Potassium enrichment stimulates the growth and reproduction of a clone of Daphnia dentifera.

Nutrient limitation commonly constrains organisms in natural ecosystems. Typically, ecologists focus on limitation by N and P. However, other nutrients can limit growth or reproduction. Here we focus on K limitation of invertebrate consumers (Daphnia dentifera) and phytoplankton in freshwater lakes. All organisms require K for several metabolic processes. In freshwater, K could limit growth because low external concentrations can increase the energetic costs of accumulating K. Furthermore, in a study linking K to disease, we previously found that K enrichment of water from one low-K lake stimulated the growth and reproduction of Daphnia. Here we test whether K could limit the production of Daphnia and phytoplankton across lakes and years. We repeated a life table experiment using water collected from a low-K lake during a different year. K again stimulated Daphnia reproduction. We also enriched water from 12 lakes with K or P and measured short-term growth of Daphnia and the resident algal community. Both nutrients increased Daphnia growth in five lakes. However, only P enhanced algal production. P stimulation of Daphnia positively correlated with algal quantity and the ratio of C to P in seston. However, K stimulation of Daphnia was not correlated with these factors or the background concentration of K. Thus, this study shows repeatable K-limited animal physiology in nature. Further, we can exclude the hypothesis that K stimulates Daphnia indirectly by enhancing algal production. These patterns call for future physiological studies to uncover the mechanistic basis of K limitation in natural systems.