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Chemical vs Biological monitoring
"The freshwater world has more than 500,000 different species of insects. They occur in habitats that range from hot springs, discarded tin cans, temporary ponds, spring seeps, wetlands, rivers, lakes, to arctic and mountain pools. In fact, if water will stand for a few days, one or another of the ubiquitous chironomids will probably be the first to inhabit that water............ Take a walk down to the lake. It doesn't matter what time it is - there will be insect activity somewhere!"
Narf, R. 1997. Midges, bugs, whirligigs and others: The distribution of insects in Lake "U-Name-It". Lakeline. N. Am. Lake Manage. Soc. 16-17, 57-62.
The benthic animals inhabiting lakes constitute an extremely diverse assemblage, both taxonomically and ecologically. There is usually a great proportion of insects present in striking contrast to what is met with in the sea. Except in the freshwater sponges and coelenterates with photosynthetic symbionts and in the groups of larger nektobenthic animals, mostly arthropods and fishes that find their food by sight, the occurrence of zoobenthos seems less affected by the light gradient than would be expected, as most groups of benthic animals have representatives that extend far into the dysphotic zone.
The number of species may be very large, particularly in the chironomids which are often the most numerous species present. The ecological specialization in the larvae is very great and of enormous importance in lake classification on account of the variation in tolerance of oxygen and other environmental conditions, so that in stratified lakes the hypolimnetic zones having different oxygen concentrations and different physical types of sediment have different chironomid inhabitants. Since the different species are in most cases at least generally and in some cases specifically determinable from their fossilized head capsules, study of these larvae can give a great deal of paleolimnological data. Fortunately they have been intensively studied, most in connection with lake classification, notably by Thienemann.
In the evolution of the freshwater biota, it is reasonable to suspect the sponges, Coelenterata, annelids, bivalves and lower gastropod molluscs, most crustacean groups, the Cyclostomata, and ultimately the fishes, as having moved from the sea to fresh water directly, while the insects entered fresh water from the land. Though the history of more phyla involves a direct aquatic path than an indirect path over land, the insects are much more prominent than the other groups in all except the largest lakes.
In striking contrast to the marine benthos, insects are extremely important and are proportionately more abundant in dilute oligotrophic lakes than in less dilute eutrophic waters. It is possible that this is related to the insects being of terrestrial origin and so less able to take up calcium and other essential substances from fresh water than are soft-bodied and other invertebrates of marine affinities; on this hypothesis, which is clearly not universally valid, insects usually obtain most of what they need by mouth but compete less well with animals having other means of absorption when the needed ions are abundant.
It has been suggested that in the less eutrophic regions, the noninsectan community, ultimately derived directly from the sea, would consist of animals that at some stages depended on dissolved inorganic ions as a source of nutrients, and that such substances would be enriched where such animals lived. The insectan communities being derived from the land would have lost this capacity and, insofar as they have become important members of the fauna of electrolyte-poor water, probably receive their inorganic nutrients from solid food blown or washed into the lake. It was further suggested that the absence of these insects in the noninsectan community is due to the high level of various invertebrates dependent on dissolved nutrients being able to produce large enough populations of predators to limit severely the survival of insect eggs.
"Though this hypothesis to me is very reasonable, it is characteristic of much of benthic ecology, in which the simplest situation depends on a complicated, often hidden, set of interspecific interactions. .............. Yet in the simplest form the hypothesis seems unlikely to be true. There is evidence that runs counter to it. All the lower insects- Odonata, Ephemeroptera, and Plecoptera- have salt-absorbing organs on the gills, and there are dragonfly nymphs that do need more electrolytes in solution than might be expected. The dragonfly Libellula as a nymph requires more salt than most other freshwater animals." .................................. G. Evelyn Hutchinson a.k.a. Father of modern Limnology and the modern Darwin (1993).
The history of biomonitoring can be traced back to Aristotle, who placed freshwater fish into seawater to observe their reactions. The first toxicity experiments were published in 1816, and described longer survival of several species of freshwater molluscs in 2% than 4% saline solutions. Studies of the survival of freshwater invertebrates exposed to metals and organic compounds appeared in the mid-1890s. The use of community structure of freshwater organisms for biomonitoring can be traced back to the pioneering work of two German scientists, R. Kolkwitz and M. Marsson, in the early 1900s. Their publication on saprobity (degree of pollution) led to the development of indicator organisms. Today, indicators are much sought after as magic bullets to summarize a wide variety of states - from biological health to economics.
A wide variety of biotic groups is used for biomonitoring. A search of the database from 1993 to July 1998 carried out by Vincent Resh and Norma Kobzina at the University of California, Berkeley confirmed that macroinvertebrates are the most popular group.
