Eight decades of phenological change for a freshwater cladoceran: what are the consequences of our definition of seasonal timing?

Summary 1. Changes in the seasonal timing of re‐occurring biological events, or phenology, are a widely reported ecological response to environmental change. Previous studies have demonstrated that plankton populations have shifted their phenology in recent decades but there is a lack of consistency with respect to the phenological metrics analysed. 2. We analysed an eight‐decade data set (1934–2009) on the seasonal dynamics of Daphnia galeata in the North Basin of Windermere, U.K. Rates of phenological change derived from ten different phenological metrics were compared. We evaluated the evidence for the effects of spring water temperature, phytoplankton phenology and over‐wintering population size on D. galeata phenology. 3. Nine of the ten phenological metrics showed statistically significant trends towards earlier seasonal timing, although rates of change varied (3.7–6.7 days per decade). Regression analyses showed a consistent effect of spring water temperature and phytoplankton phenology on the timing of D. galeata spring population development. The amount of variability explained by these drivers, the precise phytoplankton metric related most closely to D. galeata phenology and the importance of over‐wintering population size differed markedly among D. galeata metrics. 4. Hierarchal models showed that the seasonal timing of the phytoplankton peak had the most consistent effect upon D. galeata phenology and that temperatures in the month previous to the average timing of population development were influential. 5. Phenological metrics differ mathematically and conceptually. They indicate different population dynamical processes and are influenced by different ecological mechanisms. Combining information from different phenological metrics will greatly improve mechanistic understanding of the factors influencing phenological change.