Macroinvertebrates are ubiquitous and diverse, allow for the detection of a variety of perturbations in aquatic systems, and have relatively long life cycles when compared to planktons; these characteristics make temporal scales of macroinvertebrate population response an appropriate indicator for disturbance responses [1–4]. Although macroinvertebrates are usually used as bio-indicators in environmental or ecological assessments and present several advantages compared to other aquatic organisms, they are limited by various drawbacks and are influenced by human stress and natural factors such as hydrological regime, water temperature, light level, water chemistry, food resources and habitat heterogeneity; these factors present seasonal variations in communities due to changes in species related to life history strategy [5–10]. Nearly all facets of the life history of macroinvertebrates, and consequently their distribution and abundance, are influenced by water temperature, substrate type and composition, hydraulic conditions and food availability. In the natural aquatic system, the relative importance of the facets that affect macroinvertebrates and the exact pattern of seasonal variation in communities differ among study cases or geographic location [4, 11]. Therefore, seasonal changes in macroinvertebrate assemblages can be large in some systems while small in others [12]. Accordingly, the seasonal effects on macroinvertebrate community composition are important, should not be neglected and should be taken into account for biomonitoring purposes [13, 14]. In tropical, temperate and subarctic river or stream systems, seasonal variations in macroinvertebrate assemblages have been observed and well documented [8, 15–30]. However, most of these investigations were confined to streams or rivers in temperate and tropical areas, and subtropical rivers on the regional scale have received much less attention [31, 32].

In China, investigations or studies related to the seasonal variation in macroinvertebrate communities primarily involved a single waterbody, and few study cases were conducted in multiple waterbodies on a climatic regional scale. The seasonal variations in density, biomass, and diversity or taxa richness of macroinvertebrates were usually simply described or analyzed, and the conclusions varied among different study cases or bodies of water within the same regional or climatic zone. These conflicting results produced uncertainty over the bioassessment confidence and precision when assessing the aquatic ecological status with macroinvertebrate-based metrics [33–38]. In this paper, with the background of carrying out the National Water Issues Three Gorges Reservoir Special project during the Eleventh Five-Year Plan, the 27 main tributaries in the Three Gorges Reservoir Catchment (TGRC) were investigated from 2010 to 2011 to explore the temporal variations in macroinvertebrate communities on a regional scale. The objective of this study was to determine whether the macroinvertebrate communities from the tributaries in the TGRC varied as a function of seasons and to further test the temporal stability of certain metrics based on macroinvertebrates under a routine bioassessment framework. This study will provide a deeper understanding of the aquatic status in the future.

The results of our study showed that the macroinvertebrate communities in the 27 main tributaries of the TGRC varied with seasons. Among the four seasons, the taxonomic richness that was determined on the family level reached the highest point in spring, followed by winter, autumn and summer. Seasonal patterns of macroinvertebrates are largely a reflection of the seasonal trends of the predominant organisms [50]. The top 10 predominant taxa compositions showed distinct temporal variations as a function of seasons. Among them, the taxa Chironomidae, Heptageniidae, Corbiculidae and Baetidae were predominant across the four seasons and thus showed a certain extent of temporal stability. The temporal variations in communities were mainly reflected by changes in taxa proportions between seasons, which were confirmed by the results of the MRPP and ISA analyses. Moreover, the ordination plots based on NMDS analysis also revealed four distinct seasonal assemblages based on taxonomic composition. Studies showed that the taxonomic level decided the direction and the amount of seasonal variations in macroinvertebrate communities [43], and the use of the family level contributed to a decrease in temporal variation [7]. This is because representatives of families are likely to be present throughout the year, while lower taxa may be present at different times of the year due to their life cycles [43]. However, in our study, temporal variations in macroinvertebrate communities were still detected at the family level.

Strong seasonal variation among macroinvertebrates was confined primarily to streams at low altitudes (600–800 m) with the greatest monsoon rainfall [51]. Meanwhile, hydro-morphological characteristics and the water quality of aquatic systems largely determined changes in macroinvertebrate communities [17]. In the TGRC, the tributaries with elevations of 800–2000 m exhibited different hydro-morphological characteristics and water quality during the high discharge period (summer), the normal discharge period (spring and autumn) and the low discharge period (winter) [52, 53]. These differences strongly affected the distribution patterns of macroinvertebrate communities among different seasons (Table 1) because, during the wet season, high discharge usually triggers the drift behavior of some species and enhances the colonization of new species, thus bringing out the temporal variations in the abundance of the major taxa [17].

