely part of the explanation: they’re growing faster up there because it’s consistently warmer.
[Regarding food], someone is looking at macroinvertebrates since it seems like there are more bugs up there. Someone is looking at light, which is related to both turbidity and clear water, so thinking about food availability related to light. Then I was looking at food availability related to phosphorus…I was looking at phosphorus as a limiting nutrient.
Science Moab: What were you physically collecting in terms of data when you were trying to understand phosphorus as a limiting nutrient?
Brown: The data collection was a really fun part of this study. Normally, when you’re studying a limiting nutrient, you use a method called nutrient-diffusing substrata. You take these little cups with a clip lid, almost kind of like a pill bottle, and you drill a hole in the top and then you fill the cup with something like jelly, treated with nothing or added nitrogen or phosphorus. Then, you put it in the stream and you grow algae and see which one grows the most algae.
But in the lower reach of the Little Colorado River, calcium carbonate is depositing so quickly that you couldn’t leave something like that out for even two days. So we took these huge Nalgene water bottles and filled them up with the river water and some sediment from the upper and lower reaches. Half of each of those, we spiked with phosphorus.
Science Moab: What were your findings?
Brown: With the sensors, the data that we got was dissolved oxygen data. If you have algae that are photosynthesizing a lot, you’re gonna have more dissolved oxygen because that’s how they respire. I was trying to figure out if adding phosphorus created a significant change in growth in GPP (gross primary production), and if there was a difference in that change between the upper and lower reach. I found that phosphorus is a limiting nutrient. There was a significant change in GPP in both the upper and the lower reach, but I was not able to quantify if there’s more phosphorus limitation between the reaches.
So we can say phosphorus is affecting GPP and the bottom of the food chain in the upper and the lower reaches of the Little Colorado River, but we’re not able to say that one reach is more or less phosphorus-limited than the other.
Hopefully, as the rest of the study kind of comes together, that will be more clear. When you have all that calcium carbonate, there’s less phosphorus, but there might also be less stream bed stability, or different habitats or things not related to nutrients that are related to the calcium carbonate that could be affecting things. So I think the phosphorus thing will still be important as the rest of the pieces come together.
