Population Dynamics of Daphina magna in relation to Carrying Capacity in Closed Ecological SystemsReported by Yuichi “Eugene” Saito and David Barbee

Faculty Mentor: Frieda Taub

School of Aquatic and Fisheries Sciences, the University of Washington, Seattle WA 98105

Methods

Introduction

Nutrients recycle through organisms and constitute food chains and food webs. On Earth, the process of nutrient cycling takes place on a scale that is difficult to study. Closed Ecological Systems (CES) are miniature models of our planet; they function with little substance exchange. Solar energy is the only substantial input. CES are a way to study how resources are recycled on earth without the difficulty of a large scale.

The peak number of Daphnia increased as the container size increased, but the peak density of Daphnia magna (/ml) might be inversely related to size of bottles: the smaller the container is, the greater the density is. The carrying capacity of Daphnia magna to date seemed to be correlated to not only bottle sizes but also availability of gas exchanges. Rapid increase of algae results in rapid increases of Daphnia population, which was remarkably observed in the open system. Closure restricts gas exchange, especially of CO2

which is required for photosynthesis and of O2, a waste product of photosynthesis, but a requirement of Daphnia. Greater Daphnia populations in the open systems suggests that CO2 may have been limiting. The “Kent water” is the source of bicarbonate, the only significant CO2 source, other than Daphnia respiration.

Conclusion

Figure 3. A comparison of the mean Daphnia densities over 36 days between closed and open systems: Daphnia populations reached the highest peak densities (highest birthrates), but then crashed (highest death rates) in the smallest open systems. Systems with lower Daphnia peaks had greatest persistence of Daphnia populations.

Goal: Does container size affect Daphnia populations in Open and Closed

Ecological Systems?

A total of 36 ecological systems (ES) were setup in 6 closed bottles of 3 sizes with matching open bottles for comparison, so 18 Closed Ecological Systems (CES) and 18 Open Ecological Systems (OES). Data were gathered biweekly by counting the population of Daphnia magna.

We explored the concept how size and volume effect the self-sustaining closed ecological systems. We set up three sizes of CES in closed bottles consisting of inorganic medium, two species of algae, and the grazer Daphnia magna. We also set up the same sized systems in open bottles for comparison.

*Daphnia can be reproductively mature six days after birth.

Experimental Setup Summary

Total bottle volume (ml) 755-ml 272-ml 73-ml

Volume of air remaining 2% 3% 12%

Nutrient fluid volume (percent of total volume) 98% 97% 88%

Kent water (70% of fluid) 525ml 185.5ml 44.8ml

T82 w/ C&P (25% of fluid) 187.5ml 66.75ml 16ml

Ankistrodesmus (2% of fluid) 15ml 5.3ml 1.3ml

Scenedesmus (2% of fluid) 15ml 5.3ml 1.3ml

Daphnia magna 20 10 2

Number of replicates for closed cap (treatment) 6 6 6Number of replicates for loose cap (control) 6 6 6

Hours of Light:Dark

Temperature

Other Equipment

electro luminescent backlightPercival Scientific model I-36LL incubator

20 Celsius

18 to 6

PowerMac G4 w/ MacOS 9.2.2

Adobe Photoshop 6.0

Nikon Coolpix 995 digital camera with tripod

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.5

73 m l 272 m l 755 m lsize

Dap

hnia

den

sity

(/ml)

closed open

0

20

40

60

80

100

120

73 ml 272 ml 755 ml

size

nu

mb

er o

f Dap

hn

ia closed open

Figure 2. Open system: The peak mean densities increased as container size decreasedClosed system: Low mean peak density in largest (755ml) containers, but high densities in smaller (73ml, 272ml) containers

Figure1. The peak mean populations increased as container size increased in both closed and open systems.

ResultsOf the 36 ES, 29 had Daphnia populations persisted to

day 36. In these 29 ES, the population peak occurred between days 11 and 21. 6 OES and 2 CES lost the Daphnia populations by day 36.

A comparison of mean peak Daphnia populations resulted in greater the number of Daphnia as bottle size increases (Fig.1); however, the mean peak density (animals/ml) was reversed; greater density in smaller bottles (Fig.2).

Tests of mean peak density (animals/ml) differences:1. Bottle size effect: The larger the bottle, the more Daphnia per

system, but the lower the density (Daphnia /ml), p = 0.013 at = 0.05.

2. Open vs. closed treatment effect: Closed systems had lower densities (Daphnia/ml) than open systems, p = 0.001 at = 0.05.

3. Interaction effect: Significant as well, p = 0.092 at = 0.10.

A photographic process was developed to count large populations of Daphnia

The image processing increased the contrast of Daphnia magna with the surrounding medium, and allowed living individuals to be counted by eye.

Diagram of experimental setup, using a Nikon Coolpix 995 digital camera

Before and after the image analysis process: Daphnia contrast with the surrounding medium is enhanced.

73 ml CES

0

0.1

0.2

0.3

Replicate 1 Replicate 2 Replicate 3

Replicate 4 Replicate 5 Replicate 6

73 ml OES

265 ml CES

0

0.1

0.2

0.3265 ml OES

755 ml CES

0

0.1

0.2

0.3

0 10 20 30 40

755 ml OES

0 10 20 30 40

Dap

hnia

den

sity

(/m

l)

Day

Data analysis: We compared mean peak density of Daphnia among three different sizes of open and closed systems by using the two-way ANOVA statistical test to determine significance.1. Bottle size effect if there is any difference between small, medium, and large bottles.2. Open vs. closed treatment effect if there is any difference between open and closed systems.3. Interaction effect between “open effect at small, medium, and large sizes” and “closed effect at small, medium, and large sizes.”