The present study analyses the spatio-temporal variability of the macrozoobenthos from the intertidal zone of the Kneiss Islands (Gulf of Gabès, Tunisia). Samples were collected once from 34 stations, while seasonal variations were studied by sampling four times at 12 stations over the period 2013-2014. A total of 159 macrobenthos taxa associated with intertidal Zostera noltei beds are identified from the 34 stations, with a taxonomic dominance of crustaceans (32%), molluscs (29%) and annelids (27%). Abundance varies from 9244 to 36,844 ind·m -2 with a mean value of 14,346 ind·m -2 . Analysis of the trophic structure shows that the majority of stations are strongly represented by carnivores (41%), followed by the non-selective deposit feeders (16%). Cluster analysis and multidimensional scaling allow identification of three main benthic assemblages based on species abundance, corresponding to different sediment types and organic matter contents. The seasonal variability in abundance, diversity and community structure is mainly due to spring and summer recruitment. The biotic indices ( i.e. AMBI, BO2A and BENTIX) show that the intertidal area of Kneiss Islands exhibits a good ecological status.
Owing to their position at the interface of land and sea, shallow waters and coastal areas are generally highly productive and ecologically important [
In Tunisia, very few studies have been carried out on intertidal benthic macrofauna. Nevertheless, the macrobenthos of the Gulf of Gabès has been relatively more studied [
Moreover, the Kneiss Islands, located in the Gulf of Gabès (central Mediterranean Sea), represent a site of international interest in terms of their ornithological diversity (Important Bird area, SPAMI, and Ramsar site), with over 330,000 water birds counted on this wetland in spring [
Likewise, the marine ecosystems of the Kneiss Islands in the Gulf of Gabès are subject to the loss and degradation of biodiversity caused by bottom trawling [
Mosbahi et al. (2015) presented a first description of the intertidal macrozoobenthic communities in a small part of the Kneiss Islands. By extending this previous research, the present study investigates the spatial and seasonal structure of the macrobenthic communities of a large area around the Kneiss Islands over a 2-year- cycle. The main aims are: 1) to investigate the structural diversity of the benthic macrofauna based on taxonomic, trophic groups and the biodiversity parameters of the benthic macrofauna; 2) to identify the role of the main environmental factors that determine the benthic community structure and functioning; and 3) to provide a reliable assessment of the general ecological status of the macrobenthos.
Located in the north-western part of the Gulf of Gabès, between latitudes 34˚10'N - 34˚30'N and longitudes 10˚E - 10˚30'E, the intertidal zone around the Kneiss Islands extends over an area of 220 km2 (
Due to the high biodiversity of benthic communities, the foreshore of the Kneiss Islands represents the largest area in terms of water bird conservation in the Mediterranean zone (Important Bird Area in 2003) and has been designated a Ramsar site since 2007. Moreover, the tidal flats of Kneiss Islands are colonized by seagrass meadows composed of Zostera noltei Hornemann [
Samples of the macrobenthic macrofauna were collected at low tide using a corer with a sampling area of 0.0225 m2. Five replicates were carried out at each station, with four samples for biological analysis covering a total surface-area of 0.09 m2, and one sample for sediment analysis. Sampling consisted of collecting the topmost 20 cm of the sediment, being carried out at 34 stations in April 2014, in areas corresponding to fully extended Zostera noltei seagrass beds (
Sediment was sieved through a 1-mm mesh; the remaining fraction was fixed in 4% buffered formalin and stained with Rose Bengale. In the laboratory, macrofauna was sorted, identified when possible to the species level under the stereomicroscope, and counted. Species number corresponds to the total number of species recorded in 0.09 m2, and abundance to the number of individuals per 1 m2.
The topmost 3-cm sediment layer was also sampled in each replicate for granulometric analysis (median grain- size). Median grain size was determined after sieving the weighed and dried sediment through a wet column of sieves with decreasing apertures (1000, 500, 250, 125 and 63 μm). Sediment samples were then dried at 80˚C to constant weight and ground to a fine powder. The organic matter (OM) content was determined on the powder samples by “loss on ignition” at 450˚C for 4 hrs. Heavy metal contents (Zn, Pb, Cd, Fe and P) were estimated after digesting the powder sample in aqua regia (HCl−,
The original data analyzed here consists of a “stations × species” matrix (34 stations × 125 species), which was obtained after removing rare species (29 species). Species were considered rare when they occurred at less than three stations. Abundances were square-root transformed prior to analysis.
