Lacustrine Ecosystems Overview
|Aquatic ecosystems consist of
the entire drainage basin or watershed. The effects that terrestrial,
wetland and littoral biota have on the quality and quantity of inorganic and organic
loading of DOM-dissolved organic matter, or POM-particulate organic
matter to a lake can be profound. Water laden with inorganic and organic
substances flows from higher elevations to the recipient lakes through
ground water and surface streams. Often appreciable nutrient loading can
be picked up from adjacent agricultural fields and human settlements. Nutrient
organic matter content of the drainage thus originating in an entire watershed
is modified in each of the terrestrial stream, river, lake or reservoirs,
and particularly within their wetland and littoral components as it passes
through. These complex wetland littoral areas are exceedingly important
in regulating lake metabolism, and regulate the productivity of most
lakes (Wetzel 1979, 1983).
Many lakes, or bays gradually become enriched by nutrients and fill in with organic sediments, a process called eutrophication. This is a natural process resulting in ontogeny of lake ecosystem. During this process lake basins undergo succession of gradual filling with sediments which eventually become colonized by vegetation. This ultimately results in the obliteration of the aquatic ecosystem and its transformation into a terrestrial ecosystem. Human influence can, however, substantially speed up this process through cultural eutrophication.
There are three basic types of freshwater ecosystems:
In temperate lakes, shortly after spring ice break up, waters are of the maximum density i.e. at 4oC throughout the entire watercolumn. This condition is characterized by little thermal resistance to mixing which readily occurs with only a slight wind energy needed to initiate the mixing process. The lake waters are said to be in spring turnover.
As the spring progresses, surface waters are heated rapidly, increasing
the thermal resistance to mixing. This results in watercolumn temperature
stratification characterized by an upper stratum of more or less uniformly
warm, circulating and turbulent water - epilimnion, which floats
over a cold undisturbed region of watercolumn - hypolimnion. The
metalimnion (thermocline) is defined as the water stratum of steep thermal
In the winter, a reverse stratification takes place as the colder water layer at 0oC with a sheet of ice overlay the lower warmer layer at 4oC (Wetzel 1983).
Lakes can be distinguished with several common types of mixis:
Amictic lakes - are arctic lakes located within the permafrost
region. Permanently frozen over, lake waters do not mix.
Furthermore, lake mixis can be divided into holomictic in lakes in which the circulation involves the entire water column, and meromictic in lakes in which only part of water column undergoes complete mixis, and the bottom stratum never mix (Wetzel 1983).
Lake ecosystems can be divided into
zones: pelagic (open offshore waters); profundal (bottom sediments); littoral
(nearshore shallow waters); and riparian (the area of land bordering a
body of water).
Groups of living organisms have been associated with each individual zone. Plankton are small organisms with no or limited powers of locomotion that are suspended in the water; they are subject to dispersal by turbulence and other water movements. Both, the small photosynthesizing plant plankton, the phytoplankton, and animal plankton, the zooplankton, are usually denser than water and sink by gravity to lower depths. The organisms with relatively good swimming powers of locomotion are termed nekton. Bacteria and algae growing attached to substrata are collectively called periphyton, and have been further distinguished in relation to the type of substrate (sediments, rock, plant, animal, sand) upon which they grow. Benthos refers to nonplanktonic animals such as invertebrates associated with freshwater substrata at the sediment-water interface. Finally, specialized organisms adapted to the air-water interface are called pleuston. The pleuston dominated microflora is collectively termed neuston (Wetzel 1983).
The freshwater phytoplankton comprise diverse algae of almost every major taxonomic group. Photosynthetic autotrophic metabolism is the sole metabolic pathway for the synthesis of organic matter among most algae. Chlorophyll "a" occurs in all green plants such as algae, and is the primary light energy-absorbing pigment. Chlorophyll "b" is found only in green algae and euglenophytes, and chlorophyll "c" occurs in many algal groups. Since the density of most phytoplankton is greater than that of water, they tend to sink. Sinking has the advantage of disrupting the nutrient gradients surrounding the cells, but has a disadvantage of sedimenting out of the photic zone which is partly offset by upward movement and turbulent water transport (Wetzel 1983).
