Uncovering the Base of the Food Web: Primary Production Dynamics in the Colorado River

Science Center Objects

Algae, phytoplankton, and rooted macrophytes represent the base of many aquatic food webs and are known as primary producers. Through photosynthesis, these organisms convert sunlight energy into chemical energy (i.e., carbon) that in turn fuels the growth of animals such as macroinvertebrates and fish. This project uses high frequency measurements of dissolved oxygen, which is a by-product of photosynthesis, to estimate rates of primary production at six locations in the Colorado River downstream of Glen Canyon Dam. Quantifying time series of primary production is used to identify the environmental factors that control primary production. Additionally, trends in primary production may be a leading indicator of changes in fish populations and the ecosystem as a whole.

A boat on the Colorado River in front of a rock outcrop containing a concrete structure.

The Lees Ferry site pictured here is one of six sites on the Colorado River being continuously monitored for dissolved oxygen concentrations. (Credit: USGS/Freshwaters Illustrated.)

Background & Importance

Algae represent the base of the aquatic food web in Glen and Grand Canyon. Through photosynthesis, these organisms convert sunlight energy into chemical energy (i.e., carbon) that in turn fuels the growth of animals such as macroinvertebrates and fish. The rate at which these organisms photosynthesize, together with the types of organisms available (and their palatability to macroinvertebrates and fish), determines relative carbon availability for the rest of the food web. Primary production is also an important component of aquatic carbon budgets which, together with estimates of outside carbon inputs (e.g. allocthonous carbon from leaf fall etc.) and ecosystem respiration, can be used to understand ecosystem carbon balances.

General Methods

Rates of primary production in the Colorado River are estimated using high frequency measurements of water column dissolved oxygen and detailed oxygen budgeting that accounts for air-water gas exchange, rates of oxygen production owing to photosynthesis, and rates of oxygen consumption owing to respiration. Grand Canyon Monitoring and Research Center’s Water Quality Monitoring Group is continuously measuring dissolved oxygen concentrations at six locations downstream of Glen Canyon Dam at 15-minute resolution. This high frequency data is used to estimate primary production by comparing daytime oxygen levels (when photosynthesis elevates oxygen concentration) to those at night (when only physical gas exchange and decomposition processes that consume oxygen are at play). These efforts are also being informed by a larger US Geological Survey Powell Center Working Group that is focused on continental scale efforts to model stream primary production (“Continental-scale overview of stream primary productivity, its links to water quality, and consequences for aquatic carbon biogeochemistry”). Time series analysis techniques are then used to link temporal patterns in riverine primary production to potential controlling factors and to higher trophic levels.

A close-up of Cladophora glomerata, a common green alga in the Colorado River

Close-up of Cladophora glomerata, a type of alga, in the Colorado River in Glen Canyon. Cladophora is one of the dominant types of algae in Glen Canyon and contributes to overall primary production in the reach. (Credit: USGS/Freshwaters Illustrated.)

Important Results

We have developed a robust understanding of air-water gas exchange in the Colorado River. Specifically, we found that physical gas exchange is generally low in calm stretches of the river, while rates of gas exchange in large Grand Canyon rapids are some of the highest physical gas exchange rates ever measured. Building on these gas exchange results, we demonstrated that turbidity exerts a strong control on primary production in Grand Canyon. Mixing of the muddy waters from the Paria River and other tributaries with the mainstem Colorado can result in very high levels of turbidity. Under these conditions, very little light can reach primary producers and photosynthesis rates approach zero.  

Future Directions

Turbidity may be a less important control on primary production closer to Glen Canyon Dam (e.g., upstream of Lees Ferry). The Colorado River runs much clearer in this segment due to Lake Powell’s role in sequestering suspended sediments from the water column. Efforts are also underway to identify drivers of primary production dynamics in this river segment. This project could help inform our understanding of the aquatic foodbase and ultimately rainbow trout dynamics—which have been observed to be quite variable from year to year. In addition, work to model primary production further downstream is moving forward with the eventual goal of linking this information to humpback chub dynamics. Furthermore, estimates of primary production can help us quantify the extent to which the Colorado River is acting as a carbon “sink”, since photosynthesis serves to convert atmospheric CO2 into “fixed” biological material (e.g., algae).

Mixing of the muddy Paria River with the relatively clear mainstem Colorado River.

Mixing of the muddy Paria River with the mainstem Colorado River.  Suspended sediment turbidity from the Paria and other tributaries downstream of Glen Canyon is a strong control on primary production in Marble and Grand Canyon. (Credit: USGS/Freshwaters Illustrated.)