Time scales of coupled and de-coupled biogeochemical cycles
The transformation and transport of various elements are linked, both directly and indirectly, by a variety of physical, chemical, and biological processes, and intensive study of these interactions has continued to elucidate novel connections among element cycles. The robust theoretical basis for coupling of element cycles via stoichiometry and thermodynamics,and the compelling theoretical support for these mechanisms, suggest that coupled cycles should be the operant null hypothesis for the study of many biogeochemical processes, particularly macronutrients and elements with stong links to oxidation-reduction reactions. This shift in perspective should force us to ask: when, where, and how are major element cycles de-coupled? In this study, we use empirical and theoretical approaches to understand the coupling and de-coupling of C,N, and P cycles at relatively fine temporal scales. Specifically, we assess the coupling and de-coupling of C, N, and P cycles in the Ichetucknee River and other spring-fed Florida rivers at hourly, diel, and seasonal time scales, based on continous high frequency in situ measurements of inorganic nutrient concentrations. We complement this empirical study with an ecosystem model that incorporates dynamic organismal stoichiometry and kinetics to assess the time scales over which element cycles are linked and over which they diverge.
At seasonal time scales, primary productivity (C fixation) is strongly correlated with N and P uptake, but finer-scale interactions among these elements are shaped by indirect interactions. In the case of N, day-to-day variation in productivity influence the apparent magnitude of denitrification. Fine-scale P dynamics, in contrast, are influenced by geochemical interactions with photosynthetically-mediated calcite precipitation. Despite their mutual dependence on ecosystem metabolism, diel variation in N and P assimilation are distinctly out of phase, with N uptake peaking near the time of maximum photosynthesis, and maximal P uptake occuring well after sundown. Our model suggests one hypothesis to explain this de-coupling: that differential uptake kinetics can cause indepent variation in N and P uptake at fine scales. Alternatively, differential timing of N and P assimilation may reflect active regulation of the different cellular processes (synthesis of proteins and nucleic acids, respectively) that regulate N and P demand. These results illustrate that direct and indirect interactions shape interactions among element cycles at fine temporal scales, and highlight the need for theory that addresses the mechanisms that de-couple element cycles and the spatial and temporal scales over which they operate.