Seagrass demographic response to category one hurricane and the effect of spatial arrangement of propagation units in restoration efforts.

Abstract

Seagrass ecosystems provide multiple services, such as habitat for commercially and recreationally important species, improving water quality, sequestering carbon, stabilizing sediment, and reducing coastal erosion. However, since 1990, seagrass cover has declined worldwide by 7% per year. Losses have been mainly caused by coastal developments, destructive fishing methods, nutrient runoff, pollutants, invasive species, and hurricanes. The first aim of this thesis is to evaluate the effects hurricanes have on seagrass ecosystems. Demographic parameters of the local <em>T. testudinum,</em> in contrast to the invasive <em>H. stipulacea</em>, were analyzed before and after the passing of Category 1 Hurricane Fiona. Percent cover, density, and canopy height were measured with transects at three sites around Culebra, Puerto Rico. The percentage cover of <em>T. testudinum</em> significantly decreased by 45%, shoot density by 15%, and canopy height by 34%. On the other hand,<em> H. stipulacea</em> showed no significant change in any of these parameters after the hurricane. <em>H. stipulacea</em> was inhabiting under the canopy of <em>T. testudinum</em>, which could have served as protection from the blunt force of the hurricane and explain why the hurricane had no significant effect. This presents an opportunistic trait of this invasive species. The second aim focused on seagrass restoration efforts. Three distinct spatial arrangements of propagation units (aggregate, random, uniform) were evaluated over three years. After Hurricane Fiona, the aggregate treatment slightly increased shoot density by 2%, while uniform and random plummeted by 47% and 30%, respectively. Suggesting that the aggregate treatment was the more resilient spatial arrangement. The aggregate treatment has less border exposure than the others, hence less propensity to erosion and more protection for the shoots inside, an effect that would be particularly advantageous in areas subjected to high energy events. The seagrass cover in this experiment did not change significantly. A minimum density for producing positive cover growth was not determined due to the small size of the PUs in this study. Yet, 90% survival of the treatments was achieved, indicating the transplant technique works, and upscaling the experiment should yield more favorable results.