Is Nitrogen Nitrogen? Ecosystem Impacts of Anthropogenic N Sources on Algal Blooms, Hypoxia and Biogeochemical Cycling in the Neuse River Estuary, NC.
Principal investigators:
Hans W. Paerl, University of North Carolina
Marc J. Alperin, University of North Carolina
Graduate student:
Ben Peierls, University of North Carolina
student research
Nitrogen (N) limitation is a common feature of estuaries worldwide. Excessive anthropogenic N loading from unprecedented urban, agricultural and industrial growth in estuarine watersheds has led to accelerating primary production (eutrophication), accompanied by expanding harmful algal blooms. Blooms produce large accumulations of organic matter, which, in the absence of complete vertical mixing fuel bottom water hypoxia. As ecosystem stressors, hypoxia and anoxia (hypoxia) reduce and eliminate habitats, induce disease, and can be directly fatal to resident biota. In addition, sediment O2 depletion alters nutrient cycling and availability and as such is a potent biogeochemical disruptor of ecosystem function. Growing frequencies and geographic expansion of hypoxia are issues of prime concern for North Carolina and the Nation. "Nitrogen is nitrogen" is a common assumption when relating N inputs to trophic and biogeochemical responses in N-depleted aquatic ecosystems. In this project we are testing the hypothesis that contrasting biological reactivities of anthropogenic N sources, including inorganic (NO3-/NO2-, NH4+) and organic (e.g., urea, amino acids) forms, lead to physiologically and taxonomically distinct phytoplankton growth and bloom responses. These differential bloom responses yield contrasting inputs of newly-fixed carbon to the estuary which yield cascading impacts on O2 dynamics, nutrient cycling, and water/habitat quality. The responses of natural phytoplankton communities to specific nitrogenous compounds and their ramifications for community structure, development of potentially harmful taxa, subsequent impacts on hypoxia potentials, and altered nutrient cycling mediated by sediment-water column interactions are the focus of this proposal.
This research provides a multi-disciplinary (phytoplankton physiological ecology, biogeochemistry, estuarine ecology and modeling), process-level understanding of how specific N inputs modulate algal bloom and hypoxia dynamics in the N limited Neuse River Estuary, NC. This shallow, eutrophying (expanding algal blooms and hypoxia) estuary is indicative of the growing influence of anthropogenic non-point source N loading (which has increased by [about] 50% over the past 2 decades), accompanied by changes in both sources and chemical forms of N. Specifically, we have been: 1) Determining the impacts and net utilization of N sources having DIN (NO3- , NH4+ ), and DON (e.g., urea, amino acids) compositions (with and without parallel P enrichment) representative of major non-point N inputs on phytoplankton primary productivity, biomass, growth rates, and biodiversity. 2) Characterizing the relationship(s) between phytoplankton blooms, bloom taxa (algal groups), and the spatio-temporal dynamics of bottom-water anoxia/hypoxia. 3) Examining the sources and fates of phytoplankton-based detrital organic matter to understand the factors that control the efficiency with which water column productivity is incorporated into the sediment. 4) Examining the sediment's response to the input of fresh organic matter to better understand the impact of the benthic nutrient and oxygen flux on water column processes. 5) Developing a process-based conceptual model, capable of linking "new" N and P loading to alterations in estuarine phytoplankton community production, compositional responses, and secondary transport reactions. The model will help characterize and quantify hypoxia in the context of altered biogeochemical cycling, trophic state and habitat quality.
The conceptual and technical frameworks and organizational structure of this project lend themselves to student-oriented quantitative analysis and modeling efforts, which are under way. We welcome additional student involvement in the areas of multi-media (i.e., sediment-water column) and cross-disciplinary (microbial ecology-biogeochemistry-hydrodynamics) analysis and modeling. This project will provide a platform for the development of quantitative analytical and modeling expertise in ecosystem-scale responses to human vs. natural perturbations of this and similar nutrient-sensitive estuaries.
Student Research:
Is Nitrogen Nitrogen? Ecosystem Impacts of Anthropogenic N Sources on Algal Blooms, Hypoxia and Biogeochemical Cycling in the Neuse River Estuary, NC
Benjamin Peierls
Estuaries are highly productive aquatic ecosystems that, because of their position at the land-sea interface, are physically complex and dynamic. The biogeochemical functioning of these systems acts to process, store, and export watershed-based inorganic and organic matter. I am interested in what impact the microbial loop has on estuarine function and how that may vary under anthropogenic and climate-driven stressors. Specifically, I would like to evaluate the linkage between heterotrophic bacterioplankton and autotrophic phytoplankton with respect to organic matter utilization and remineralization in the Neuse River and Pamlico Sound Estuary, North Carolina. For instance, how much of estuarine phytoplankton production is lost to microbial sinks versus how much is available to higher trophic levels? It would be difficult to measure all the relevant ecosystem components and processes, so a simplified model might be used to identify the variables most critical to answering this and other questions. Modeling could also help to extrapolate small-scale, microbial processes up to whole system behavior and could provide a virtual laboratory to test the effect of estuarine perturbations.
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