Oregon Continental Shelf Research Articles

The role of seasonal hypoxia and benthic boundary layer exchange on iron redox cycling on the Oregon shelf

AbstractWidespread hypoxia occurs seasonally across the Oregon continental shelf, and the duration, intensity, and frequency of hypoxic events have increased in recent years. In hypoxic regions, iron reduction can liberate dissolved Fe(II) from continental shelf sediments. Fe(II) was measured in the water column across the continental shelf and slope on the Oregon coast during summer 2022 using both a trace metal clean rosette and a high‐resolution benthic gradient sampler. In the summer, Fe(II) concentrations were exceptionally high (40–60 nM) within bottom waters and ubiquitous across the Oregon shelf, reflecting the low oxygen condition (40–70 μM) at that time. The observed inverse correlation between Fe(II) and bottom water oxygen concentrations is in agreement with expectations based on previous work that demonstrates oxygen is a major determinant of benthic Fe fluxes. Rapid attenuation of Fe(II) from the benthic boundary layer (within 1 m of the seafloor) probably reflects efficient cross‐shelf advection. One region, centered around Heceta Bank (~ 44°N) acts a hotspot for Fe release on the Oregon continental shelf, likely due to its semi‐retentive nature and high percent mud content in sediment. The results suggest that hypoxia is an important determinant of the inventory of iron is Oregon shelf waters and thus ultimately controls the importance of continental margin‐derived iron to the interior of the North Pacific Basin.

Seasonal, Annual, and Decadal Distribution of Three Rorqual Whale Species Relative to Dynamic Ocean Conditions Off Oregon, USA

Whale populations recovering from historical whaling are particularly vulnerable to incidental mortality and disturbance caused by growing ocean industrialization. Several distinct populations of rorqual whales (including humpback, blue, and fin whales) migrate and feed off the coast of Oregon, USA where spatial overlap with human activities are on the rise. Effective mitigation of conflicts requires better foundational understanding of spatial and temporal habitat use patterns to inform conservation management. Based on a year-round, multi-platform distance sampling dataset (2016-2021, 177 survey days, 754 groups observed), this study generated density models to describe and predict seasonal distribution of rorqual whales in Oregon. Phenology analysis of sightings revealed a peak of humpback whale and blue whale density over the Oregon continental shelf in August and September respectively, and higher fin whale density in the winter (December). Additionally, we compared rorqual sighting rates across three decades of survey effort (since 1989) and demonstrate that rorqual whales are strikingly more prevalent in the current dataset, including distinct increases of blue and fin whales. Finally, density surface models relating whale densities to static and dynamic environmental variables acquired from data-assimilative ocean models revealed that summer and spring rorqual distribution were influenced by dynamic oceanographic features indicative of active upwelling and frontal zones (respectively 27% and 40% deviance explained). On the continental shelf, blue whales were predicted to occur closer to shore than humpback whales and in the more southern waters off Oregon. Summer and spring rorqual models, and humpback whale models, showed predictive performance suitable for management purposes, assessed through internal cross-validation and comparison to an external dataset (388 groups observed). Indeed, monthly hotspots of high predicted rorqual whale density across multiple years were validated by independent sightings (80% overlap in the summer model). These predictive models lay a robust basis for fine-scale dynamic spatial management to reduce impacts of human activities on endangered populations of rorqual whales in Oregon.

