Geology topics

Craig M. Robertson

Publications and source records attributed to Craig M. Robertson.

5 recordsLinked to original sources

USGS2021

The role of habitat heterogeneity and canyon processes in structuring sediment macrofaunal communities associated with hard substrate habitats in Norfolk Canyon, USA

Topographic and hydrodynamic complexity in submarine canyons promotes steep gradients in food availability and geophysical parameters which affect ecological assemblages and beta diversity. While habitat heterogeneity in submarine canyons is known to support diverse and abundant megafaunal communities, due to difficulty in sampling little is known about infaunal communities adjacent to hard substrate habitats, their contribution to canyon assemblages, and overall deep-sea diversity. Sediments were collected in three distinct habitat types: within Norfolk Canyon (western Atlantic) adjacent to hard substrate habitats including canyon walls and large boulders, along the main axis of the canyon, and on the adjacent continental slope to quantify macrofaunal (>300 μm) density, diversity and community composition, and sediment geochemical parameters including grain size, organic content, and stable isotope composition. While macrofaunal densities were similar among habitats sampled at comparable depths, diversity was higher near the hard substrate environments. Discrete communities were present in each habitat type, while annelid functional composition was similar between hard substrate adjacent and canyon axis habitats. Although diversity and abundance of hard substrate adjacent sediment communities did not change with depth, communities were driven by sediments with low organic matter content, whereas canyon and slope community assemblages were best explained by depth and higher organic content. Beta diversity among hard substrate adjacent sediments and canyon axis communities was high, with 27% of canyon taxa and 10% of regional taxa only occurring in hard substrate habitats. Given the thousands of submarine canyons worldwide, our results highlight the overall importance of substrate habitat heterogeneity within canyons in supporting deep-sea benthic diversity and suggest that both within-canyon and regional diversity are underestimated.

Norfolk Canyon↗

USGS2020

Submarine canyons influence macrofaunal diversity and density patterns in the deep-sea benthos

Submarine canyons are often morphologically complex features in the deep sea contributing to habitat heterogeneity. In addition, they act as major conduits of organic matter from the shallow productive shelf to the food deprived deep-sea, promoting gradients in food resources and areas of sediment resuspension and deposition. This study focuses on the Baltimore and Norfolk canyons, in the western North Atlantic Ocean, and investigates how different biogeochemical drivers influence canyon and slope macrofaunal communities. Replicated sediment cores were collected along the main axes (~180–1200 m) of Baltimore and Norfolk canyons and at comparable depths on the adjacent slopes. Cores were sorted, assessing whole community macrofaunal (>300 μm) abundance, diversity and standing stocks. Canyon communities were significantly different from slope communities in terms of diversity, abundance patterns and community assemblages, which were attributed to high levels of organic matter enrichment within canyons. There was a significant departure from the expected density-depth relationship in both canyons, driven by enhanced abundances between 800 and 900 m canyon depths, which was characterised as a deposition zone for organic matter. Bathymetric zonation, sediment dynamics, organic enrichment, and disturbance events were clear factors that structured the benthic communities in both Baltimore and Norfolk canyons. Coupling family-level community data, with sediment grain-size and biogeochemistry data explained community dynamics across depth and biogeochemical gradients, providing further evidence that canyons disrupt macrofaunal diversity and density patterns in the deep-sea benthos.

Deep Sea Research Part A, Oceanographic Research P↗

USGS2020

Benthic infaunal communities of Baltimore and Norfolk Canyons

The imperative for finding, cataloging, and understanding continental margin diversity derives from the many key functions, goods and services provided by margin ecosystems and by an increasingly deleterious human footprint on our continental slopes (Levin and Dayton 2009). Progress in seafloor mapping technology and direct observation has revealed unexpected heterogeneity, with a mosaic of habitats and ecosystems linked to geomorphological, geochemical, and hydrographic features that are capable of influencing biotic diversity (Levin and Sibuet 2012). Submarine canyons are dramatic and widespread topographic features crossing continental and island margins in oceans, connecting shelf-margins to deep ocean basins (Harris and Whiteway 2011). Their importance as biodiversity hotspots has continued to emerge over the last two decades as research efforts have increased. Understanding the physical parameters within a canyon system is a primary factor for understanding habitat variability and ecological patterns within the confines of canyon systems (Levin et al. 2001). Margin sediments exhibit ubiquitous depth zonation (Carney et al. 2005), with a diverse suite of species that occupy restricted bathymetric ranges along any given section of the margin. Major shifts in composition among taxa are observed at the shelf-slope transition zone (canyons <500 m), along the upper slope (1,000 m), and at the lower slope transition zone (<3,000 m) (Gibson et al. 2005). In the deep sea, macrofaunal assemblages are generally limited by the availability of allochthonous organic material (Rowe et al. 1982, Billet et al. 1983, Rex et al. 2005, Smith et al. 2008) where macrofaunal densities usually decline with depth and distance from the shore (Rowe et al. 1982, Houston and Haedrich 1984, Rex et al. 2005). However, canyon fauna can experience enhanced food supply through the resuspension and deposition of organic-rich sediments, delivered by increased current velocities within the confines of the canyon (Rowe 1971, Shepard et al. 1974). As a result, canyons are often reported as sustaining enhanced abundances and biomass compared with nearby open slope habitats at similar depths (Vetter and Dayton 1998, Duineveld et al. 2001, De Leo et al. 2010) as well as enhancing regional (γ) and local (α) biodiversity (Hecker et al. 1983, Vetter and Dayton 1998, De Leo et al. 2010, Vetter et al. 2010). Furthermore, enhanced habitat heterogeneity can also be a major structuring agent of ecological assemblages, promoting beta (β) diversity (McClain and Barry 2010) in canyon environments. Canyon systems have often been described as biodiversity hotspots, especially at mid-slope depths (Levin and Sibuet 2012) where physical processes, characterized by complex patterns in hydrography, promote topographically induced upwelling, enhanced mixing via internal tides, and the focusing of tidal bores (Vetter and Dayton 1998, Cacchione et al. 2002). Additionally, sediment transport and accumulation (García et al. 2008) represent important influential ecological drivers. Factors such as substrata heterogeneity (Levin and Sibuet 2012) and concentration of organic matter (De Leo et al. 2010) have been suggested to explain higher faunal diversity, abundance, and benthic productivity found in canyon systems compared with surrounding areas. Bathymetric patterns of species diversity have been attributed to changes in sediment characteristics (Etter and Grassle 1992), productivity, currents, oxygen, disturbance, and the interplay of biotic effects with depth and latitude (Levin et al. 2001, Carney et al. 2005). Recent studies report on the uniqueness of canyon benthic communities and habitats and the view that no two canyons are alike (Cunha et al. 2011). Certain submarine canyons may maintain 436 characteristic and unique faunas, but more often canyon macrofaunal assemblages show high dominance and locally reduced biodiversity (Rowe 1971, Gage 1997, Curdia et al. 2004, Cun

