U.S. Department of the Interior

U.S. Geological Survey

pH Regulates Phosphorus Cycling in the 

Colorado River

Bridget R. Deemer

Robin Reibold, Anna Fatta, Jessica Corman, Charles Yackulic, and Sasha Reed

Glen Canyon Dam Adaptive Management Program

Annual Reporting Meeting

January 11, 2022, 3:30-3:45pm 
Project E- Controls on Ecosystem Productivity: 

Nutrients, Flow, and Temperature

 Element E.1.- Phosphorus budgeting and controls of phosphorus 

availability

 FY2021 E.1. budget: $168,074

 LTEMP resource goal: Natural Processes

 Science hypotheses:

 H4: A large fraction of the sediment P pool is calcite bound. 

 H6: Lower pH leads to elevated water column P bioavailability due to P release 

from calcium carbonates in the sediment. 
Variable phosphorus release from Glen Canyon 

Dam controls tailwater food webs

Korman et al. 2021
Phosphorus budgeting 

project aims to 

constrain the role of 

tributary inputs

 Sampled one 5 hour 700 cfs

storm in August 2018

 The one storm contributed 

~15% of the monthly TP 

loading to Marble Canyon

 Extrapolating to the other 

storm that month, the Paria

River could have 

contributed 50% of the 

August 2018 TP loading
Ongoing Work by Tom Sabol & Somer 

Morris to Build a Total P Budget

 ISCO automatic sampling to 

capture storm flow on the 

Paria and Little Colorado 

rivers

 Community science sampling 

to capture smaller tributaries 

and mainstem sites through 

time 

Type Number of Sites or 

Storms

Total 

Number of 

TP Samples

Total Number 

of Dissolved P 

Samples

Total Number 

of Paired Silt 

and Clay 

Samples

Paria Baseflow 1 15 15 0

Paria Storm 6 (storms) 23 17 *

Little Colorado River 5 (storms) 34 12 *

Community Science Mainstem 8 (sites) 129 0 13

Community Science Tributary 21 (tributaries) 53 0 51

Preliminary Data, Do Not Cite
For this talk: How important might Colorado 

River sediments be as a phosphorus pool?

P

P

p

p

 In lakes, internal 

phosphorus 

loading is broadly 

acknowledged as 

important for 

ecology

 Could this be 

important in the 

Colorado River? 

If so, what 

controls it?
Coupled Nutrient Cycling

 Acidic conditions (e.g. that arise from decomposition) can 

cause dissolution of calcite & release of SRP

CaPO4

− + HH2O + CCCC2 + CCCCCCCC3 ⇌ CCCC2+ + 2HH 3

−+ Ca2+ +PO4

-

High pH pH Low pH
Glen Canyon Dam Has 

Changed Downstream pH 

Conditions Considerably 

Preliminary Data, Do Not Cite

Variable water levels and variable 

productivity in Lake Powell leads to more 

fluctuation in outflow pH than before 

river damming

Dip in pH corresponds to high P and 

good food base, and high fish condition 

and recruitment in Lees Ferry and near 

the Little Colorado River (2012)
Water Levels Influence the pH of Dam Releases

Higher photosynthesis, 

higher pH

Higher respiration, lower 

pH

Preliminary Data, Do Not Cite
Study Questions

Q1: Is P bioavailability in the Colorado River pH-driven?

Q2: Can sediments override dam loading in terms of a P 

source under certain conditions? 
Sediment Samples

 Glen Canyon: tailwater reach

 Paria River: sediment from storm event 

incubated with overlying Colorado River 

water

 Pearce Ferry: influenced by Kanab Creek 

flood 

Preliminary Data, Do Not Cite
Sediment Incubation Design

 Temperature controlled incubator for 

24 hours in the dark at ~13 degrees C

 1 set of initials (5x)

 7 treatments (5x each):

 Control

 CO2 amendment

 Oxygen free headspace

 4 different pHs

Preliminary Data, Do Not Cite
Conceptual Model

Declining pH and/or declining dissolved oxygen 

may cause the release of sediment bound 

phosphorus

When additional phosphorus is released to the 

water column we expect:

- Elevated cellular P (total protein)

- Elevated SRP & TP

- Elevated Ca (if calcite mediated)

- Reduced “P-tase” (alkaline 

phosphatase is an enzyme that cleaves P from 

organic molecules to make it more available for 

uptake)

Preliminary, Do Not Cite
Expected Results

U.S. Department of the Interior

U.S. Geological Survey

pH

Total Protein

pH

Alkaline P-tase

pH

SRP

pH

Calcium

pH

DIN:SRP
Biological 

Response

pH

Total Protein

pH

Alkaline P-tase

Higher total protein in the bottles 

with lower pH

Less indicator of phosphorus 

limitation (alkaline phosphatase) in 

the bottles with lower pH 

Preliminary Data, Do Not Cite
Chemical Response

 Elevated calcium in 

low pH treatments 

from all 3 sediment 

types

 Elevated SRP & TP 

in low pH treatments from 

Paria and Pearce

 No similar changes 

in nitrogen concentrations 

across pH

pH

SRP

pH

Calcium

Preliminary Data, Do Not Cite
P limitation is alleviated in Paria and Pearce 

Ferry Incubations under low pH

pH

DIN:SRP

Preliminary Data, Do Not Cite
Estimating the Potential Effect on the P Budget

 Average loading from dam is ~65 kg SRP d-1

 SRP loading from Pearce under pH 6 treatment is ~31 kg d-1
Study Questions

Q1: Is P bioavailability in the Colorado River pH-driven?

Data thus far seems to support pH-driven P release.

Q2: Can sediments override dam loading in terms of a P source under 

certain conditions?– Downstream on an event-based/daily timescale, yes
Future Directions

High Temperature Temperature Low Temperature

CaPO4

− + HH2O + CCCC2 + CCCCCCCC3 ⇌ CCCC2+ + 2HH 3

−+ Ca2+ +PO4

-

High pH pH Low pH
Acknowledgements

 Co-authors: Robin Reibold , Anna Fatta, Sasha Reed, Jessica Corman, Charles 

Yackulic

 Lab Nutrient Analyses: Adam 

Baumann

 Paria River sediment 

collections: Joel Unema

 Boat support: Clay Nelson 

 Logistical help: Kim Dibble, 

Tom Sabol, Erika Geiger, Ann- Marie Bringhurst, Seth Felder
U.S. Department of the Interior

U.S. Geological Survey

Questions?