Coexistence of different phytoplankton species is a conspicuous feature of fresh waters. Although a few species commonly dominate a phytoplanktonic assemblage, a number of rarer species coexist among the dominant species. Many environmental factors interact to regulate spatial and seasonal growth and succession of phytoplankton populations. The vertical distribution of photosynthesis is strongly related to available light, as high light intensities on the surface inhibit photosynthesis. Algae also have definite temperature optima and tolerance ranges, which interact with other parameters to cause seasonal succession. Major changes in seasonal succession of phytoplankton are nutrient related; particularly, the availability of phosphorus, nitrogen, and silica. In temperate fresh waters, distinct seasonal patterns and successions of algal populations and biomass are observed in phytoplankton communities. Growth is greatly reduced during winter as both, light and temperatures are low. A spring biomass maximum is commonly observed as light conditions improve. Lower phytoplankton biomass often occurs during summer as nutrients become depleted by the spring blooms, and a second autumnal maximum typically develops as the fall circulation brings about more nutrients from the sediments to the watercolumn. As nutrient limitations of the phytoplankton of infertile (oligotrophic) waters reduces the phytoplankton biomass, nutrient inputs from external sources such as runoff from rest of the watershed, or loading from antropogenic sources may cause eutrophic conditions and high productivity, and eventually excessive biomass (algal blooms). Even under hypereutrophic conditions, however, a point may be reached when self shading inhibits further increases in productivity, regardless of nutrient availability (Wetzel 1983).
Grazing of phytoplankton by rotifers and microcrustacea can influence algal populations and their succession. Selective grazing based on algal cell size, particularly by microcrustaceans, can alter seasonal succession of phytoplankton. Grazing losses can be highly significant during certain periods of the year, during which time the algal populations are severely reduced. Horizontal variations in primary productivity of phytoplankton can be large. Spatial variations increase in significance in small lakes. In larger water bodies it occurs in the vicinity of littoral zones and inlet areas of most lakes as morphometric complexity of lake basins and therefore potential for nutrient retention increases (Wetzel 1983).
Protozoa are simple, relatively large 2um-3mm unicellular organisms which belong to animal kingdom Protista. Most freshwater porotozoa are attached to benthic substrata. They feed on bacteria and small algae. Little is known about the population dynamics and productivity of planktonic protozoans, but many protozoan populations exhibit summer maxima and their population densities are positively correlated with summer increases in temperature and abundance of food items on the surficial sediments. Under certain circumstances, they can form a substantial component of pelagic zooplankton communities. Many are meroplanktonic, in that only a portion of the life cycle is planktonic. These forms spend the rest of their life cycle in sediments, often encysted throughout the winter (Wetzel 1983).
Most rotifers are sessile and associated with the littoral zone. Some are planktonic; these species can form major components of the zooplankton. Many rotifers may have complex life cycles divided between diapause, asexual and sexual mode of reproduction. Most are non-predatory omnivores feeding on bacteria, phytoplankton and detrital particulate organic matter. A few rotifers are predatory on protozoa, rotifers, and small crustaceans (Wetzel 1983).
Cladoceran zooplankton, like daphnia, are small (0.2 to 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. They 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 zooplankton as well as detrital particles with prehensile legs (Wetzel 1983).
Planktonic copepods consist of two major groups; the calanoids and the cyclopoids. These two groups are separated on the basis of body structure, length of antennae and legs. Cyclopoid copepods are raptorial; they seize food particles and draw them to the mouth. Many cyclopoids are carnivorous on other zooplankton. Some are herbivorous on a variety of unicellular an filamentous algae. The locomotion is by movement of appendages which results in short, jerky swimming movements. Calanoid copepods swim more continuously in rotary motions, setting up currents carrying particles to modified structures of the maxillae (Wetzel 1983).
Many zooplankters, particularly the Cladocerans exhibit marked diurnal vertical migrations. The adaptive significance of diurnal migrations is unclear, but likely evolved as a mechanism for fish predator avoidance. In addition, the horizontal spatial distribution of zooplankton throughout the epilimnion (upper water layer) is uneven and patchy. Planktivorous fish can be important in regulating the abundance and size structure of zooplankton populations. Prey are visually selected. In other cases, gill rakers of certain fish collect zooplankton as water passes through the mouth and across the gills. Fish select large zooplankters and can eliminate large cladocerans from lakes. When selection by fish is not in effect, large zooplankters are present. In this case, smaller-sized zooplankton are generally not found to co-occur with the larger forms as a result of size-selective predation by the larger forms of invertebrates (copepods, phantom midge larvae, predaceous Cladocera, etc.)(Wetzel 1983).