There are compelling reasons for the apparent popularity of freshwater macroinvertebrates in current biomonitoring practice; they offer a number of advantages:
The RCA is a significant development in biomonitoring because it solves the problem of trying to locate nearby control or reference sites when studying an impacted system, a problem that bedevils traditional approaches. Rather than using upstream reference sites in a river system or next-bay-over reference sites in a lake, an array of biologically similar, least-impacted sites scattered throughout a catchment or region is used. Once the reference condition has been established, any site suspected of being impacted can be assessed by comparison to the reference data and its status determined. The reference condition database could exist in perpetuity.
Two pattern recognition techniques (using the computer software package PATN) are employed in the analysis: cluster analysis and ordination. The ordination vector scores from the original axes of the pattern analysis are correlated (using CORR in SAS) with environmental variables which are anticipated to be least affected by anthropogenic activities (e.g. alkalinity, depth, silt, sodium, etc.). Multiple discriminant analysis (MDA) is used to relate the site groupings from the pattern analysis to the environmental variables and to generate a model that can be used to predict community assemblages and functional responses at new sites with unknown but potential contamination. The predicted community assemblages and functional responses are then compared with the actual benthic communities and responses at a site, and the need for remedial action is determined.
The predictive capability of this discriminant model was confirmed by performing several validation runs on subsets of data. An example of the use of the model for sediment in Collingwood Bay (an area of concern designated by the IJC in Georgian Bay, Lake Huron) is presented and the technique is shown to be more precise in determining the need for remediation than the currently used provincial sediment quality criteria based on Screening Level Concentration (SLC) and laboratory toxicity tests.
Specimens were identified to the family level.
|Collection of data on invertebrate assemblages and habitat characteristics at a range of reference sites
|Classification of reference sites
|Sites are grouped a priori based on their geophysical attributes; final classification is based on species composition
|Sites are classified into groups using clustering methods based on the similarity of their species composition.
|Selection of reference sites for comparison with a test site
|Based on the geographical or physical attributes of the site
Based on a subset of sites with the highest probability using a discriminant model
Based on all sites but weighted by the probability of group membership
|Based on quartile distributions of additive metrics
Based on comparison of test-site and reference-site group in taxa ordination space using probability ellipses constructed around reference sits
Based on the probability of expected taxa occurrences from all weighted reference-site groups
Conclusions of the authors:
"Multimetrics are attractive because they produce a single score that is comparable to a target value and they include ecological information. However, not all information collected is used, metrics are often redundant in a combination index, errors can be compounded, and it is difficult to acquire current procedures. Multivariate methods are attractive because they require no prior assumptions either in creating groups out of reference sites or in comparing test sites with reference groups. However, potential users may be discouraged by the complexity of initial model construction. The complementary emphases in the multivariate methods examined (presence/absence in AusRivAS cf., abundance in BEAST) lead us to recommend that they be used together, and in conjunction with, multimetric studies."
In the past three decades, the study of aquatic insects has been revolutionized. Not only have pure research studies shown the pivotal role played by the larvae of aquatic species in the breakdown of terrestrial leaf litter and the pathways by which this plant energy is incorporated into the tissue of fishes, birds and other vertebrates but a host of more applied research has revealed the importance of aquatic insects in the spread of diseases, in the biological assessment of water quality, and in the reconstruction of past environments on earth. Most recently, aquatic insects are being used, increasingly, to test many elegant hypotheses in contemporary ecological theory.
On land and in freshwater benthic habitats, it is the crustaceans that are in the minority, in some cases no doubt, because of the superior competitive abilities of insects. A seed of contention, however, is perhaps sown by the fact that insects are not as abundant in ancient Lake Baikal, coincident with a huge array of endemic amphipod species, as they are in other large lakes.
Since the majority of marine insects are found in what have been termed "bridging habitats" (e.g. estuaries, saltmarshes, the intertidal zone and mangrove swamps), it could be argued that we are currently looking not at the end of an evolutionary pathway leading freshwater insects into marine habitats, but at early steps in the journey.
The littoral habitat of lakes usually supports larger and more diverse populations of benthic invertebrates than do the sublittoral and profundal habitats. The vegetation and substrate heterogeneity of the littoral habitat provide an abundance of microhabitats occupied by a varied fauna, which in turn enhances invertebrate production. The littoral habitat is also highly variable due to seasonal influences, land use patterns, riparian variation, and direct climatic effects producing high-energy areas. The epifauna species composition, number of individuals, areal extent, and growth form vary with the species composition of the macrophyte beds, making it difficult to determine the benthic status accurately.
