Understanding the temporal dynamics of biocommunities is a foundational topic in community ecology (Rull, 2012). On short-term scales, studying the seasonal variations in fish community structures and their ecological processes and drivers can provide guidelines for biological conservation and fishery resource management (Galib et al., 2016). Several methods for describing biological community structures have been recorded in the literatures and have been extended from early taxonomic structures to functional structures (Lai et al., 2022; Yuan et al., 2024). Numerous taxonomic structure indicators of biocommunities can also be applied to measure their functional structure. For example, functional alpha diversity is derived from the calculation principle of taxonomic alpha diversity, which is used to describe the heterogeneity of interspecific functional traits within a community (Mammola et al., 2020). Moreover, beta diversity can also be computed with the basis of species occurrence and biological functional traits to describe the dissimilarity of pairwise communities in their taxonomic and functional structures (Villéger et al., 2013). However, both alpha and beta diversities were studied based on taxonomic level of organisms in several previous studies because of shortage in data of functional traits and tools of algorithms (Li et al., 2018; Cunha et al., 2019). Recently, considering that the functional traits of organisms largely control their responses to environmental disturbances (He et al., 2022), a growing number of researchers have investigated community dynamics simultaneously used species- and trait-based approaches to capture comprehensive information about community dynamics (Li et al., 2023; Yuan et al., 2024).

Against the background of the sustained decline in biodiversity due to natural and anthropogenic disturbances, quantifying the ecological processes that drive changes in biocommunities has become a key issue in conservation biology (Chase, 2003). Numerous existing studies have focused on species gains and/or losses, which result in community reorganization and increases or declines in species number (Liu et al., 2023; Coulon et al., 2024). Baselga (2010) decomposed beta diversity into turnover and nestedness proportions to describe the above processes of changes in biocommunities (i.e., species gains and/or losses). However, different species occupy different ecological niches in ecosystems because of variations in their functional traits (Miller et al., 2023). Therefore, species gains and/or losses, which are the processes of changes in the taxonomic structure of biocommunities, inevitably cause changes in their functional structure (Halpern and Floeter, 2020). Accordingly, beta diversity calculated based on functional traits can also be decomposed into the above two components to describe the ecological processes of changes in functional structures of biocommunities (Villéger et al., 2013). For instance, Castro et al. (2023) revealed that both taxonomic and functional beta diversities of the fish communities in continental Chile declined throughout their history (pre-European) to their current composition. The decline in taxonomic beta diversity included species gains and losses. The species with gains were mainly six exotic Salmonidae with similar functional traits, and those with losses were endangered and vulnerable native species with unique functional traits, which caused the functional nestedness proportion was higher than the functional turnover proportion.

Although the ecological processes of turnover and nestedness jointly regulate biocommunity structure, their relative importance largely depends on species composition and species-environment relationships (He et al., 2022). How organisms respond to environmental changes have been demonstrated to be determined by their functional traits because traits characterize the ecological roles played by organisms and interactions between species and environmental variables (Violle et al., 2009). Therefore, trait–based approaches can reveal community assembling mechanisms and provide better understanding of community dynamics and their ecological processes than taxonomic methods (White et al., 2018). Recently, more and more studies profoundly revealed the spatiotemporal dynamics of fish communities and their driving mechanisms in various ecosystems by using trait-based approaches; these works have largely contributed to biological conservation and fishery resource management in their localities (Barnett et al., 2019; Sarremejane et al., 2020).

Zhoushan Fishing Ground (ZFG) is an important fishery economic zone in China and has rich species (Zhou et al., 2013). In recent few decades, the area has undergone multiple pressures, such as aquaculture, climate change, and other natural and anthropogenic disturbances, resulting in a serious decline in its fishery resources (Ni and Lu, 2003; Wang et al., 2023). In order to maintain and restore the fishery resources in the ZFG, a series of fishery management policies have been sequentially proposed, including the most famous “summer moratorium of marine fishing”. The seasonal patterns of fish communities are an important basis for fishery resource evaluation and ecosystem-based fishery management (Luo et al., 2024). However, although several studies have revealed the seasonal variations in the fish communities by using taxonomic methods, how their functional structure change with seasons is still unclear. In addition, to the best of our knowledge, there were no studies on the processes of seasonal changes in the fish communities. The knowledge gap seriously hinder the ecosystem-based fishery management in the study area. Therefore, a thorough knowledge of seasonal variations in fish communities in the ZFG is urgently required.