Gathering local ecological knowledge to augment scientific and management understanding of a living coastal resource: The case of Oregon’s nearshore groundfish trawl fishery

Globally, coastal nearshore regions are an intersecting point for human and biological productivity, often serving as hotspots for subsistence, commercial, and recreational fishing activities. Despite this, many nearshore areas remain poorly understood, monitored or managed. This case study examined the nearshore sector of Oregon’s groundfish trawl fishery, which exists in shallow estuarine and continental shelf habitats common along the West Coast of North America; areas that are important for early life history stages of many commercial and recreational fisheries. The West Coast groundfish fishery includes over 90 different species, 40 of which occur within Oregon’s nearshore (here defined as the portion of the shelf extending seaward to a water depth of 200 m). The very shallow portions of the Oregon Coast (the area of the shelf inshore of 55 m) have been subject to limited scientific survey monitoring, and much of the details of the ecology, health, and processes in these habitats remain poorly understood. The utilization of the nearshore region by the commercial groundfish trawl fleet is also minimally documented despite the fact that experiential knowledge (local ecological knowledge [LEK]; trawl logbooks, fish tickets, interviews) exists. This research explored the capacity of capturing LEK sources to inform and enhance understanding of the drivers of effort and the vitality of nearshore fishery resources. Our approach used statistical analysis and mapping of nearshore trawl effort from 1981 to 2017 and gathered semi-structured interviews of intergenerational fishermen to bolster data-poor areas. Insights provided by sampling strategies and historical to current knowledge of access to groundfish assemblages provide informed baselines for future management. Spatial mapping results revealed a decline in trawl effort on the Oregon continental shelf thought time. Logbook and interview data assessment illuminated market and ecological drivers of fishing behavior as well as a unique sector of the groundfish fleet in Oregon: the beach fleet, with unique market and socio-economic challenges. Findings indicate a mixed-methods approach can provide a more thorough assessment of long-term interest in Oregon’s nearshore groundfish fishery. Ensuring better understanding of coastal interfacing regions such as Oregon’s nearshore insights potential for better conservation and utilization of marine resources and improved monitoring in resource limited management contexts.

Benthic assemblages of mega epifauna on the Oregon continental margin

Environmental assessment studies are usually required by a country's administration before issuing permits for any industrial activities. One of the goals of such environmental assessment studies is to highlight species assemblages and habitat composition that could make the targeted area unique. A section of the Oregon continental slope that had not been previously explored was targeted for the deployment of floating wind turbines. We carried out an underwater video survey, using a towed camera sled, to describe its benthic assemblages. Organisms were identified to the lowest taxonomic level possible and assemblages described related to the nature of the seafloor and the depth. We highlighted six invertebrate assemblages and three fish assemblages. For the invertebrates within flat soft sediments areas we defined three different assemblages based on primarily depth: a broad mid-depth (98–315 m) assemblage dominated by red octopus, sea pens and pink shrimps; a narrower mid-depth (250–270 m) assemblage dominated by box crabs and various other invertebrates; and a deeper (310–600 m) assemblage dominated by sea urchins, sea anemones, various snails and zoroasterid sea stars. The invertebrates on mixed sediments also were divided into three different assemblages: a shallow (~100 m deep) assemblage dominated by plumose sea anemones, broad mid-depth (170–370 m) assemblage dominated by sea cucumbers and various other invertebrates; and, again, a narrower mid-depth (230–270 m) assemblage, dominated by crinoids and encrusting invertebrates. For the fish, we identified a rockfish assemblage on coarse mixed sediments at 170–370 m and another fish assemblage on smaller mixed sediments within that depth range (250–370 m) dominated by thornyheads, poachers and flatfishes; and we identified a wide depth-range (98–600 m) fish assemblage on flat soft sediments dominated by flatfishes, eelpouts and thornyheads. Three of these assemblages (the two broad fish assemblages and the deep flat soft sediments invertebrate assemblage) seem to represent deeper examples of assemblages already known on the Oregon continental shelf, especially on soft sediments, while the assemblages in the pockmarks habitat (the narrower depth ranges) might be unique to the area. This diversity of assemblages in a relatively small section of the Oregon continental upper slope and shelf shows the importance of environmental assessment studies in helping limit future impacts of industrial activities on benthic communities.

