Diel Vertical Migration in Zooplankton: Experimental Investigations
Using Video-Microscopy and Plankton Mini-Towers
(Romberg Tiburon
Center for Environmental Studies and The
Department of Biology, San Francisco
State University, San Francisco, CA 94132)
Abstract - Introduction
- Methods - Results - Summary
and Conclusions - Acknowledgments -
References Fig.1
- Fig.
2 - Fig.
3 - Fig.
4 - Fig.
5
ABSTRACT
Diel vertical migration (DVM) is one of the most common yet variable behaviors
exhibited by pelagic organisms. For example, DVM in marine zooplankton
is known to be affected by the quantity and quality of both biotic (e.g.,
predators and food) and abiotic (e.g., light) environmental conditions,
often making for temporally and spatially variable behavior. Understanding
the effects of these environmental variables, both separately and in combination,
on DVM of zooplankton is one of the major long-term goals of our laboratory
group. To this end, and as a complement to our on-going field studies,
we have developed a new experimental system in the laboratory which combines
video-microscopy with a series of 2-m high columnar tanks. Each tank is
equipped with infra-red microscopic video cameras on motorized systems
that scan the full vertical range of each tank at pre-selected intervals.
Each tank has a series of valves to allow for adding or withdrawing water
samples for manipulation and/or analysis of nutrients, chlorophyll, etc.
A light simulator with a dawn/dusk wheel adjusts the light over each 24-hr
period. Zooplankters > 0.25 mm are seen as shadows and recorded on a VCR.
Using Acartia spp. as test organisms, we present results of three
recent sets of experiments: first, the occurrence of an endogenous vertical
migration rhythm; second, the role of vertically heterogeneous food resources
(or "thin layers") in modulating the vertical distribution of zooplankton;
and third, the role of predator-mediated chemical exudates in triggering
migration behavior in zooplankton.
INTRODUCTION
What processes regulate the distribution and migration behavior of zooplankton?
This has been a central question of study by oceanographers and aquatic
ecologists for more than a century. While much progress has been made both
in field and laboratory studies, efforts have been hampered greatly by
the difficulty of monitoring the fine-scale vertical distribution of planktonic
organisms over a naturally relevant depth.
For instance, field studies, especially those in advective environments,
suffer from not being able to track the same individuals over time, and
from changing environmental conditions (food abundance, predator composition,
salinity, etc.) over the period of observation. Laboratory observations,
on the other hand, have often suffered from artificiality of environemntal
conditiosn, and inparticular, use of unrealsitically small spatial scales
(e.g., container size). Mesocosms, or enclosures, are sometimes a good
compromise between the better control and replication afforded by experimental
manipulation on the one hand, and the natural environmental conditions
of a field study on the other.
Irrespective of which approach is employed, one is still left with the
need to monitor the full vertical extent of the organism over a naturally
occuring depth range, something not easily done with conventinal sampling
techniques. With this need in mind we designed and constucted the following
experimental system combining video-microscopy with a series of 2-m high
columnar tanks, or "mini-towers". We have used this system to investigate
several questions about where, when and why planktonic organisms are disturbed
in the water column, and which environmental factors -both biotic (food,
predators) and abiotic (light, salinity, temperature, etc.) - regulate
vertical distribution and migration behavior.
METHODS
Our experimental set-up consists of a series of columnar tanks equipped
with infra-red video cameras that scan the full vertical range of each
tank once per hour and thereby determine the vertical distribution of organisms
over a several day period (Figs. 1
& 2). 
Each
custom Plexiglas tank (Fig.
1a) is 210 cm tall x 7.6 cm deep x 5.1 cm wide and fitted with eight
sets of valves (Fig
1b.) (one input and one outflow) spaced evenly at 28 cm intervals to
allow for adding or withdrawing water samples for manipulation or analysis
(e.g., nutrients or chlorophyll). A natural light simulation system (Fig
1c.) is positioned above the tank and comprises a light source (65
watt GE Grow Bulb) and dusk/dawn simulation wheel made of continually increasing
layers of neutral density blocking gel (ranging from 10-90% transmission).
Each dawn or dusk rotation of the wheel takes 1 hr, leaving 11 hrs of "daylight"
and 11 hrs of complete darkness (although this can easily be adjusted to
give more natural diel cycles of light, depending on latitude and season).
The light source for filming is an infrared light-emitting diode (LED)
(Fig.
1d.). A plano-convex lens (Fig.
1e.) mounted behind the experimental tank converts the point light
source of the LED to a columnated light source illuminating the tank from
behind. A Cohu monochrome video camera (Fig.
1f.) fitted with a macro/zoom lens and is lined up with the light source
and mounted in the front of the tank. This arrangement provides for a depth
and width of video field equal to the tank dimensions while allowing easy
recognition of zooplankters as small as 0.25 mm. Zooplankters are seen
as silhouettes and recorded on a VCR with a date/time recorder. This entire
assemblage of camera, lens and LED light source is mounted on a motorized
linear bearing/rail system (Fig.