Here, we calculated taxonomic fiversity (TD), functional diversity (FD), and functional redundancy (FR) of the fish communities in the ZFG and tested their changes between autumn 2023 and spring 2024. We investigated the ecological processes of the seasonal changes in the fish communities by beta diversity and its turnover and nestedness proportions for the pairwise communities between spring and autumn. In addition, we explored the trait-environment relationships by using RLQ and fourth-corner methods. Given that the seasonal dynamics of the species and functional trait composition in the ZFG which demonstrated by Li et al. (2023), we hypothesized that TD and FD would be significantly different between spring and autumn, while FR would be determined by the functional traits of gain and loss species between the two seasons. Given that there are interspecific differences of functional traits of fishes (Miller et al., 2023), we hypothesized that the taxonomic and functional structures would be change with seasons via different ecological processes. The results are expected to reveal seasonal variations in the fish communities and their drivers in the ZFG to provide information for fishery management planning.

In this work, we found that in the ZFG, TD and FR significantly differed between spring 2024 and autumn 2023, but FD did not significantly differ between the two seasons. The seasonal changes in diversity indices result from the seasonal changes in fish community structures which are a universal phenomena in various ecosystems (Galib et al., 2016; Li et al., 2023). Numerous biotic and abiotic drivers, including variations in physio-ecological characteristics of living things and environmental gradients, cause seasonal dynamics of fish communities (Loher and Seitz, 2006; Lai et al., 2022). In some fishery economic zones, seasonal fishing is another important driver of the seasonal patterns of the fish communities (Xing et al., 2020). Notably, TD and FD changed differently with seasons in this study. Consistent with our result, some studies also found that TD and FD respond differently to natural and anthropogenic disturbances and often exhibit different spatiotemporal dynamics (Li et al., 2023; Yuan et al., 2024). Their results may be attributed to the inconsistent processes of TD and FD in fish assembly (Cilleros et al., 2016). Given that the species-environmental relationships are dependent on the functional traits of species, FD is always highly sensitive to environmental changes (Kendzierska and Janas, 2024). Considering that whether a species can successfully settle in an ecosystem is rely on the species-environmental relationships, TD is closely related to environmental characteristics (Agiadi et al., 2024). Therefore, TD and FD can be considered as the integrated effects of environmental filtering. Importantly, the seasonal pattern of FR was consistent with TD, and both of them were significantly higher in autumn than that in spring. This result suggested that the species that occurred only in autumn had similar functional traits in contrast to the species shared by the two seasons. This finding can also demonstrated by the absence of significant changes in FD. Additionally, although TD in spring was lower than that in autumn, functional space showed opposite seasonal patterns. This result also indicates that species in autumn shared more similar functional traits than those in spring. These results demonstrated that FR and FD lacked necessary connections because of the various functional traits of different fishes (Zeng et al., 2022). In accordance with the ecological meaning of FR, fish communities with high FR values are expected to be influenced negligibly by species loss (Aune et al., 2018). In addition, it is widely believed that FR is closely positively related to ecosystem stability (Biggs et al., 2020). Given the high FR in autumn 2023, we propose that in the ZFG, the fish community in autumn has higher resistance to environmental changes than that in spring.

Studing the various aspects of beta diversity can deeply promotes our insights into the intricate processes of the changes in fish communities (Novillo and Ojeda, 2020). The mean values of T.jac and F.jac in the ZFG were high, indicating that the taxonomic and functional structures substantially differed between spring and autumn. Figure 4 illustrates that the dissimilarity of the taxonomic structure between any pairwise communities of spring and autumn concentrated in the part of larger T.jtu, whereas functional beta diversity showed a disperse distribution. These results suggested that the dissimilarity of the taxonomic structures between spring and autumn was mainly driven by species turnover (the process contains both species gain and loss with similar number), whereas that of functional structures was driven by both functional space turnover and expansion or contraction between the two seasons (Villéger et al., 2013). In the ZFG, we identified 21 and 36 species occurred only in spring and autumn, respectively. Some seasonally reproductive species cyclically migrate into or out the area in specific seasons (Goertler et al., 2021). Other species only appear in one season, possibly for foraging or overwintering needs (Chen, 2022). However, species replacement did not result in an equal degree of functional space replacement, with T.jtu accounting for 89.9% of T.jac and F.jtu only accounting for 57.9% of F.jac. These results indicated that approximately half of the species involved in turnover had similar functional trait values. Beta diversity indices are effective for deeply understanding the mechanisms of biocommunity construction and have implications for predicting the future processes of biotic homogenization (Castro et al., 2023). By combining beta diversity and its turnover and nestedness proportions, TD, FD, and FR indices, we found complex relationships between fish communities in spring 2024 and autumn 2023, which can provide important guidelines for biodiversity sustainable management in the ZFG.