The sublittoral habitat, below the area of dense macrophyte beds, but above typical thermoclines, lacks the heterogeneity of the littoral habitat; However it is also less subject to littoral habitat variables and influences. The sublittoral habitat is rarely exposed to severe hypoxia but might also lack the sensitivity to toxic effects that is found in the profundal habitat. The sublittoral habitat supports diverse infaunal populations, and standardized sampling is easy to implement because a constant depth and substrate can be selected for sampling. Therefore, the sublittoral habitat is the preferred habitat for surveying the benthic assemblage in most regions.
The profundal habitat, in the hypolimnium of stratified lakes, is more homogeneous due to a lack of habitat and food heterogeneity, and hypoxia and anoxia in moderately to highly productive lakes are common. The profundal habitat is usually dominated by three main groups of benthic organisms including chironomid larvae, oligochaete worms, and phantom midge larvae (Chaoborus). Many species of chironomids and tubificid oligochaetes are tolerant to low dissolved oxygen, such that these become the dominant profundal invertebrates in lakes with hypoxic hypolimnia. As hypoxia becomes more severe tubificids can become dominant over chironomids. In cases of prolonged anoxia, the profundal assemblage might disappear entirely. If hypoxia is rare in reference lakes of the region, and if toxic sediments are suspected to occur in some lakes, then the profundal habitat might be preferred for the region.
Benthic macroinvertebrates are moderately long-lived and are in constant contact with lake sediments. Contamination and toxicity of sediments will therefore affect those benthic organisms which are sensitive to them. Acidification of lakes is accompanied by shifts in the composition of benthic assemblages to dominance by species tolerant of acidic conditions. Effects of rapid sedimentation are less well-known but appear to cause shifts toward lower abundances and oligotrophic species assemblages as well as more motile species.
Benthic macroinvertebrates are present year-round and are often abundant, yet not very motile. However, the benthos integrate environmental conditions at the sampling point.
Reference sites must be carefully selected because they will be used as a benchmark against which test sites will be compared. The conditions at reference sites should represent the best range of minimally impaired conditions that can be achieved by similar lakes within the region. The reference sites must be representative of the region, and relatively least impacted compared to other lakes of the region.
Sites that are undisturbed by human activities are ideal reference sites. However, land use practices and atmospheric pollution have so altered the landscape and quality of water resources nationally that truly undisturbed sites are rarely available.
Stringent criteria might require using park or preserve areas for reference lakes. Criteria for reference lakes will also pertain to the condition of the watershed, as well as the lake itself.
An alternative to characterizing present-day reference conditions is to estimate historic or prehistoric pristine conditions. In many lakes, presettlement conditions can be inferred from fossil diatoms, chrysophytes, midge head capsules, cladoceran carapaces, and other remains preserved in lake sediments. Fossil diatoms are established indicators of historical lake alkalinity, salinity, and trophic state. Diatom frustules, composed of silica, are typically well preserved in lake sediments and easy to identify. However, remains of other organisms are problematic because of incomplete preservation.
Many species of macroinvertebrates are diagnostic of certain kinds of habitats and their water quality. They are known as indicator organisms, that is organisms that become numerically dominant only under a specific set of environmental conditions. The most common usage of benthic organisms is as indicators of water quality, especially trophic status of lakes, calcium hardness, alkalinity, pH and conductivity. Stream organisms that exhibit adaptations to life in flowing waters are indicators of stream environments. These organisms exhibit clues that they are from erosional substrates in stream environments. In contrast, organisms that live in depositional substrates (e.g. pools of streams, sediments of lakes) have features characteristic of lentic environments. Some benthic organisms are restricted to temporary ponds and each species has one or more adaptations to survive a period of drought."
Species are thought to adopt one of three life history strategies in order to live in stable and unstable environments: (i) r-selection; (ii) K-selection; or (iii) bet-hedging. Stable environments are those in climates that are relatively constant and/or predictable, as in tropical climates. Unstable and/or unpredictable environments are characteristic of variable climates, like those in the temperate zone. Stable environments are characterized by species with a K-strategy, while fluctuating environments are characterized by species with an r-strategy. Advocates of r- and K- selection deal with models in which fecundity and mortality schedules fluctuate. Bet-hedging is advocated when fluctuations in these life history traits occur.
The physiological and ecological tolerances and requirements describe the "hardiness" of a species. The more hardy a species is, the greater its ability to adapt to quickly changing environments. "Weed" species are not likely to become endangered or extinct. They are widely distributed and if pollution or intentional destruction by humans eradicates them in one part of the country, other populations will perpetuate the species. If humans alter the rate of change in habitat quality, pollution (or eutrophic) indicator species have less potential to become extinct than do clean water (or oligotrophic) indicator species.
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