Estimation of ravine sediment production and sediment dynamics in the lower Le Sueur River watershed, Minnesota

This study focuses on quantifying explicitly the sediment budget of deeply incised ravines in the lower Le Sueur River watershed, in southern Minnesota. High-rate-gully-erosion equations along with the Universal Soil Loss Equation (USLE) were implemented in a numerical modeling approach that is based on a time-integration of the sediment balance equations. The model estimates the rates of ravine width and depth change and the amount of sediment periodically flushing from the ravines. Components of the sediment budget of the ravines were simulated with the model and results suggest that the ravine walls are the major sediment source in the ravines. A sensitivity analysis revealed that the erodibility coefficients of the gully bed and wall, the local slope angle and the Manning’s coefficient are the key parameters controlling the rate of sediment production. Recommendations to guide further monitoring efforts in the watershed and increased detail modeling approaches are highlighted as a result of this modeling effort.

Estimation of Ravine Sediment Production and Sediment Dynamics in the Lower Le Sueur River Watershed, Minnesota

This study focuses on quantifying explicitly the sediment budget of deeply incised ravines in the lower Le Sueur River watershed, in southern Minnesota. High-rate-gully-erosion equations along with the Universal Soil Loss Equation (USLE) were implemented in a numerical modeling approach that is based on a time-integration of the sediment balance equations. The model estimates the rates of ravine width and depth change and the amount of sediment periodically flushing from the ravines. Components of the sediment budget of the ravines were simulated with the model and results suggest that the ravine walls are the major sediment source in the ravines. A sensitivity analysis revealed that the erodibility coefficients of the gully bed and wall, the local slope angle and the Manning’s coefficient are the key parameters controlling the rate of sediment production. Recommendations to guide further monitoring efforts in the watershed and increased detail modeling approaches are highlighted as a result of this modeling effort.

The effects of carbon dioxide fertilization on the ecology of tropical seagrass communities

Increasing atmospheric CO2 concentrations associated with climate change will likely influence a wide variety of ecosystems. Terrestrial research has examined the effects of increasing CO2 concentrations on the functionality of plant systems; with studies ranging in scale from the short-term responses of individual leaves, to long-term ecological responses of complete forests. While terrestrial plants have received much attention, studies on the responses of marine plants (seagrasses) to increased CO 2(aq) concentrations remain relatively sparse, with most research limited to small-scale, ex situ experimentation. Furthermore, few studies have attempted to address similarities between terrestrial and seagrass responses to increases in CO2(aq). The goals of this dissertation are to expand the scope of marine climate change research, and examine how the tropical seagrass, Thalassia testudinum responds to increasing CO 2(aq)concentrations over multiple spatial and temporal scales. ^ Manipulative laboratory and field experimentation reveal that, similar to terrestrial plants, seagrasses strongly respond to increases in CO 2(aq) concentrations. Using a novel field technique, in situ field manipulations show that over short time scales, seagrasses respond to elevated CO2(aq) by increasing leaf photosynthetic rates and the production of soluble carbohydrates. Declines in leaf nutrient (nitrogen and phosphorus) content were additionally detected, paralleling responses from terrestrial systems. Over long time scales, seagrasses increase total above- and belowground biomass with elevated CO2(aq), suggesting that, similar to terrestrial research, pervasive increases in atmospheric and oceanic CO2(aq) concentrations stand to influence the productivity and functionality of these systems. Furthermore, field experiments reveal that seagrass epiphytes, which comprise an important component of seagrass ecosystems, additionally respond to increased CO2(aq) with strong declines in calcified taxa and increases in fleshy taxa. ^ Together, this work demonstrates that increasing CO2(aq) concentrations will alter the functionality of seagrass ecosystems by increasing plant productivity and shifting the composition of the epiphyte community. These results have implications for future rates of carbon storage and sediment production within these widely distributed systems.^

Mangrove production and carbon sinks: A revision of global budget estimates

Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of the available data on carbon fluxes in mangrove ecosystems. A reassessment of global mangrove primary production from the literature results in a conservative estimate of ∼218 ± 72 Tg C a−1. When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at ∼112 ± 85 Tg C a−1, equivalent in magnitude to ∼30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.