Introduction
    Drainage Basin Characteristics
    Lake Basin Characteristics
    Water Quality
    Biological Characteristics (Introduction)
    Biological Characteristics (Plants)
    Biological Characteristics (Invertebrates)
    Biological Characteristics (Fish)
    Biological Characteristics (Wildlife)
    References

Biological Characteristics (Invertebrates)

Invertebrates

Animals that eat plants are primary consumers. On land, primary consumers include cows, deer, mice, insects and humans. In Alberta lakes, most of the primary consumers are invertebrates, small "bugs and worms", that either drift freely in the water as zooplankton, or live among the aquatic macrophytes or on the sediments as benthic invertebrates. Not all of these small creatures are peaceful plant-eaters, some are voracious predators (secondary consumers) on other invertebrates, whereas others eat organic debris. The zooplankton form a vital link in the chain of productivity in lakes. They transform the algae, the minute producers of the open water, into food for other life forms, including fish.

Zooplankton

Most people who use and enjoy lakes would never realize that there is a multitude of tiny animals suspended in the open water of the lake. These are the zooplankton, a collective word for several types of small invertebrate animals that inhabit the open water. Most range in length from 0.1 to about 3 mm. By peering into the water on the shady side of a boat on a sunny day, one can see the larger zooplankters jerkily swimming past.

TYPES OF ZOOPLANKTON  There are three main groups of these organisms in Alberta lakes. The rotifers are very small (0.05 to 0.5 mm) and soft-bodied (Fig. 42), but many have a thickened cuticle called a lorica, which may have facets, spines or extensions. They have no swimming legs, but move through the water by beating tiny hairs called cilia, which are arranged in a circle at the forward end of the animal. When the cilia beat, they appear to spin like a wheel, and rotifers are sometimes called "wheel animals". Most rotifers feed on bacteria, small algal cells and organic matter, but a few feed on other tiny animals.

The copepods are crustaceans, and are thus related to shrimp. Most adult copepods range between 0.5 and 4 mm in length, and they have swimming and grasping legs and antennae (Fig. 43). Most copepods, like most of the freshwater zooplankton, carry their eggs until they hatch. Newly hatched copepods look quite different from adults. Because their outer skin is hard, as it is in shrimp, copepods must go through a series of 11 to 13 moults before they reach adulthood. There are two main types of copepods in the zooplankton. Calanoid copepods have long antennae and feed mainly on algae. Cyclopoid copepods, with short antennae, are fast-moving little animals; the adults often feed on other animals, but immature stages and some adults eat algae.

Cladocerans, sometimes called water fleas, are also crustaceans (Fig. 44). They have large antennae used for swimming and a thin, bivalved shell that does not cover the head. Their five or six pairs of legs are used for funneling and filtering food particles. Adults of most species range in size between 0.2-mm and 3-mm long. Most of the year, when food is plentiful in the lake water, cladocerans produce female young only, without the benefit of fertilization by males. But when conditions become unfavourable, such as when their food supply dwindles, males are produced and mating occurs. A different kind of egg results; they are enclosed in a heavy-walled case and shed during the next moult. These resting eggs can withstand freezing and drying and allow the population to pass through adverse times in a protected state. Sometimes the lakeshore is strewn with these tiny saddle-shaped egg cases, or they look like large pepper grains on the surface of the water. After a period of time-usually a minimum of three weeks-and in response to little-understood conditions, these eggs hatch, and the young cladocerans renew the population cycle.