The size of macrophyte patches, and the littoral zone of lakes and streams varies greatly in relation to the open water pelagial region. In smaller lakes, however, the wetland and littoral flora and its epiphytes are commonly a dominant source of synthesized organic matter.
Four groups of aquatic macrophytes can be distinguished on the basis of morphology and physiology. Of those, the emergent macrophytes produce erect, linear leaves from an extensive rhizome/root base, and are physiologically similar to terrestrial plants. The rooting tissues grow in an anaerobic substratum and therefore, must obtain oxygen for respiration from aerial organs. The rates of transpiration by emergent and floating macrophytes are often extremely high, and result in water losses to the atmosphere that are greater than evaporation from an equivalent area of water. The vegetative evapotranspiratory losses of water can be sufficiently large to appreciably reduce water levels of the fresh waters and surrounding terrestrial areas (Wetzel 1983).
Nutrients for growth are assimilated from the sediments by emergent and rooted floating-leaved plants, and from water in the free-loating submersed macrophytes. As in terrestrial plant communities, light availability is a major factor regulating the growth and competitive interactions of aquatic macrophytes. The productivity of aquatic macrophytes has been evaluated from changes in biomass over time, and it was found that the growth is generally higher on organic-rich soil than on sandy sediments. On a unit area basis, the net primary productivity of aquatic macrophytes is among the highest of any community in the biosphere. Emergent macrophyte productivity is the highest (1500-4500 g C m-2 year-1); submersed macrophytic productivity is considerably less (50-1000 g C m-2 year-1), but often equals or exceeds that of phytoplankton (50-450 g C m-2 year-1). The phytoplankton productivity is generally lower in littoral zones containing stands of aquatic vegetation. This can be largely attributed to the competition for nutrients and carbon with the macrophytes. It follows that wetland and littoral regions of freshwater ecosystems are commonly intensely metabolically active owing to the presence of aquatic macrophytes, and they are frequently the primary source of organic matter to fresh waters. The synthesis of organic matter by emergent vegetation and its decomposition therefore accounts for high loading of DOM and inorganic nutrients to the lakes, and the timing of loading corresponds to the stages of wetland plant growth, senescence, and decomposition. By the same token however, as water laden with nutrients and DOM enters the lake from external sources, the chemical loading to the lake is greatly modified by the metabolic activity of their littoral submersed vegetation and their epiphytic microflora which function as a selective metabolic sieve (Wetzel 1983).
The benthic invertebrates of fresh
waters are diverse. This diversity, and extreme heterogeneity in distribution,
modes of feeding, reproduction, and morpological and behavioural characteristics
make it difficult to generalize. Complex general patterns of coexistence,
interrelationships, and community productivity emerge, however, as vertebrate
predation upon benthic animals is an important regulator of spatial and
temporal population structure and dynamics. Benthic community structure
in lakes and especially in the littoral zone usually consists of a rich
fauna with high oxygen demands. Heterogeneity of substrata is great in
the littoral, thus benthic animal species diversity is greater in the littoral
than in the more homogeneous profundal zone. Two maxima in abundance and
biomass of benthic animals are often observed; one in the littoral zone,
the other in the lower profundal zone. As lakes become more productive,
the number of benthic animals adapted to hypolimnetic conditions of reduced
oxygen and increased decompositional end products declines. By the same
token however, submersed macrovegetation can be eliminated as a result
of light attenuation. Maximum abundance and biomass of benthic animals
may then shift to the profundal zone. With further eutrophication and intensive
organic matter decomposition in the profundal zone, much of the benthic
fauna of the profundal zone can also be eliminated, and a shift occurs
in the percentage composition of two dominant benthic groups. A decrease
in a dipteran chironomid larvae, and an increase of more tolerant oligochaete
worms. The dipteran phantom midge Chaoborus is another major component
of the profundal benthic fauna of lakes. Chaoborus larvae migrate
into the open water at night and prey heavily and selectively on zooplankton.
Protozoans also form a part of zooplankton community in lakes, and as such, were discussed earlier. The turbelarian flatworms are well represented in lake sediments. Most are restricted to quiescent shallow areas of lakes and low velocity streams where water velocity is reduced. Flatworms prey upon other small invertebrates. Their productivity is directly correlated with general fertility and productivity of fresh waters (Wetzel 1983).
