Litter Decomposition And Macroinvertebrates Colonization During Winter In A Stream Of The Changbai Mountains

In the boreal region,the headwater stream under surface ice still maintains its fluidity during winter,and some macroinvertebrates overwinter in the form of the larva.The remaining litters on the riverbed are important for the overwinter survival of macroinvertebrates.The life history of some macroinvertebrates?shredders?is often highly synchronized with the decomposition process of litter on the riverbed.However,there were few studies on characteristics of macroinvertebrate communities and the ecological process they involved in the icy streams during the winter process?pre-freezing period,freezing period,and thawing period?.We know little about the process of litter decomposition and macroinvertebrates colonization during winter,which greatly limits our knowledge and understanding of the winter ecological process in the headwater stream,so relevant researches need to be carried out urgently.This study was conducted in a headwater stream of the Longwan National Nature Reserve in the Changbai Mountains.Firstly,fundamental researches are conducted to explore the characteristics of macroinvertebrate communities in winter,the composition of macroinvertebrate communities in riffles and pools during winter?pre-freezing period,freezing period,and thawing period?,and the distribution characteristics of litter patches on riverbed during winter and the effect of litter patches on macroinvertebrate communities.On the basis of these,the in-situ control experiments were carried out using litter decomposition bags?A.mono,T.amurensis and Q.mongolica litter?to explore the decomposition of single and mixed leaf litter and the colonization of macroinvertebrates during winter,as well as the effect of different habitats on leaf litter decomposition and macroinvertebrates colonization.The main results are summarized as follows:?1?The cold temperature and freezing have caused habitat quality deterioration,which led to the great difference between the macroinvertebrate communities in winter and that in summer and autumn,and disrupted the stability of functional feeding groups structure.The density,species richness,and Margalef diversity index of macroinvertebrates all decreased to the lowest in winter.A total of 32 taxa of macroinvertebrates were identified in winter,pertaining to 2classed,6 orders,and 18 families,and aquatic insects were dominant.The density of macroinvertebrates was 2057(ind.m^-2),among which,the density and relative abundance of Plecoptera were the highest.In terms of the functional feeding groups,there were more collector-gatherers and predators,followed by shredders,collector-filterers,and scrapers.?2?There were significant differences in the composition of macroinvertebrate communities between riffles and pools.More taxa with swimming ability?such as Pteroptera,Ephemeroptera,and Gammarus sp.?were distributed in riffles,while more taxa without swimming ability?such as Chironomidae spp.?colonized in pools.The species density,shredder density,species richness,and Shannon-Wiener index of macroinvertebrates in riffles were significantly higher than in pools during winter.The differences in habitat heterogeneity and stability affected by characteristics of physical structure,such as hydraulic conditions and sediment composition,maybe the main explanation for the differences in macroinvertebrate communities.?3?There were significant differences in the accumulation characteristics of litter patches between riffles and pools.Comparing with the litter patches in the pools,the detritus area of litter patches in the riffles was smaller,the depth was lower and the velocity was higher;the abundance and relative abundance of leaf litters were higher,and the relative abundance of CPOM was lower.The main functional feeding groups in the two litter patches were collector-gatherer,predators,and shredder,but there were significant differences in macroinvertebrate community composition between riffle litter patches and pool litter patches.More predators?Chloroperlidae sp.?,collector-gatherers?Ephemeterella sp.?,and shredders?Gammarus sp.and Nemoura sp.?with swimming ability colonized in riffle litter patches,but more shredders?Nemotaulius sp.and Hexatoma sp.?without swimming ability colonized in pool litter patches.On the whole,the macroinvertebrate density and shredder density in riffle litter patches were higher than in pool litter patches,and the shredder density was significantly positively correlated with the abundance and relative abundance of leaf litters in litter patches.Therefore,the hydraulic conditions and composition characteristics of litter patches affected the composition of macroinvertebrate communities in the headwater stream during winter.In addition,compared with riverbed,macroinvertebrate density was higher in litter patches,and community structure was more stable during winter,indicating that litter parches were stable habitats,and could play an important role in maintaining the abundance and stability of macroinvertebrate communities in winter.?4?The litter of A.mono?k=0.0152?,T.amurensis?k=0.0162?and mixed species litter?k=0.0104?decomposed at rapid decomposition rates,and Q.mongolica?k=0.0064?decomposed at a moderate decomposition rate during winter.Macroinvertebrates community colonization on litter bags changed through time regardless of leaf species.Litter bags of A.mono,T.amurensis and mixed species supported higher abundances of macroinvertebrates in the freezing period,while litter bags of Q.mongolica supported stable abundances of macroinvertebrates during winter.Shredders colonized on the litter bags and reached maximum abundance in the freezing period,during which they significantly promoted leaf litter mass loss,indicating that this period is the crucial stage for macroinvertebrates to consume leaf litter during the winter.The Gammarus sp.was found to be the most important shredder species for leaf litter decomposition,and showed significant feeding selectivity for different species of litter.The decomposition rates of A.mono and T.amurensis were 1-fold faster than those of Q.mongolica,implying that the litter of the first two species was preferred by the shredders in this stream.The mixture of the litter of A.mono,T.amurensis,and Q.mongolica had little effect on the litter decomposition processes during winter.However,there were significant differences in macroinvertebrate communities on different litter bags,highlighting the importance of leaf litter diversity for the support of macroinvertebrate communities.?5?The litter decomposition rates were significantly different between the two habitats,and the decomposition rates were faster in riffles.The colonization process of macroinvertebrates was also different between riffles and pools.Macroinvertebrate abundance and shredder abundance colonized on litter bags of A.mono and T.amurensis in riffles reached the highest in the freezing period,which was significantly higher than that in pools.However,macroinvertebrate abundance remained stable on litter bags in pools during winter.The faster velocity and higher shredder abundance in riffles were the main reasons for the faster decomposition rate of litters.At the same time,the litter decomposition rate could affect its palatability as food and stability as habitat,which also affect the colonization process of macroinvertebrates.Based on the above obtained outcomes from our experiments,the process of litter decomposition and macroinvertebrate colonization in the boreal stream was elucidated.We acquired further knowledge and insights into the characteristics of macroinvertebrate communities and their interaction with litter decomposition during winter in the boreal stream.This study was aimed to provide a scientific theoretical basis for the in-depth understanding of the structure and function of the stream ecosystem in the cold regions;to deliver practical reference value for biodiversity protection,and evaluation,and ecological restoration of the river ecosystem.