**Trophic Features, Benthic Recovery, and Dominance of the Invasive** *Mytilopsis Sallei* **in the Yundang Lagoon (Xiamen, China) Following Long-Term Restoration**

### **Paolo Magni 1,\*, Serena Como 1, Maria Flavia Gravina 2,3, Donghui Guo 4, Chao Li 4 and Lingfeng Huang 5,\***


Received: 20 June 2019; Accepted: 12 August 2019; Published: 15 August 2019

**Abstract:** A comprehensive set of physicochemical variables in near-bottom water and surface sediments, as well as the soft-bottom macrozoobenthic assemblages were investigated at six sites across the Yundang Lagoon (Southeast China) in November 2012. This lagoon was severely damaged in the 1970s due to domestic and industrial pollution and land reclamation and underwent a massive restoration effort over the past 30 years. Our objectives were to: (1) assess the current trophic and environmental condition of the lagoon; (2) investigate the pattern of spatial variation in the macrozoobenthic assemblages; and (3) assess the benthic recovery in relation to the main environmental gradients and the presence of invasive alien species. Nutrient, chlorophyll-*<sup>a</sup>*, biological oxygen demand (BOD5), chemical oxygen demand (CODMn), and total organic carbon (TOC) concentrations were lower than those reported in previous decades, ye<sup>t</sup> organically-enriched conditions occurred at an inner site. From azoic conditions in the 1980s and a few benthic species reported prior to this study, we found a significant increase in benthic diversity with 43 species heterogeneously distributed across the lagoon. The invasive bivalve *Mytilopsis sallei* was the dominant species, which was associated with the richest benthic assemblage. However, *M. sallei* is a pest species, and its spatiotemporal distribution should be carefully monitored. These results highlight the central role of the macrozoobenthos in providing important ecological information on the current status of the Yundang Lagoon and as an effective biological tool to follow the recovery's progress and the future evolution of this highly valued ecosystem.

**Keywords:** macroinvertebrates; biodiversity; eutrophication; saprobity; organic pollution; spatial variation; sediments; coastal lagoons; invasive alien species