Studies showed that the highest concentrations of individuals from most major taxonomic groups were found during the egg hatching season [6]. Mayflies (Ephemeroptera) were more abundant during the dry season and decreased during the wet season as a result of being washed away [17]. The rational explanation for this phenomenon is that the dry season, with low current and relatively high water temperature, is often the major hatching period for mayflies and thus results in the rapid growth in individuals [17]. Our study showed that the percentage of the taxa Heptageniidae and Baetidae, which were involved in the top 10 predominant taxa composition, was highest (53.57%) in autumn (the normal discharge period) and lowest (26.42%) in summer (the high discharge period). Our results were slightly different from the previously described observational results and inferred that autumn was the important hatching period for mayflies.

Seasonal patterns in climate, such as precipitation and insolation, result in within-year changes in the flow and temperature of aquatic systems. These changes greatly influence the timing of emergence, reproduction, growth and development of many aquatic macroinvertebrates and, in turn, influence the seasonal replacement of organisms [54]. Thus, among the numerous environmental conditions, discharge and water temperature are often considered the important factors that contribute to the marked seasonality in assemblage composition [7, 24, 25, 27, 55]. According to the theory “intermediate disturbances”, the season variations in discharge and water temperature enhance species richness and maintain maximum species density. Discharge variation as a function of seasons contributes to the changes of available habitats by affecting areas of wetted perimeter in river systems, while water temperature variation as a function of seasons results in disturbances to many species with different life history strategies [24]. During the wet season, high discharge events can cause severe population losses and changes in the community composition and structure of macroinvertebrates; spate-induced disturbances also induce high variation in wet-season samples [31, 32]. The fluctuations in densities of macroinvertebrates are associated with the unpredictable floods and different life histories of most macroinvertebrates in tropical areas. At temperate latitudes, loss of organisms through flushing during the spring thaw or summer emergence of aquatic insects caused the density fluctuations of macroinvertebrates; in the tropical areas, however, the interruption of flow or low flow resulted in a marked reduction in density [17]. Streams in monsoonal environments are subject to extreme seasonal variation in flow. Predictable floods and spates in these areas are qualified as disturbances due to their large magnitude and geomorphological effect [31]. In the TGRC tributaries, which were located in a subtropical region with altitudes from 800 to 1000 m, discharge variation among seasons was still an important factor that caused the density fluctuations in macroinvertebrates. The discharge of these tributaries is often large due to floods that occur frequently in summer, and the density of macroinvertebrates in summer declined by scouring when compared to other seasons. The significant differences in community composition among seasons reflected the different life history strategies of most macroinvertebrate taxa [56].

Temporal changes in macroinvertebrate composition have been related to life history patterns in the community, and this relationship has been hypothesized to have evolved in response to food availability and seasonal changes in physicochemical factors [9]. Seasonal abundance of food could strongly influence the life cycles of the aquatic community and could result in the seasonal variation in macroinvertebrate community composition [6, 57]. The variations of food abundance as a function of season can result in significant seasonal variations in the abundance of certain taxa [7]. In tropical areas, the end of each dry season and the start of the rainy season experience the accumulation of allochthonous materials such as leaves and twigs. The accumulation of these materials in riverbeds provides shelters, or habitats, and food sources for macroinvertebrates [17]. In the TGRC tributaries, leaves and twigs from mountain trees along the riverbeds served as important shelters for some taxa, especially mayflies and caddisflies, and the attached algae were important food resources for scrapers. It was possible that these factors were the reasons why the percentages of EPT taxa were high in all seasons and exhibited significant temporal variations (Fig. 4a). In subarctic and temperate streams, the abundance of macroinvertebrates was low in mid-summer, increased in late summer and autumn due to recruitment from the hatching and growth of shredders that followed autumnal leaf input, and sharply declined in spring due to associated adult emergence [19]. In our study, the abundance of macroinvertebrates was the lowest in summer, increased in autumn and winter, and then decreased in spring. The temporality of macroinvertebrates in these subtropical tributaries was fundamentally similar to the seasonal patterns in subarctic and temperate streams. In subtropical areas, such as Georgia, USA, the number of EPT taxa showed high variation among seasons, while chironomid richness demonstrated decent seasonal stability [2]. In tropical forest streams, such as those in Malaysia, relatively more EPT individuals were found during the dry season when compared to the wet season and the seasonality impact on EPT taxa was extremely evident [58]. In this study, the percentages of EPT taxa, some mayflies such as Baetidae, Caenidae and Ephemerellidae, and caddisflies such as Hydropsychidae showed significant temporal variation (p < 0.01). The percentage of Chironomidae, however, displayed nonsignificant temporal variation (p = 0.079). This finding was related to the seasonal changes in the life cycles of particular taxa, resulting in the seasonality changes in relative abundances [59]. In subtropical monsoonal climate streams, small body sizes, short life cycles and continuous reproduction could be important strategies that ensure the rapid resilience and persistence of macroinvertebrate assemblages through time [31]. These factors may explain why the percentage of Chironomidae abundance showed nonsignificant temporal variation in the TGRC.

