Environmental Engineering Reference
In-Depth Information
near or in the metalimnion for much of the year. However, this phytoplankton also has gas vacuoles
through which they can control their depth by regulating their buoyancy. In the fall, as temperatures
cool and mixing occurs to a greater depth, P. rubescens blooms are more frequently reported in the
surface waters (Figure 15.16). So, bloom density may not actually increase, but simply become more
visible as this phytoplankton moves up in the water column.
In addition to seasonal variations in vertical positions in the water column, variations over diel
(24 hour) periods in response to light are also well known, particularly for dinolagellates. This
movement is known as diel vertical migration. DVM is ubiquitous among plankton organisms
(Hutchinson 1957), so it is common in both phytoplankton and zooplankton and must then provide
some ecologic advantage. One beneit for phytoplankton, for example, may be increased exposure
to light near the surface during the daytime and then increased nutrient concentrations in deeper
waters at night (Eppley et al. 1968).
15.3.1.2 Zooplankton
15.3.1.2.1 Classiication and Importance
Zooplankton comprise a very diverse group of organisms, which includes the primary consum-
ers that graze on phytoplankton. Zooplankton provide the link between the primary producers
and higher consumers. As such, they inluence both the quantity and species composition of the
phytoplankton community and of the secondary consumers. In addition, zooplankton are integral
to the recycling of nutrients obtained from grazing on phytoplankton, which is important to both
communities. For example, Axler et  al. (1981) found that nitrogen regeneration via zooplankton
excretion and microbial mineralization was critical for phytoplankton growth in the epilimnion of
Castle Lake, California, since the levels of dissolved inorganic nitrogen were insuficient to main-
tain measured rates in the absence of regeneration.
As seen in the previous section, phytoplankton are typically taken to represent planktonic plants.
However, phytoplankton are made up primarily of members of the kingdom Protista, which are
not true plants, and also includes bacteria (the blue-green algae or Cyanobacteria), which are in a
separate domain from plants. Similarly, zooplankton literally refers to planktonic animals (“zoo”:
animal). However, some of the important zooplankton are also members of the kingdom Protisa
rather than Animalia. So, the zooplankton may be classiied based on their location and feeding
habits as well as by their phylogeny.
Planktonic animals are dominated by four different general groups (Wetzel 2001). They include
members of the kingdom Protista (Protists) and three groups in the kingdom Animalia: the Rotifers
(Phylum Rotifera), the Cladocerca, and the Copepods (both in the phylum Arthropoda). Note that
Wetzel (2001) indicated that Protista is not a natural taxonomic group and is being abandoned.
15.3.1.2.1.1 Prot ist s The majority of the protist zooplankton is protozoans or related forms.
They include the lagellates, which move using lagella, and the ciliates, which move using cilia,
with the lagellates being the most abundant. In addition to the planktonic forms, the lagellates
include forms important for other reasons. For example, Giardia causes gastrointestinal trouble and
Trichomonas causes venereal disease. Perhaps one of the best known of the protistan zooplankton is
the Euglena, which include dinolagellates ( Ceratium and Peridinium ), chrysomonads ( Dinobryon ,
Mallomonas , and Synura ), euglenids ( Euglena ), volvocids ( Volvox and Eudorina ), choanolagellates
( Astrosiga ), and others (Wetzel 2001; Figure 15.18).
Wetzel (2001) indicated that while the actual biomass of protistan zooplankton is small in rela-
tion to other zooplankton, their generation rates are high. As a result, they are the most important
microbial consumers and have a substantial impact on the utilization and cycling of nutrients and
organic materials.
15.3.1.2.1.2 Rot ifers Rotifers form an important group of soft-bodied invertebrates of which
most are sessile (ixed or attached to some substrate, such as to macrophytes), but some important
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