Assessing Sources and Ages of Organic Matter Supporting River and Estuarine Bacterial Production: A Multiple-Isotope (Δ 14C, δ 13C, and δ 15N) Approach

We used radiocarbon $(\Delta {}^{14}\text{C})$ and stable isotopic (δ 13C, δ 15N) signatures of bacterial nucleic acids to estimate the sources and ages of organic matter (OM) assimilated by bacteria in the Hudson River and York River estuary. Dualisotope plots of Δ 14C and δ 13C coupled with a three-source mixing model resolved the major OM sources supporting bacterial biomass production (BBP). However, overlap in the stable isotopic (δ 13C and δ 15N) values of potential source end members (i.e., terrestrial, freshwater phytoplankton, and marsh-derived) prohibited unequivocal source assignments for certain samples. In freshwater regions of the York, terrigenous material of relatively recent origin (i.e., decadal in age) accounted for the majority of OM assimilated by bacteria (49-83%). Marsh and freshwater planktonic material made up the other major source of OM, with 5-33% and 6-25% assimilated, respectively. In the mesohaline York, BBP was supported primarily by estuarine phytoplankton-derived OM during spring and summer (53-87%) and by marsh-derived OM during fall (as much as 83%). Isotopic signatures from higher salinity regions of the York suggested that BBP there was fueled predominantly by either estuarine phytoplankton-derived OM (July and November) or by material advected in from the Chesapeake Bay proper (October). In contrast to the York, BBP in the Hudson River estuary was subsidized by a greater portion (up to ∼25%) of old (∼24,000 yr BP) allochthonous OM, which was presumably derived from soils. These findings collectively suggest that bacterial metabolism and degradation in rivers and estuaries may profoundly alter the mean composition and age of OM during transport within these systems and before its export to the coastal ocean.