論文の公開元へA study on temporal scales in the variation of a zooplankton community in a small lake
概要
Ecological communities are assembled by complex interaction processes such as niche-related selection, dispersal, and ecological drift. Since zooplankton is an important component in freshwater ecosystems as consumers of phytoplankton and prey of planktivorous fish, many studies have been conducted to elucidate factors determining the patterns of the community and population dynamics. However, although insight into these processes strongly depends on the observational scale. Most previous studies on freshwater zooplankton have focused on the spatial scales, creating a void in knowledge related to temporal scale focused zooplankton dynamics. Furthermore, the timing of iterating biological and seasonal environmental events is changing on a global scale caused by increasing temperature and other climatic drivers. Accordingly, studies on phenology patterns of aquatic organisms in temperate ecosystems are important to identify driving forces affecting the community structures and thus population dynamics of zooplankton at various temporal scales. In this thesis, therefore, I focused on various aspects of temporal changes in the zooplankton community in Lake Hataya Ohnuma, a small mountain lake in Yamagata, Japan, to elucidate if the driving forces affecting the structure vary depending on temporal scales.
In the 2nd chapter, I examined the temporal β-diversity of the zooplankton to examine how compositional changes in the community vary depending on time scales. The zooplankton community was monitored for nine years from 2008 to 2016 with monthly samplings. We estimated the inter- annual, inter-seasonal, intra-seasonal, and inter-month β-diversity with the magnitude of sampling errors and examined relative contributions of these time scales on changes in the zooplankton community structure. The temporal β-diversity was high at the inter-seasonal scale than at the intra- seasonal and inter-annual time scales, suggesting that taxonomic turnover in the zooplankton community was regulated by seasonally changing parameters such as water temperature. Indeed, the temporal β-diversity is related significantly to water temperature, although no significant relationship was detected between the temporal β-diversity and chlorophyll-a concentrations. In addition, the large inter-month β-diversity with significant inter-annual β-diversity indicated that the phenology of the zooplankton community changed depending on years, especially before and after 2012. These results indicate that temporal β-diversity is a useful gauge for detecting phenological changes and quantifying plankton communities' temporal stability at various time scales.
In the 3rd chapter, I examined the temporal β -diversity of the zooplankton community in Lake Hataya Ohnuma if the relative importance of abiotic and biotic factors regulating the zooplankton community varies depending on the temporal scales. The analysis of temporal β-diversity at various temporal scales with environmental variables showed that water temperature was a prime factor regulating the inter-seasonal variations of the community structure, while algal food conditions were more important in the intra-seasonal variations. Algal food conditions were also important for the variation in the community structures in the same month but different years. Temporal dissimilarity in terms of body size tended to be larger in inter-annual scale than inter-seasonal scale, suggesting that predation pressure can be a factor explaining temporal variations of the zooplankton community over a single year. These results show that the relative importance between biotic and abiotic factors regulating the zooplankton community change depending on the temporal scales examined.
In the 4th chapter, I examined the pattern of temporal dynamics of the zooplankton community in Lake Hataya Ohnuma. Specifically, I analyzed whether it exhibited synchrony or compensatory dynamics if the relative importance of these patterns changed depending on time scales and what abiotic and biotic factors contributed to these patterns. For these objects, I utilized a wavelet analysis and computed a wavelet modulus ratio to quantify synchrony and compensation dynamics among zooplankton species along with different time scales. Results revealed that patterns of synchrony and compensation varied according to the timescale focused. In Lake Hataya Ohnuma, the zooplankton community exhibited compensatory dynamics at seasonal and sub-annual times scale during the stagnant period when water temperature variation was high, especially in years with low food conditions and low predation pressures by fish. The results showed that the wavelet analysis was useful to unveil the pattern of community dynamic and to identify its temporal scales intrinsic within plankton communities, which would often be hidden by a large difference in the temporal variations among zooplankton species.
In the last chapter, I examined a subset of the zooplankton community studied in the previous chapters to explore ecological mechanisms causing the species-specific difference in the temporal patterns. For this object, I examined the genetic composition of the rotifer community in lake Hataya Ohnuma and investigated whether genetically distant species occupy different temporal niche spaces or vice versa. I also examined the temporal niche similarity among the rotifer species and which environmental conditions might promote these covariances in species occurrences. The analyses showed that phylogenetic distance among the rotifer species correlated significantly and positively with temporal niche overlap, suggesting that genetically similar species occupy different temporal niche spaces probably due to a result, or avoidance, of competitive interactions. A pair of rotifer species that showed significant niche segregations preferred different temperatures from each other, although these species generally showed a preference for similar food quantity and quality. The result implies that water temperature is an important prime factor determining patterns of temporal dynamics at the population as well as community levels and the variations of these patterns among different time scales.
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