Animals - Freshwater Ecology: Concepts and Environmental Applications - page 172

METHOD 9.1.
Sampling Zooplankton
A variety of zooplankton nets and sampling methods have been devised.
Techniques depend on the species of interest. Zooplankton populations are
patchy over space and time, so it is necessary to sample at a variety of
depths in a lake to obtain adequate population estimates and to ensure that
all species are captured.
For very small species, such as rotifers, that are able to pass through
larger nets, samples are collected with a Van Dorn or similar sampler (see
Chapter 6), preserved, and concentrated by settling or use of a fine filter in
the laboratory. For larger species, more water must be processed and a va-
riety of net configurations can be used with standardized mesh sizes.
Details for obtaining a volumetric sample are presented in Wetzel and
Likens (1991). One approach is to filter a volume of water through a net
in the lake; another to remove a set volume of water from the lake and fil-
ter it at the surface. Metered nets often are used for obtaining quantitative
zooplankton samples. These nets have a flow meter on the mouth and are
fish and large invertebrates. Many have desiccation-resistant stages and are
found in temporary pools in which they can escape predation. Brine shrimp
(Artemia salina) are tolerant of very saline waters and can establish dense
populations in lakes that are too saline to contain fish. Brine shrimp are
commonly used as food for aquarium fish and aquaculture.
Species of Cladocera (Fig. 9.11E) are often extremely important zoo-
plankters in lakes. Thus, cladocerans are frequently studied aquatic inver-
tebrates. Many methods have been developed to sample the cladocerans and
other zooplankton from lakes (Method 9.1). Cladocerans have a small, cen-
tral compound eye and a carapace that is used as a brood chamber. Adults
range from 0.2 to 18 mm in length. The thorax has four to six pairs of legs
that beat continuously and create water currents that bring in algae, proto-
zoa, and detritus. The food is filtered onto the many fine setae on the legs
and moved toward the mouth. In planktonic species, the large, second an-
tennae are used for swimming, and the animals move with a jerky motion.
Reproduction in cladocerans is parthenogenic for most of the year in
most species. Thus, females are encountered most often in nature. At times,
often when conditions are suboptimal for growth, males and sexual fe-
males are produced, and sexual reproduction occurs. The fertilized eggs are
encased in a resistant ephippium that can remain dormant until conditions
once again are favorable for growth and reproduction. There are several
juvenile instars that generally appear similar to adults.
A change in body form across seasons called cyclomorphosis is one as-
pect of cladoceran biology that has received considerable attention. The
variation in shape is diverse (Fig. 9.12) and can be related to temperature,
varied predation pressure, other abiotic factors, or synergistic effects of
several factors (Yurista, 2000). Finally, cladocerans exhibit a variety of be-
haviors including swarming, predator avoidance, mating, and feeding se-