Master of Science (MS)
Oceanography and Coastal Sciences
The upper Breton Sound estuary was hydrologically reconnected to the Mississippi River via the Caernarvon freshwater diversion structure in 1991. The Caernarvon structure can provide controlled freshwater pulses to the upper Breton Sound estuarine ecosystem, replicating historic freshwater pulsed events, although the original authorization was to control salinity isohalines at specific locations in the estuary. However, unlike historic freshwater pulsed events prior to the construction of levees, the current freshwater pulse contains an unprecedented amount of inorganic nitrogen, predominately as nitrate (annual average 71.4 µM NO3-). Denitrification is a microbial process, which can potentially remove excess nitrate entering coastal Louisiana ecosystems due to these riverine pulsed events. This study presents the first 15N isotope pairing technique (IPT) denitrification estimates from coastal sediments in Louisiana, and evaluates the influence water temperature and different habitat types have on denitrification rates. Three IPT assumptions were tested in the current study and were fulfilled; however, the fourth assumption, the influence of annamox, was not evaluated. The three IPT assumptions evaluated were fulfilled in the benthic and marsh habitat sediments. However, the marsh habitat sediments from upper Breton Sound provide a dilemma for the current IPT design; 15NO3- diffusion will not reach deep into the plant rhizosphere where optimal conditions persist for coupled nitrification-denitrification activity. There was a significant interaction between habitat (marsh and benthic) and water temperature (8 and 22 °C) treatments on denitrification rates. Mean total denitrification (direct + coupled denitrification) estimates from a 70 µM 15NO3- incubation concentration were 17.5 (± 3.1) and 5.1 (± 1.5) ìmol N m-2 h-1 for benthic and marsh habitat sediments at 22 °C, and were 7.8 (± 1.9) and 2.1 (± 0.45) ìmol N m-2 h-1 for benthic and marsh habitat sediments at 8 °C, respectively. Overall, total denitrification rates ranged from 0.28 to 284.1 ìmol N m-2 h-1 for both habitats at 22 °C over a 2 to 200 µM 15NO3- incubation concentration range. Coupled denitrification comprised the majority of the total denitrification rate measured. Mean direct denitrification rates did not exceed 2 ìmol N m-2 h-1, and suggests direct denitrification measured by the isotope pairing technique in my study is not a major pathway for NO3- removal in upper Breton Sound benthic and marsh habitats.
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Lenaker, Peter L., "Applying the isotope pairing technique to evaluate how water temperature and habitat type influence denitrification estimates in Breton Sound, Louisiana" (2009). LSU Master's Theses. 488.
Twilley, Robert R.