ENCM 21062 - Ecology Laboratory
Practical No. 13
Freshwater Zooplankton
Introduction
Zooplankton of inland waters are dominated by four major groups of organisms: (a) protists
that include protozoa and functionally similar protists, (b) rotifers, and two subclasses of crustaceans,
(c) the cladocerans and (d) the copepods.
Protistan zooplankton are the most important microbial consumers and have major functions
in organic carbon utilization and nutrient recycling. Most protists feed on bacteria, algae, particulate
detritus and other protists. Most rotifers are sessile and are associated with the litoral zone; some are
completely planktonic; these species can form major components of the zooplankton. Most rotifers
are non-predatory and omnivorously feed on bacteria, small algae, and detrital particulate organic
matter. Most food particles eaten are small (<12 µm in diameter). Most cladoceran zooplankton are
small (0.2-3.0 mm) and have a distinct head; the body is covered by a bivalve carapace. Locomotion
is accomplished mainly by means of the large second antennae. Most cladocerans feed on particles
filtered from the water by means of setae and hairs on five pairs of legs. A few cladocerans are
predaceous and seize other plankton and detrital particles with prehensile legs. Planktonic copepods
consist of two major groups, the calanoids and the cyclopoids. These two groups are separated on the
basis of the body structure, length of antennae, and legs. Cyclopoid copepods are raptorial; they seize
food particles and draw them to the mouth. Some cyclopoids are carnivorous on other zooplankton;
some are herbivorous on a variety of unicellular and filamentous algae. Calanoid copepods swim
more continuously in rotary motion.
Learning outcomes
At the end of this practical session, student will be able to,
1. recognize major categories of freshwater zooplankton.
2. enumerate zooplankton using a Sedgewick-Rafter chamber.
Zooplankton Enumeration
Principles
The abundance of plankters is usually too low for accurate direct microscopic enumeration,
so plankters must usually be concentrated before counting. Techniques for concentrating plankton
include capture by plankton nets, centrifugation, sedimentation, membrane filtration, and sand
filtration (American Public Health Association et al., 1971). The plankton net is the most efficient
device for concentrating zooplankton.
Special apparatus
The following items are needed:
Sedgewick-Rafter counting cell
1 ml large bore microbiological pipette
Microscope
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Counting the plankters
Mix the concentrated samples thoroughly and then pipette 1.00 ml of concentrated sample
into the Sedgewick-Rafter counting chamber and carefully position the cover glass over the chamber
without forming an air bubble. This can usually be accomplished by laying the cover glass
diagonally across the chamber and slowly rotating the cover glass as the sample is introduced from
the pipette. Place the counting chamber beneath the microscope, select a random microscope field,
and identify and count plankters. Repeat this procedure at least ten times.
Calculation
The Sedgewick-Rafter counting chamber contains exactly 1.00 ml (50 mm long × 20 mm
wide × 1 mm deep) and has a surface area of 1000 mm2. The exact area viewed within the grid is
known, so the number of organisms counted in a given number of grids can be expanded to the total
number of organisms per milliliter of the concentrated sample. This permits calculation of the
number of organisms per milliliter of the original sample.
Question:
Describe how zooplankton contributes to the energy flow and food webs of the freshwater
ecosystem.
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