Estuaries are highly productive, yet threatened ecological systems that deliver a range of ecosystem services and benefits to human society. However, the health of many estuaries is under threat from high levels of anthropogenic activities and pollution associated with the dense human populations that occur in these regions (Adger et al., 2005). For example, sediment and organic material deposited by rivers in estuaries include nutrients, heavy metals, microplastics, and other contaminants (Chen et al., 2012; Han et al., 2017; Xu et al., 2018) that have led to biodiversity loss and reduced ecological resilience in estuarine environments (Lotze et al., 2006; Liu et al., 2018a).

The recognition of the essential role of estuaries for humans and other organisms has led to the initiation of ecological restoration programs (Lotze et al., 2006), where rehabilitation or construction oyster reefs represent an effective estuarine restoration technique. The biological structure formed by a large number of oysters is called an oyster reef, which is an important coastal habitat, because they provide many ecological services and bring multiple ecological benefits (Grabowski et al., 2022), such as water purification through water filtration and the removal of phytoplankton and heavy metals, and habitat provisioning through creation of three-dimensional habitats for macrobenthos and fish (Breitburg et al., 2000; Coen and Luckenbach, 2000; Grabowski and Peterson, 2007; Thomsen et al., 2007; La Peyre et al., 2014; Liu et al., 2018b). Thus, construction of oyster reefs has been increasingly used for shoreline protection and ecological restoration (La Peyre et al., 2014; Chakraborty, 2016; Walles et al., 2016).

The Yangtze Estuary is the largest in Asia and is one of the most globally important ecotones (Chen et al., 2012) that provides vital ecological services, including breeding grounds for aquatic organisms and stopover and overwintering sites for millions of migratory birds (Xu and Zhao, 2005). However, it is affected by rising sea-level and anthropogenic pressures that have culminated over centuries of overexploitation, reclamation, and pollution (He et al., 2011; Kirwan and Megonigal, 2013; Liu et al., 2016b; Yu et al., 2018); consequently, eutrophication and reduction in biodiversity have led to a deterioration in the ecological health of the Yangtze Estuary (Chai et al., 2006; Chen et al., 2012; Huang et al., 2021; Huang et al., 2022). In response, an oyster reef restoration project was initiated in the region (Quan et al., 2009), based on previous studies that showed the construction of artificial oyster reefs in the Yangtze River estuary was feasible and effective in the restoration of the ecosystem (Quan et al., 2009; Quan et al., 2012; Lv et al., 2016a; Liu et al., 2018b).

It is likely that artificial and restored oyster reefs may be affected by human activities and pollution, such as over-fishing of the nursery beds and oil contamination, therefore, monitoring of such restoration schemes is crucial for their success. Macrobenthos represents a vital component of oyster reefs, and are ideal indicators for use in ecosystem health monitoring schemes due to their limited mobility, ease of collection, and high sensitivity (Borja et al., 2000; Borja et al., 2003; Gamito and Furtado, 2009; Liu et al., 2019); however, studies of oyster reefs have tended to focus on taxonomic macrobenthos community structure, abundance, biomass, diversity, and ecosystem health (Quan et al., 2009; Quan et al., 2012; Lv et al., 2016a; Liu et al., 2018b) that may be more appropriate in the evaluation of biodiversity rather than ecosystem health (Cummins et al., 2005; Peng et al., 2013). Macrobenthos functional groups represent taxa that have similar ecological characteristics, so assessment of their responses to environmental change is more meaningful than that of individuals or taxonomic families, because it provides a more accurate indication of ecosystem-level responses to ecological perturbations (Yang et al., 2017).

Ecological restoration may trigger the emergence of novel functional groups and turnover of existing functional groups (Lv et al., 2018). Previous studies of the Yangtze Estuary oyster reef have quantified biological resources, including macrobenthos and nektons, macrobenthic taxonomic diversity and ecological health (Quan et al., 2007; Quan et al., 2009; Quan et al., 2012; Lv et al., 2016a; Liu et al., 2018b); however, assessment of macrobenthos functional groups remains lacking. Here, we identified macrobenthos functional groups to assess the ecological status of the artificial oyster reef in the Yangtze estuary. Specifically, the objectives were to (1) assess the composition and abundance of macrobenthos functional groups; (2) measure long-term variation in macrobenthos functional group composition; (3) test the relationship between macrobenthos functional groups and environmental variables; and, (4) use macrobenthos functional groups to assess ecosystem health.

Restored and artificial oyster reefs play a vital role in improving the ecological health status of estuarine environments, because they improve water quality and provide a three-dimensional structure that may be colonized by macrobenthos and fish (Breitburg et al., 2000; Thomsen et al., 2007). In the first year (2004) since its creation, the artificial oyster reef in the Yangtze estuary was found to have become host to three functional groups (comprising five species) of macrobenthos; the species richness of the functional groups rapidly increased with time, with the exception of 2008, before it peaked in 2011 and remained stable from 2012. This increase in functional group species richness occurred after oyster abundance had stabilized (2011) (Liu et al., 2018b) and prior to the increase in macrobenthos taxonomic diversity that occurred from 2012 to 2016-2017 (Liu et al., 2018b). These results support previous studies that have indicated the oyster reefs in the Yangtze Estuary be develop into self-sustaining systems (Walles et al., 2016; Liu et al., 2018b).

