<p>Interspecific synchrony (i.e. synchrony among species pairs) influences community synchrony (i.e. synchrony among all species) and thereby the temporal variability of the entire community and ecosystem functioning. Yet, the mechanisms driving synchrony remain difficult to identify. Recent studies suggest that both the timescale and the functional similarity, determined by functional traits, influence interspecific synchrony, but both have rarely been considered together. Furthermore, plankton population dynamics differ among seasonal phases representing contrasting environmental conditions, but interspecific synchrony across phases remain understudied. Thus, we analysed 20 years of phytoplankton dynamics in Lake Constance and explored (i) how community and interspecific synchrony vary across timescales; (ii) the functional similarity-interspecific synchrony relationships across timescales; and (iii) how interspecific synchrony and its relationship with functional similarity depend on seasonal phases. Community and interspecific synchrony varied across timescales: they were notably highest at the annual scale and lowest at longer scales. Furthermore, we found positive functional similarity-interspecific synchrony relationships depending on the timescale and the seasonal phase. Functional similarity explained up to 20% of variance in interspecific synchrony at the subannual and annual timescales, and up to 14% between late spring and summer at shorter timescales. Overall, this study highlights that timescale, functional similarity and seasonal phase are crucial factors to understand interspecific synchrony. Additionally, investigating synchrony at the species pair level provides insights into community synchrony, which is key to predict community stability.</p>