Monitoring and Evaluating Marine Debris in a Fine-Scale Convergence Phenomenon Around Makai Pier, Windward, Oahu
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Lockett, Brittney
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Marine Science
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Marine debris accumulation in large-scale convergence zones such as the Great Pacific Garbage Patch, and in submesoscale convergence features such as internal waves and estuarine fronts is well documented, however, concentrated features on the finescale are just beginning to be studied. This study quantifies densities of microplastic (0.5 - 5 mm) and organic debris accumulation in a nearshore, finescale phenomenon around Makai Pier on four different days with different environmental conditions: light wind, average wind, above average wind, and above average north swell. Additionally, semi-quantitative patterns of marine debris were documented via weekly surveys by assigning ordinal values to various debris types and in different areas around the pier, resulting in a two-year time series of observations. Microplastic and organic debris were quantified within features (areas exhibiting concentrated debris), the swash zone along the shore, and in areas away from concentrated features (the background) around the pier by conducting manta net tows and collecting sieve samples through the surface of the water, and these estimates were used to calibrate the semi-quantitative data from the time series. To effectively compare the strength of converging mechanisms that this phenomenon demonstrates, we use the concentration factor as a dimensionless measurement, representing the ratio of the density of debris inside a convergence feature to the concentration of debris in the background. Microplastic densities within concentrated features reached ≥ 1000 #/m² on twenty-five percent of targeted survey days, with over fifty percent of the targeted days reaching densities ≥ 100 #/m². The concentration factor of debris within features was estimated to range from 1 to 10?, with wave height being the most significant environmental predictor. Microplastic densities in the swash zone were consistently greater than in the background, yielding concentration factors between 1 and 100. These results are some of the first to quantify highly concentrated debris within features on the order of meters, and the observed microplastic densities are, to the best of our knowledge, the highest densities in surface waters recorded in the literature making Makai Pier an excellent candidate for efficient marine debris removal.
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Hawaii Pacific University