Info

Moss

Fontinalis (willow moss)

Source: Home and Goldman (1994).

Source: Home and Goldman (1994).

periodically to substrates by several "toes." A few have a ring of cilia that gives the impression of circular motion (accounting for the name) as it sweeps in suspended matter for food. Some are parasites or predators.

Arthropods include two important aquatic groups: the crustaceans and the insects. Both have exoskeletons composed of chitin. Most of the larger Zooplankton are crustaceans

Figure 15.9 Protozoans. Amoebae: (a) Amoeba; ciliates: (b) Paramecium, (c) Frontonia; stalked ciliates: (d) Carchesium, (e) Epistylis, (f) Vorticella; flagellates: (g) Peranema, (h) Chilomonas, (i) Astasia. (From Standard Methods, 10th ed. # 1955 American Public Health Association.)

Figure 15.10 Planktonic crustaceans: (a) the cladoceran Daphnia; (b) the copepod Cyclops; (c) the ostracod Cipridopsis. (From Standard Methods, 10th ed. # 1955 American Public Health Association.)

Figure 15.10 Planktonic crustaceans: (a) the cladoceran Daphnia; (b) the copepod Cyclops; (c) the ostracod Cipridopsis. (From Standard Methods, 10th ed. # 1955 American Public Health Association.)

(Table 15.6). Cladocerans, such as Daphnia, and copepods, such as Cyclops, combine with protozoans to form the majority of aquatic zooplankton.

The presence of some of the insect larvae or nymphs are indicators of a healthy aquatic environment (Table 15.7 and Figure 15.12). Examples include nymphs of stoneflies, mayflies, and caddisfly larvae. Recall that a nymph is an immature stage of that minority of insects that go through gradual metamorphosis. It is usually similar in form to the adult, except possibly lacking the adult's wings, coloration, and sexual maturity. The immature form of insects that go through complete metamorphosis, called a larva, may be very different in form (often, a worm or caterpillar) and habitat from the adult. The caddisfly larvae may be housed in a case made of bits of plant matter, as shown in Figure 15.12.

The insects occupy trophic levels between the primary producers and the fish. Hatching of adult insects may occur en mass and results in a feeding frenzy among the fish.

Benthos The benthos contains organisms on and beneath the surface of the sediment. Their food web is based on detritivores: bacteria, fungi, protozoans, and invertebrate animals. The source of the energy is the primary productivity of the water column, for lakes, or the watershed, for rivers. Transport of pollutants through the pores of the sediments cannot be predicted using the diffusion methods from groundwater pollution hydrology. This is because burrowing organisms actively mix and transport chemicals in sediment (and the sediment itself), move water through the sediment, and provide channels for increased circulation and diffusion.

Two distinct benthic environments are the littoral and the profundal, according to whether they are above or below the thermocline, respectively. The littoral zone tends to have higher species diversity, although the profundal zone may have greater total numbers of organisms. The littoral benthos is subject to significant daily and seasonal changes. The profundal benthos may experience seasonal changes in oxygen in eutrophic lakes.

The littoral benthos consist of relatively more herbivores, grazers, and filterers than the profundal. Macrophytes can greatly increase the available habitat. Wave action and the proximity of the surface usually ensure an ample oxygen supply.

The profundal benthos consist mainly of worms and mollusks in four groups: oligo-chaete worms (only in cold waters), amphipods (crustaceans, including mysid shrimp), insect larvae, and clams (Figure 15.13). Most of these are detritus feeders, although there may be some predators, such as the midges. Most of the food input to the profundal benthos comes from the spring algae bloom, especially diatoms, which sink readily. The profun-dal benthos can store large amounts of lipids to get them through the rest of the year.

Figure 15.11 Other invertebrates: (1) hydrachnid, or water mite; (2) water spider; (3) gasterotrich, Chaetonotus; (4) coelenterate, Hydra; (5) tardigrade, Macrobiotus; (6) bryozoan, Plumatella; (7), bristle worm, Nais; (8) sewage worm, Tubifex; (9) leech, Clepsine; (10) flatworm, Planaria; (11) colonial rotifer, Conochilus; (12) nematode worm; (13) freshwater sponge; (14) gommules and spicules from the sponge. (From Standard Methods, 10th ed. # 1955 American Public Health Association.)

Figure 15.11 Other invertebrates: (1) hydrachnid, or water mite; (2) water spider; (3) gasterotrich, Chaetonotus; (4) coelenterate, Hydra; (5) tardigrade, Macrobiotus; (6) bryozoan, Plumatella; (7), bristle worm, Nais; (8) sewage worm, Tubifex; (9) leech, Clepsine; (10) flatworm, Planaria; (11) colonial rotifer, Conochilus; (12) nematode worm; (13) freshwater sponge; (14) gommules and spicules from the sponge. (From Standard Methods, 10th ed. # 1955 American Public Health Association.)

TABLE 15.5 Some Important Aquatic Protozoans

Phyla

Habitat

Examples

Flagellates (some are algae)

Lakes; oceans; some are parasites

Synura (Chrysomonad)* Ceratium (Dinoflagellate)* Euglena* Oikomonas

Ciliates

Ponds, streams, detritus, some parasitic

Paramecium Vorticella

Amoeboid

Globigerina (Foraminifera) Actinosphaerium (Radiolaria) Vampyrella (algal parasite)

Sporozoans (all parasitic)

Aquatic organisms

Henneguya (on fish) Plasmodium (human malaria)

Source: Home and Goldman (1994).

Source: Home and Goldman (1994).

The differences in adaptation between littoral and benthic organisms can be represented by a comparison of the littoral amphipod Gammarus pulex and the benthic chiro-nomid midge Chironomus anthracinus. Midges are larvae of predatory gnats. They feed on rotifers and burrow into the sediments at night. When the water is saturated, both species use oxygen at the rate of about 250 mg/g. But for Gammarus the rate decreases proportionally as oxygen saturation in its environment decreases. At 30% of saturation its respiration rate drops from to about 100 mg O2 per gram of organism. The midge, on the other hand, maintains a fairly constant respiration rate down to 30% saturation; 100 mg/g

TABLE 15.6 Common Aquatic Crustaceans

Taxonomic

Most Common

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