Some indices try to integrate the whole or most of the environmental information in a single value. Nevertheless, these indicators are rarely used in a generalized way because they have usually been developed to be applied in a particular system or area, which turns them dependent on the type of habitat and seasonality. Moreover, their application usually requires a large amount of data of different nature. The most commonly used indices are listed in the following:
Pollution coefficient. This index, proposed in 1982 by J. Satsmadjis, takes into account the relationship between the infauna and sediment structure. The calculation involves the following assumptions: (1) the number of species increases linearly with the number of individual animals; (2) sediment structure can be represented by a single value (the sand equivalent, s') based on the relative percentages of sand and silt; and (3) faunal abundance is related to s and depth. These following integrated equations are used:
S' = s + t/(5 + 0.2s) i0(-0.0187s'2 + 2.63s' -4)(2.20-0.0166h) g' = i / (0.0124i + 1.63)
where P is the coefficient ofpollution; S9 the sand equivalent; S the percent sand; t the percent silt; i0 the theoretical number of individuals; i the number of individuals; h the station depth; g9 the theoretical number of species; and g the number of species.
Coefficients of 1.5-2.0, 2.0-3.0, 3.0-4.0, and 4.0-8.0 are assumed to indicate slight, moderate, heavy, and very heavy pollution, respectively.
Benthic index of environmental condition. In this index, proposed by Engle in 1994, the expected diversity is calculated throughout Shannon-Wiener index adjusted for salinity:
Benthic index of environmental condition
= (2.3841 x proportion of expected diversity)
+ (-1.6728 x proportion of total abundance of tubifids) + (0.6683 x proportion of total abundance of bivalves)
Expected diversity = 0.75411 + (0.00078 x salinity) + (0.00157 x salinity2) + (-0.00030 x salinity3)
This index was developed for estuarine macrobenthos in the Gulf of Mexico in order to discriminate between areas with degraded environmental conditions and areas with nondegraded or reference conditions. The final step in its development involved the estimation of discriminating scores for all sample sites and normalizing calculated scores to a scale of 0-10, setting the break point between degraded and nondegraded reference sites at 4.1. Therefore, values lower than 4.1 indicate degraded conditions, values higher than 6.1 indicate nondegraded situations, and values between 6.1 and 4.1 reveal moderate disturbance.
Trophic index (TRIX). This index, introduced by R. A. Wollenweider in 1998, is based on chlorophyll a, oxygen saturation, mineral nitrogen, and total phosphorus. The parameters, to be included in this trophic index, were selected as directly related to eutrophication phenomena and the index aims at characterizing the trophic state of coastal waters. It is expressed by the following equation:
n in which k = 10 (scaling the result between 0 and 10), n = 4 (number of variables integrated), Mi is the measured value of variable i, Ui the upper limit of variable i, and Li the lower limit of value i.
The resulting TRIX values are dependent on the upper and the lower limits chosen and indicate how close the current state of a system is to the natural state. However, the comparison of TRIX values obtained for different areas becomes more difficult. In general, when a wide, more general range is used for the limits, TRIX values for different areas are more easily compared to each other.
Ecological functional index (EQI). This index, used by E. A. Fano in 2003, is based on the characteristics of the primary producers and benthic faunal communities and has been designed to assess the ecological quality of the coastal lagoons. The parameters (macrofaunal abundance; number of taxa; taxonomic diversity; functional diversity; macrofaunal biomass; phytoplankton biomass; and macro-phytal biomass) are taken into account. Each one of these attributes, which are expressed by heterogeneous units, is then transformed onto a dimensionless quality scale ranging from 0 to 100, simply by assigning 100 to the highest value, and by normalizing all the other values to 100. Once all attributes are expressed by means of this scale, they are combined to obtain the integrated index, whose maximum theoretical value varies from 700 to 800, depending on whether macroalgae are present in a particular habitat. These values would correspond to the optimum condition of the index, irrespective of the units and magnitudes used to measure the different individual attributes. Obviously, the closer the actual values are to, let us say, 800, the better the condition of the environment.
EQI also allows comparisons to be made between sites from different lagoons (nEQJ). Data sets from the different lagoons are merged into a worksheet so that the value of each attribute can be rescaled, using the same quality scale of 0-100 on the complete data set. Finally, scores are summed and divided by the number of attributes measured in each different lagoon. In this way, the use of EQI can derive a series of continuous values, from 0 to 800 (nEQI: from 0 to 100). The result obtained is a functional classification of the sites within a lagoon or between different lagoons.
Up to now, this index has only been applied in a few Mediterranean coastal lagoons.