The free living nematodes (roundworms), are widely distributed in fresh waters and can constitute a significant component of the benthic fauna. Nematode feeding habits are diverse; some species are strict herbivores, others are strict carnivores on other small animals, and still others are detrivorous on dead particulate organic matter. In the temperate zone, population dynamics of many nematode species exhibit three generations per year, with maximum production during the winter and spring periods with reduced fish predation and increased food abundance. The highest densities and productivity of nematodes are commonly found in littoral substrata of productive lakes (Wetzel 1983).
Two major groups of of aquatic annelids (segmented worms) form
a significant components of benthic fauna. Oligochaete worms are
diverse, and occur in a spectrum of fresh waters, from unproductive to
extremely eutrophic lakes and rivers. A large number of species of oligochaetes
coexist in sediments of lakes, but species abundance patterns changes at
different depths as the particle size and the organic content of the
sediment change. As lakes become organically more polluted (eutrophic)and
dissolved oxygen concentrations become reduced or eliminated, an abundance
of tubificid oligochaetes is commonly found concomitant with a precipitous
reduction and exclusion of most other benthic animals. As long as some
oxygen is periodically available, and toxic products of anaerobic sedimentary
metabolism do not accumulate, the rich food supply and freedom from competing
benthos and predators permit rapid growth. Oligochaete density can be very
large (thousands per m2), but productivity can vary greatly
from year to year due to population dynamics changes of predators such
as chironomid midge larvae.
The bivalved microcrustacean ostracods are widespread in fresh waters. Because of their small size, occurrence in surficial sediments, and difficult taxonomy, little is known of ostracod ecology, population dynamics, or productivity. They occur in the surficial sediments (0-5 cm) where they feed by filtration on bacteria, algae, detritus, and other microorganisms, and their densities are known to increase in more productive waters (to >50000 m-2). Ostracod reproduction is parthenogenetic for much of the ostracod life cycle. One to three generations may occur per year (Wetzel 1983).
The representatives of four groups of malocostracean crustaceans can form major components of the benthic fauna of some fresh waters (Wetzel 1983):
Aquatic insects are extremely diverse. Some orders of insects are entirely aquatic, others inhabit fresh waters only during certain life stages. Association of insects with a particular substrate is usually directly related to their feeding on that substrate or on the associated microflora. Most insects tend to be nonselective in their food habits, but a few species feed specifically on a given species of food substrate. Facultative feeding invertebrates ingest a wider array of food and tend to inhabit a greater diversity of stream and lake habitats. The biomass of aquatic insects is relatively constant if food supplies are constant. The insect biomass turnover is controlled by water temperature, food availability, feeding rate and respiration, but growth efficiency of carnivores tend to be less than that of herbivores or detrivores (Wetzel 1983).
Fish communities form an integral component of fresh water systems, and their impact on the operation of the system in terms of carbon flux and nutrient regeneration can be significant. For example, the shift of fish species feeding on larger sized food organisms to planktivorous species can have marked effects on zooplanktonic composition and productivity. This change can in turn influence the species composition of phytoplankton and consequently the entire lake productivity at the primary level. Another conspicuous example occurs during eutrophication of lake systems in which the fish species change dramatically from salmonid and coregonid species of quite stringent low thermal and high oxygen requirements to warm water species that are increasingly tolerant of eutrophic conditions. The warm water fish are restricted to the epilimnion during summer stratification. Certain fish, such as the carp, have omnivorous feeding habits and can be very effective in modifying the littoral substrata to the point where many submersed macrophytes are eliminated. Such feeding activities often disturb the sediments resulting in increased turbidity, reduced transparency and decline of phytoplanktonic as well as submersed macrophyte productivity (Wetzel 1983).
There are several physical factors influencing the distribution and abundance of fishes in lakes and reservoirs. They are the temperature, light, water movement, water level fluctuations, lake surface area, depth and the bottom substrate (Moyle and Cech 1988).
Based on the temperature, we can distinguish three different
lake systems. Cold-water lakes are characterized by the presence of salmonid
fishes. In warm-water lakes we can primarily find centrarchids such
as black, largemouth, smallmouth basses and sunfishes, percids (perch,
walleye), esocids (pike), or cichthyids (white, yellow and
striped basses). Two-story lakes then, have warm-water fishes in the upper
epilimnion, and the cold-water species occupy the lower hypolimnion.