Benthic macrofauna communities are defined quantitatively by selected descriptors which reflect the numerical importance of the species. Specific richness S (number of species in each station), abundance A (ind・m−2), Shannon-Weaver’s diversity index H’ [
Identified species are classified here into trophic groups according to [
Regarding the seasonal parameters (
Multivariate analysis is performed to study spatial variability of the macrozoobenthic communities associated with Zostera (Zosterella) noltei seagrass. A square root transformation is applied to the abundance matrix (data for each station are pooled prior to undertaking further analyses), before calculating the Bray-Curtis similarities using the statistical package PRIMER® 6.0 [
Stations | Long E | Lat N | Sed | S | A | J’ | H’ | AMBI | BO2A | BENTIX | T˚C | Sal | pH | OM |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
IK1 | 32.621306˚ | 38.04531˚ | FS | 27 | 17,600 | 0.87 | 4.14 | 1.18 | 0.003 | 4.65 | 23.4 | 39.2 | 8.69 | 1.59 |
IK2 | 32.621554˚ | 38.03952˚ | MS | 24 | 12,977 | 0.80 | 3.67 | 1.77 | 0.037 | 4.04 | 23.5 | 39.3 | 8.69 | 2.43 |
IK3 | 32.621609˚ | 38.03262˚ | FS | 29 | 13,377 | 0.90 | 4.37 | 1.57 | 0.001 | 4.97 | 23.8 | 39.5 | 8.67 | 1.34 |
IK4 | 32.621499˚ | 38.02711˚ | FS | 22 | 12,133 | 0.84 | 3.76 | 0.46 | 0.001 | 5.52 | 23.8 | 39.7 | 8.67 | 1.32 |
IK5 | 32.620837˚ | 38.02380˚ | FS | 39 | 22,444 | 0.86 | 4.57 | 1.54 | 0.047 | 4.42 | 23.9 | 39.3 | 8.67 | 2.28 |
IK6 | 32.620037˚ | 38.02297˚ | FS | 36 | 14,888 | 0.87 | 4.54 | 1.93 | 0.064 | 4.22 | 23.2 | 39.6 | 8.64 | 3.23 |
IK7 | 32.619375˚ | 38.02904˚ | FS | 35 | 36,844 | 0.68 | 3.53 | 1.88 | 0.001 | 3.97 | 22.9 | 39.2 | 8.64 | 2.03 |
IK8 | 32.619209˚ | 38.03538˚ | FS | 25 | 11,422 | 0.90 | 4.18 | 1.75 | 0.067 | 4.08 | 22.8 | 39.5 | 8.65 | 2.52 |
IK9 | 32.618906˚ | 38.03980˚ | FS | 28 | 13,111 | 0.88 | 4.23 | 1.20 | 0.036 | 5.04 | 22.8 | 39.2 | 8.59 | 2.85 |
IK10 | 32.618989˚ | 38.04807˚ | FS | 31 | 11,733 | 0.88 | 4.37 | 1.67 | 0.012 | 4.39 | 22.9 | 39.2 | 8.65 | 1.43 |
IK11 | 32.619816˚ | 38.05055˚ | MS | 22 | 12,755 | 0.87 | 3.91 | 2.18 | 0.067 | 3.58 | 22.7 | 39.2 | 8.71 | 1.84 |
IK12 | 32.620478˚ | 38.05387˚ | FS | 34 | 14,622 | 0.89 | 4.53 | 1.35 | 0.046 | 4.71 | 23.2 | 39.3 | 8.64 | 3.66 |
IK13 | 32.617913˚ | 38.01249˚ | MS | 30 | 10,355 | 0.86 | 4.22 | 1.43 | 0.045 | 4.71 | 23.8 | 39.5 | 8.05 | 2.54 |
IK14 | 32.618630˚ | 38.00393˚ | MS | 33 | 9800 | 0.87 | 4.40 | 1.52 | 0.006 | 4.55 | 22.8 | 39.4 | 8.26 | 1.85 |
