Millennial-aged organic carbon subsidies to a modern river food web

Recent studies indicate that highly aged material is a major component of organic matter transported by most rivers. However, few studies have used natural C-14 to trace the potential entry of this aged material into modern river food webs. Here we use natural abundance C-14, C-13, and deuterium (H-2) to trace the contribution of aged and contemporary organic matter to an important group of consumers, crustacean zooplankton, in a large temperate river (the Hudson River, New York, USA). Zooplankton were highly C-14 depleted (mean Delta C-14 = -240 parts per thousand) compared to modern primary production in the river or its watershed (Delta C-14 = -60 parts per thousand to +50 parts per thousand). In order to account for the observed C-14 depletion, zooplankton must be subsidized by highly aged particulate organic carbon. IsoSource modeling suggests that the range of the aged dietary subsidy is between similar to 57%, if the aged organic matter source was produced 3400 years ago, and similar to 21%, if the organic carbon used is >= 50 000 years in age, including fossil material that is millions of years in age. The magnitude of this aged carbon subsidy to river zooplankton suggests that modern river food webs may in some cases be buffered from the limitations set by present-day primary production.