Nitrogen Biogeochemistry in the Oligohaline Zone of a New England Estuary

We investigated nitrogen cycling in the oligohaline zone (the low-salinity region where river water first enters the estuary) of the Parker River estuary in northeastern Massachusetts. We introduced an isotopic tracer (15N-NO3−) for 27 days in August 1996 to help determine how watershed-derived nitrogen moves through the upper estuary. The amount of tracer added was sufficient to enrich nitrate isotopically by ∼100‰ in the vicinity of the addition but did not influence nitrate concentration appreciably. During typical summer low-flow conditions as occurred during the addition period, essentially all riverine nitrate (including the nitrate tracer) was rapidly removed from the water column by the planktonic diatom Actinocyclus normanii. Export of tracer down-estuary was low during the isotope addition period, in part because of low river discharge. Instead, most of the nitrogen originally assimilated by A. normanii was transferred to sediments in the oligohaline zone. Nitrogen demand by phytoplankton during summer exceeded riverine supply by an order of magnitude. The additional nitrogen came mainly from the regeneration of benthic nitrogen, although some may have come from groundwater. The whole-ecosystem isotope tracer approach applied here was a powerful means of investigating the fate of watershed-derived nitrogen in the upper estuary.