IK15 | 32.619347˚ | 37.99593˚ | MS | 33 | 10,622 | 0.81 | 4.08 | 1.45 | 0.033 | 4.98 | 22.8 | 39.5 | 8.36 | 4.32 |
IK16 | 32.617554˚ | 38.00449˚ | MS | 37 | 10,711 | 0.88 | 4.60 | 1.29 | 0.050 | 5.24 | 23.1 | 39.5 | 8.40 | 3.25 |
IK17 | 32.618189˚ | 37.99566˚ | MS | 32 | 9511 | 0.88 | 4.40 | 1.92 | 0.050 | 4.24 | 23.3 | 39.5 | 8.49 | 2.92 |
IK18 | 32.618851˚ | 37.98904˚ | MS | 30 | 10,222 | 0.92 | 4.53 | 1.43 | 0.042 | 4.67 | 23.2 | 39.5 | 8.54 | 3.94 |
IK19 | 32.616313˚ | 38.00780˚ | MS | 38 | 11,800 | 0.85 | 4.49 | 1.83 | 0.044 | 4.17 | 23.1 | 39.5 | 8.55 | 1.75 |
IK20 | 32.617278˚ | 37.99207˚ | MS | 38 | 10,000 | 0.90 | 4.76 | 1.23 | 0.041 | 5.14 | 23.3 | 39.5 | 8.53 | 3.61 |
IK21 | 32.618244˚ | 37.97993˚ | MS | 36 | 9244 | 0.82 | 4.25 | 1.77 | 0.008 | 4.20 | 23.4 | 39.5 | 8.72 | 2.24 |
IK22 | 32.615485˚ | 38.00421˚ | MS | 38 | 12,222 | 0.80 | 4.23 | 1.41 | 0.035 | 5.13 | 23.7 | 39.5 | 8.64 | 4.38 |
IK23 | 32.616727˚ | 37.98518˚ | MS | 42 | 10,977 | 0.79 | 4.26 | 1.80 | 0.047 | 4.75 | 22.6 | 39.6 | 8.53 | 3.31 |
IK24 | 32.617692˚ | 37.97414˚ | MS | 36 | 9422 | 0.85 | 4.40 | 1.19 | 0.009 | 5.09 | 23.5 | 39.5 | 8.56 | 3.82 |
IK25 | 32.620398˚ | 38.06897˚ | Mud | 41 | 15,600 | 0.84 | 4.53 | 1.66 | 0.030 | 4.45 | 23.8 | 40 | 8.66 | 11.75 |
IK26 | 32.619169˚ | 38.07494˚ | Mud | 59 | 15,288 | 0.88 | 5.21 | 1.59 | 0.019 | 4.62 | 23.6 | 40 | 8.79 | 17.88 |
IK27 | 32.618607˚ | 38.07600˚ | Mud | 56 | 20,266 | 0.79 | 4.62 | 1.93 | 0.030 | 4.21 | 22.9 | 39.5 | 8.81 | 12.51 |
IK28 | 32.617939˚ | 38.07389˚ | Mud | 50 | 14,911 | 0.89 | 5.03 | 1.47 | 0.022 | 4.61 | 23.1 | 39.3 | 8.79 | 19.12 |
IK29 | 32.617869˚ | 38.06651˚ | Mud | 55 | 19,777 | 0.82 | 4.76 | 1.93 | 0.014 | 4.18 | 22.7 | 39.1 | 8.72 | 12.48 |
IK30 | 32.617809˚ | 38.06019˚ | Mud | 46 | 15,511 | 0.87 | 4.80 | 1.60 | 0.025 | 4.64 | 22.8 | 39.2 | 8.95 | 17.15 |
IK31 | 32.617834˚ | 38.05597˚ | Mud | 63 | 19,400 | 0.84 | 5.02 | 1.50 | 0.010 | 4.58 | 23.6 | 40.1 | 8.87 | 13.85 |
IK32 | 32.617236˚ | 38.04860˚ | Mud | 55 | 14,533 | 0.89 | 5.18 | 1.64 | 0.014 | 4.33 | 23.7 | 40.2 | 8.87 | 9.23 |
IK33 | 32.616991˚ | 38.03384˚ | Mud | 57 | 18,022 | 0.83 | 4.86 | 1.70 | 0.017 | 4.23 | 24.1 | 40.5 | 8.53 | 17.75 |
IK34 | 32.616639˚ | 38.02401˚ | Mud | 49 | 15,688 | 0.83 | 4.66 | 1.55 | 0.016 | 4.69 | 24.4 | 39.4 | 8.85 | 13.13 |
Characteristics | Stations | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