Benthic oxygen consumption rates during hypoxic conditions on the Oregon continental shelf: Evaluation of the eddy correlation method

Three stations, at ∼80 m water depth on the Oregon shelf between 44.7°N and 43.9 N, were studied under hypoxic conditions in late spring and summer of 2009 to determine benthic oxygen consumption rates. Oxygen fluxes were derived from eddy correlation (EC) measurements made from an autonomous lander deployed for 11–15 h at a time. Average oxygen consumption rates ranged from 3.2 to 9.8 mmol m−2 d−1 and were highest at the southernmost station. Methods for separating eddy components and rotating coordinates were examined for effects on EC fluxes. It was found that oscillations at frequencies associated with surface and internal waves made significant contributions, but horizontal component biasing could be minimized by wave‐based rotation methods. Additional measurements included benthic boundary layer properties, and sediment permeability and profiles of sediment organic C, chlorophyll‐a, excess 210Pb and % fines. Comparative flux estimates were determined from benthic chamber measurements and microelectrode profiles at two of the stations. The chamber O2 consumption rates exceeded the EC fluxes by factors of 1.2–1.8, which may reflect enclosure effects, the different spatial and temporal scales of the measurements, and/or inhomogeneous benthic respiration rates. The magnitudes of the fluxes by either method, however, are low for shelf depths. Thus, for benthic O2 consumption to contribute to Oregon shelf hypoxia, bottom waters must be slowly renewed and minimally ventilated by along‐ or across‐shelf advection and turbulent mixing. Circulation studies indicate these conditions are favored by increased near‐bottom stratification during persistent summer upwelling‐ relaxation cycles.

Observations of mixed layer restratification by onshore surface transport following wind reversal in a coastal upwelling region

Observations, from the Oregon continental shelf, describe the slumping of a coastal upwelling front in response to a reversal of winds from upwelling‐ to downwelling‐favorable. Initially, the front outcropped in a surface mixed layer of depth 10–20 m with a pronounced cross‐shelf density gradient. Following wind reversal, both the unbalanced cross‐shelf pressure gradient and wind‐driven Ekman transport drove a rapid onshore movement of near‐surface water, causing the mixed layer to restratify. At the surface, the cross‐shelf density gradient steepened to become a discontinuous front, which propagated onshore at 0.43–0.60 m s−1 as the head of a buoyant gravity current. Internal waves were driven ahead of the front on the base of the former mixed layer. An injected dye tracer revealed that surface water from inshore of the strongest frontal gradient detached from the surface as the gravity current passed over the top of it. This water largely retained the low potential vorticity signature that it had taken on in the mixed layer as it spread across the shelf in a weakly stratified, subsurface layer. Restratification and frontogenesis were most likely gravity‐driven in the first hours following wind reversal, with the contribution from Ekman transport becoming increasingly significant.

Degradation of phytoplankton-derived organic matter: Implications for carbon and nitrogen biogeochemistry in coastal ecosystems

Experiments were conducted using seawater from the Oregon continental shelf to determine: (1) rates of phytoplankton-derived particulate organic matter (POM) and dissolved organic matter (DOM) degradation by natural microbial communities, and (2) whether inorganic nutrients or flagellate grazing limit the bacterial response to, and subsequent degradation of, the DOM. In the initial seawater samples, nutrients were depleted and organic matter concentrations were elevated above concentrations found in upwelled water, indicative of recent bloom conditions. In whole water treatments incubated for 3 d, an average of 24% of the total organic C and 33% of the POC was degraded, with some portion of the POC being converted to DOC. In treatments incubated after POM was removed by filtration, DOC degradation was initially rapid and then proceeded at a slower rate. After 3 d, an average of 41% of the DOC was degraded. Selective degradation of the C-component of both the POM and DOM relative to the N-component was observed. Reductions in flagellate grazing resulted in increases in bacterial abundance and enhanced DOC degradation, while inorganic nutrient amendments had little effect. Overall, these results suggest that a fraction of the phytoplankton-derived POM and DOM can be rapidly degraded, contributing to oxygen consumption on the continental shelf. The long degradation time of a less labile DOC fraction relative to potential offshelf transport mechanisms suggests that Oregon's coastal waters may be a source of DOC to adjacent offshore waters of the North Pacific.