ZOOPLANKTON IN ALBERTA LAKES  There area few species of zooplankton in Alberta lakes that are both abundant and widespread. For example, many of the dominant species in deep, unproductive Cold Lake are the same as those in shallow, highly productive Cooking Lake. The ubiquitous cyclopoid copepods Diacyclops bicuspidatus thomasi and Mesocyclops edax are perhaps the most wide spread and common species of copepod in Alberta lakes. As adults, they feed on other zooplankton. Diacyclops bicuspidatus thomasi has some notoriety in Alberta, because it is the intermediate host for Triaenophorus crassus, the parasitic worm that infests cisco and lake whitefish. There is usually one or sometimes two dominant calanoid copepod species in each lake as well. In typical freshwater lakes, Diaptomus oregonensis is the most common calanoid copepod (as in Baptiste, Tucker, and Lessard lakes), but in more saline lakes Diaptomus sicilis is prevalent (as in Gull, Miquelon and Garner lakes), and some lakes have both species (Wabamun, Amisk, Lac La Biche, Hastings). Rotifers are sometimes dominant in some lakes, as in Crowsnest Lake when it was sampled in 1976. The most common species of rotifer in Alberta lakes are Keratella cochlearis and K. quadrata, but other species may become very abundant at times. The predaceous Asplanchna priodonta is one of the largest rotifers in Alberta lakes. Cladocerans, notably Daphnia pulex, Daphnia pulicaria and Daphnia galeata mendotae, may dominate the zooplankton in early summer, to be replaced by other cladocerans such as Diaphanosoma leuchtenbergianum, Chydorus sphaericus and Bosmina longirostris later in the summer and in early autumn.

FOOD AND SEASONAL CYCLES  Most zooplankters eat phytoplankton, the algae of the open water, but filter-feeders like the Cladocera will eat any appropriate-sized organic matter. A few species of rotifers and copepods feed on other zooplankton but the only predaceous cladoceran in Alberta lakes is Leptodora kindtii, of fantastic appearance and relatively enormous size-up to 18 mm (Fig. 45).

Populations of the various species of zooplankton change over the seasons in response to changing light conditions, water temperature, dissolved oxygen, food supply and predation, as well as to inherent growth and behavioural factors. In the spring, when zooplankton populations are increasing and small-celled types of algae are present, grazing zooplankton may greatly reduce the quantity of algae in the water. Sometimes the water will become crystal clear, but is teeming with large Daphnia, as has been observed in Tucker and Nakamun lakes, for example. As the phytoplankton is grazed, some of the less edible species of algae may increase at the expense of the edible ones, and eventually the water becomes green again.

Population changes of zooplankton over seasons have been little-studied in Alberta lakes, but it is likely there are general similarities among similar types of lakes. One of the few studies in Alberta was conducted in 1981 in Wabamun Lake. The graph in Figure 46 shows how populations of algae and large grazing zooplankton changed over the seasons in this lake. Phytoplankton were already abundant in the water when the ice left the lake in late April, and the cladocerans Daphnia galeata mendotae and Bosmina longirostris took advantage of this large supply of food. Their populations increased dramatically, so that by June they were very abundant. The June samples indicated an equally dramatic decline in phytoplankton. By July, blue-green algae such as Anabaena were becoming abundant, and the Daphnia and Bosmina populations declined. In many eutrophic lakes, species of Daphnia do not seem to thrive when blue-green algae are abundant, either because the algal colonies are too large for them to eat, or because they may not be able to digest some types of blue-green algae even though the cells are small. In August, blue-green algae declined and there was another pulse of zooplankton, particularly the cladocerans Chydorus sphaericus and Daphnia retrocurva, and numerous immature copepods. By September, the zooplankton population had declined, and filamentous blue-greens (Lyngbya birgei) and diatoms (Melosira italica, Stephanodiscus niagarae) were abundant.

Seasonal cycling in zooplankton and phytoplankton is much more complicated than the above description would suggest. Water temperature plays a role, because higher temperatures increase the rate of metabolism in all these organisms. When the water is warm, zooplankton need more food, but they can also reproduce faster. Predation - one animal eating another - also plays a role.

Fish are important predators on the zooplankton. Small crustaceans are the main food of most types of young fish and a few species of adult fish, such as cisco and various minnows. The decline in the Daphnia population in Wabamun Lake in July 1981 may have resulted partly from the abundance of young fish, which typically feed in the open water at that time of year.

