New strategies for measuring rates of environmental processes in rivers, lakes, and estuaries

A central goal in limnology is measurement of physical, biogeochemical, and biological process rates. We can measure process rates from the temporal and spatial patterns they create in a measured variable, and we use 3 approaches for making those measurements: the fixed-site approach for detecting temporal pattern at a location, the snapshot approach for detecting spatial pattern at an instant in time, and the flow path approach for detecting temporal pattern as it changes through space. To compare and contrast these approaches, we present patterns in temperature collected simultaneously based on all 3 approaches. Translating these patterns into process rates requires different assumptions for each approach, and these assumptions lead to uncertainty in process rates. We propose that these assumptions and related uncertainty can be reduced by making simultaneous measurements based on all 3 approaches. Each approach fills gaps in the spatial and temporal patterns measured by the others, and these patterns can be combined to derive a process rate. We develop a conceptual theory to support this strategy for measuring process rate based on 2 criteria: the mixing time of a water body and the analytical limitations of the measurement. This new strategy for measuring process rates in aquatic environments has the potential to increase the resolution of rate measurements, reduce their uncertainty, and enhance limnologists' ability to resolve process rates from an increasing flow of environmental data.