Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Human activities have increased the supply of nitrogen (N) and phosphorus (P) to coastal waters worldwide, threatening coastal wetlands with excess nutrient loading and subsequent eutrophication. In this dissertation, I present results from two decade-scale fertilization experiments in a Sagittaria lancifolia dominated oligohaline marsh that examined the species-, community-, and ecosystem-level effects of nutrient enrichment. My objectives were to determine (1) which nutrient limits primary production, (2) how increased supply of the limiting nutrient affects plant community structure and function, both above- and belowground, and (3) whether nutrient over-enrichment compromises ecosystem stability. Overall, significant changes in plant growth occurred with N enrichment only. Aboveground, N enrichment stimulated primary production and altered plant tissue nutrient ratios, nutrient resorption efficiencies, and species dominance. Belowground, excess N simultaneously increased live root biomass accumulation in unexploited soil and reduced in situ live root standing crop. The rate of marsh elevation change was unaffected by nutrient enrichment due to an apparent compensatory effect on marsh accretionary processes whereby nutrient-induced shallow subsidence, attributed to reduced live root standing crop, was balanced by nutrient-enhanced soil accretion resulting from greater organic matter accumulation at the soil surface. In addition, the structural integrity of the soil matrix did not deteriorate under elevated nutrient conditions; decomposition rates were similar to control plots, and although root standing crop was reduced, the root system were evidently stronger as soil shear strength tended to increase rather than decrease. Based on these results, I conclude that this oligohaline marsh is N-limited, and that N enrichment beyond the assimilation capacity of the vegetation drives changes in plant community structure and function caused by altered plant nutrient cycling. Eutrophic conditions were both beneficial and detrimental to ecosystem function, and therefore represent an unlikely destabilizing mechanism in this coastal marsh and possibly others due to counterbalancing effects on plant growth above and below the soil surface. However, additional long-term research is required in a diverse range of habitats and environmental settings before broad-based, general conclusions concerning the effects of nutrient enrichment on coastal wetland stability can be made with a high degree of certainty.
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Graham, Sean A., "Decade-scale Nutrient Enrichment Effects on Wetland Plant Community Structure, Function, and Stability" (2013). LSU Doctoral Dissertations. 3620.