1g.). This allows for viewing of the entire height of the tank up to
once every six minutes, but is usually run at a rate of once per hour.
Components of the system are run via a computer controlled timer. 
Live zooplankton were collected from San Francisco Bay before each experiment.
Individual adult female Acartia were sorted and placed into the
tanks at stocking densiites of 25-30 individuals per tank (ca. 4-5 individuals
per liter) and each experiment was run for several hours to several days
. Video tapes were analyzed by noting the exact depth (+/- 1 cm) of each
organism in each vertical scan of each tank, such that a weighted mean
depth (Bollens et al. 1994) or vertical "center of mass" is calculated
every interval.
RESULTS
This system has been used to address a broad range of questions concerning
the vertical distribution and migration of marine zooplankton. We provide
results below for three such questions: endogenous migration behavior;
response to vertical patches - or "thin layers" - of food; and response
to chemical exudates of predators.
ENDOGENOUS RHYTHM OF MIGRATION BEHAVIOR.
Figure
3 shows the vertical migration behavior of Acartia spp. over
48 hrs in both the presence of a diel light cycle and in total darkness.
There are two important things to note from this figure: first, that a
migration seems to occur approximately every 6 hrs, or on a semi-diurnal
cycle, coincident with the local tides in the region of San Francisco Bay
from which the copepods were collected. The second main point is that this
occurs even in total darkness, evidence of an endogenous rhythm of migration
behavior in Acartia spp.
RESPONSE TO "THIN LAYERS" OF FOOD.
Figure
4. In these experiments we manipulated both the amount and vertical
distribution of food (Thalassiosira weisflogii) in the tanks. This
was accomplished by establishing several layers of varying density seawater
(by adding salts or de-ionized water) and then injecting cultured algae
into these layers. Thickness of these layers varied from 15 to 110 cm,
with concentrations of food ranging from 0 to 180 mg chlorophyll/liter.
Data are presented as percentage of the water column total abundance of
copepods or food (total pigments) occurring within any given depth interval.
Four depth intervals for each of five sampling periods are shown in Note
the positive correlation between food abundance and copepod abundance;
the inference being that copepods congregate where their food is most abundant,
i.e., "thin layers" have a concentrating effect on zooplankton.
RESPONSE TO PREDATOR EXUDATES.
Figure
5. In these experiments we examined the possibility that Acartia
spp. are responding to the presence of chemical exudates (or kairomones)
of zooplanktivorous fish (threespine sticklebacks, Gasterosteus aculeatus).
Recent studies have shown several species of zooplankton to respond to
predator exudates, including cladocerans (Dodson, 1988; Ringelberg, 1991a;1991b;
Dawidowicz and Loose, 1992; De Meester, 1993; Loose, 1993; Loose et al.,
1993), chaoborid larvae (Dawidowicz et al., 1990; Tjossem, 1990; Dawidowicz,
1993), brine shrimp larvae (Forward and Hettler, 1992; Forward and Rittschof,
1993) and freshwater copepods (Neill, 1990; 1992). Our past work (Bollens
and Frost 1989, Bollens et al. 1994, 1995), however, implicated visual
or mechanical cues, not chemical exudates, as responsible for triggering
changes in vertical distribution and migration in Acartia. In our
more recent experiments utilizing this new experimental system (Fig.
5), we found no differences in the vertical distribution of Acartia
spp. in the presence or absence of caged fish. These results corroborate
our earlier findings on Acartia.
SUMMARY & CONCLUSIONS
The system we have developed allow us to continuously monitor the vertical
distribution and migration of zooplankters in well controlled and well
replicated tanks that allow us to manipulate the environmental conditions
such as food, predators, light, salinity, etc. This new tool has proven
invaluable in allowing us to test several hypotheses previously put forward
to explain where, when and why zooplankters are positioned in the water
column. The results presented above highlight three interesting phenomena.
First, that an endogenous rhythm, apparently on a semi-diurnal tidal cycle,
is present in Acartia spp. from San Francisco Bay. Second, the occurrence
of thin layers, or vertical patches of food, seems to act as an inducement
for copepods to congregate at these depths. Third, that predator exudates
do not seem to affect the vertical distribution or migration behavior of
Acartia, such as has been shown for some other zooplankters.
ACKNOWLEDGMENTS
This work was generously sponsored by the Office of Naval Research in the
form of a Young Investigator Award to S. M. Bollens. We also wish to thank
the staffs of the Romberg Tiburon Center and Friday Harbor Laboratories,
as well as C. Speekmann, C. Morgan, S. Lowe, L. Lougee, B. Frost, J. Cordell,
and W. Kimmerer for assistance and advice and W. Lampert and the Max Planck
Institute for inspiration on our 'mini-towers'.
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Last Updated 4-3-98 by Sean Avent.
