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We used radiocarbon ([[DELTA].sup.14]C) and stable isotopic ([[delta].sup.13]C, [[delta].sup.15]N) 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 [[DELTA].sup.14]C and [[delta].sup.13]C coupled with a three-source mixing model resolved the major OM sources supporting bacterial biomass production (BBP). However, overlap in the stable isotopic ([[delta].sup.13]C and [[delta].sup.15]N) 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.

Food webs in aquatic systems can be supported both by carbon from recent local primary productivity and by carbon subsidies, such as material from terrestrial ecosystems, or past in situ primary productivity. The importance of these subsidies to respiration and biomass production remains a topic of debate. While some studies have reported that terrigenous organic carbon supports disproportionately high zooplankton production, others have suggested that phytoplankton preferentially support zooplankton production in aquatic ecosystems. Here we apply natural abundance radiocarbon (Δ14C) and stable isotope (δ13C, δ15N) analyses to show that zooplankton in Lake Superior selectively incorporate recently fixed, locally produced (autochthonous) organic carbon even though other carbon sources are readily available. Estimates from Bayesian isotopic modeling based on Δ14C and δ13C values show that the average lake-wide median contributions of recent in-lake primary production and terrestrial, sedimentary, and bacterial organic carbon to the bulk POM in Lake Superior were 58 %, 5 %, 33 %, and 3 %, respectively. However, isotopic modeling estimates also show that recent in situ production contributed a disproportionately large amount (median, 91 %) of the carbon in mesozooplankton biomass in Lake Superior. Although terrigenous organic carbon and old organic carbon from resuspended sediments were significant portions (median, 38 %) of the available basal food resources, these contributed only a small amount to mesozooplankton biomass. Comparison of zooplankton food sources based on their radiocarbon composition showed that terrigenous organic carbon was relatively more important in rivers and small lakes, and the proportion of terrestrially derived material used by zooplankton correlated with the hydrologic residence time and the ratio of basin area to water surface area. [ABSTRACT FROM AUTHOR]

Food webs in aquatic systems can be supported both by carbon from recent local primary productivity and by carbon subsidies, such as material from terrestrial ecosystems or past in situ primary productivity. The importance of these subsidies to respiration and biomass production remains a topic of debate, but they may play major roles in determining the fate of organic carbon and in sustaining upper trophic levels, including those contributing to economically important fisheries. While some studies have reported that terrigenous organic carbon supports disproportionately high zooplankton production, others have suggested that phytoplankton preferentially supports zooplankton production in aquatic ecosystems. Here we apply natural abundance radiocarbon (Δ14C) and stable isotope (δ13C, δ15N) analyses to show that zooplankton in Lake Superior selectively incorporate recently-fixed, locally-produced (autochthonous) organic carbon even though other carbon sources are readily available. Estimates from Bayesian isotopic modeling based on Δ14C values show that the average lakewide median contributions of recent in situ algal, terrestrial, sedimentary, and bacterial organic carbon to the bulk POM in Lake Superior were 23 %, 28 %, 15 %, and 25 %, respectively. However, the isotopic modeling estimates show that recent in situ production (algae) contributed a disproportionately large amount (median, 40-89 %) of the carbon in zooplankton biomass in Lake Superior. Although terrigenous organic carbon and old organic carbon from resuspended sediments were significant portions of the available basal food resources, these contributed only a small amount to zooplankton biomass (average lakewide median, 2% from sedimentary organic carbon and 9% from terrigenous organic carbon). Comparison of zooplankton food sources based on their radiocarbon composition showed that terrigenous organic carbon was relatively more important in rivers and small lakes, and the proportion of terrestrially-derived material used by zooplankton correlated with the hydrologic residence time and the ratio of basin area to water surface area. [ABSTRACT FROM AUTHOR]

We used radiocarbon (Δ14C) and stable isotopic (δ13C, δ15N) 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. Dual-isotope plots of Δ14C and δ13C coupled with a three-source mixing model resolved the major OM sources supporting bacterial biomass production (BBP). However, overlap in the stable isotopic (δ13C and δ15N) 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. [ABSTRACT FROM AUTHOR]