**1. Introduction**

Coastal lagoons are typically human-managed, highly productive ecosystems which provide multiple uses and services [1–3]. However, in the last few decades, engineering interventions have increasingly affected lagoons' morphology, hydrodynamics, and sedimentology, resulting in excessive

inputs of nutrients and organic matter (OM) that have led to eutrophication and organic over-enrichment of the sediments [4–7]. These factors have profoundly impaired the biological structure, trophic status, and functioning of coastal lagoon systems worldwide [8–11]. As an example, dystrophic events are the cause of anoxia, sulfide development, mass mortalities, and shifts to alternative ecohydrological states characterized by reducing conditions and lower pH [12–15]. Within this context, the concept of "saprobity", originally developed for rivers and lakes more than a century ago [16,17], applies well to coastal lagoons as a "state descriptor" of the ecosystem's condition resulting from the input and decomposition of OM and the removal of its catabolites [18]. Thus, both the trophic features (i.e., the amount of nutrients and OM in water and sediments) and the degree of saprobity (i.e., the balance between input of OM and other processes such as mineralization, sinking, dilution and export of OM) are instrumental for assessing the natural conditions and the environmental quality of a lagoon. Contrary to estuaries where the salinity gradient is dominant [19], the seawater renewal and hydrodynamics strongly govern the land-sea gradient in coastal lagoons [18,20]. Several field studies have described how species or groups of benthic organisms reduce their abundance in a given order as environmental stress increases, and how the number of taxonomic groups is reduced as stress increases [21–25]. Similarly, the main e ffect of saprobity on the macrozoobenthic assemblages is that the number of species that can cope with it decrease progressively as saprobity increases, leading to a reduction of species richness and diversity [18,26]. Yet, OM input associated with low saprobity due to high oxygen availability or low by-product (e.g., ammonia and sulfide) concentrations can promote the vitality of the biocoenosis, increasing biomass and abundance.

In China, many coastal areas are experiencing major urban and economic development, which is often coupled with growing anthropogenic pressures such as land reclamation, which often causes loss of coastal wetlands and serious environmental problems [27]. Monitoring and assessment of man-made pollutants and their sources are, thus, crucial for evaluating the environmental quality of these areas, including coastal lagoons, located at the interface between continental and marine ecosystems. Accordingly, a substantial body of legislation exists to address environmental protection in China, such as laws on Water (21 January 1988) and Environmental Protection (26 December 1989), Sea Water Quality GB 3097-1997, Environmental Quality for Surface Water GB 3838-2002, and Provisions for Monitoring of Marine Culture and Propagation Areas (1 April 2002) [28]. The Yundang Lagoon is an urban water body located in the highly populated and industrialized city of Xiamen (Southeast China). This lagoon, a natural bay open to the sea and historically rich in marine species, was severely damaged in the 1970s due to domestic and industrial pollution and land reclamation for agriculture. In particular, the Yundang Lagoon was reduced in size by more than 90% (from 10 km<sup>2</sup> before 1970 to 0.8 km<sup>2</sup> by 1988) and su ffered decreased water flow and enhanced siltation, anoxia, loss of habitat and species, and the eventual collapse of the whole ecosystem [29–31]. Thus, an integrated managemen<sup>t</sup> program aimed at restoring the ecosystem was put in place in the late 1980s, which included clean-up activities, wastewater treatment, flood prevention, and sludge dredging. The lagoon was also opened to the outer bay of the Xiamen Sea through a diversion canal inlet in the early 1990s, coupled with new restoration actions including a mangrove plantation and phytoremediation techniques [29,32]. One of the main current aspirations of residents and the governmen<sup>t</sup> is to restore the ecosystem services that a healthy Yundang Lagoon could provide [29,33].

Environmental and ecological studies in Chinese coastal lagoons are sparse, but growing steadily, owing to the scientific and socio-economic interest in these highly productive but fragile ecosystems [27,31]. Within this framework, in the present study the environmental and benthic features of the Yundang Lagoon were investigated as a unique case-study of a formerly "dead" lagoon which underwent a massive restoration effort over the past 30 years. We hypothesized that the long-term restoration interventions may have led to improved environmental and biological conditions of the lagoon. To address these issues, we analyzed a comprehensive set of physicochemical variables in water and sediments, as well as the macrozoobenthic assemblages, at six different sites across the Yundang Lagoon. Our objectives were to: (1) evaluate the trophic features of the lagoon and possible areas of major environmental concern; (2) investigate the pattern of spatial variation in the macrozoobenthic assemblages; and (3) assess the benthic recovery in relation to both the main environmental (e.g., trophic and saprobity) gradients and the presence of invasive alien species.

### **2. Materials and Methods**
