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Ref.: MsC Thesis, University of Algarve, March 2018
The aim of this dissertation is to evaluate different acoustic methods to characterise air bubbles and their application in the estimation of oxygen bubbles produced by marine plants during the photosynthesis process. Several methods are described in the literature to estimate the amount and distribution of air bubbles in marine waters however, the existing methods focus essentially on bubbles at the ocean's surface. Under certain conditions, the oxygen released by marine plants during photosynthesis occurs in the form of bubbles. The estimation of this bubbles is difficult and is often considered underestimated by conventional methods. Acoustic methods can be used to estimate the production of bubbles with greater precision and, moreover, to learn the dynamics of their production. It is necessary to evaluate the oxygen transfer process of the plants to the water, the model of acoustic propagation in seagrass fields, the configuration of the system, methods to filter the influence of unwanted parameters on the received signal (e.g., temperature changes, noise, tide, sound speed, salinity), and, to characterise environmental and biological noise. In this work, I propose to evaluate a suitability and generic bubble estimation method described in the literature, or the development of new methods for the monitoring of bubbles released by marine plants, in particular, seagrass Cymodocea Nodosa. The reliability of a generic bubble estimation method described in the literature, as well as, new methods for monitoring the bubbles released by marine plants will be tested. All the tests were conducted with the Cymodocea Nodosa seagrass species, in tanks on IPMA-EPPO. Combining acoustic with other techniques (CTD data, tide height) will allow the development of a robust and accurate acoustic measurement system. The acquired signals can be processed to estimate the amount of oxygen bubbles produced in that environment. With the use of this measurement system, I believe that this innovative acoustic method can be used to accurately quantify the ecosystem metabolism and that it will represent an important tool to manage and monitor the production of coastal areas by integrating spatial and temporal scales.
Keywords: signal processing, acoustic measurements system, underwater bubbles, acoustic oceanography, oxygen production, oxygen bubbles, seagrass meadow.