High-Frequency Internal Waves on the Oregon Continental Shelf

Abstract Measurements of vertical velocity by isopycnal-following, neutrally buoyant floats deployed on the Oregon shelf during the summers of 2000 and 2001 were used to characterize internal gravity waves on the shelf using measurements of vertical velocity. The average spectrum of Wentzel–Kramers–Brillouin (WKB)-scaled vertical kinetic energy has the level predicted by the Garrett–Munk model (GM79), plus a narrow M2 tidal peak and a broad high-frequency peak extending from about 0.1N to N and rising a decade above GM79. The high-frequency peak varies in energy coherently with time across its entire bandwidth. Its energy is independent of the tidal energy. The energy in the “continuum” region between the peaks is weakly correlated with the level of the high-frequency peak energy and is independent of the tidal peak energy. The vertical velocity is not Gaussian but is highly intermittent, with a calculated kurtosis of 19. The vertical kinetic energy varies geographically. Low energy is found offshore and nearshore. The highest energy is found near a small seamount. High energy is found over the rough topography of Heceta Bank and near the shelf break. The highest energy occurs as packets of high-frequency waves, often occurring on the sharp downward phase of the M2 internal tide and called “tidal solibores.” A few isolated waves with high energy are also found. Of the 1-h periods with the highest vertical kinetic energy, 31% are tidal solibores, 8% are isolated waves, and the remainder of the periods appear unorganized. The two most energetic tidal solibores were examined in detail. As compared with the steady, propagating, two-dimensional, inviscid, internal-wave solutions to the equations of motion with no background shear [i.e., the Dubreil–Jacotin–Long (DJL) equation], all but the most energetic observed waveforms are too narrow for their height to be solitary waves. Despite the large near-N peak in vertical kinetic energy, the M2 internal tide contributes over 80% of the energy, ignoring near-inertial waves. The tidal solibores make a very small contribution, 0.5%, to the overall internal-wave energy.

Comparison of acoustic and net sampling systems to determine patterns in zooplankton distribution

An analysis was conducted of the strengths and limitations of acoustic methods to determine biomass and taxonomic distribution patterns over the Oregon continental shelf. Measurements of volume backscatter from two different acoustic instruments were compared with each other and with predicted volume backscatter calculated from a coincident net tow. Spatially and temporally coincident data were collected from a six‐frequency bioacoustic system (Tracor Acoustic Profiling System (TAPS)) and from a 1 m2 Multiple Opening Closing Net and Environmental Sensing System (MOCNESS). The combined net/acoustic tows were conducted over the Oregon continental shelf in August 2001, during both day and night. The TAPS was mounted on the upper frame of the MOCNESS and ensonified a volume of water directly in front of the net mouth. A four‐frequency echosounder (Hydroacoustic Technologies Inc. (HTI)) with downward looking transducers was towed off the port side of the ship during the MOCNESS/TAPS tows. Fine‐scale vertical distributions of zooplankton that were resolved acoustically were not apparent within the integrated depth strata of the individual MOCNESS net samples. The taxonomic and size composition of the zooplankton community had strong effects on volume backscatter (SV), although the magnitude of SV was not linearly related to zooplankton biomass. Predicted SV calculated from scattering models and the contents of the MOCNESS nets agreed best (sometimes within 5 dB) with measured SV from the TAPS at 420 and 700 kHz and measured SV at 420 kHz from the HTI.

