Flows and phosphorus: the influence of 

Lake Powell on primary production 

dynamics in the Colorado River

Glen Canyon Dam Adaptive Management Program 

Annual Reporting Meeting, January 20, 2021

Bridget R Deemer
Project Information

 Project E: Nutrients and Temperature as Ecosystem 

Drivers: Understanding Patterns, Establishing Links and 

Developing Predictive Tools for an Uncertain Future

 Project Element E.2: Linking Temperature and Nutrients 

to Metabolism and Higher Trophic Levels

 Project Objective: Determine drivers of ecosystem 

metabolism (including primary production and 

respiration) throughout the Cre

 LTEMP Resource Goals Addressed or Studied: Natural 

Processes

 Restore, to the extent practicable, ecological patterns and 

processes within their range of natural variability, including the 

natural abundance, diversity, and genetic and ecological integrity of 

the plant and animal species native to those ecosystems.
Project Information, cont.

 Project Title: Lake Powell Water Quality Monitoring Program

 Project Elements: 

 Determination of water-quality status and trends in Lake Powell and GCD 

releases

 Documentation of the effects of the structure and operation of GCD on 

the quality of water in Lake Powell and GCD releases

 Improving Access to Historical Dataset

 Project Objectives: 

 Determination of water-quality status and trends in Lake Powell and GCD releases

 Documentation of the effects of the structure and operation of GCD on the quality of water in 

Lake Powell and GCD releases

 Integration with GCDAMP information needs and downstream monitoring programs

 Funding Amount and Source: $212,955 for CY 2020, 5 Year Interagency 

Agreement R18PG00108 (ends Dec 31, 2022)

 Cooperators: Bureau of Reclamation, National Park Service
Conceptual model (1)
Gross Primary Production (GPP) and 

the Food Web

 Total amount of oxygen produced during 

photosynthesis in the river

 Some of this carbon is available to higher 

trophic levels, some of it is consumed by 

bacterial and autotrophic respiration
Oxygen Data for Modeling GPP

 Lees Ferry (two station)

 30 Mile (two station)

 60 Mile

 Bright Angel

 National Canyon

 Diamond Creek

 10 MiniDOT sites

Appling & Yackulic et al. 2018 JGR Biogeosciences
Conceptual model (2)
Management Change in Recent 

Years: Bug Flows

• “Give bugs 

the weekends 

off”

• Weekend 

stable low 

flows MayAugust

• Eggs laid on 

weekends 

never dry
Increased Light Intensity at Depth

0

6/28

500

1000

1500

2000

2500

6/30 7/2 7/4 7/6 7/8

Light Intensity (PPFD μmol m-2

s-1)

-5

-4

-3

-2

-1

M. Yard- Unpublished Data

Unpublished data, subject to 

change, do not cite.
Do Bug Flows Enhance GPP?

 During clear-water times of tributary 

quiescence, changes in discharge are a more 

important control on turbidity

 Do the low and steady flows in May and June 

of 2018 and 2019 lead to clearer water and 

higher rates of GPP?
Study Design

4 YSIs

7 MiniDOTs

11 reaches 

120-400km 

downstream

MiniDOT

Gage & YSI

Unpublished data, subject to 

change, do not cite.
Sensor Maintenance
Modeling Controls on GPP

 Used YSI data to interpolate discharge and 

turbidity at MiniDOT sites

 Compare GPP estimates from one “weekend” 

and 4 “weekdays” 

 Mixed effects models to quantify the role of 

depth, turbidity, month, site, weekend

Unpublished data, subject to 

change, do not cite.
Weekend Flows Support Higher GPP 

Weekend 

GPP is 

~145% of 

Weekday 

GPP 

Across all 

Reaches

Unpublished data, subject to 

change, do not cite.
Much of This Effect Is Turbidity Driven

45% whole river increase in GPP on weekends

Deemer, Yackulic and others, unpublished Unpublished data, subject to 

change, do not cite.

15
Both Fines and 

Sand Drive 

Turbidity 

Reductions

 ~10% reduction in water 

depth

 ~22% reduction in 

discharge

 ~30% reduction in turbidity
Conceptual model
Phosphorus- building block of all life

Redfield Ratio:

106C : 16N : 1P

166C : 22N : 1P
SRP is difficult to measure

Unpublished data, subject to 

change, do not cite. SRP (mg/L)

Yackulic and Deemer, unpublished
N:P Ratios In Dam Outflows

Deemer and others, unpublished Unpublished data, subject to 

change, do not cite.
Conceptual model (3)
Dam SRP concentrations are related 

to aquatic insect drift in Glen Canyon

Korman et al. 2020

Yackulic and Deemer unpublished
Rainbow trout recruitment models fit 

for 2017 annual reporting meeting

Model using SRP and existing rainbow trout population size as covariates 

previously outperformed model used in LTEMP EIS.

Out of sample R2: 0.85 Out of sample R2: -0.33

Yackulic, preliminary data, do not cite
We Can Detect Changes in SRP 

Within Lake Powell Water Column

Month

Average gradient of 0.002 mg/L SRP 

per 10 m drop below penstock Unpublished data, subject to 

change, do not cite.
Disconnect Between SRP and GPP in 

Glen Canyon

Yackulic and Deemer, unpublished Unpublished data, subject to 

change, do not cite.
Conceptual model (4)
Next Work Plan: Role of Macrophytes 

vs Diatoms in Glen Canyon GPP

 Hypothesis 1: SRP is 

disconnected from Glen 

Canyon GPP because 

macrophytes are accessing 

different (sediment) P pool

 Hypothesis 2: SRP does 

predict diatom (labile) GPP 

production 

 Research Question: Is GPP 

by macrophtyes and 

diatoms additive? Or is 

there a shading effect? 
Dam Management Can Influence 

Both Light and P

Water withdrawal 

height effects SRP 

especially during 

spring

Discharge effects 

turbidity during 

times of tributary 

quiescence
Lake Powell Database

 Transfer from formerly maintained Microsoft 

Access database to a SQL Server Database is 

nearly complete

 Goals:

 Write a data paper describing and publishing the 

updated dataset

 Make data download linked to visualizations that 

are publicly available
Tableau Visualization Platform

Hensleigh, Voichick, Deemer, unpublished
Acknowledgements

 GPP modeling/data: Charles 

Yackulic, Bob Hall, Ted 

Kennedy, Jeff Muehlbauer, Mike 

Dodrill, Maite Arroite, Mike Yard, 

David Topping, Nick Voichick, 

Tom Gushue, David 

Goodenough

 Glen Canyon GPP: Kim Dibble, 

Tom Sabol, David Goodenough

 Lake Powell Monitoring Group: 

Nick Voichick, James 

Hensleigh, Robert Radtke, Jeff 

Arnold, and Jeremiah Drewel

 Logistics & Lab Support: Carol 

Fritzinger, Dave Foster, Seth 

Felder, Amy Stephenson, and 

Ann Marie Bringhurst
Questions