Nitrogen flow through the food web in the oligohaline zone of a new England estuary

We introduced tracer levels of N-15-enriched nitrate into the oligohaline r each of the Parker River estuary (Massachusetts, USA) to study the transfor mation and fate of watershed-derived nitrogen during the biologically produ ctive summer period. Intensive temporal and spatial measurements were made of the isotopic label in a wide range of organisms during the N-15 amendmen t 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 produ cers, and detritus derived mostly from the surrounding marsh. The planktoni c centric diatom Actinocyclus normanii was the primary vector of tracer nit rogen to benthic and water-column organisms. Fauna that attained the highes t N-15-tracer content (planktonic copepods, planktivorous juvenile fishes, several macrofaunal crustaceans) derived a major part of their incorporated nitrogen (similar to 50% or more) from the bloom-forming planktonic diatom through consumption of freshly produced cells and deposited diatom detritu s, or through processing of diatoms by. an herbivorous intermediary. Of the nitrogen incorporated by intertidal sedimentary biota (pennate diatoms, ha rpacticoid copepods, oligochaetes), benthivorous fishes (mummichog, white s ucker), and sand shrimp, 10-30% derived from planktonic diatoms, often thro ugh a delayed remineralization and re-incorporation loop. Fauna that assimi lated little label (spionid polychaete, anthurid isopod, American eel) obta ined >90% of their nitrogen from a pathway based on plant detritus, or from older, nonlabeled diatom detritus. Assimilation and remineralization by be nthic organisms of nitrogen incorporated in A. normanii demonstrated an une xpectedly strong linkage of water-column and benthic production. Benthic or ganisms continued to show tracer enrichment up to 2 mo after the end of tra cer addition, indicating that deposited planktonic diatoms constituted a be nthic food and nitrogen reserve. The N-15-tracer illuminated food web pathw ays and revealed the speed with which nitrogen moves through this estuarine ecosystem.