Interdisciplinary oceanography in kelp forest environments

Kelp forests, located on all mid-latitude coasts, provide habitats and ecosystem services (nutrient regulation, carbon sequestration, fisheries) worth billions of dollars annually. Macrocystis pyrifera, or giant kelp, is a three-dimensionally complex algae that dominates rocky reefs in the California Current (from Alaska to Baja). This thesis explores the relationship between giant kelp, hydrodynamics, and ecology. First, trends in increased temperature and decreased dissolved oxygen (DO) are explored around Isla Natividad, a Mexican fishing co-op community dependent on the health and sustainability of numerous fisheries including green abalone. Green abalone is a high-value benthic organism that is sensitive to environmental changes and an indicator species for ecological health. In an eight-year study, which compared the sheltered and exposed sides of the island, observations show that the quantity of warming events is dictated by the El Niño Southern Oscillation (ENSO) cycle and occurs more frequently on the sheltered side. However, on the island's protected side, regular internal bores, formed by wind and internal tides, bring in cool water at consistent intervals. This process could provide a thermal haven for abalone and enhance their resilience to higher temperatures during the phases of easing and resetting the internal bores. Vizcaíno Bay, adjacent to Isla Natividad, is characterized by its entrapment of upwelling, leading to heightened primary productivity and elevated levels of DO, compared to the exposed side of the island. Hypoxic events are consistent from year to year, but durations are increasing while DO concentrations are lethally decreasing, particularly on the exposed side. Shifts in seasonality of the hypoxic events now overlap with warming events, generating multiple stressor events. For any abalone that have increased resistance to warming or hypoxic events, scientific studies indicate that individual resistance to adverse conditions will not increase resistance under multiple ecological pressures. Given the dynamics on the sheltered side of the island, abalone populations are not only more likely to survive, but the temperature and DO regimes allow this subpopulation to build resiliency to future conditions. A targeted experiment in 2018 has led to an increased understanding of local hydrodynamics that can drive abalone larval dispersal, which is especially significant for the sheltered side of the island. The presence of alongshore tidal currents, coupled with periods of intensified cross-shore exchange prompted by pressure differentials, may facilitate the extended dispersal of larvae. Considering these observations, Punta Prieta, a Marine Protected Area (MPA) located on the sheltered side, is presumed to play a crucial role in fostering the recruitment and survival of green abalone populations in that area, given the large radius of dispersal. Thick kelp forests actually impede currents and slow down water flow, leading to longer water residence times and reduced influx of advected nutrients. Additionally, widespread upwelling in the area leads to decreased benthos DO levels. Kelp plants are not rigid; they are flexible yet buoyant and therefore respond to the flow and impose variable drag. An experiment in Monterey, California, was designed to study how mature kelp plants move with and without the surface canopy layer. Kelp motion depends on the strength of currents, waves, and the presence of the surface canopy. The surface canopy of kelp not only reduces the plant's tilt due to strong currents and diminishes the kelp's oscillatory motion caused by wave orbitals. However, in strong currents, when the kelp is fully extended (pronated) and lacks a surface canopy, the plant's full extension in the current results in reduced oscillatory movement due to waves. Lastly, a convolutional neural network algorithm to detect sea otters was developed for use by USFW for oil spill reconnaissance and annual surveys. By modifying YOLOv5 for tiny objects, adding other ocean background images, including image augmentation, and hyperparameter tuning, the model's final F1 score and mAP increased by more than 15\%, even with the relatively small data set. This thesis highlights the importance of interdisciplinary science, and how ecology and policy can frame physical oceanographic research.

Keywords: Engineering.
Advisor: Monismith Stephen Gene
Fringer Oliver B. (Oliver Bartlett)
Koseff Jeffrey Russell
Micheli Fiorenza