3.3 Benthic macroinvertebrates as water quality
⎯ They are relatively sedentary and therefore representative of the area where they are collected
⎯ Samplings can be done with material low cost
⎯ They have high taxonomic diversity and high diversity also in the types of food and in the life cycles
As mentioned above, this community presents a great variety of tolerance ranges to disturbances: there are species that, when disturbed, can disappear or reduce their abundance while the most tolerant species may even increase their densities when others have already disappeared. This property has allowed the development of biotic indices, based on the tolerance of the different taxa of of macroinvertebrates to pollution or human disturbances 17. For instance, among the less tolerant groups are the larvae belonging to the orders Ephemeroptera, Trichoptera and Plecoptera and larvae or adults of some families of Coleoptera: that means they can show a high sensitivity to pollution and degradation of aquatic ecosystems.
One of the issues that have been founded in this study was the low presence of the most sensitive families mentioned above. In wetlands and in particular in rice paddies, due to their high eutrophication, EPT (Ephemeroptera, Plecoptera and Trichoptera) almost do not appeared; instead the dominant groups encountered were families most tolerant to pollution and human disturbances.
Biological indices in wetlands, associated with macroinvertebrates, are not yet widely diffused. In this study the following biotic indices have been applied: Shannon and Simpson diversity indices, QAELS (Índex de Qualitat de l’Aigua d’Ecosistemes Lenítics Soms) and IMN (Índice del Modo de Nutrición).
17 Alonso, A. y Camargo, J., (2005). Evaluating the effectiveness of five mineral artificial substrates for the sampling of benthic macroinvertebrates. Journal of Freshwater Ecology 20: 311-320.
3.3.1 Shannon-Wiener Index
Diversity indices use the community structure in term of its diversity to estimate the water quality: it is a valid alternative to the study of “indicator species” presence, especially in this study, conducted in such a eutrophic ecosystem. Generally, good water quality is synonym with a diversified community. The diversity indices which are widely used are the Shannon-Weaver Index and the Simpson Index; they incorporate, in a single value, richness and abundance of the species.
The Shannon-Wiener Index H’ (1949) 18, or Shannon Diversity Index (SDI), describes the diversity as a measure of uncertainty to predict which species would belong to an individual chosen at random from a sample of S species and N individuals. Therefor, H’=0 when the sample contains only one species and H’ will be massive when all the species S are represented by the same number of individuals ni (where “i” is the species considered), that means that the community has a perfectly equitable abundance distribution.
𝐻. = − (𝑝1 × 𝑙𝑜𝑔6𝑝1)
8
19:
where pi is the proportional abundance of the i-th species, representing the probability that an individual of the species i is present in the sample (being then the sum of pi equal to 1).
𝑝1 = 𝑛1 𝑁
This index has been widely used since it was developed and its results are considered as good indicators of biodiversity.
18 Developed by Claude Shannon in 1949.
Table 3.2: Relations between Shannon Diversity Index and Pollution level
SDI Diversity Level Pollution Level
3.0-4.5 High Slight
2.0-3.0 Moderate Light
1.0-2.0 Less Moderate
0.0-1.0 Very less Heavy pollution
3.3.2 Simpson index
The Simpson Index SDi 19 indicates the probability of finding two individuals of different species in two successive random “extractions without replenishment”; it gives a higher weight to abundant species underestimating rare species, taking values between 0 (low diversity) up to a maximum of [1-1/S].
𝐷<1 = 𝑝16
<
19:
Simpson index is expressed as a measure of dominance and it is related to relative abundance and species richness. However, the dominance and the diversity are proprieties opposed to each other. An appropriate transformation to obtain an index positively correlated with diversity is needed, since diversity the most common form in ecology:
𝑆>1 = 1 − 𝐷<1 = 1 − 𝑝16
19 Developed by Edward H. Simpson in 1949
3.3.3 QAELS
Biological quality indicator based on benthic macroinvertebrates are less developed in lakes and wetlands.
