Primary colonization and small-scale dynamics of non-indigenous benthic species: a case study

An investigation aimed at recording NIS settlement by the employment of a bare artificial substratum constituted by caged lightweight expanded clay (EC) has been carried out in a confined marine basin. Packaged substratum, submitted to different organic treatments and a control was positioned in spring both in the water column and into the bottom sediment. Two months experiment provided 17 non-indigenous (NIS) versus 154 indigenous species (IS). Eleven “slow” colonizing NIS occurred at mean density of 0.2 ± 0.08 ind. L⁻¹, whereas six species of “fast” colonizing NIS reached a mean density of 34.5 ± 13.0 ind. L⁻¹ (individual organisms) and a coverage of 37.1 ± 31.6 cm² L⁻¹ (modular organisms). PCoA plots ordered assemblages according to time (from 48% up to 75% total variation) and to organic treatment (about 22%) both for individual and modular organisms. In general, organic contamination exerted negative effect both on NIS and IS, and the reference condition was preferred. Among individual NIS, the best colonizer in terms of density and biomass was primarily the sphaeromatid Paracerceis sculpta for both sediment-buried (64.78 ind-cm² L⁻¹ and 217.43 mg L⁻¹) and suspended artificial substratum (143.94 ind-cm² L⁻¹ and 483.14 mg L⁻¹). The amphinomid Linopherus canariensis prevailed in the buried artificial substratum (52.27 ind-cm² L⁻¹ and 92.81 mg L⁻¹ respectively). Sessile and modular NIS appeared more susceptible to the initial organic contamination and the artificial substratum suspended into the water column was more suitable for these colonizers. Celleporaria brunnea (79.74 ind-cm² L⁻¹), Paraleucilla magna (66.69 ind-cm² L⁻¹), Hydroides elegans (12.49 ind-cm² L⁻¹) prevailed in the mean surface coverage among other NIS and IS species. The organic enrichment reduced to four up to five times the colonization rate of the first two species.

Experimental data showed the great competitive capacity of NIS in the early stages of fouling, both under control and organically-enriched conditions, although this type of disturbance affected the whole fouling assemblage.