IK1 | IK2 | IK3 | IK4 | IK5 | IK6 | IK7 | IK8 | IK9 | IK10 | IK11 | IK12 | ||
T (˚C) | A | 27.2 | 26.6 | 26.7 | 26.4 | 26.8 | 26.3 | 26.5 | 26.7 | 26.9 | 26.6 | 26.4 | 26.2 |
W | 13.0 | 13.3 | 13.5 | 13.9 | 13.2 | 13.1 | 13.5 | 12.3 | 13.0 | 13.3 | 13.5 | 13.1 | |
S | 29.7 | 30.3 | 29.6 | 30.0 | 30.0 | 31.0 | 30.3 | 30.2 | 29.7 | 30.0 | 30.0 | 29.9 | |
Sal | A | 41.3 | 40.7 | 40.4 | 41.0 | 40.5 | 40.9 | 39.7 | 41.8 | 41.4 | 41.1 | 41.0 | 40.5 |
W | 38.9 | 38.2 | 38.3 | 39.0 | 38.3 | 38.8 | 39.2 | 39.1 | 38.9 | 38.3 | 38.5 | 38.3 | |
S | 39.5 | 39.5 | 40.1 | 39.9 | 39.9 | 40.0 | 39.9 | 40.3 | 40.7 | 39.9 | 39.7 | 39.3 | |
pH | A | 8.74 | 8.78 | 8.85 | 8.10 | 8.94 | 8.94 | 8.93 | 8.92 | 8.99 | 8.91 | 8.12 | 8.00 |
W | 8.05 | 7.84 | 8.20 | 7.81 | 8.30 | 7.50 | 8.00 | 7.91 | 7.92 | 8.10 | 8.23 | 8.40 | |
S | 8.34 | 8.47 | 8.75 | 8.46 | 8.52 | 8.85 | 8.20 | 8.78 | 8.36 | 8.55 | 8.71 | 8.66 | |
OM (%) | A | 1.70 | 2.21 | 2.01 | 1.86 | 2.61 | 4.02 | 2.42 | 2.19 | 2.90 | 1.69 | 2.48 | 5.28 |
W | 1.62 | 1.98 | 1.79 | 1.93 | 2.11 | 3.68 | 2.68 | 1.98 | 2.64 | 1.82 | 2.18 | 4.88 | |
S | 1.89 | 2.56 | 1.58 | 2.01 | 3.12 | 3.12 | 2.71 | 2.42 | 2.86 | 2.1 | 2.10 | 4.12 | |
Hm (μg∙g−1) | Zn | 546.4 | 465.25 | 638.75 | 529.75 | 543.25 | 655.5 | 181.49 | 557 | 266.71 | 565.00 | 422.38 | 388.42 |
Pb | 18.76 | 31.53 | 39.65 | 34.23 | 33.76 | 37.75 | 31.16 | 36.52 | 27.12 | 44.48 | 32.82 | 36.98 | |
Fe | 129.87 | 138.6 | 154.2 | 558.9 | 561.2 | 770.1 | 515.4 | 438.7 | 269.1 | 339.9 | 386.24 | 288.60 | |
Cd | 0.186 | .912 | 0.737 | 0.527 | 0.496 | 0.629 | 0.539 | 0.417 | 0.480 | 0.464 | 0.682 | 0.583 | |
P | 478.35 | 528.31 | 311.42 | 548.12 | 462.32 | 386.23 | 386.12 | 498.61 | 792.01 | 687.42 | 724.32 | 698.43 |
The granulometric analyses reveal that there are three main types of sediment: medium sand, fine sand, and mud (
For the physical and chemical parameters, only one measurement was carried out for the whole set of 34 sampled stations in April 2014 (
ANOVA shows that temperature, salinity and pH differ significantly between seasons, with temperatures varying between 12.3˚C in winter (IK8) and 31˚C in summer (station 6), salinity between 38.2 in winter (IK2) and 41.8 in autumn (IK5), pH between 7.84 (IK2) in winter and 8.99 in autumn (IK9) and organic matter between 1.32 (IK4) and 5.28% (IK12). The Tukey test reveals that the temperature and salinity differ significantly between the four seasons, whereas the pH differs only in winter.