Water salinity is a key factor that affects the distribution of macrobenthos in estuarine habitats (Holland et al., 1987; Liu et al., 2018a), where species richness, abundance, and biomass increase with increase in salinity (Yuan et al., 2002). In this study, the increase in salinity (from S5 and N6 to S8 and N9) resulted in an increase in species richness, abundance, and biomass of macrobenthic functional group (P< 0.05; Figure 7 ), indicating salinity affects the distribution of macrobenthos functional groups in the artificial Yangtze Estuary oyster reefs. Adaptation to salinity varies among species (Yuan et al., 2002); for example, Phascolosoma esculenta, which was only collected from the high salinity sites (S8 and N9), belongs to the D functional group, and has a survival salinity of 5-30 ‰, with a salinity range for embryo development of 17.10-30.15 ‰ (Jin et al., 2011). We found that functional group D, whose main species comprise polychaetes and Sipuncula that are sensitive to fluctuations in environmental conditions (Dauvin and Ruellet, 2007; Cai, 2015), was absent from the moderate salinity study sites (S5 and N6). There are marked seasonal changes in natural environmental variables, such as salinity and dissolved oxygen, and tidal conditions in the area where study sites S5 and N6 were located: it is possible that mud trapped at the bottom of the oyster reef is removed by tidal action, resulting in a lack of food source required by functional group D.

Salinity not only influences estuarine macrobenthos distribution patterns, but also affects growth and survival of oysters (Hulathduwa and Brown, 2006; Liu et al., 2018b). Effects of salinity on oyster size and abundance vary with region, therefore oyster substrate of the artificial reefs is likely to similarly vary with region, and may lead to diverse distribution patterns of macrobenthos functional groups. Our RDA confirmed that salinity, barnacle abundance, and oyster abundance (substrate parameters) were the main factors that influenced the distribution of macrobenthos functional groups in the Yangtze River estuary artificial oyster reefs ( Figure 7 ). Although there were differences in growth and abundance of oysters between regions and at different tidal zones within a region, there was no difference in macrobenthos functional group species richness, abundance or biomass, with the exception of species richness of group D among the tidal zones. These results indicate that while there are differences in substrate habitat with salinity and among tidal zones within a salinity range, the distribution pattern of macrobenthos functional groups varies with salinity, but not with different tidal zone, such that tidal zone and associated effects on substrate elicit little effect on the artificial oyster reef macrobenthic functional groups in the Yangtze Estuary. This finding is inconsistent with those of other local natural habitats, such as Jiuduansha and Quanzhou Bay (Zhu and Lu, 2003; Zhuo et al., 2014), possibly because the habitat of artificially constructed oyster reef lacks diversity. We suggest enrichment and diversification of artificial oyster reef habitats may improve the recruitment of functional group types to increase biodiversity and ecosystem stability.

In general, healthy environments contain most types of feeding guild, whereas degraded environments tend to be dominated by a few trophic groups, with others absent (Gamito and Furtado, 2009; Peng et al., 2013). In this study, we found one or two groups were absent from the moderate salinity study sites (S5 and N6), where they included functional groups that play an important role in estuarine food web nutrient cycling and energy flow. For example, Pl is the main source of nutrition for predators and omnivores, and plays an important role in controlling the growth of plankton, such as cyanobacteria ( Figure 2 ). The absence of this functional group from the moderate salinity study sites indicates compromised ecological health, as supported by our results for j _FD, and previous results for AMBI, M-AMBI, and ABC curve (Liu et al., 2018b). Our ANOSIM 2 analysis showed site differences in ecological health, where it was better in the high salinity study sites. These results indicate that 14 years after construction, the habitat status of the artificial oyster reefs in the Yangtze Estuary was good or high, and was better under high salinity conditions (S8 and N9) than under moderate salinity (S5 and N6). Long-term artificial oyster reef monitoring confirms the feasibility of artificial oyster reef construction in the Yangtze estuary, and can be applied to estuarine areas or tidal flats within the same salinity range, such as east shoal of Nanhui, which are mainly affected by reclamation (Liu et al., 2018a). Coupled with the complex habitat of natural intertidal zone, it is inevitably more conducive to the ecological restoration of artificial oyster reef in these areas.

We found that macrobenthos functional group structure was more heterogeneous under high salinity conditions (S8 and N9) than under moderate levels of salinity (S5 and N6). Substrate parameters and environmental variables, such as dissolved oxygen, pH, temperature, and salinity, were similar between S5 and N6, and S8 and N9. However, functional group species richness, abundance, and biomass, and j _FD of the southern dike (S5 and S8) was higher than in the north dike (N6 and N9), possibly due to site conditions. The southern region of the oyster reef in the Yangtze Estuary is proximate to the ecologically protected Jiuduansha Nature Reserve (Zhang et al., 2018) that may affect the structure and ecological environment of macrobenthos (Lv et al., 2016b). In contrast, the lack of such protection measures in the north dike may render the oyster and macrobenthos in this area susceptible to human activities, such as overfishing. Therefore, we suggest the establishment of a nature reserve to protect this important oyster reef habitat in the Yangtze estuary.

In summary, artificial oyster reef was feasibility and has positive effect on the ecological health in the Yangtze estuary. Further, the ecological health was worse in medium salinity than high salinity, may due to the absence of functional groups. The release of species of absent functional group can be taken in the medium salinity region. The artificial oyster reef may be disturbed by human activities, thus a nature reserve should be established to protect this important habitat in the Yangtze estuary.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

YNZ: Conceptualization, Methodology, Resources, Formal analysis, Investigation, Data curation, Visualization, Writing - original draft, Writing - review and editing. HW: Conceptualization, Methodology, Resources, Visualization, Data curation, Writing - review and editing. YLZ: Conceptualization, Methodology, Resources, Visualization, Funding acquisition, Writing - review and editing. JD: Writing - original draft, Writing - review and editing. ZZ: Investigation, Data curation, Writing - review and editing. HZ: Conceptualization, Methodology, Writing - review and editing. ZL: Conceptualization, Methodology, Resources, Visualization, Supervision, Writing - review and editing, Project administration, Funding acquisition. All authors contributed to the article and approved the submitted version.