Dynamic metal speciation analysis in aquatic ecosystems is emerging as a powerful basis for development of predictions of bioavailability and reliable risk assessment strategies. A given speciation sensor is characterized by an effective time scale or kinetic window that defines the measurable metal species via their labilities. Here we review the current state of the art for the theory and application of dynamic speciation sensors. We show that a common dynamic interpretation framework, based on rigorous flux expressions incorporating the relevant diffusion and reaction steps, is applicable for a suite of sensors that span a range of time scales. Interpolation from a kinetic spectrum of speciation data is proposed as a practical strategy for addressing questions of bioavailability. Case studies illustrate the practical significance of knowledge on the dynamic features of metal complex species in relation to biouptake, and highlight the limitations of equilibrium-based models.