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.
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Lake
Waban at Wellesley College, MA, where I conduct some
of my experiments.
Photo
courtesy of Wellesley College
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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.
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Students
standing next to a black tubular enclosure used to
test effects of ANL on vertical migration of zooplankton.
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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.
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Moore
and students on Lake Waban in Wellesley, MA, July 2004.
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Artificial
Light | Zooplankton & Vertical
Migration | Experiments |
