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Item Open Access Spatial Relationships among Hydroacoustic, Hydrographic and Top Predator Patterns: Cetacean Distributions in the Mid-Atlantic Bight(2016) LaBrecque, ErinEffective conservation and management of top predators requires a comprehensive understanding of their distributions and of the underlying biological and physical processes that affect these distributions. The Mid-Atlantic Bight shelf break system is a dynamic and productive region where at least 32 species of cetaceans have been recorded through various systematic and opportunistic marine mammal surveys from the 1970s through 2012. My dissertation characterizes the spatial distribution and habitat of cetaceans in the Mid-Atlantic Bight shelf break system by utilizing marine mammal line-transect survey data, synoptic multi-frequency active acoustic data, and fine-scale hydrographic data collected during the 2011 summer Atlantic Marine Assessment Program for Protected Species (AMAPPS) survey. Although studies describing cetacean habitat and distributions have been previously conducted in the Mid-Atlantic Bight, my research specifically focuses on the shelf break region to elucidate both the physical and biological processes that influence cetacean distribution patterns within this cetacean hotspot.
In Chapter One I review biologically important areas for cetaceans in the Atlantic waters of the United States. I describe the study area, the shelf break region of the Mid-Atlantic Bight, in terms of the general oceanography, productivity and biodiversity. According to recent habitat-based cetacean density models, the shelf break region is an area of high cetacean abundance and density, yet little research is directed at understanding the mechanisms that establish this region as a cetacean hotspot.
In Chapter Two I present the basic physical principles of sound in water and describe the methodology used to categorize opportunistically collected multi-frequency active acoustic data using frequency responses techniques. Frequency response classification methods are usually employed in conjunction with net-tow data, but the logistics of the 2011 AMAPPS survey did not allow for appropriate net-tow data to be collected. Biologically meaningful information can be extracted from acoustic scattering regions by comparing the frequency response curves of acoustic regions to theoretical curves of known scattering models. Using the five frequencies on the EK60 system (18, 38, 70, 120, and 200 kHz), three categories of scatterers were defined: fish-like (with swim bladder), nekton-like (e.g., euphausiids), and plankton-like (e.g., copepods). I also employed a multi-frequency acoustic categorization method using three frequencies (18, 38, and 120 kHz) that has been used in the Gulf of Maine and Georges Bank which is based the presence or absence of volume backscatter above a threshold. This method is more objective than the comparison of frequency response curves because it uses an established backscatter value for the threshold. By removing all data below the threshold, only strong scattering information is retained.
In Chapter Three I analyze the distribution of the categorized acoustic regions of interest during the daytime cross shelf transects. Over all transects, plankton-like acoustic regions of interest were detected most frequently, followed by fish-like acoustic regions and then nekton-like acoustic regions. Plankton-like detections were the only significantly different acoustic detections per kilometer, although nekton-like detections were only slightly not significant. Using the threshold categorization method by Jech and Michaels (2006) provides a more conservative and discrete detection of acoustic scatterers and allows me to retrieve backscatter values along transects in areas that have been categorized. This provides continuous data values that can be integrated at discrete spatial increments for wavelet analysis. Wavelet analysis indicates significant spatial scales of interest for fish-like and nekton-like acoustic backscatter range from one to four kilometers and vary among transects.
In Chapter Four I analyze the fine scale distribution of cetaceans in the shelf break system of the Mid-Atlantic Bight using corrected sightings per trackline region, classification trees, multidimensional scaling, and random forest analysis. I describe habitat for common dolphins, Risso’s dolphins and sperm whales. From the distribution of cetacean sightings, patterns of habitat start to emerge: within the shelf break region of the Mid-Atlantic Bight, common dolphins were sighted more prevalently over the shelf while sperm whales were more frequently found in the deep waters offshore and Risso’s dolphins were most prevalent at the shelf break. Multidimensional scaling presents clear environmental separation among common dolphins and Risso’s dolphins and sperm whales. The sperm whale random forest habitat model had the lowest misclassification error (0.30) and the Risso’s dolphin random forest habitat model had the greatest misclassification error (0.37). Shallow water depth (less than 148 meters) was the primary variable selected in the classification model for common dolphin habitat. Distance to surface density fronts and surface temperature fronts were the primary variables selected in the classification models to describe Risso’s dolphin habitat and sperm whale habitat respectively. When mapped back into geographic space, these three cetacean species occupy different fine-scale habitats within the dynamic Mid-Atlantic Bight shelf break system.
In Chapter Five I present a summary of the previous chapters and present potential analytical steps to address ecological questions pertaining the dynamic shelf break region. Taken together, the results of my dissertation demonstrate the use of opportunistically collected data in ecosystem studies; emphasize the need to incorporate middle trophic level data and oceanographic features into cetacean habitat models; and emphasize the importance of developing more mechanistic understanding of dynamic ecosystems.
Item Open Access Submesoscale Biophysical Interactions on the Gulf Stream: Eddies, Fronts, and New Observational Methods(2022) Gray, Patrick CliftonOur oceans are a key part of the Earth system, an underappreciated bastion of our carbon cycle, and the home of incredible biodiversity, yet marine ecosystems are extremely challenging to model with a range of feedbacks that are not understood. Particularly poorly understood are linkages between physics and biology at the submesoscale (horizontally O(0.1-10) km, vertically O(.1) km, temporally O(1) day) that could help explain broad scale properties in ocean biology. Persistent fronts of western boundary currents like the Gulf Stream are hotspots for submesoscale dynamics that may influence phytoplankton productivity and diversity with ramifications across marine ecosystems. In this dissertation I: 1) review the use of drones and ocean color remote sensing for observing biology at the submesoscale and below, and articulate a vision for addressing key observational needs with drones, 2) develop more robust methods for retrieving ocean color from drones, 3) integrate observations from new and existing sampling technologies to investigate phytoplankton enhancement and shifts in phytoplankton community composition on the Gulf Stream front, and 4) with a frontal eddy as a mesocosm investigate community composition in the eddy vs shelf and Gulf Stream water and then more broadly understand their impact on the marine ecosystem of the Gulf Stream and mid-Atlantic Bight. This work revealed seasonality contrasting that of the typical North Atlantic blooms, but more similar to Sargasso Sea water at this latitude with peak biomass in winter. We found shifting nutrient limitations and observed chlorophyll-a enhancement on the front in a large minority of samples with an indication of winter being more likely for enhancement. We speculate about linkages between this enhancement and ageostrophic circulation on the front but did not directly demonstrate this. Community composition shifts were present in all transects across the Gulf Stream front, though no consistent pattern emerged, with gradual shifts, step changes, and anomalies at the front. Finally the in-depth frontal eddy investigation revealed a different community, dominated by Prochlorococcus, and many optical indicators of a post-bloom shift to a microbial loop environment within the eddy. In summary the major goals of this work were to understand the interplay of ocean physics and marine ecosystems at the fine-scale. Towards this goal, I develop more robust methods for measuring ocean color from drones and then focus on the Gulf Stream, parsing out connections between the front and chl-a and biodiversity and then focus on a frontal eddy as a mesocosm for physical-ecological interactions and for their ecological impact on the mid-Atlantic Bight.