LB 2-20 - A theoretical and empirical approach to understanding the impacts of dormancy on the ecological dynamics of phytoplankton

Phytoplankton contribute significantly to global biogeochemical cycles, produce ~50% of the world’s oxygen, and form the bottom of the marine food web. Their abundance and productivity are strongly influenced by their environment, including temperature. Dormancy enables individuals of many phytoplankton species to survive stressful or even otherwise fatal environmental conditions by reducing metabolic costs and often by changing their morphology. Dormancy is an especially important process to understand as more species encounter increasing temperatures and larger thermal fluctuations. The consequences of dormancy may be complex: dormant phytoplankton are more likely to survive harsh conditions, but if they stay dormant too long they may be unable to germinate. However, there is little experimental work quantifying either the viability of dormant individuals, the external cues causing dormancy and germination, or how quickly they occur. This information is critical to understanding and modeling the effects of dormancy on phytoplankton population and community dynamics. The goal of our study is to better understand how dormancy affects the ecology of phytoplankton, which we achieve by combining experiments and theory. Experimentally, we sought more information about viability of dormant individuals as well as abiotic cues for germination/encystment. We collected alpine lake sediment cores from Yosemite National Park in California and then experimentally germinated phytoplankton from different sediment depths (ranging in age from near present day to more than a century old) under a range of thermal environments. Older phytoplankton had significantly lower germination rates, and these rates varied with temperature and phytoplankton functional group. Theoretically, we examined the effects of different dormancy strategies on phytoplankton population dynamics. We developed an ecological model of phytoplankton dynamics under variable temperatures, for structured populations that engage in dormancy. We examine competition between phytoplankton with two different strategies: constant rates of germination/encystment and temperature-dependent rates (qualitatively related to our empirical results). Across three different fluctuating temperature regimes (optimal, cold, hot), we found that a temperature-dependent dormancy strategy is competitively superior only under conditions that are optimal on average. Our work shows that dormancy can strongly affect population dynamics, and that these effects vary depending on environmental conditions and the complexity of dormancy strategies species use. Additional experiments and theory are needed to study the diversity of dormancy responses across taxa, and to determine how interactions between dormancy and environment may influence future phytoplankton dynamics under climate change.