Baltimore Canyon, Norfolk Canyon↗

USGS2017

Seasonal variability in particulate matter source and composition to the depositional zone of Baltimore Canyon, U.S. Mid-Atlantic Bight

Submarine canyons are often hotspots of biomass and productivity in the deep sea. However, the majority of deep-sea canyons remain poorly sampled. Using a multi-tracer approach, results from a detailed geochemical investigation from a year-long sediment trap deployment reveals details concerning the source, transport, and fate of particulate matter to the depositional zone (1318 m) of Baltimore Canyon on the US Mid-Atlantic Bight (MAB). Both organic biomarker composition (sterol and n-alkanes) and bulk characteristics (δ13C, Δ14C, Chl-a) suggest that on an annual basis particulate matter from marine and terrestrially-derived organic matter are equally important. However, elevated Chlorophyll-a and sterol concentrations during the spring sampling period highlight the seasonal influx of relatively fresh phytodetritus. In addition, the contemporaneous increase in the particle reactive elements cadmium (Cd) and molybdenum (Mo) in the spring suggest increased scavenging, aggregation, and sinking of biomass during seasonal blooms in response to enhanced surface production within the nutricline. While internal waves within the canyon resuspend sediment between 200 and 600 m, creating a nepheloid layer rich in lithogenic material, near-bed sediment remobilization in the canyon depositional zone is minimal. Instead, vertical transport and lateral transport across the continental margin are the dominant processes driving seasonal input of particulate matter. In turn, seasonal variability in deposited particulate organic matter may be linked to benthic faunal composition and ecosystem scale carbon cycling.

Deep-Sea Research Part I: Oceanographic Research P↗

USGS2016

Macrofaunal communities associated with chemosynthetic habitats from the U.S. Atlantic margin: A comparison among depth and habitat types

Hydrocarbon seeps support distinct benthic communities capable of tolerating extreme environmental conditions and utilizing reduced chemical compounds for nutrition. In recent years, several locations of methane seepage have been mapped along the U.S. Atlantic continental slope. In 2012 and 2013, two newly discovered seeps were investigated in this region: a shallow site near Baltimore Canyon (BCS, 366–412 m) and a deep site near Norfolk Canyon (NCS, 1467–1602 m), with both sites containing extensive chemosynthetic mussel bed and microbial mat habitats. Sediment push cores, suction samples, and Ekman box cores were collected to quantify the abundance, diversity, and community structure of benthic macrofauna (>300 μm) in mussel beds, mats, and slope habitats at both sites. Community data from the deep site were also assessed in relation to the associated sediment environment (organic carbon and nitrogen, stable carbon and nitrogen isotopes, grain size, and depth). Infaunal assemblages and densities differed both between depths and among habitat types. Macrofaunal densities in microbial mats were four times greater than those present in mussel beds and slope sediments and were dominated by the annelid families Dorvilleidae, Capitellidae, and Tubificidae, while mussel habitats had higher proportions of crustaceans. Diversity was lower in BCS microbial mat habitats, but higher in mussel and slope sediments compared to NCS habitats. Multivariate statistical analysis revealed specific sediment properties as important for distinguishing the macrofaunal communities, including larger grain sizes present within NCS microbial mat habitats and depleted stable carbon isotopes (δ 13 C) in sediments present at mussel beds. These results suggest that habitat differences in the quality and source of organic matter are driving the observed patterns in the infaunal assemblages, including high β diversity and high variability in the macrofaunal community composition. This study is the first investigation of seep infauna along the U.S. Atlantic slope north of the Blake Ridge Diapir and provides a baseline for future regional comparisons to other seep habitats along the Atlantic margin.

Deep-Sea Research Part II: Topical Studies in Ocea↗