Grantee Research Project Results
Final Report: Tracing the Fate of Nitrogen Inputs from Watersheds to Estuaries
EPA Grant Number: R824767Title: Tracing the Fate of Nitrogen Inputs from Watersheds to Estuaries
Investigators: Deegan, Linda A. , Peterson, Bruce J.
Institution: Marine Biological Laboratory
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 1996 through December 1, 1997
Project Amount: $232,323
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
Description and Objective of the Research: The uptake and processing of watershed-derived, dissolved inorganic nitrogen (DIN) within the estuarine food web is partly conditioned by hydrologic and biotic interactions occurring where watershed inputs first enter tidal waters. In this low salinity, oligohaline transition zone, transformations of nutrients and organic matter can profoundly affect nitrogen cycling, export, and storage. In addition, this upper estuarine reach is of critical importance to many species of invertebrates and fish. Despite the potential importance of the upper estuary to N-cycling and secondary productivity, this region has received relatively little direct study, and is conspicuously absent in most models of estuarine nitrogen flow or trophic structure. Our overall goal was to determine the biogeochemical fate of watershed-derived nitrate during the biologically productive period of sumrner and early fall. We introduced tracer levels of a stable isotopic compound (15N--enriched nitrate) into the oligohaline reach of the Parker River estuary (Massachusetts, USA) to study the transformation and fate of watershed-derived nitrogen during summer 1996. The focus of the study reported here is the structure of the food web in the upper estuary, and the pathways along which the tracer nitrogen was transferred to higher trophic levels. A companion study (Holmes et al. accepted) addressed nitrogen cycling and the longer-term fate of DIN in the upper estuary.Summary/Accomplishments (Outputs/Outcomes):
Intensive temporal and spatial measurements were made of the isotopic label in a wide range of organisms during the 15N amendment and for two months thereafter. The tracer helped determine the relative trophic importance of three major sources of organic nitrogen fueling the food web of the upper Parker River estuary - pianktonic 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 highest 15N-tracer content (planktonic copepods, planktivorous juvenile fishes, crustacean epifaunal primary consumers), derived a major part of their assimilated nitrogen (50% or more) from the bloom-forming planktonic diatom. The trophic pathway was either through direct consumption of freshly-produced cells or through processing of diatoms by an herbivorous intermediary. Intertidal sedimentary biota (pennate diatoms, harpacticoid copepods, oligochaetes), benthivorous fishes (mummichog, white sucker), and sand shrimp received 10--30% of their assimilated nitrogen from planktonic diatoms, often through a delayed remineralization and re-incorporation loop. Fauna that assimilated little label (spionid polychaete, anthurid isopod, American eel) obtained more than 90% of their nitrogen from a plant detritus-based pathway, or from older, non-labeled diatom detritus. Consumption, remineralization, and uptake of A. normanii by benthic organisms demonstrated an unexpectedly strong linkage of water column and benthic production. Benthic organisms continued to show tracer enrichment up to two months after the end of tracer addition, indicating that deposited planktonic diatoms constituted a benthic food and nitrogen reserve. The 15N-tracer illuminated estuarine food web pathways and revealed the speed with which nitrogen moves through the estuarine ecosystem.The upper Parker River estuary serves as a rich feeding ground for abundant, small migratory fishes. Much of the advantage of feeding in this reach is conferred by the high production of Actinocyclus normanii. An important trophic link between this diatom and secondary consumers is the planktonic copepod Eurytemora affnis. This species thrived in the upper estuary during summer bioom conditions of A. normanii when it produced at least two cohorts. E. affnis was an important food for juveniles of anadromous fishes such as alewife and white perch. 15N-tracer evidence indicates that Atlantic silverside, a summer migrant from the polyhaline estuary into tidal freshwaters, also fed on diatom-based food sources (Table 3). Alewife and Atlantic silverside support production of several estuarine and coastal fish species, including striped bass and bluefish. Conversely, the most abundant fish of the upper estuary, juvenile mummichog and white sucker, benefit relatively little, and only indirectly, from freshly-deposited planktonic diatoms. Their major source of organic nitrogen is largely independent of new planktonic primary production, suggesting the importance of detrital material derived from other sources, such as the surrounding marsh or older, buried phytoplanktonic detritus, in fueling their meiofaunal prey. However, the primary consumers among the crustacean macrofauna of the subtidal upper estuary thrive on the plentiful, labile planktonic food source. In late summer, grass shrimp attain high densities (> 30 individuals/m2), and are the preferred prey of many fish predators, including rainbow smelt and juvenile striped bass. Amphipods are eaten by larger white sucker and mummichog, and mud crabs are vulnerable to large white perch and small striped bass. Mud crab exoskeletal remains were abundant in the scat of river otter of the upper Parker River estuary.
Conclusions:
The addition of the 15N-NO3- tracer to the upper Parker River estuary proved a powerful tool that in conjunction with other isotopic, behavioral, life-historical, and distributional observations, helped elucidate whole-ecosystem food web structure and trophic dynamics in this critical, understudied estuarine region. Several other studies have used 15N additions to trace nitrogen flow in freshwater ecosystems; however, our study is the first to use this approach in an estuary. This study has built upon previous work describing the isotopic uniqueness of biota in the upper Parker River estuary. Much of this distinctiveness can now be ascribed to the influence of Actinocyclus normanii in both the planktonic and benthic food webs. In the present study, we identify A. normanii as a major component of the assimilated diet of several dominant primary consumers, including planktonic copepods, benthic amphipods, grass shrimp, and mud crabs.The upper reaches of many coastal plain estuaries support high productivity of autochthonous planktonic diatoms, including those of the German North Sea and the western tributaries of Chesapeake Bay. The similarity of biota in the upper reaches of the Parker River and German North Sea estuaries is remarkable, including dominance of Actinocyclus normanii and Eurytemora affnis in the plankton, and identical (Gammarus tigrinus) or ecologically-similar (Palaemon /ongirostris replacing Palaemonetes spp.) crustacean primary consumers in the benthos. It is likely that the tidal freshwater and oligohaline reaches of other estuaries that support phytoplankton blooms also contain consumer species that exploit the seasonally high planktonic production.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 8 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Holmes RM, Peterson BJ, Deegan LA, Hughes JE, Fry B. Nitrogen biogeochemistry in the oligohaline zone of a New England estuary. Ecology, February 2000;81(2):416-432. |
R824767 (Final) |
not available |
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Hughes JE, Deegan LA, Peterson BJ, Holmes RM, Fry B. Nitrogen flow through the food web in the oligohaline zone of a New England estuary. Ecology, February 2000;81(2):433-452. |
R824767 (Final) |
not available |
Supplemental Keywords:
RFA, Scientific Discipline, Water, Geographic Area, Nutrients, Water & Watershed, Ecology, Hydrology, State, Chemistry, Biology, EPA Region, Watersheds, nutrient supply, oligohaline zone, bioassessment, fate and transport, bacteria, fish, nitrogen inputs, Massachusetts (MA), nitrogen oxide, Land Margin Ecosystem Research, aquatic ecosystems, water quality, Region 1 , aquatic biotaRelevant Websites:
http://www.mbl.edu/htmi/ECOSYSTEMS/Research/research.htmlProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.