B-IBI.The following metrics are used to estimate the B-IBI values, used by S. B. Weisberg in 1997: Shannon-Wiener index; total species abundance; total species biomass; percent abundance of pollution-indicative taxa; percent abundance of pollution-sensitive taxa; percent biomass of pollution-indicative taxa; percent biomass of pollutionsensitive taxa; percent abundance of carnivores and omnivores; and percent abundance of deep-deposit feeders.
The scoring of metrics to estimate B-IBI is carried out by comparing the value of a metric from a given sample of unknown sediment quality to thresholds established from reference data distributions (see Table 5). This index was developed to establish ecologic status of Chesapeake Bay, and therefore it is habitat type and season specific, advisable to use in spring only.
This index was modified by Van Dolah in 1999 and it was calculated using the average score of the following metrics: mean abundance; mean number of taxa; percentage of abundance of the top two numerical dominants; and percentage abundance of pollution sensitive taxa.
Biotic integrity for fishes. A fish-based index of biotic integrity (IBI) was developed by M. McGinty and C. Linder in 1997 for tidal fish communities of several small tributaries of the Chesapeake Bay. Nine metrics (number of species; number of species comprising 90% of the catch; number of species in the bottom trawl; proportion of carnivores; proportion of planktivores; proportion of benthivores; number of estuarine fish; number of anadromous fish; and total fish with Atlantic menhaden removed) are used to calculate the index taking into account species richness, trophic structure, and abundance.
The quantification of the different metrics utilized to estimate the index is carried out by comparing the value of a metric from the sample of unknown water quality to thresholds established from reference data distributions.
Table 5 Thresholds used to score each metric of the B-IBI
Tidal freshwater Shannon-Wiener Abundance (m-2) Biomass (gm-2)
Abundance pollution indicative taxa (%)
Oligohaline Shannon-Wiener Abundance (m-2) Biomass (gm-2)
Abundance pollution indicative taxa (%) Abundance sensitive taxa (%)
Low mesohaline Shannon-Wiener Abundance (m-2) Biomass (gm-2)
Abundance pollution indicative taxa (%)
Biomass pollution sensitive taxa (%)
Biomass >5 cm below sediment-water interface (%)
High mesohaline sand Shannon-Wiener Abundance (m-2) Biomass (gm-2)
Abundance pollution indicative taxa (%) Abundance sensitive taxa (%) Abundance carnivores and omnivores (%)
High mesohaline mud Shannon-Wiener Abundance (m-2)
Biomass pollution indicative taxa (%) Biomass pollution-sensitive taxa (%) Abundance carnivores and omnivores (%) Biomass >5 cm below sediment-water interface (%)
Polyhaline sand Shannon-Wiener Abundance (m-2) Biomass (gm-2)
Abundance pollution indicative taxa (%) Biomass pollution-sensitive taxa (%) Abundance deep-deposit feeders (%)
Polyhaline mud Shannon-Wiener Abundance (m-2) Biomass (gm-2)
Biomass pollution indicative taxa (%) Biomass pollution-sensitive taxa (%) Abundance carnivores and omnivores (%) Taxa >5cm below sediment-water interface (%)
500-1000 or >4000-10000 0.25-0.5 or >3-50 25-75
500-1500 or >3000-8000 0.5-3 or >25-60 25-75 10-40
500-1500 or >2500-6000 1-5 or >10-30 10-20 40-80 10-80
1000-1500 or >3000-5000
1000-1500 or >2500-5000
1500-3000 or >5000-8000
1000-1500 or >3000-8000
<500 or >8000 <0.5 or >60 >75 <10
<1000 or >5000
<1500 or >8000
<1000 or >8000
This index is extensively treated in Biological (health) between the potential and actual fish assemblages
Integrity. and is calculated using the formula:
Fish health index (FH/). This index, proposed by J. A. Cooper in 1993, provides a measure of the similarity FHI = 10(/)[ln(P)/ln(Pm„x)]
where J is the number of species in the system divided by the number of species in the reference community; P is the potential species richness (number of species) of each reference community; and Pmax is the maximum potential species richness from all the reference communities. The index ranges from 0 (poor) to 10 (good).
The FHI was used to assess the state of South Africa's estuaries, and although it has proved to be a useful tool in condensing information on estuarine fish assemblages into a single numerical value, it is based only on presence/ absence data, and consequently does not take into account the relative proportions of the various species present.
Estuarine ecological index (EBI). This index, introduced by L. A. Deegan in 1993, includes the following metrics: total number of species; dominance; fish abundance; number of nursery; number of estuarine spawning species; number of resident species; proportion of benthic associated species; and proportion of abnormal or diseased fishes. The expected trends of the selected metrics as a response anthropogenic stress (direction of change) are listed in the Table 6.
The usefulness of this index requires that it reflects not only the current status of fish communities but also its applicability over a wide range of estuaries, although this is not entirely achieved.
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