It is known that the presence or absence of fish can greatly influence the types of zooplankton present in lakes and ponds. Fish tend to eat the largest-sized animals they can find, and therefore will remove large-bodied forms such as Daphnia and Diaptomus. Smaller-sized zooplankton, such as Chydorus, Bosmina and some species of cyclopoid copepods, will then become abundant.

Light conditions may also affect the population structure. A copepod or a cladoceran living several metres below the surface of the lake would experience very different light conditions on a day that the water was relatively free of algae, compared to a day when it looked like pea soup. Chydorus sphaericus leaves its normal home in the shallow areas among weed beds to move into the open water during the time when blue-green algal blooms develop, perhaps in response to the different light conditions. The types of zooplankton that prefer the open water of lakes will also migrate away from shore if they happen to drift into shallow water.

Copepods and cladocerans also migrate vertically in response to light conditions. During the day, most species tend to stay in deep water, but as darkness approaches they move toward the surface of the lake. The clearer the lake water, the greater the extent of vertical migration. In a murky, green lake, the migration up and down might cover only a metre or so but in a clear lake, such as the Kananaskis Lakes, zooplankton might migrate tens of metres everyday. Light - both the colour and intensity - is the cue that stimulates cladocerans and copepods to move up or down. Such migratory behaviour has adaptive significance. It may allow these animals to avoid predators, obtain high quality food, or grow more efficiently at the lower temperatures near the bottom.

ZOOPLANKTON STUDIES  In spite of the importance of zooplankton in transforming algae into food for other life forms including fish, they are often neglected in lake studies, perhaps because their life cycles and population structure are complex and highly variable over the seasons and even from year to year within the same lake. In addition, analysis of zooplankton samples to determine biomass and species composition is very labour intensive.

To sample the zooplankton, a biologist drops a specially-designed net (Wisconsin net) over the side of a boat to the desired depth, and then pulls it up to the surface. The mesh size is usually 64 to 80 µm, which retains all but the smallest animals (a few rotifers will go through mesh of this size range). The animals in the sample are counted and wet or dry biomass may be determined. The volume of lake water that passed through the net is estimated so the number or weight of zooplankton per unit volume of water can be determined. Sometimes a different piece of equipment, called a plankton trap, is used. It is lowered down in an open position, then closed at a particular depth. The water in the trap runs out through a mesh-enclosed "bucket", similar to that used on the end of the Wisconsin net. The sample is reported as number of zooplankton per litre of lake water, or dry or wet weight (biomass) per volume of water for that particular day.

Although there have been few detailed assessments of population dynamics of the zooplankton in Alberta, several inventories of zooplankton species have been conducted as part of limnological or fisheries studies. These include baseline surveys on several lakes in the Beaver River drainage basin (Cold, Tucker, Ethel and Moore lakes). In addition, Alberta Environment has assessed biomass and abundance of zooplankton in Baptiste, Wabamun, Gull, Beaverhill, Cooking, Miquelon and Hastings lakes. The University of Alberta studied the relative importance of zooplankton in 15 central Alberta lakes and conducted intensive studies of plankton dynamics in Narrow and Amisk lakes. The most extensive inventory of zooplankton in Alberta covered mountain lakes, many of which are not included in the Atlas.

P.A. Mitchell

Benthic invertebrates

Benthic invertebrates are small animals without backbones that live in or on the bottom mud, on aquatic plants, on or under rocks, on sunken or floating trees, and among the debris on the bottoms of lakes or streams. There are many kinds of benthic invertebrates that inhabit our lakes, but most of them go unnoticed by the casual observer. These animals play a critical role in lakes because, as a group, they eat almost any form of organic material, for example, bacteria, small algae, filamentous algae, large aquatic plants, decaying plant material, microscopic animals, other macroinvertebrates or large dead animals. As well, they are the major food of many fish, waterfowl and shorebirds. Despite their importance in aquatic ecosystems, invertebrate communities have not been intensively studied in Alberta lakes.