Lagrangian trajectories on the Oregon shelf during upwelling

Neutrally buoyant, isopycnal-following floats were deployed on the Oregon continental shelf during the upwelling seasons of 2000 and 2001 and were carried southward by the mean current. The floats made CTD profiles and obtained GPS fixes twice daily, thus providing a hydrographic section along a known track. The floats followed the water accurately while at depth, but were displaced from the trajectories of the deep water during semidiurnal surfacings. These effects were large for water depths shallower than 100m, but small on the rest of the shelf. Float trajectories, corrected for advection while on the surface, showed significant error when near the shore, but little net effect offshore. Some floats moved onshore and upward along the sloping isopycnals as expected during upwelling. Although the position of the isopycnal could be predicted accurately from the wind, the motion along the isopycnal showed significant fluctuations unrelated to the wind. These may be due to barotropic shelf waves. Some floats moved southward, roughly following the isobaths around Heceta Bank to Cape Blanco. Here they underwent large vertical and cross-shelf excursions and eventually moved offshore. Two floats passed through this region 25 days apart following different trajectories, indicating an unsteady flow. Overall, these data show the expected mix of a classical upwelling circulation in the north, an offshore jet with eddies in the south, and a strong influence of topography on both the mean flow and its fluctuations.

Internal solitary waves of elevation advancing on a shoaling shelf

A sequence of three internal solitary waves of elevation were observed propagating shoreward along a near‐bottom density interface over Oregon's continental shelf. These waves are highly turbulent and coincide with enhanced optical backscatter, consistent with increased suspended sediments in the bottom boundary layer. Non‐linear solitary wave solutions are employed to estimate wave speeds and energy. The waves are rank ordered in amplitude, phase speed, and energy, and inversely ordered in width. Wave kinetic energy is roughly twice the potential energy. The observed turbulence is not sufficiently large to dissipate the waves' energy before the waves reach the shore. Because of high wave velocities at the sea bed, bottom stress is inferred to be an important source of wave energy loss, unlike near‐surface solitary waves. The wave solution suggests that the lead wave has a trapped core, implying enhanced cross‐shelf transport of fluid and biology.

Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Abstract Detailed observations of the structure within internal solitary waves propagating shoreward over Oregon's continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By as...

Internal hydraulic flows on the continental shelf: High drag states over a small bank

Observations of currents, hydrography, and turbulence provide unambiguous evidence for hydraulic control of flow over an isolated three‐dimensional topographic feature on Oregon's continental shelf. The flow becomes critical at the crest of the bank, forming a strong supercritical downslope flow in the lower layer. Farther downstream, internal hydraulic jumps form as the bottom flow becomes subcritical. As a consequence, turbulence is greatly enhanced in the bottom boundary layer, in the sheared interface above the swiftly flowing bottom current, and in the internal hydraulic jump. The dissipation rate of turbulent energy is consistent with the mean energy removal rate for a hydraulic jump in an idealized two‐layer flow. This enhanced turbulence constitutes a “high drag” state of the flow in which the form drag introduced by the topography exerts significant influence on the flow around it and mixing is increased 102–103 times the background values.

Downwelling Circulation on the Oregon Continental Shelf. Part I: Response to Idealized Forcing

Time-dependent downwelling on the Oregon continental shelf is studied with a two-dimensional approximation, that is, spatial variations across shelf and with depth, using the Blumberg-Mellor, finite-difference, stratified, hydrostatic, primitive equation model. The time-dependent response of a coastal ocean at rest to constant, downwelling-favorable wind stress is examined. Topography and stratification representative of the Oregon continental shelf are used for the basic case experiment. The wind stress forces onshore flow in a turbulent surface boundary layer. The compensating flow below the surface layer advects the density field downward and offshore and accelerates an alongshore current in the form of a vertically and horizontally sheared coastal jet. The dominant feature of the response flow field is a downwelling front that moves slowly offshore, leaving behind an inshore region where the density is well mixed. The downwelling front in the density field is concentrated near the bottom, while the front in alongshore velocity extends over the full depth and is nearly vertical, separating weak alongshore velocities inshore from the coastal jet offshore. The front contains strong vertical motion from the surface to the bottom and some recirculation. Much of the offshore flow from the base of the front is characterized by time- and space-dependent fluctuations involving spatially periodic separation and reattachment of the bottom boundary layer and accompanying recirculation cells. This flow has positive potential vorticity and appears to be finite-amplitude slantwise convection resulting from a hydrostatic symmetric instability. Additional experiments show the dependence of the response flow field an the magnitude of the wind stress, the initial stratification, and the shelf topography. Experiments with the vertical turbulent kinematic viscosity and diffusivity parameterized by a different turbulence closure scheme, as a function of a local Rich-ardson number, or as constants show dependence of the response flow field on the choice of turbulence submodel. The occurrence of a well-mixed region inshore and the existence of time- and space-dependent fluctuations associated with slantwise convection in the near-bottom offshore flow appear to be robust features of the two-dimensional downwelling response.

