[Truncated abstract] Biogeochemical processes in seagrass sediments influence growth and nutrition of seagrasses. This thesis investigates the below-ground interactions between biotic and abiotic factors that influence seagrass nutrition and growth, with focus on a small species of seagrass, Halophila ovalis (R. Br.) Hook ƒ., from the Swan River Estuary, Western Australia. Seagrass showed significantly lower growth and an increase in leaf nitrogen and phosphorus concentrations with increased organic matter loading. With maximal light reduction, lower growth rates and average leaf weights were observed, and leaf nitrogen and phosphorus concentrations were higher. Light reduction was also shown to increase bioavailability of inorganic nutrients within porewater of seagrass sediment . . . Sulphide was hypothesised to have an inhibitory effect on nutrient uptake of Halophila ovalis. Below-ground sulphide inhibits the photosynthetic efficiency of photosystem II at sulphide concentrations greater than 1 mM. Sulphide exposure enhanced phosphate uptake, with no significant effect on ammonium uptake of H. ovalis. This thesis demonstrates that biogeochemical processes both constrain the potential growth of seagrasses and influence the nutrient status of seagrass tissue. Consideration of the influence of sulphide stress on seagrasses is likely to be particularly important for anthropogenically influenced aquatic systems, where inputs of organic matter are enriched relative to pristine ecosystems. A better understanding of biogeochemical processes will allow researchers to predict how future changes in sediment chemistry will influence seagrass meadows.

Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 118-124). Also available on the World Wide Web.

The Florida Everglades is a massive and highly managed subtropical wetland ecosystem, strongly influenced by anthropogenic control of freshwater distribution and highly susceptible to a changing climate, including rising sea-level and changes in temperature and rainfall. Shifting hydrologic regimes impact ecosystem function and biogeochemistry, which in turn control the sources, fate, and transport of organic matter. As a master environmental variable, it is essential to understand how organic matter dynamics will respond to changes in the balance between freshwater and saltwater associated with landscape-scale Everglades restoration efforts and climate change. The research comprising this dissertation improves current understanding of the linkages between organic matter and hydrology in the Everglades across a broad range of temporal and spatial scales. A range of research tools, including stable molecular biomarkers, water quality sensors, data synthesis and multivariate statistics were utilized. Biomarkers were used to track particulate organic matter mobilization in response to experimentally manipulated flows and provided initial evidence that sheet flow restoration can re-engineer landscape microtopography, influencing both ecosystem structure and organic matter inputs to Everglades National Park (ENP). Short-term and long-term temporal studies indicated the quantity and quality of dissolved organic carbon responds to changes in freshwater flow to marshes and mangrove forests in ENP, and that spatial patterns and trends are driven by a complex mixture of managed and natural surface water inputs (i.e., rainfall and water management inflows) as well as groundwater discharge. Application of climate scenario forecasting to relationships established between organic matter and hydrologic drivers predicted reductions in dissolved organic carbon export from ENP and changes in organic matter molecular composition. Furthermore, high-frequency measurements showed hydrologic connectivity of freshwater and estuarine organic matter pools at sub-monthly time-scales. In summary, the work presented here clearly indicates strong yet spatiotemporally complex relationships between changes in water and the sources and transport of organic carbon through the Everglades.

Invasive plant species are major threats to the biodiversity and ecosystem stability. The purpose of this study is to understand the impacts of invasive plants on soil nutrient cycling and ecological functions. Soil samples were collected from rhizosphere and non-rhizosphere of both native and exotic plants from three genera, Lantana, Ficus and Schinus, at Tree Tops Park in South Florida, USA. Experimental results showed that the cultivable bacterial population in the soil under Brazilian pepper (invasive Schinus) was approximately ten times greater than all other plants. Also, Brazilian pepper lived under conditions of significantly lower available phosphorus but higher phosphatase activities than other sampled sites. Moreover, the respiration rates and soil macronutrients in rhizosphere soils of exotic plants were significantly higher than those of the natives (Phosphorus, p=0.034; Total Nitrogen, p=0.0067; Total Carbon, p=0.0243). Overall, the soil biogeochemical status under invasive plants was different from those of the natives.

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