Macroinvertebrates are sensitive to watershed conditions and exhibit sufficient stability in assemblage structure over time; these characteristics make them useful as long-term monitors of aquatic system health and indicators of water quality [3]. Seasonal changes in macroinvertebrate taxon abundances that are related to life history introduce temporal variation into macroinvertebrate community structure and can potentially confound bioassessments [56]. In bioassessment framework, seasonal variability should be taken into account for providing acceptable levels of confidence and precision [14, 60]. Some metrics did not change significantly with seasons because certain representative species from different orders were present in all seasons [13]. Furthermore, certain metrics with pronounced seasonal variability lead to the greatest criticism in biomonitoring [61]. Ignoring natural seasonal variability in metrics can confound the detection of anthropogenic environmental change and affect the consistency and efficacy of metrics [2, 4, 6, 62]. In the streams of central Europe, a region dominated by a temperate climate, most metrics based on macroinvertebrates exhibited significant differences among seasons [6]. In the streams located in central western Georgia, USA, an area characterized by subtropical climate, the metrics related to compositional and functional feeding group measurements also showed high seasonal and annual variation [2]. In this paper, natural fluctuation of aquatic ecosystems (temporal effects) resulted in variations in metrics such as EPT%, Baetidae%, Caenidae%, Ephemerellidae% and Hydropsychidae%. These results indicate that seasonal effects should not be neglected under biomonitoring framework.

In climate regions with relatively high seasonal environmental variability or complex climatic patterns, the macroinvertebrate fauna was always characterized by the stochastic, unstable community structure in aquatic systems; within these systems, organisms had fast growth and high colonization capacity, and showed strong temporal variation or seasonality [31, 55]. For instance, the species richness and abundance were significantly higher during the dry season than during the wet season in tropical streams [22, 58]. Considerable seasonal and annual variations were observed in the densities of the major macroinvertebrate groups in the arctic and alpine river systems [63]. In the temperate zone, macroinvertebrate communities varied seasonally and displayed significant seasonality [64]. In the TGRC tributaries, Chironomidae, Heptageniidae and Baetidae were the three most numerous families across seasons, but their changes in abundance as a function of season could induce strong temporal variations among all macroinvertebrates. This would cause temporal variations in a series of metrics based on macroinvertebrates, and further demonstrates that the seasonal effects in biomonitoring studies should not be ignored in future studies. In the subtropical monsoonal climate streams of South America, community composition and abundance of macroinvertebrates differed significantly between seasons [31]. This variation was supported by our results, which showed that the effect of seasons on macroinvertebrate communities were very strong in subtropical monsoonal areas.

The temporal variations in macroinvertebrate communities in the main tributaries of the TGRC were significant, and Chironomidae, Heptageniidae, Corbiculidae and Baetidae dominated the communities across seasons. The changes in taxa proportions caused the temporal variations in the communities, and the temporal variations could be detected at the family level. The different hydro-morphological characteristics and water quality during different seasons strongly affected the distribution patterns of macroinvertebrate communities, and autumn was the important hatching period for mayflies in the TGRC. Discharge variation among seasons was an important factor for the density fluctuations of macroinvertebrates. In the TGRC tributaries, the temporal variation patterns of macroinvertebrates were fundamentally similar to the seasonal patterns in subarctic and temperate streams. Natural fluctuations in aquatic ecosystems (temporal effects) resulted in variations in some macroinvertebrate-based metrics and indicated that seasonal effects should not be neglected under biomonitoring framework. Furthermore, our results also supported the observation that seasonal effects on macroinvertebrate communities were very strong in subtropical monsoonal areas.