Land-use change, urban development, and climate change are altering the magnitude and nature of riverine inputs of organic matter and inorganic nutrients to the coastal zone and oceans throughout the world. Such long-term, and probably irreversible, changes in riverine inputs can be expected to trigger corresponding changes in both global and coastal carbon budgets and in coastal food webs, eutrophication, and anoxia and species diversity. A mechanistic understanding of these changes requires a substantial improvement in our ability to treat quantitatively the transformation of organic matter during riverine transport.
In our prior separate research, we found that the bioavailability of DOM in a southern river is related to the aliphatic carbon content of the DOM, as indicated by its average elemental composition. We further found with a bioenergetic model that the rate and efficiency at which many simple organic substrates are oxidized and incorporated into microbial biomass are closely related to the degree of reduction of those substrates, as deduced from their elemental composition. Preliminary joint research showed that these two predictors of bioavailability of DOM are strongly correlated (as expected from theory) and about equally successful.
We plan to address 3 research questions: 1. How does the bioavailability of riverine DOM vary in stream and rivers? 2. Can we predict the bioavailability of DOM from simple measures of bulk composition? 3. Can bioavailability of DOM be predicted from a simple measurement of chemical oxygen demand? With the use of our bioenergetic model of microbial growth and techniques to describe the degree of substrate reduction of labile and bulk DOM pools, we will examine relationships between the composition and utilization of bulk and labile pools. Our goal is to test our model that was developed for known labile DOM pools to predict the rates of bulk organic matter utilization and microbial growth from simple measurements of the composition of the bulk DOM pool. This is a successful model of organic matter utilization that provides a causal, mechanistic understanding microbial processes on the basis of simple measures of chemical composition.
Our work to date is preliminary since DOM samples were collected from only a few sites, a systematic approach to investigate a spectrum of DOM sources was not employed, and the techniques to perform the experiments as well as the theories regarding bioavailability were under development at the time. We are now in a position to rigorously test our models and propose to do so by conducting experiments in streams and rivers of greatly contrasting characteristics. We will measure organic matter composition (aliphatic, aromatic and excess carbon and degree of substrate reduction) and bioavailability (microbial growth, growth efficiency and organic matter oxidation). Based on our models, a strong correlation should be observed between DOM bioavailability and aliphatic C content and degree of substrate reduction of the DOM.
This proposal was submitted to the EGB Program, and is being jointly funded by:
1. Division of OCE - (Rice) 2. Division of DEB - (Firth)
