Organic matter budgets and carbon and nitrogen economies

Stable isotope data for functional groups and model predictions

The goal of this workshop is to compare the use of organic matter sources in food webs across a wide variety of estuaries. Our null hypothesis was that the isotopic composition of consumer organisms would reflect organic matter in proportion to its contribution to the organic matter budget of the estuary. To test this hypothesis, we completed stable isotope (13C, 15N and 34S) analysis of producer and consumer organisms in estuaries with organic matter budgets dominated by algal, riverine and marsh inputs. We chose organisms which belonged to functional groups in the food web.

The workshop is still in progress as we are completing comparisons of observed isotopic values to isotopic values predicted by a model based on the organic matter budgets and trophic fractionation. In addition, we are using the observed isotopic values of the functional groups to back-calculate their respective relative sources of organic matter. For the back-calculation we are contrasting the use of two isotopes (13C, 15N) vs three isotopes (13C, 15N and 34S).