RLQ and fourth-corner analyses results revealed that close correlations among L, R, and Q. Species gain or loss is associated with their functional traits and environmental changes (He et al., 2022). In our study, all five environmental variables exhibited significant changes between spring 2024 and autumn 2023. If environmental changes exceed the tolerance ranges of a species, the species will no longer survive in the area. In the meanwhile, other suitable species may migrate into and resettle in the area because of environmental filtering (Kraft et al., 2015). Therefore, fish community assembly and their responses to natural and anthropogenic pressures are mainly determined by the trait-environmental relationships (Baptista et al., 2021).

Additionally, the ordination of biotic and abiotic variables in the two-dimensional coordinate system based on the first two RLQ analyses indicated that water temperature, salinity and DO are important variables influencing functional structure of the fish communities. According to previous studies, functional structure of biocommunities is tightly related to the trait-environment relationships (Araújo et al., 2024). In this study, high LM was significantly correlated with low water temperature and high DO and salinity, as well as the presence of species in spring. LM is a common index of sexual maturity characteristics and is often used as an important basis for determining the minimum allowable length for fishing (Stark, 2012). A high LM value indicates that a species has a large individual size and is often categorized as a cold-water species (Fontoura et al., 2009). This finding may account for the significant negative relationship between LM and water temperature. A higher Q/B value was significantly correlated with low water temperature and high DO, as well as the presence of species in autumn. Q/B is an index of trophic interaction and can be used to describe food consumption by a species (Wiff et al., 2017). The natural phenomenon where species in low water temperature areas often require more food intake to maintain their functions may explain the negative relationship between Q/B and water temperature. Within a certain range, high DO can promote the metabolism of organisms, thus accounting for the positive relationship between DO and Q/B (Tran-Duy et al., 2011). High TL was significantly correlated with low water temperature and high DO, as well as the presence of species in spring. TL indicates the position of species in food chain, providing information on predator–prey interactions (Pasquaud et al., 2010). Previous studies have demonstrated that increasing water temperature is more favorable to species at low TLs than to those with large body sizes (Daufresne et al., 2009; Lefort et al., 2015). Thus, continued shortening in the food chain and cascading of ecological effects caused by global warming should be emphasized (Guiet et al., 2016). The relationship between TL and DO may be mainly impacted by water temperature, with high water temperatures always accompanied with low DO. In fact, the impacts of environmental changes on organisms are very complex as there are close relationships between different environmental variables, which thereby expanding or compressing the tolerance of some functional traits to environmental changes (Elsdon and Gillanders, 2004). Therefore, to capture more comprehensive the trait-environment relationships in natural ecosystems, the interactive effects of various environmental variables and their collaborative impacts on biological functional traits are needed to further studied.

The study revealed the different variations in the taxonomic and functional structures of the fish communities in the ZFG with seasons, as well as the resulting different seasonal variations in the TD, FD, and FR. Overall, both the taxonomic and functional structures exhibit high dissimilarities between spring and autumn. The variations in the taxonomic structures were mainly caused by species turnover, whereas that in the functional structures were caused by functional turnover and nestedness. Despite the important discoveries that taxonomic and functional structures underwent different changes, further studies on long temporal scales are needed because our data were limited to 2023–2024. The seasonal changes in the fish communities were closely related to the trait-environment relationships. In specific, LM, Q/B, and TL were significantly positively related with DO, and significantly negatively related with water temperature. Given that the FR showed significantly higher in autumn than in spring, we propose that the fish communities in autumn had higher resistance to environmental changes. This study provides substantial insight into the short-term variations in the fish communities in the ZFG, which provide guidelines for fishery management in the study area. In future, we will conduct further research on the long-term dynamics of the fish communities and their ecological processes to reveal the mechanisms of the fish communities assembly in the ZFG.