Community Structure of Methane Cold Seeps in the Western Atlantic and Eastern Pacific
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Understanding to what extent a particular ecosystem can influence the larger environment is the driving question behind my research. To begin examining this topic, several factors should be considered including 1) the community composition within a certain ecosystem 2) what factors drive community assembly, to predict the composition and organization of other related communities, and 3) the extent of influence of the ecosystem on the surrounding environments.
Chemosynthetic ecosystems are systems that are dependent on chemicals as the base of the food chain, rather than light as in photosynthesis. These systems are frequently biological hotspots within the ocean, where benthic communities can research a higher density than surrounding regions. Methane seeps, where methane and other hydrocarbons migrate through sediments to the seafloor, are one of the major forms of chemosynthetic ecosystems. The known environmental influence of seeps grows stronger as the number of discovered seeps increases, making understanding their breath of impact increasingly relevant.
Data collected in this study utilized two types of underwater research tools, an autonomous underwater vehicle (AUV) and a remotely operated vehicle (ROV). The study begins by examining seep fields about 200 nm off the South Carolina coast, at the Blake Ridge (~2150 m depth) and Cape Fear (~2580 m depth) Diapirs. Geophysical and photographic data were collected during surveys were used to examine the relationship between biomass-dominant invertebrates (mussels, Bathymodiolus heckerae, and clams, Vesicomya cf. venusta) and seafloor physiography. Concentric zonation of mussels and clams at each of the four sites within the seep field suggests the influence of chemical gradients on megafaunal distribution. Distributions of dominant seep features (bivalves, carbonates, bacterial mats) were used to define the active seep site. The relationship between seeps and nearby non-endemic fauna is examined in this study, with a focus on trophic guilds. Geospatial mapping indicated that non-seep-endemic taxa (those not hosting chemoautotrophic endosymbionts) either show positive association (e.g., squat lobsters, cake urchins), negative association (e.g., sea urchins, certain sea cucumbers), or no distributional bias (e.g., sea stars, certain fish) to the presence of a seep.
Further investigation into these faunal relationships may improve understanding of predictive community assembly rules, as well as clarifying the services that seeps provide to the larger ocean ecosystem. Data collection and analytical techniques used here yielded high-resolution habitat maps that can serve as baselines to constrain temporal evolution of seafloor seeps, and to inform ecological niche modeling and resource management.
Another aspect of the study is how seep communities differ when in an extremely low oxygen environment. Methane seeps are typically biological hotspots on the seafloor, with dense faunal communities relative to background (non-chemosynthetic) areas (Carney, 1994). However, some areas of the ocean are extremely low in oxygen, leading to decreases in overall fauna diversity, which can sometimes also affect seep communities. The eastern Pacific contains extensive oxygen minimum zones (OMZs), areas where the dissolved oxygen concentration falls below 22 μM, or 0.5 mg/L (Helly and Levin, 2004; Karstensen et al., 2008), as opposed to averages of 180-270 μM (4-6 mg/L) typically found in the ocean. These zones can intersect the continental slope, affecting benthic organisms, including those found at methane seeps in the region.
The Redondo Knoll Seep (~900 m depth), located just ~30 km off the California coast, shares features of both a seep and an oxygen minimum zone. High-resolution imaging providing a highly detailed photo mosaic and 1 cm resolution bathymetric maps, allowed for a comprehensive site view to further the geologic and microbial examinations. Particularly notable were the extensive microbial mats and, due to its location near the core of a local OMZ (averaging <1 μM oxygen), the lack of endemic seep megafauna or other non-endemic fauna.
While both seeps and OMZs are common by themselves, only a small number of studies have examined them together, such as off the coast of Chile (Sellanes et al., 2010), Pakistan (Fischer et al., 2012; Himmler et al., 2015), and Oregon (Levin et al., 2010). Consequently, there is still much to learn about the ecosystem and organisms at these combination sites.
Overall, this study considers the sphere of influence methane seeps provide the surrounding area through examination of the relationship between endemic fauna, their geologic habitat, and non-endemic fauna, with the intention to use these interactions to better inform seep roles in the environment in the face of anthropogenic disruptions, such as deep-sea mining and climate change, and as well as sites to consider for astrobiological studies.
Wagner, Jamie Katherine Sarbo (2019). Community Structure of Methane Cold Seeps in the Western Atlantic and Eastern Pacific. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/18784.
Dukes student scholarship is made available to the public using a Creative Commons Attribution / Non-commercial / No derivative (CC-BY-NC-ND) license.