Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Biology; and the Woods Hole Oceanographic Institution), 2005. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references. / The goals of this thesis were to quantify the behavior of gastropod larvae (mud snails Ilyanassa obsoleta) in turbulence, and to investigate how that behavior affects larval supply in a turbulent coastal inlet. Gastropod larvae retract their velums and sink rapidly in strong turbulence. Turbulence-induced sinking would be an adaptive behavior if it resulted in increased larval supply and enhanced settlement in suitable coastal habitats. In laboratory experiments, mud snail larvae were found to have three behavioral modes: swimming, hovering, and sinking. The proportion of sinking larvae increased exponentially with the turbulence dissipation rate over a range comparable to turbulence in a tidal inlet, and the mean larval vertical velocity shifted from upward to downward in turbulence resembling energetic nearshore areas. The larval response to turbulence was incorporated in a vertical advection-diffusion model to characterize the effects of this behavior on larval supply and settlement in a tidal channel. Compared to passive larvae, larvae that sink in turbulence have higher near-bed concentrations throughout flood and ebb tides. / (cont.) This high larval supply enables behaving larvae to settle more successfully than passive larvae in strong currents characteristic of turbulent tidal inlets. A study was conducted at Barnstable Harbor, MA to estimate the responses of larvae to turbulence in the field. Gastropod larvae from different coastal environments had genus-specific responses to turbulence, suggesting that larvae use turbulence for large-scale habitat selection. On ebb tides, mud snail larvae had a similar response to turbulence as in the laboratory, with greater sinking velocities in strong turbulence. Behavior estimates differed for flood and ebb tides, indicating that additional physical cues influence behavior. Turbulence-induced sinking behavior would enhance retention and promote settlement of mud snail larvae in habitats like Barnstable Harbor. / by Heidi L. Fuchs. / Ph.D.

La disminución de las reservas de agua dulce debido al cambio climático y sus efectos cada vez más imprevisibles, requiere de la construcción de nuevas presas y el mantenimiento de la capacidad de almacenamiento de los embalses existentes. Los proyectos de represamiento, generalmente consideran una vida útil de 50 años, producto de los sedimentos que se acumularán en el reservorio. Sin embargo, luego de transcurrido ese tiempo es inviable suspender las actividades generadas alrededor de una presa. Por esto, se requiere prolongar la vida útil de los embalses, mediante prácticas de manejo de sedimentos utilizando las estructuras que componen las presas, lo que involucra menores costos respecto de otras alternativas. Las prácticas de manejo de sedimentos más difundidas son el sluicing, el flushing y la ventilación de corrientes de turbidez. Se requiere una revisión de estas prácticas para implementarlas en el Perú, lo cual se desarrolla en el presente estudio, donde se realiza la evaluación de la presa Congalla. Este proyecto entró en funcionamiento en octubre del 2016, el cual requirió para su construcción de una particular obra de desvío, consistente en un ducto de barriles metálicos. Este ducto luego se convirtió en la descarga de fondo, con 100 m de longitud y 0.60 m de diámetro interno, mucho más de lo necesario técnicamente, pero más económico que otras opciones. Lo que se busca es aprovechar el área de la sección del conducto, para verificar su utilidad para la eliminación de sedimentos. Se ha realizado un análisis de información en gabinete y un arduo trabajo de campo en las temporadas de estiaje y lluvias, en los meses de agosto del 2018 y marzo del 2019 respectivamente. De acuerdo a los cálculos, luego de dos años y medio, se tiene una acumulación de sedimentos de 2793.24 m3 utilizando fórmulas empíricas y de 1633.49 m3 según mediciones efectuadas. Realizando la apertura de la válvula de la descarga, se han obtenido resultados de hasta 1.08% de eliminación de los sedimentos calculados mediante las mediciones, el cual es un buen indicador de la eficacia de limpieza. Los efectos negativos en el ecosistema, de los sedimentos liberados aguas abajo de la presa, fueron indetectables. / Decrease in freshwater reserves due to climate change and its increasingly unpredictable effects, requires the construction of new dams and maintenance of storage capacity of existing reservoirs. Damming projects, generally consider a useful life of 50 years, as a result of sediments that will accumulate in the reservoir. However, after that time has elapsed, it is not feasible to suspend the activities generated around a dam. Therefore, it is necessary to extend the useful life of reservoirs, through sediment management practices using the structures that make up the dams, which involves lower costs compared to other alternatives. Most widespread sediment management practices are sluicing, flushing, and venting of turbidity streams. It is required a review of these practices to implement them in Peru, which is developed in the present study, where evaluation of the Congalla dam is carried out. This project came into operation in October 2016, which required for its construction a particular diversion work, consisting of a metal barrel pipeline. This pipeline later became the bottom discharge, with a length of 100 m and an internal diameter of 0.60 m, much more than technically necessary, but cheaper than other options. What is sought is to take advantage of the duct section’s area, to verify its usefulness for elimination of sediments. An analysis of information has been carried out in office and arduous fieldwork in dry and rainy seasons, in months of August 2018 and March 2019 respectively. According to calculations, after two and a half years, there is an accumulation of sediments of 2793.24 m3 using empirical formulas and 1633.49 m3 according to measurements made. By opening the discharge valve, have been obtained results of up to 1.08% removal of sediments calculated through measurements, which is a good indicator of cleaning efficacy. Negative effects on the ecosystem, from the sediments released downstream from the dam, were undetectable. / Tesis