Although benthic invertebrates inhabit all lakes in Alberta, the number of animals and the number of types (or species) that occur depends on a large number of factors, including water depth, the amount of nutrients in the water, the presence of oxygen throughout the year, the presence of fish, whether large plants are present, and the connections between water bodies as the last ice sheets retreated. Water depth, lake area and the fertility of the lake are important factors in determining the size of the various depth zones of a lake and the abundance of invertebrates. Lakes can be divided into three depth zones: littoral, sublittoral, and profundal. Each of these zones and the benthic invertebrates that might be found there are discussed separately and summarized in Table 7.

The littoral zone can be defined as the area from the water's edge down to the maximum depth at which large aquatic vegetation can grow. The lake bottom in this zone can consist of soft mud, sand or rocks. Typically, there is also a large amount of plant debris, including leaves, stems, twigs and even whole trees, that may fall in the lake. Because there are hiding places and abundant food, the littoral zone supports a far greater quantity and diversity of invertebrates than the other depth zones. Furthermore, water temperatures and dissolved oxygen concentrations are higher in the littoral zone than in deeper areas, so invertebrates and their food grow more quickly. A sample of plants and sediments from the littoral zone of any Alberta lake could contain an abundance of different species. Scuds (amphipods) (Fig. 47) are usually the most abundant invertebrates in the littoral zone in productive Alberta lakes such as Tucker, Ste. Anne, Gull and Utikuma.

The littoral zone is also home for the invertebrates that are usually among the most conspicuous to recreationists. Water beetles and water boatmen are easily seen dodging among the aquatic plants near shore. Young dragonflies, damselflies and mayflies hatch and feed as nymphs in the littoral zone; when they mature, they emerge as winged insects to dart and hover along the shoreline (Fig. 48, Fig. 49). Snails, or at least their shells, are often found along the beach after a windy day. Leeches swim in the littoral zone, or hide in the sediment; some species suck blood and intimidate swimmers, but many species, especially the large ones, feed on other invertebrates and are as harmless to humans as earthworms.

Lakes that naturally contain large fish such as yellow perch, northern pike or suckers, probably also contain large clams, usually Anodonta grandis. Large clams are intolerant of low oxygen concentrations, and therefore are usually not found in lakes where summer or winter fish kills occur. Additionally, larval clams require fish as a host for a short time. Most benthic surveys of Alberta lakes either have ignored these large clams or have only sampled deep waters where they do not live. The biomass of clams can be several times greater than that of all other invertebrates combined. Large clams have been found in Long Lake (near Athabasca) and Narrow Lake but they also occur in Baptiste Lake, Amisk Lake, Lac La Biche, Lac Ste. Anne, and many other lakes. Crayfish (Orconectes virilis) are another large type of invertebrate, but they only occur naturally in Alberta in the drainage system of the Beaver River. Crayfish feed at night, hide under rocks and logs during the day, and are very fast moving, so they are seldom seen or collected in benthic surveys. They have been reported only from Amisk Lake but probably occur in other lakes in the Beaver River basin, for example, North Buck, Pinehurst, Moose and Marie lakes. Although crayfish can be eaten, the populations in Alberta lakes are probably too small and slow growing to permit any significant harvest.

The sublittoral zone is the most difficult zone in a lake to define. It begins at the point where large plants disappear and continues down to the lower boundary of all plant life. Benthic algae and photosynthetic bacteria often grow on the sediments in the sublittoral zone. The lower boundary is usually not obvious, especially in shallow, well-mixed lakes. Sublittoral zone sediments are made up of finer particles than littoral sediments, but partly decayed plant material still occurs, particularly near the edge of the weed beds. Lakes that support large numbers of snails and clams often have areas where many empty shells accumulate. There are fewer species of invertebrates present in the sublittoral zone than the littoral zone, largely because food is less abundant and hiding places are fewer. The most common groups of animals are midge larvae, aquatic earthworms and fingernail clams (Fig. 50). Some lakes also have moderate numbers of mayfly nymphs and unionid clams in the area between the lower edge of the weedbeds and the top of the thermocline. As lakes cool in the fall, many scuds and snails migrate down out of the littoral zone and into the upper part of the sublittoral zone, perhaps to avoid being frozen into the ice. In the spring, these animals migrate back up into the littoral zone.