The QAELS index (Índex de Qualitat de l’Aigua d’Ecosistemes Lenítics Soms) has been elaborated to determining the ecologic state of shallow lagoon systems in Cataluña 20. It combines aspects of taxonomic richness and abundance. It responds to the following formula:
𝑄𝐴𝐸𝐿𝑆 = 𝐴𝐶𝐶𝑂 + 1 × 𝐿𝑜𝑔(𝑅𝐼𝐶 + 1)
where ACCO is an index based on the abundance of cladocereans, copepods and ostracods while RIC is an index based on the insects and crustaceous richness.
In detail,
𝐴𝐶𝐶𝑂 = 𝑘1 × 𝑛1
F
19:
i : indicator taxon
j : number of indicator taxa
ni : relative abundance of ith taxon, 𝑛1 = GH
GIJI
Ntot : sum of abundance of all indicators taxa ki : quality value of ith taxon
ki values are shown in figure 7.2 (Appendix B). For this study, the option “temporary freshwater wetland” has been chosen, because fields do not remain flooded all the time.
Moreover, the RIC index is based on the insect and crustacean richness:
20 Agència Catalana de l’Aigua. (2004). Caracterització, regionalització i elaboració d’eines d’establiment de l’estat ecològic de les zones humides de Catalunya.
RIC = N° of crustacean genera + N° of Heteroptera and Coleoptera adult genera + N° of larval and insect families
Considering microcrustaceans can be very useful for the determination of the ecological status of lagoons and wetlands: in addition to be good indicators they leave identifiable remains in sediments.
The obtained value corresponds to a certain quality class (Table 3.5).
Table 3.3: classification of water quality according to QAELS value (ACA,2004)
QAELS Water Quality
>8.0 Very Good
6.0-8.0 Good
4.0-6.0 Moderate
2.0-4.0 Poor
>2.0 Very Poor
3.3.4 IMN
The IMN (Índice del Modo de Nutrición) is based on the feeding mode of aquatic macroinvertebrates and their relationship to the trophic structure of the community.
The heterogeneity of organisms is directly related to the diversity of the sampling point:
therefore, a heterogeneous environment would correspond to an environment of high diversity, in such a way that, if the environment is altered, a decrease in the diversity in the types of feeding of macroinvertebrates would be obtained. The best condition of the ecosystem is caused by the presence of all trophic groups in proportions ranging from 5 % to 15 %.
This index is applicable to all aquatic systems from rivers to lagoons and wetlands ; 21 for this reason it has been used in this study, although it is not yet widespread.
To applicate the IMN, an identification at family level is needed, following the classification in nutritional groupings proposed by Tachet et al. 22 (Figure 7.3, Appendix B), which are created in function of the mandibular apparatus or in the form of macroinvertebrates’ food catching .
Once the trophic affinity has been assigned to each macroinvertebrate and the relative percentage of each nutritional groups, this index considers positive the presence of a high number of nutritional groups (and considers that these nutritional groups oscillate between 0 and 40), increasing the positive score as the percentage of them increases. At the same time, the absence of certain groups or a proportion of them greater than 40 would imply negative scores, which increase as the percentage and the number of groups not existing increase. This process results in a positive and a negative score. By subtracting the negative from the positive, the final IMN value is obtained.
As for the other indices mentioned above, the value obtained corresponds to a certain quality class:
Table 3.4: Ranges of food web and trophic quality accorded to de IMN value
IMN Class Food Web Diversity Level Interpretation
>70 I Very High Unpolluted
55-69 II High Clean but slightly impacted
40-54 III Moderate Moderately impacted
20-39 IV Less Impacted
0-19 V Very Less Heavily impacted
21 Rueda, J., López, C. y Hernández, R., (2005). Evaluación de la calidad de los ecosistemas acuáticos a partir del modo de nutrición (IMN) de sus macroinvertebrados. Didáctica de las ciencias experimentales y sociales.
22 Tachet, H., Bournaud, M. y Richoux. 1987. Introduction à l‟étude des invertébrés des eaux douces, systématique élémentaire et aperçu écologique. Université de Lyon. Association Française de Limnologie. 155 pp.