As regards the heavy metals, Zn varies between 181.49 and 638.75 μg∙g−1, Pb between 18.76 and 44.42 μg∙g−1, Fe between 129.87 and 770.10 μg∙g−1, while Cd levels do not exceed 1 μg∙g−1. Phosphate contents fluctuate between 311.42 (IK3) and 792.01 μg∙g−1 (IK11) (
The taxonomic identification of the collected invertebrates produced a list of 159 taxa belonging to six zoological groups. Crustaceans are dominant (32% of the total number of taxa), followed by molluscs (29%) and annelid polychaetes (27%). The three other phyla identified (echinoderms, cnidarians and tunicates) account for only 12% of the total number of taxa. The faunistic parameters show a wide range of variability, from 9244 to 36,844 ind∙m−2 for abundance (with a mean abundance of 14,346 ± S.D. 3540 ind∙m−2), 22 to 63 taxa for specific richness, 0.68 to 0.90 for evenness and 3.53 to 5.18 bits./ind. for the Shannon index (
For the 12 stations monitored seasonally over one year (
AMBI, BO2A and BENTIX indices are consistent with the results classifying all stations as having good ecological status (
The dendrogram and MDS ordination (
SIMPER analysis (
Group | I | II | III |
---|---|---|---|
Group mean similarity (%) | 36.11 | 40.85 | 56.90 |
Main species (the % contribution to the similarity within each group is given for each species) | Melinna palmate: 47.70 Euclymene lumbricoides: 44.96 Bittium reticulatum: 42.15 Arenicola marina: 39.19 Euclymene oerstedi: 35.25 Loripes lucinalis: 31.05 Ruditapes decussates: 26.05 Cirratulus cirratus: 19.93 Cerastoderma glaucum: 13.6 Cerithium scabridum: 7.17 | Melinna palmata: 52.81 Cymadusa filosa: 50.42 Tricolia speciosa: 47.46 Lumbrineris tetraura: 44.30 Loripes lucinalis: 40.85 Euclymene oerstedi: 37.08 Potamides conicus: 32.99 Cirratulus cirratus: 27.25 Scrobicularia plana: 20.56 Cerithium scabridum: 10.42 | Nereis cultifera: 54.22 Gammarus insensibilis: 51.02 Cerastoderma glaucum: 47.38 Arenicola marina: 43.40 Pseudocuma longicornis: 38.98 Cirratulus cirratus: 29.51 Bittium reticulatum: 24.38 Cerithium scabridum: 19.14 Scrobicularia plana: 13.72 Potamides conicus: 7.54 |
This study reports the spatial and temporal variability of benthic macrofauna communities over the whole intertidal zone of the Kneiss Islands in the Gulf of Gabès, i.e. an inventory of species associated with Zostera eelgrass, distributed in the intertidal ecosystem, along with the spatial distribution of macrobenthic fauna assemblages, as well as the seasonal variations according to the main environmental changes and anthropogenic pressures, especially traditional human activities.
In the present study, three distinct macrofaunal assemblages can be recognized in the apparently homogeneous Zostera noltei meadow. The assemblages are characterized by spatial and temporal changes in the population, and their distribution pattern seems to be entirely governed by the physico-chemical and edaphic characteristics of the environment. The faunal assemblages of groups II (12 stations) and III (10 stations) make up the two most representative aspects of the Z. noltei macrobenthic communities. They comprise 65% of stations, being densely vegetated compared to the poorly vegetated stations of group I. Granulometric analyses show that the sediments on the foreshore of the Kneiss Islands are generally fine grained. Seagrass cover and different sediment characteristics indicate that these two factors play an important role in the establishment of different macrozoobenthic assemblages within a Zostera noltei habitat, thus confirming the results found by [
In this study, we identify 159 taxa, associated with intertidal Zostera noltei seagrass beds, which are unequally distributed among the sampling stations. The benthic community in the Kneiss Islands seems to be characterized by a structure and organization closely similar to ecosystems in other parts of the Mediterranean, dominated mainly by crustaceans, molluscs, polychaetes, cnidarians and echinoderms. This pattern has also been observed in other Mediterranean coastal sites, such as in the Kerkennah Islands [
The dominant macrozoobenthic assemblages within the intertidal area colonized by the Zostera noltei seagrass beds of the Kneiss Islands (
During the study period, the macrozoobenthic composition was subject to predictable seasonal changes: the main community structure parameters fluctuated according to the typical seasonal cycle of coastal temperate waters [
Taken together, the calculated values of the various biotic indices show a certain degree of intercorrelation among themselves. Indeed, the majority of these indices classify all stations colonized by intertidal eelgrass meadows as having a good ecological status, with some having a high quality status (
Long-term studies are essential for understanding the natural variability of species composition, dominance structure and the functional diversity of benthic communities in relation to natural environmental drivers, all of which are affected by climate change [
Finally, it would be of great interest to carry out the same type of study on the subtidal zones of the Kneiss Islands, characterized by the presence of submerged tidal channels. This particular Mediterranean ecosystem is subject to a range of anthropogenic pressures, which lead to a decrease in sediment grain-size from downstream to upstream under the action of tidal currents [
The authors thank M. Carpenter for the English revision, as well as the fishers of the Kneiss Islands for their help during the sampling and the Laboratory team of GeoResources Materials, Environment and Global Changes (FSS) for their assistance during sediment analysis.
Nawfel Mosbahi,Jean-Philippe Pezy,Jean-Claude Dauvin,Lassad Neifar, (2016) Spatial and Temporal Structures of the Macrozoobenthos from the Intertidal Zone of the Kneiss Islands (Central Mediterranean Sea). Open Journal of Marine Science,06,223-237. doi: 10.4236/ojms.2016.62018