The profundal zone is the area below the deepest extent of all plant growth, including algae and photosynthetic bacteria; it is dark and temperatures are characteristically low. Shallow lakes may not have a true profundal zone. However, if some of the bottom area has fine-grained sediments and is free of all conspicuous plants and algae, this area is often considered the profundal zone. The profundal sediments are very fine-grained and vary in consistency from soft muck to a poorly defined, jellylike material. The primary source of food for invertebrates in this zone are organisms or organic particles settling out of the water column. Some of this organic debris originates on land or in the littoral zone; this material decreases with the distance from shore. Very few types of invertebrates live in the profundal zone, because the habitat is fairly uniform (fine sediments), the food supply is sparse and the water is relatively cold. In lakes where oxygen concentrations are very low or nil during all or part of the year, this area may contain few or no organisms. Some species of midge larvae (Fig. 51) can withstand very low dissolved oxygen concentrations and are the dominant invertebrate in the profundal sediments of Alberta lakes. If oxygen concentrations are not greatly depleted, aquatic earthworms can also be found. At certain times of the year, phantom midge larvae also inhabit the profundal zone. Young phantom midge larvae are free-swimming, whereas older larvae rest in the profundal sediments during the day and migrate into the water column at night to feed on small zooplankton. Some of the large, cold, well-oxygenated lakes in Alberta, such as Cold, Athabasca and Lesser Slave lakes, also contain two unusual invertebrates, opossum shrimp (Mysis relicta) and deepwater scuds (Diporeia hoyi).These two crustaceans usually occur together and are important prey of deepwater fish such as lake trout. Deepwater scuds spend most of their time on the bottom and feed on organic matter in the sediments. Opossum shrimp are found on or near the bottom during the day where they also feed on organic matter in the sediments. Opossum shrimp are strong swimmers, and at night they migrate up into the water column where they actively search for and prey upon all sizes of zooplankton. These shrimp have been stocked into the Upper and Lower Kananaskis Lakes, Spray Lakes Reservoir and Crowsnest Lake to serve as food for young lake trout and rainbow trout. It is too early to determine if the introductions to Crowsnest Lake and Spray Lakes Reservoir have been successful, but rainbow trout in the Kananaskis Lakes make extensive use of opossum shrimp as food. Fisheries managers cannot introduce opossum shrimp into lakes indiscriminately because they are voracious feeders on plankton and have greatly altered the zooplankton community, to the detriment of fish, in some lakes in the western United States and Scandinavia.

STUDIES OF BENTHIC INVERTEBRATES  Benthic invertebrates are very difficult to study. Their abundance varies widely in space and time, therefore many samples must be collected to obtain an adequate estimate of abundance. This is especially true for the littoral zone community. Hence, estimates of invertebrate abundance are only available for a few Alberta lakes, for example, Narrow, Seibert, Sturgeon and Wolf lakes and Lac Ste. Anne. Only for Narrow Lake are there sufficient data to allow examination of the variation in abundance of invertebrates with both space and time. Sampling in most Alberta lakes has been of the survey type, which identifies the most abundant species present in a lake, but does not allow comparison of invertebrate abundance among lakes. Some benthic surveys have only sampled one depth zone of the lake or samples were collected randomly all over the lake and then average abundance was calculated. Whole-lake comparisons require that estimates of abundance in the littoral, sublittoral and profundal zones be multiplied by the area of the respective depth zones. These values are then added together and the resulting number is divided by the total area of the lake. Thus, random sampling can result in highly biased estimates of invertebrate abundance and one could wrongly conclude that one lake is more productive than another, when in fact the opposite is true. Throughout the Atlas, efforts have been made to show when and how many samples were collected, which depth zones were sampled and how often samples were collected for any estimates of invertebrate abundance.

J.M. Hanson