Studying population dynamics in Daphnia pulex through fine scale temporal sampling

Daphnia, a common aquatic microcrustacean, can alternate between asexual and sexual reproduction. It is hypothesized that at the beginning of a growing season, sexually produced dormant eggs hatch, introducing large amounts of genetic diversity. Over time, selection for the most fit clones causes genetic diversity to decrease over time. At the end of the growing season, as conditions deteriorate, Daphnia switch to sexual reproduction, producing dormant resting eggs for the next growing season before dying out over the winter. This traditional view of Daphnia life history is frequently assumed, but lacks empirical evidence to support this notion. We tested these assumptions by sampling 8 ponds in Dorset, United Kingdom every 2-3 weeks over the course of a single growing season (March – August). Samples were sorted based on life stage. We found a more complex process at play that what was previously asserted. There was high variability in life history between ponds even within close proximity. Some ponds were ephemeral and were recolonized by hatching at the beginning of the growing season, while other ponds overwintered and reproduced asexually throughout the entire year. Additionally, the summer of 2018 was the hottest in the United Kingdom since 1976, causing many ponds to dry out early. Prior to drying out, these ponds became primarily populated with a different, though ecologically similar, genus, Simocephalus, suggesting competition had a substantial impact on the Daphnia populations. Our sampling efforts provide valuable insight into Daphnia population dynamics and provide a baseline for comparison with future generations.