5

9

13

DO

(m

g/L

)

all in situ data

hand probe

plateaus

Dissolved oxygen over three days

5

10

15

Day 1 Day 2 Day 3

Time

DO

(m

g/L

)

all in situ data

maximumrate

extrapolations

We compared several methods for calculating rates of NPP and R:• Maximum rate: regression of region of linear increase or decrease in continuous probe data• Extrapolations: from dawn and dusk values calculated based on afternoon DO increase• Plateaus: from peaks and troughs in continuous probe data• Hand probe: from dawn-dusk measurements with hand-held probe

maximum rate

extrapolations plateaus hand probe

Day 1 0.57 1.41 0.31 0.39Day 2 0.49 0.45 0.71 0.30 0.20 0.29 0.09

Day 3 0.15 0.45 0.26 0.10 0.35 0.08 0.19

R NPP R NPPR

0.43

R NPP

A comparison of methods for measuring aquatic total system metabolism in six experimental wetland cells

Janet Ackerman, Kristin Braziunas, Polly GibsonSystems Ecology (ENVS 316), Fall 2006, Oberlin College

In light of the current emphasis on wetland restoration and the growing understanding of the ecological importance of wetlands, ecologists need simple and accurate tools for assessing wetland function. One important measure of function is aquatic total system metabolism (TSM) - the productivity (photosynthesis) and respiration of aquatic organisms living in the wetland. Since aerobic metabolism produces or consumes oxygen, measuring the concentration of dissolved oxygen (DO) in the water column is a convenient way to assess wetland TSM. DO generally increases over the course of a day as photosynthesis produces oxygen and then decreases overnight as respiration consumes it.

The best methodology for measuring TSM is unclear. There are two common tools for measuring DO in aquatic systems: in situ probes, which automatically log DO at specified intervals over a long period of time, and hand-held probes, which are typically used to measure DO at a much lower frequency. The hand probe is cheaper and simpler to use, but the more frequent measurements possible with the in situ probe produce a more complete data set. This project seeks to compare the data produced by these two techniques so that future projects can select the method best suited to their goals .

• Are the hand-held and the in situ DO probes reliable (do they read consistent measurements)?

• How accurately do hand probe measurements at dawn and dusk capture wetland productivity in comparison with in situ readings? Are there better times to take measurements?

BACKGROUND

METHODS

Our study took place in one of six ¼-acre experimental wetland cells near Oberlin, OH in late fall 2006.

We measured DO in the wetland over four relatively sunny days with both an in situ probe and a hand probe. All measurements were taken at a marker in the northwest corner of the cell at a depth of approximately 33 cm.

Janet and Kristin measure DO using the hand-held probe

We calculated net primary productivity (NPP) and respiration (R) from both the in situ and hand probe data. We compared these measurements of TSM to determine the suitability of using dawn and dusk to capture metabolic activity.

We also compared in situ and hand probe DO readings to determine the reliability of the probes.

RESULTS

We thank John Petersen for his dedication and support throughout the course of this project and the environmental studies department at Oberlin College for providing us with equipment to conduct our research. We also thank the New Agrarian Center and the residents of the George Jones Farm for sharing their space with us. Finally, we thank our fellow classmates for their valuable input, especially the Wetland Nutrients group for sharing their research.

CONCLUSIONS

• Measurements of DO are consistent between the hand-held and the in situ DO probes.

• Rates of productivity and respiration within this wetland cell vary over the course of the day. This makes it difficult to use twice-daily measurements to calculate accurate rates.

• Dawn-dusk measurements with the hand-held probe provide a reasonable estimate of daily change in DO. However, taking two afternoon measurements of DO may be a better way to accurately determine a steady rate of productivity.

FUTURE RESEARCH• Our study was limited to a few short, fall-season days. Future

research should collect data over a longer time period to get a more complete picture of patterns in DO fluctuations. Data collection during the summer is especially important, when long, sunny days foster high productivity.

• Other studies could quantify spatial variation within a single wetland cell or measure average daily productivity as a function of light.

y = 0.99x - 0.12R

2 = 0.97

6

8

10

12

6 8 10 12DO (mg/L) according to in situ probe

DO (mg/L) according to hand probe

The high R² value indicates a strong linear relationship between the two sets of measurements.

The slope is close to 1 and the y-intercept is small, meaning that the two probes gave us almost identical DO values.

QUESTIONS

ACKNOWLEDGEMENTS

DATA COLLECTION

METHODOLOGICAL ANALYSIS Comparison of dissolved oxygen values from two probes

Calculated Metabolic rates (mg/L/hr)