Indicators Based on the Abundance of Selected Species

These biological indicators cover the presence or absence of selected species. Indicator species have to be selected in order to be representative for a certain phenomenon or to be sensitive to distinct changes. Hence, their appearance and dominance is associated with a certain environmental situation. The usage of this kind of indices implies on the one hand a certain level of uncertainty; on the other, the results can directly be linked to biological phenomena which is useful for proper management of resources. In the following, some examples will be outlined.

Saprobic classification

The saprobe system is a collection of organisms, that gives information about the degree of water pollution. The different pollution intensities (saprogenic stages) are related to certain indicator organisms (e.g., bacteria, fungi, algae, amoeba, mussels, worms, insect larvae, and fishes) and range from polysaprobic (very highly polluted), a-mesosaprobic (highly polluted), ^-mesosaprobic (medium polluted) to oligosaprobic (rather clean and clear water).

Bellan's pollution index

Bellan considers aquatic species like Platynereis dumerilii, Theosthema oerstedi, Cirratulus cirratus, and Dodecaria concharum as water pollution indicators. Clear water is indicated by species like Syllis gracillis or Typosyllis prolifera. The equation of the Bellan's pollution index can be formulated as follows:

IP = y ^ dominance of pollution indicator species/dominance of pollution (clear) water indicators.

If the index value is bigger than 1, a pollution disturbance in the community is indicated.

AZTI Marine Biotic Index

For this indicator the soft bottom macrofauna is distinguished into five groups in accordance to their sensitivity to increasing stress:

1. species that are very sensitive to organic enrichment and eutrophication and that are only present under unpolluted conditions;

2. species that are indifferent to organic enrichment, occur always in low densities and show no significant variations over time;

3. species tolerant to excess organic matter enrichment (these species can also be found under normal conditions but usually their populations are supported by organic enrichment);

4. second-order opportunist species, very often small polychaetes; and

5. first-order opportunist species (deposit feeders).

AMBI (biotic coefficient) = {(0 x %I) + (1.5 x % II) + (3 x % III) + (4.5 x % IV) + (6 x % V)}/100

The AZTI marine biotic index (AMBI) results can be classified as: normal (AMBI coefficient between 0.0 and 1.2), slightly polluted (1.2 and 3.2), moderately polluted (3.2 and 5.0), highly polluted (5.0 and 6.0), or very highly polluted (6.0 and 7.0). The AMBI has been considered useful in terms of the application of the European Water Framework Directive to coastal ecosystems and estuaries.

Bentix

The Bentix is based on the AMBI but has only three groups in order to avoid errors in the grouping of species and to make the calculation of the index easier:

• Group I. Species generally sensitive to disturbances.

• Group II. Species that are tolerant to stress or disturbance. Populations may respond to organic enrichment or other source of pollution.

• Group III. First-order opportunistic species (pioneer, colonizers or species which are tolerant to hypoxia).

The Bentix results can be classified as: normal (4.5-6.0), slightly polluted (3.5-4.5), moderately polluted (2.5-3.5), highly polluted (2.0-2.5), or very highly polluted (Bentix = 0).

Macrofauna monitoring index

This index comprises 12 indicator species. Each of them is assigned a score, based primarily on the ratio of its abundance in control versus impacted samples. The index value is the average score of those indicator species which are present in the sample.

Benthic response index

The Benthic response index (BRI) is calculated as abundance weighted average pollution tolerance of species that occur in a sample. This is similar to the weighted average approach used in gradient analysis.

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