Model Studies of Near-Inertial Waves in Flow over the Oregon Continental Shelf

Abstract Two-dimensional, primitive equation model studies of wind-forced flow over a continental shelf Show that, under upwelling conditions, high levels of near-inertial wave energy are found in the interior over the shelf. The regions of elevated wave energy, with maximum wave amplitudes of around ±0.2 m s−1, persist for up to two weeks and have spatial scales of 20–40 m vertically and 5–20 km horizontally. Relatively high dissipation levels are associated with these concentrations of wave energy. When forced with downwelling-favorable winds, model results show very little subsurface inertial energy on the shelf. A comparison of inertial wave propagation using the primitive equation model and a linearized version of the model demonstrates strong dependence on the background flow field. The behavior of inertial waves using the linearized model is completely different: very little subsurface inertial energy is seen on the shelf in either upwelling or downwelling conditions, except where the bottom slope ...

Upwelling Circulation on the Oregon Continental Shelf. Part II: Simulations and Comparisons with Observations

Sixty-day simulations of flow on the Oregon continental shelf are performed using the Blumberg and Mellor sigma coordinate, primitive equation model. The model is two-dimensional (an across-shelf section) with high spatial resolution and realistic shelf topography. Forcing consists of surface heat flux, either hourly or low-pass filtered wind stress, and in one case, a constant alongshore pressure gradient. Model results are compared with current and hydrographic measurements from the CUE-2 program. The horizontal scale of the alongshore coastal jet is significantly influenced by the structure of the initial density and velocity fields. The model successfully reproduces the vertical shear in the alongshore velocity field v, but the model's mean v field is too strongly southward, and the variance in both the vvu and v fields is underpredicted. Inclusion of the alongshore pressure gradient, while improving prediction of the mean alongshore velocities, does not improve the model-data correlation. The time-mean model density agrees with observations at middepths over the shelf, but shows larger values than observed near the surface. The results demonstrate the importance of including a surface heat flux and of specifying realistic initial density and alongshore velocity fields.

Upwelling Circulation on the Oregon Continental Shelf. Part I: Response to Idealized Forcing

Upwelling Circulation on the Oregon Continental Shelf. Part I: Response to Idealized Forcing

Biology of an active methane seep on the Oregon continental shelf

Biology of an active methane seep on the Oregon continental shelf

Biology of an active methane seep on the Oregon continental shelf

Biology of an active methane seep on the Oregon continental shelf

Universal similarity and the thickness of the viscous sublayer at the ocean floor

Experiments conducted on the Oregon continental shelf in June 1979 indicate that the boundary layer flow at the seafloor was hydrodynamically smooth. Fine‐resolution velocity profiles are used to test the assumption that the flow behaved like a universally similar, neutrally buoyant flow over a smooth wall. The non‐dimensional thickness of the viscous sublayer is far more variable than has been observed in laboratory studies over perfectly smooth walls. This variability may conceivably be related to upstream changes of surface roughness or to the presence of distributed roughness elements. Laboratory results indicate that the near‐bed flow adjusts extremely slowly to upstream roughness changes and suggests that flow near the sublayer in the ocean may be influenced by roughness changes as far as tens of meters upstream.