Deciphering chrysophyte responses to climate seasonality

Climate change involves alterations in seasonality as well as shifts in mean annual temperature. Cold temperate lakes show strong seasonality, with winter ice cover and alternating mixing and stratification periods during the ice-free season. These physical changes are ultimately related to seasonal weather variation and also drive annual phytoplankton succession. Therefore, phytoplankton remains in lake sediment records are potentially useful for reconstructing past seasonal climate signals. With the exception of investigations on varved sediments, however, little research has been carried out on the subject. Here we present two lines of evidence demonstrating that chrysophyte stomatocysts can be useful for inferring past climatic seasonality. First, we show that marked seasonal stomatocyst replacement is related to periods of the main physical processes in the lake. Second, using instrumental climate data and microfossils in sediment cores, we show that two main components of stomatocyst variability over the last ~150 years are related to seasonal fluctuations. The first of these components is related to stomatocysts present during summer stratification that respond to general warming trends, particularly in July and August. The second component relates to stomatocysts typically present in the spring and autumn mixing periods. Numbers of these two stomatocyst types vary inversely in response to variation in spring (April-May) temperatures. The number of stomatocyst types affected by spring temperatures is much greater than those related to summer temperatures. We provide evidence that chrysophyte stomatocysts are an excellent proxy for spring air temperature reconstructions, with little influence from summer or general annual trends. We also show that with relatively little effort, it is possible to categorise the chrysophyte stomatocyst assemblage of a lake to carry out detailed investigation of the record of seasonal changes preserved within the sediment.