*3.2. Meiofauna*

A total of 16 meiofaunal taxa were found: Plathelminthes, Nemertina, Nematoda, Kinorhyncha, Bivalvia, Polychaeta (adults and nectochaetes), Oligochaeta, Copepoda (adults and juveniles), Ostracoda, Amphipoda, Cladocera, Isopoda, Tanaidacea, Insecta, Halacaroidea, and Pycnogonida. The dominant taxa were generally nematodes (from 41% at C2 in winter 2011 to 86% at C3 in summer 2010), copepods (from 4% at C3 in summer 2010 to 33% C3 in winter 2011), and ostracods (from 0% at C2 in summer 2010 to 12% at C3 in winter 2011).

The PERMANOVA carried out on the structure of the meiofaunal assemblage indicated highly significant differences only among sites (*p* < 0.001), while no significant differences emerged among periods or site × period interactions (Table 2). In particular, pairwise comparisons highlighted significant differences between C3 and the other two sites (C1 and C2). The aforementioned variations were tested by means of the SIMPER analysis, which highlighted that five main groups, i.e., Nematoda, Copepoda, Ostracoda, Nauplii, and Halacaroidea, contributed up to the 95% of the cumulative similarity, with an individual contribution varying from 58% for Nematoda to 2.3% for Halacaroidea (Table 3; Figure 4). All the other taxa contributed to less than 2%.

**Figure 4.** Meiofauna community structure in the sampling sites C1, C2, and C3.



**Table 3.** Results of the Similarity Percentage (SIMPER) analysis showing the average dissimilarity (Av. Dissim.), % contribution of each taxon (Cont. %), % cumulative for the pair comparisons (Cum. %), and average abundances of each sites (Av. ab.). The overall average dissimilarity is 70.9, 76.0, and 82.6 for C1 vs. C2, C1 vs. C3, and C2 vs. C3, respectively.


Variation in the meiofaunal abundance, number of taxa, diversity, evenness, and the Ne/Co ratio are shown in Figure 5. While differences between sites in abundance, evenness, and the Ne/Co ratio were high, those in taxon number and diversity were not. In particular, abundance was the highest at C3, which also showed the most marked standard error, and consequently the lowest evenness. The Ne/Co ratio was higher in summer, but always lower in winter both at C2 and C3, which coincided, with the drop in salinity. Finally, the most confined and organically enriched site C2 showed the most variation in the Ne/Co ratio up to various orders of magnitude, particularly in summer. The 2-way PERMANOVA conducted individually on each biotic measure showed significant differences among sites in the total abundance, taxon number, evenness and the Ne/Co ratio, while no difference was found for the diversity index (Table 4). The pairwise comparisons among the three sites are showed in Table 4 and highlighted higher and significantly differences especially of C3. PERMANOVA did not reveal significant differences of meiofaunal abundance, taxon richness, Shannon diversity, and Ne/Co ratio among the periods investigated, while some differences of evenness (*p* < 0.05) were found (Table 4).

**Figure 5.** Mean values (n = 6, ±SE standard error) of meiofaunal community synthetic measures at the sampling sites C1, C2, and C3 in summer (Su) and winter (Wi).

The nMDS on the structure of assemblage similarly highlighted three major groups corresponding to the three study sites, with a partial overlap between C1 and C2, and a major separation of C3 from both C1 and C2 (Figure 6). In this analysis, the environmental variables superimposed to the biotic data showed that salinity and DO were the main responsible factors for the separation of C3 from C1 and C2 (on the right-hand side) and that Wc, OM, Chl-*<sup>a</sup>*, and phaeopigments contributed mostly to the separation between C1 and C2 (Figure 6). These results demonstrated two main gradients influencing the spatiotemporal variation in the meiofaunal assemblages in the Cabras Lagoon. Temperature, on the contrary, did not make a relevant contribution to the ordination of the sites as indicated by its short segment.


*Water* **2019**, *11*, 1488

**Figure 6.** MDS on meiofaunal abundance (stress = 0.09) with superimposed environmental.
