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Experiments
Artificial Light | Zooplankton & Vertical Migration | Experiments


Hypothesis/ Experiment

Our hypothesis regarding how artificial light at night (ANL) affects zooplankton migration behavior grew out of a student project conducted in my Advanced Freshwater Ecology course. For this project, students compared patterns of zooplankton vertical migration on a cloudy and clear night in a Boston suburban lake exposed to ANL. On cloudy nights, the intensity of sky glow from ANL is at least 3 to 4 times brighter than on clear nights, so the students hypothesized that zooplankton migration to the surface waters on a cloudy night would be more reduced in amplitude (i.e., zooplankton would move a shorter distance upwards) than on a clear night. Although their results for a clear and for a cloudy night did not differ significantly, there was a trend in the data suggesting the effect of ANL was stronger on cloudy nights.

Lake Waban

Lake Waban at Wellesley College, MA, where I conduct some of my experiments.

Photo courtesy of Wellesley College


In the late 1990’s, three summer research students and a high school teacher working with me evaluated the effects of ANL more rigorously by conducting a field manipulation in two lakes. The students and I tested the hypothesis that ANL suppresses the vertical migration behavior of zooplankton by deploying tubular enclosures (diameter = 0.5 m; length = 4 to 7 m) in an urban and a suburban lake in the Boston, MA area. These enclosures allowed us to manipulate the underwater light regime. Some of the enclosures, constructed of black plastic, eliminated ANL, and we compared the pattern of vertical migration in these enclosures to that in clear control enclosures and in the open water of each lake. Enclosures were open at the bottom and allowed zooplankton and fish to ascend up into them. Three replicate samples of zooplankton were collected at night at each of three depths inside the enclosures and at the open lake locations. Samples were collected with Kemmerer bottles (4.2 or 3.2 l) and filtered through a plankton net (10 or 20 µ mesh) to concentrate the plankton. Back in the lab, crustacean zooplankton were identified and counted.

Black Enclosure

Students standing next to a black tubular enclosure used to test effects of ANL on vertical migration of zooplankton.

 

Results

We found that movements of Daphnia and Bosmina were significantly greater in amplitude (e.g., 2 to 3 meters higher) and magnitude (e.g., 10 to 20% more individuals) within the black enclosures (Moore et al. 2001).
Because Daphnia and Bosmina responded in a more normal fashion within the black enclosures – which simulated a dark night – we concluded that ANL reduces both the amplitude and magnitude of the zooplankton movements near the surface. As a result, we speculate that increased amounts of surface algae may be left unconsumed by the zooplankton and potentially lead to algal blooms and a change in water quality. But, this has yet to be confirmed.

Future Work

ANL may be altering the spatial distribution (i.e., physical location), diel movements (i.e., drift of stream insects, vertical migration of zooplankton and fish), and possibly the demography of many types of aquatic organisms. Topics for future research include expanding existing measurements of ANL, testing for cascading of ecological effects, probing ecological effects of glare in nearshore areas, and exploring the potential for evolutionary responses to ANL in waterways with different exposure histories.

Moore and students on boat

Moore and students on Lake Waban in Wellesley, MA, July 2004.

 


Artificial Light | Zooplankton & Vertical Migration | Experiments


Image in header from © T. Credner & S. Kohle, AlltheSky.com

 

Created By: Bing Li '05 and Zsuzsa Moricz '06
Maintained By: Marianne Moore
Date Created: July 6, 2004
Last Modified: August 6, 2004
Expires: June 1, 2005