Meiofaunal assemblages of the bay of Nisida and the environmental status of the Phlegraean area (Naples, Southern Italy)

Nisida is an islet of volcanic origin of the Phlegraean archipelago (Southern Italy) that has been included in the General Reserve of the Regional Park of Phlegraean Islands. The islet has been inaccessible both by land and by sea for a long time and, thus, has maintained its naturalistic value almost unchanged, in contrast with the close Bagnoli area, in which a steel industry has been active for several decades. An investigation was carried out in 2014–2015 at the bay of Nisida (Porto Paone). The main aims of our study were to document the structure of the meiofaunal assemblage, to identify the environmental factors that lead to its spatial and temporal variations, and to evaluate the ecological quality of this area, so bridging a gap in the scientific knowledge of Porto Paone. According to the diverse substrate typology and hydrodynamic regime, four stations were established to evaluate the meiofaunal assemblage over a period of one year. A high number of meiofaunal taxa (21) and a high biodiversity of meiofauna were recorded: the total meiofaunal density was higher in the eastern sector of the bay, where the effects of currents increase the abundance of temporary meiofauna in the sediment by increasing the supply of planktonic larvae of macrofauna (i.e., annelids, molluscs). PERMANOVA revealed significant differences of the assemblage structures at both the temporal and spatial scales. Summer conditions appeared to favor an increase of several taxa, which may be related to both an enhancement of the trophic sources, in turn promoting more trophic lifestyles, and to the biological cycles of individual taxa. Among the environmental variables, sediment variations, temperature, and chlorophyll-a (Chl-a) concentration appeared to be important in influencing the meiofaunal distribution. The results suggest that Porto Paone is characterized by a great richness (21 vs. 8 total taxa), especially if compared with the close, highly impacted area of Bagnoli. According to the current classification of environmental quality based on the meiofaunal richness, Porto Paone may be classified with a high ecological quality.