The interactions between chemical factors such as lake water chemistry, pH, or dissolved oxygen concentration and fish diversity and production can be complex. They are largely determined by the composition of surrounding rocks, soils, and plant communities as well as the lake water's trophic status. Such chemical factors, however, have a rather subtle effect on temporal changes in the fish community composition, yet, the composition of fish fauna of most lakes is rarely constant. It is likely due to change as a lake ages, as the climate fluctuates from year to year, and as the abundance of food organisms changes from season to season (Moyle and Cech 1988).
Biological factors such as predation,
competition and symbiosis are exceedingly important determinants of fish
community composition and ultimately the lake productivity. As stipulated
predation by fish is a major factor influencing the composition
of the biotic communities of lakes, from plants, through invertebrates
to fish. Grazing and rooting about by fish, particularly carp, can greatly
influence the amount and species of aquatic plants growing on lake bottoms,
as well as the species and numbers of invertebrates associated with the
plants (Straškraba 1965 in Moyle and Cech 1988). Most fish that browse
on invertebrates associated with aquatic plants or the bottom are selective
as to what species they prey on, and such selective predation will greatly
affect the composition of the invertebrate community (Stein and Kitchell
1975). Predation also has a major impact on the structure of fish communities
and populations in lakes. Under more or less stable environmental condition,
self-regulating predator-prey systems can theoretically develop. Steady
cropping of a prey species by a predator will reduce intraspecific competition
and consequently increase growth rates of the prey. Such rapid growth may increase
the reproductive potential of the prey by allowing the prey to mature at earlier
age and by increasing the egg production, since the increase of fecundity in fishes
tends to have an exponential relationship to fish length. If the increased reproduction
by the prey results in a surplus of prey fish, two things may happen simultaneously;
the predator population may increase, and the intraspecific competition
among the prey may increase resulting in slower growth and lower reproduction.
These two factors would result in a rapid decline in the prey population
followed by a decline in the predator population to a former level. This
cycle could repeat itself. While this population regulatory mechanism could
well operate in lakes, in reality it is likely to be modified as predator
switch prey and the reproduction and growth of both, predator and prey,
is influenced by changing environmental conditions (Moyle and Cech 1988).
Fish Zones in lakes tend to be rather arbitrary. However, fish species in lakes do tend to sort themselves out along environmental gradients during summer months, when most individual and population growth occurs. as a result, distinct clusters of species tend to be associated with the broad habitat types within lakes. Within each fish zone the species further tend to segregate by microhabitat and food preferences.
Living organisms constitute only a part of the total lake organic matter. Most organic matter is non living and is collectively called detritus. Detritus consists of all dead particulate and dissolved organic matter. Much of the newly synthesized organic matter by photosynthesis is not consumed by animals, but upon senescence enters the detrital pool and is decomposed (Wetzel 1983).
The ecosystem relationships can be viewed in terms of individual trophic levels. The trophic structure of a community refers to the pathways by which energy is transferred and nutrients cycled. Photosynthetic organisms are primary producers, and represent the first level (L1) of the trophic structure. The L1 organisms are eaten by primary consumers or herbivores (L2), which in turn are successively consumed by secondary (L3), tertiary (L4) etc., consumers (carnivores). Therefore in aquatic ecosystems, photosynthesizing algae extract nutrients dissolved in water. They are consumed by grazing zooplankton. Herbivorous zooplankton in turn become food for carnivorous zooplankton. All zooplankton then, is eaten by larger invertebrates, as well as small or planktivorous fish. All invertebrates and small fish in turn become food for larger invertebrates, amphibians, larger fish, aquatic birds, or mammals. Larger, rooted plants on the other hand extract their nutrients from the sediments through their root system. They are grazed upon by larger herbivores such as larger invertebrates, amphibians, reptiles, fish or aquatic birds (Wetzel 1983).
It follows that production is the amount of new organic biomass thus formed over a period of time. It includes any losses from respiration, excretion, secretion, injury, death, and grazing. Finally, productivity refers to an average rate of production over a distinct period of time (per day, year,... etc.). Estimates of primary productivity by photosynthesis can be obtained directly by following changes in oxygen production, or rates of inorganic carbon assimilation. Secondary productivity by invertebrates and vertebrates is based on changes in numbers, biomass, and growth rates (Wetzel 1983).
The number of species in the community (species richness S) increases with the complexity of food webs and with the extent of organismal niche overlap. Various indices of diversity have been used. The Shannon diversity index [H'] is commonly used and accounts for both, relative abundance as well as the number of species (Shannon and Weaver, 1949 in Wetzel 1983).