Nitrogen Flow through the Food Web in the Oligohaline Zone of a New England Estuary

We introduced tracer levels of15N-enriched nitrate into the oligohaline reach of the Parker River estuary (Massachusetts, USA) to study the transformation and fate of watershed-derived nitrogen during the biologically productive summer period. Intensive temporal and spatial measurements were made of the isotopic label in a wide range of organisms during the15N amendment and for 2 mo thereafter. The tracer helped determine the relative trophic importance of three major sources of organic nitrogen supporting the food web of the upper Parker River estuary: planktonic and benthic primary producers, and detritus derived mostly from the surrounding marsh. The planktonic centric diatom Actinocyclus normanii was the primary vector of tracer nitrogen to benthic and water-column organisms. Fauna that attained the highest15N-tracer content (planktonic copepods, planktivorous juvenile fishes, several macrofaunal crustaceans) derived a major part of their incorporated nitrogen (∼ 50% or more) from the bloom-forming planktonic diatom through consumption of freshly produced cells and deposited diatom detritus, or through processing of diatoms by an herbivorous intermediary. Of the nitrogen incorporated by intertidal sedimentary biota (pennate diatoms, harpacticoid copepods, oligochaetes), benthivorous fishes (mummichog, white sucker), and sand shrimp, 10-30% derived from planktonic diatoms, often through a delayed remineralization and re-incorporation loop. Fauna that assimilated little label (spionid polychaete, anthurid isopod, American eel) obtained >90% of their nitrogen from a pathway based on plant detritus, or from older, nonlabeled diatom detritus. Assimilation and remineralization by benthic organisms of nitrogen incorporated in A. normanii demonstrated an unexpectedly strong linkage of water-column and benthic production. Benthic organisms continued to show tracer enrichment up to 2 mo after the end of tracer addition, indicating that deposited planktonic diatoms constituted a benthic food and nitrogen reserve. The15N-tracer illuminated food web pathways and revealed the speed with which nitrogen moves through this estuarine ecosystem.