With funding from the Ecosystem Studies Program of the National Science Foundation (DEB), IES researchers Drs. Caraco and Cole, with colleagues Dr. James Bauer (Virginia Institute of Marine Science) and Dr. Peter A. Raymond (Yale University) are asking: "How old is the terrestrial organic matter that is transported and metabolized by large rivers?"

Intriguingly, recent radiocarbon data demonstrate that at least for some rivers, the average age of dissolved and particulate organic C can be quite old, centuries to millennia older than the leaves on the trees or the forest floor. The C budgets of rivers suggest that about half the terrestrial inputs are metabolized during the short time (weeks) that this organic matter resides in the river during transport. This rapid metabolism of terrestrial C could imply that terrestrial organic matter or considerable age fuels the food web in a river. Alternatively, if younger organic C is selectively metabolized, older, (¹⁴C-depleted) organic would make up a disproportionate fraction of the dissolved and particulate material transported to the coastal zone.

The Hudson River at Rhinecliff, New York.

The goal of this work is to determine the delivery and fates of Aged Terrestrial Organic C (ATOC) in rivers. Put simply, the researchers seek to find out what happens to old (¹⁴C depleted) terrestrial organic matter once it enters a river. How variable is the delivery of ATOC seasonally and among tributaries feeding a major river system? How much of the ATOC is exported? How much ATOC is decomposed within the river and what regulates the balance between decomposition and export? Finally, does ATOC enter the riverine food web and to what extent does it fuel riverine metabolism, relative to younger terrestrial inputs or riverine primary production? A combination of observational, experimental and modeling approaches are being used to answer these questions. The researchers are focusing on the Hudson River because prior work has shown that for this river, terrestrial organic matter of considerable age (1000 to 5000 years old) dominates C inputs to this system.

This study will significantly enhance the scant database on the age distribution of organic matter in rivers; the fate of differently aged organic matter within rivers; and the delivery of ATOC to the coastal zone. Further, this work will provide better understanding of the conditions that promote decomposition or export of this old, terrestrial C. Finally this work will facilitate the exploration of the value of ambient ¹⁴C as a food web tracer by tracking its abundance in specific riverine organisms.