Term of Award

Summer 2014

Degree Name

Master of Science in Biology (M.S.)

Document Type and Release Option

Thesis (open access)

Department

Department of Biology

Committee Chair

Risa A. Cohen, PhD

Committee Member 1

Daniel Gleason

Committee Member 2

Christine Hladik

Abstract

Salt marsh macrophytes, such as Spartina alterniflora, play a critical role in uptake and transformation of inorganic nitrogen before it reaches coastal waters, thereby reducing the potential for eutrophication. Although nitrogen availability typically limits S. alterniflora growth, it may be possible to exceed the nitrogen uptake capacity of S. alterniflora. Increasing either nitrogen concentrations or salinity are key factors regulating S. alterniflora nitrogen uptake. Investigating the effects of nutrients and salinity on S. alterniflora is important given that increases in inorganic nitrogen supply to surface waters from agriculture and urbanization occur simultaneously with freshwater withdrawals that reduce flow and increase salinity. Spartina alterniflora nitrogen uptake in response to increasing inorganic nitrogen (ammonium, NH4+) (0, 10, and 100 µM), and salinity (20, 30, and 40 psu) treatments in a fully crossed factorial design were measured in greenhouse microcosms with tidal simulation in Statesboro, GA from April-October, 2013. Prior to the factorial study, a three month pilot study comparing S. alterniflora growth in novel tidal simulator design and salt marsh field plots revealed tidal simulation did not affect plant height, stem density, or above and belowground biomass. After 48 hours the highest water column NH4+ uptake occurred at the lowest salinity (20 psu) and highest ammonium concentrations (100 µM) tested. After 6 months of NH4+-15N additions, above and belowground S. alterniflora plant tissue δ15N increased proportionally with NH4+ additions and was reduced by 50% with salinity increases from 20 to 40 psu across all NH4+ addition levels. Furthermore, S. alterniflora above and belowground biomass and main shoot height was reduced with increasing salinity from 20 to 40 psu and not significantly affected increasing NH4+ additions. However, at high salinity (40 psu) biomass reductions were mitigated by intermediate (10 µM) NH4+ additions by a 50% increase over 0 and 100 µM NH4+ additions. Stem density and main shoot height measured weekly also reflected mitigation by intermediate (10 µM) NH4+ additions at elevated salinity. That S. alterniflora nitrogen uptake and biomass decrease with increasing water column salinity suggests alteration of coastal salinity may reduce nitrogen uptake capacities of S. alterniflora dominated salt marshes. Thus estuarine water column salinity should be considered when regulating inorganic nitrogen loads in aimed at conserving salt marsh nutrient retention.

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