Organic matter cycling in the York River estuary, Virginia: An analysis of potential sources and sinks

A study of the organic matter (OM) sources and biogeochemical and physicochemical sinks was undertaken in the York River estuary, Virginia. The reactivity of dissolved organic carbon (DOC) was enhanced from ∼25--68% by the combined effects of exposure to natural sunlight and bacterial decomposition. In contrast, sunlight exposure decreased the bioreactivity of DOC in the higher salinity lower York by a factor of five. The combined effects of photochemical and bacterial processing were found to modify both the bioavailability and metabolic fate of OM (e.g. respiration vs. biomass). Stable isotopic (delta13C, delta15N) and radiocarbon (Delta14C) values of bacterial nucleic acids were used to estimate the sources and ages of OM assimilated by bacteria in the York and Hudson River estuaries. Bacterial production in freshwater regions of the York was fueled by OM of young, terrigenous origin which accounted for 42--89% of OM assimilated. The remainder (11--58%) of OM assimilated was derived from freshwater algae. In the mid-salinity York, bacterial production was supported by phytoplankton-derived OM in the spring and summer (93--100%) and marsh-derived OM in the fall (73--100%). Isotopic values of bacteria in the lower York suggested production was supported by phytoplankton-derived OM (86--100%) in July and November and algal and marine-like OM (50--69%) in October. In contrast to the young (10--20 yr) OM assimilated by bacteria in the York, production in the Hudson River was subsidized by old (∼1200 BP) terrigenous OM. Higher C:N ratios, lower delta13C and delta 15N values and depletions of total lipid and lipid compound classes in high molecular weight dissolved organic matter (HMW DOM (≥3kDa)) relative to particulate organic matter (POM), suggested differences in the reactivity and cycling of these two OM fractions. Within the dissolved pool, polyunsaturated fatty acids (FA) were a strong predictor of DOC decomposition in bioassays. FA and sterol distributions suggest that POM is derived from phytoplankton/zooplankton sources, while HMW DOM has a bacterial and vascular plant signature. Thus, the physical form of OM (particulate vs dissolved) may affect both the distribution and biogeochemical processing of OM such that terrigenous DOM may be exported, while POM is retained within the estuary