Stream flow intensity of the Saavanjoki River, eastern Finland, during the past 1500 years reflected by mayfly and caddisfly mandibles in adjacent lake sediments

Climate-change projections suggest altered precipitation, temperature, and hydrological patterns for the near future. To better understand these future changes, long-term evidence from paleoarchives is necessary to determine the natural variability and climate relations in surficial hydrology. In this study, we aim to demonstrate that distribution of sedimentary mouthparts (mandibles) of aquatic mayfly (Ephemeroptera) and caddisfly (Trichoptera) larvae are dependent on stream conditions and this relationship can be used in paleohydrological studies to reconstruct past changes in stream flow. The results from a river–lake continuum in eastern Finland showed that the surface sediment accumulation of mayfly and caddisfly mandibles was strongly related to stream flow with highest abundances found from the riverine sediments. A sediment core taken from the deep basin of the lake revealed mandible accumulation patterns that correlated with a previous independent paleohydrological reconstruction. We interpret that the primary forcer for mandible deposition to lake sediments is spring floods caused by snow melt, and hence the mandible accumulation, represents amount of winter precipitation. Subsequently, the results indicated strong stream flow and wet winters during the Medieval Climate Anomaly (MCA) and weaker stream flow and dry winters during and after the Little Ice Age (LIA). This wintertime reconstruction contrasts the general summertime trends of dry MCA and wet LIA. The present results confirm the usability of mayfly and caddisfly mandibles in paleohydrological studies and provide evidence for past hydrological dynamics that can be valuable when addressing the potential magnitude of future changes.