Little Colorado River
The Little Colorado River (LCR) is a culturally and spiritually significant drainage for many people in the southwest. The LCR also provides sediment load to the main stem of the Colorado River for the production of sandbars downstream AND it is an important spawning ground for and home to the largest population of Humpback Chub. We talk with Phoebe Brown, river guide and researcher, about her studies that involve understanding the interactions of phosphorus and the growth rates of the Humpback Chub.
Meet the Scientist: Phoebe Brown
Phoebe Brown is originally from Salt Lake City and is a recent graduate of Middlebury College, VT. By great luck she was able to conduct undergraduate research on the Little Colorado River in Grand Canyon, AZ. Her work focused on ecosystem nutrient limitation and is part of a larger project surrounding research on the endangered Humpback Chub. From this, she has become fascinated by questions about conserving native desert fishes and broader research on the intricacies of desert aquatic systems. As a river guide on the Green and Colorado Rivers, she holds a great interest and care for the aquatic and riparian ecosystems of the Colorado Plateau, and is currently celebrating her bachelor’s degree by spending some much needed outdoors time in the area. Phoebe hopes to continue working somewhere in the field of desert aquatics and is (tentatively) planning on graduate school in the next few years, where hopefully she can continue merging her place-based and academic interests.
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Interview Excerpt - Studying Growth Rates of the Humpback Chub
The Little Colorado River (LCR) 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 and a white hue adds to the river’s amazing color. The Little Colorado River can be divided into the upper and lower reach which are divided by a series of travertine waterfalls. The LCR is also one of the last remaining places where you can find the endangered humpback chub. We are talking today with Phoebe brown 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 LCR?
Brown: The Humpback Chub populations declined during the 1990s and so in 2003, Arizona Fish and Wildlife researchers started taking Humpback Chub from the mouth or from the lower reach and they put Humpback Chub 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 figuring out why? so someone is looking at macro invertebrates, it seems like there’s more bugs up there. Someone is looking at light, which is related to both turbidity and clear water. So thinking about just like food availability related to light, and then I was looking at food availability related to phosphorus. 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 done research on temperature with the LCR, because right now most of the perennial water in the LCR is coming from a spring and there’s times of year when actually the water coming out of, you know, the water in the upper reach that’s closer to the spring is actually warmer than the water in the lower reach closer to the river. it’s likely that part of the explanation is that they’re growing faster up there because it’s consistently warmer. So, we were looking at this food based thing, because it has been established in other places and 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’d use a method called nutrient diffusing substrata. You take these like 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 either nothing or added nitrogen or phosphorus, or a combination. 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 that 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 up there the upper and lower reaches and then half of each of those, we spiked with phosphorus. So we added phosphorus, and then I had an oxygen sensor, a light sensor and a temperature sensor in there.
Science Moab: What were your findings?
Brown: With the sensors, the data that we got was oxygen, dissolved oxygen data. So 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 was there 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 that there’s more phosphorus limitation between the reaches. So, we can say phosphorus is affecting GPP and kind of the food base, the bottom of the food chain, in the upper and the lower reaches of the LCR. 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 habitat 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.
