Keywords

1 Introduction

Freshwater macroinvertebrate communities constitute a key element for both food web structure and ecosystem health. However, freshwaters are often heavily impacted by anthropogenic activities altering water quality and macroinvertebrate diversity [1]. Water quality represents a big challenge for the future, considering global change and human activity increase, particularly in dry countries such as those located in the Southern Mediterranean Basin.

The goals of this study were: (1) to assess the spatial and seasonal variation of macroinvertebrate assemblages of three streams located in northern Tunisia, and (2) to examine which abiotic factors determine the spatial and temporal structure of these macroinvertebrate assemblages.

2 Materials and Methods

Three pristine sites (P), located in the upstream part of each stream, and three sites altered by anthropogenic activities (A), located in the downstream reach, were selected for this study and sampled seasonally in 2013: Kasseb (ST1-P, ST2-A), El Bey (ST3-P, ST4-A), and Lebna (ST5-P, ST6-A). Salinity (S), dissolved oxygen (DO), and pH were measured with standard portable equipment (WTW, MPP350). The [NO3−], [NH4+], [PO42−], [COD], and [BOD5] were determined in the laboratory. Macroinvertebrates were collected using kick net and Surber sampler, stored in 70% ethanol and transported to the laboratory for the identification of taxa which was made using a binocular stereomicroscope and available keys [2]. The STATICO method [3], was carried out to analyze the two series of data in a spatial–temporal framework, where the spatial or the temporal sampling repetition provides a third dimension to the dataset. Analyses were performed using the ade4 package [4], of R software [5].

3 Results

3.1 Macroinvertebrates and Environmental Parameters

A total of 41 taxa of macroinvertebrates were collected in the 24 samples from 6 sampling sites. While richness tended to be higher in the pristine site, abundance was higher in the altered ones. The taxonomic group with highest taxa richness in pristine sites was Coleoptera (with 10 taxa) followed by Heteroptera (7 taxa). Compared with Coleoptera and Heteroptera, Odonata (3), Ephemeroptera (3), Crustacea (4), and Mollusca (3) were relatively poor. However, in the three altered sites, Annelida were represented by eight taxa, and Diptera by three species. The dominant macroinvertebrate taxa were also different. Thus, the dominant taxa at pristine sites were Ostracoda (15.61%), and Anopheles sp (36.81%) at altered sites. Environmental factors showed spatial differences between sampling sites and temporal differences among seasons (Table 1). The pH values varied from 5.09 to 8.05 (mean = 7.25 ± 0.86). All sites had mean salinity between 0.55 PSU and 1.88 PSU, the lowest in winter (0.46 PSU at site 2) and the highest in summer (2.1 PSU at site 6). Lower mean DO and higher mean [NO3−], [NH4+], [PO42−], [COD], and [BOD5] were found at the three altered sites than at the three pristine sites.

Table 1 Mean and standard deviation values of environmental variables
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3.2 Community Analysis and Relationship with Environmental Parameters

3.2.1 Interstructure

The factor map of the interstructure, with the first principal component explaining 60.3% of the total inertia and the second 16.3%, identified two main groups of seasons (Fig. 1a). This suggested that only the first axis might be chosen for the compromise analysis. The correlation circle showed all the sampling dates displaying the same sign on the first interstructure factor (axis 1), indicating a positive correlation between the corresponding set of matrices (Fig. 1a).

Fig. 1
figure 1

Results of the STATICO method. The four plots are as follows: a The interstructure plot. b Compromise analysis principal axes map (environmental variables). c Compromise analysis principal axes map (aquatic macroinvertebrates taxa). d Typological values of the four tables (cos2 and table weights)

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To build the compromise, the weights of each pair of tables ranged from 0.438 to 0.571 (Fig. 1d). The Spring–Summer seasons were different from the Autumn–Winter seasons. This means that the co-structures between environmental parameters and macroinvertebrate communities were different in Spring–Summer and Autumn–Winter.

3.2.2 Compromise

Out of the eight environmental factors taken into account, only a few were found to be correlated with the structure of macroinvertebrate assemblages (Fig. 1b). The first axis described a positive association with COD, BOD5, [PO42−], and [NH4+], and a negative association with pH and DO. The second axis was mainly positively associated with the salinity but negatively associated with [NO3−]. The correspondence with the macroinvertebrate taxa allowed identifying those taxa more clearly related to the environmental gradients (Fig. 1c). On the first axis, the most abundant taxa (Oligochaeta Tubificidae and Diptera Culicidae) were associated with sites having higher BOD5 and COD, and were opposed to the other taxa, as Ostacodes, associated with sites characterized by higher DO.

4 Discussion

Water chemistry is an important factor in shaping macroinvertebrate assemblages [6]. The spatial patterns of macroinvertebrate assemblages at the six sites were highly persistent along the four seasons. Both the abundance and richness of Diptera and Oligochaeta were positively related to pollution parameters COD and BOD5. The environmental factors which most influenced the macroinvertebrate assemblages also changed with seasons. Furthermore, a specific differentiation is related to upstream sites (unpolluted situation: high oxygen concentration and high pH) occupied mainly by some insect groups (e.g., beetles, bugs, mayflies), and downstream sites (highly polluted situation: high concentrations of ammonium and phosphates, high rate of chemical oxygen demand, and biological oxygen demand), where highly pollutant resistant species as Oligochaeta and the Diptera Anopheles sp. proliferated.

5 Conclusions

Macroinvertebrate assemblages varied both spatially and temporally in response to the abiotic and biotic factors. According to the spatial distribution, macroinvertebrates could be classified into two groups based on their sensitivity to pollution, being Diptera, and noninsect taxa such as Annelids (Oligochaetes) which are mostly associated with pollution parameters (COD and BOD5).