Heceta Bank Research Articles

Long-term laboratory observations of Euphausia pacifica fecundity: comparison of two geographic regions

We tested the assumption that female Euphausia pacifica produce multiple broods of eggs during their spawning season by maintaining individual females under laboratory conditions at 10.5°C and comparing the egg production of females collected from 2 populations, 1 from Heceta Bank off central Oregon (44° N, n = 27 individuals), and the other from waters near Santa Barbara, California (34° N, n = 12 individuals). We documented the production of multiple, distinct broods by daily observation of females over periods from 1 to 9 mo. Median brood sizes were not significantly different between regions: 102 eggs for females from Heceta Bank and 116 eggs for females from Santa Barbara. Median interbrood period (IBP) was significantly different: 4 d for Santa Barbara females and 5 d for Heceta Bank females. Brood size and IBP were highly variable within and among individuals. The total number of eggs produced female -1 in the experiment ranged from 378 to 8600, and IBPs ranged from 1 to 79 d. A brood of eggs represented an average of 9% of the female's weight in carbon, equivalent to 0.5 to 6% of body weight d -1 (average of 1.7% d -1 ).

Flow‐topography interactions in the northern California Current System observed from geostationary satellite data

Data from Geostationary Operational Environmental Satellites are used to study the seasonal evolution of temperature fronts in the northern California Current System (CCS), focusing on the interactions with topographic features. Fronts first appear close to the coast in response to upwelling winds, moving offshore with the continuous input of energy to the system. Late in the upwelling season (after July), the upwelling front is persistently found over deeper waters south of Heceta Bank, Oregon, than north of it, suggesting that the equatorward jet separates from the shelf at Heceta Bank. Inshore of the upwelling front, weak gradients are found on the Bank. The interaction of the equatorward flow with Heceta Bank and Cape Blanco, Oregon, farther south, substantially increases the mesoscale activity and oceanic frontal habitat downstream to the south in the CCS, where fronts are persistently found greater than 100 km from the coast.

On the spatial structure of coastal circulation off Newport, Oregon, during spring and summer 2001 in a region of varying shelf width

A time series of hourly surface current maps in the shelf waters off Newport, Oregon, was made during April–September 2001 using five SeaSonde HF current mappers, during Coastal Advances in Shelf Transport (COAST). The surface currents responded rapidly to the changing winds, in repeated patterns that were strongly affected by bottom topography. An equatorward current jet repeatedly formed in response to upwelling winds, its strength, but not its trajectory, covarying with the meridional wind stress. Near Cape Foulweather (44.8°N), where the shelf begins to widen, the jet rotates, weakens, and trends offshore to the south first along, then across, the isobaths. Below Cape Foulweather, inshore of the jet, a lee region of generally weak currents was commonly observed. The equatorward jet core was most commonly observed near the 80‐m isobath between 45.0°N and 44.4°N, but transited offshore between spring and summer over Heceta Bank. At Newport (44.6°N), it was rarely observed less than 8 km from the coast. A second, inshore, equatorward jet, previously unknown, was observed repeatedly south of Waldport (44.4°N). Sustained downwelling wind episodes produced poleward currents, though less responsively north of Cape Foulweather. Strongest poleward flow was generally trapped near the coast. Surface currents were correlated with the northward wind, except regionally far from shore over Heceta Bank, responding within half a day. Response to the wind varies spatially, being intensified in the narrow shelf (northern) region. The equatorward jet persists through periods of zero wind forcing. Coastal sea level covaries with the meridional wind and the primary mode of ocean current response.

Modeling of ecosystem processes on the Oregon shelf during the 2001 summer upwelling

Three‐dimensional ecosystem response to wind forcing on the continental shelf off Oregon is studied using a five‐component nitrogen‐based ecosystem model coupled to a high‐resolution circulation model. We investigate, in particular, the influence of time‐dependent winds and alongshore variations in shelf topography for summer 2001 during the time period of the Coastal Ocean Advances in Shelf Transport (COAST) field experiment. Over summer 2001 the average southward, upwelling‐favorable wind leads to the development of a southward coastal jet and upwelled cold nutrient‐rich water over the shelf. A strong influence of the shelf topography is found in the observed and modeled nutrients and plankton, with significant spatial variability associated with interactions of the shelf flow with Heceta Bank. The model results show that the largest mean chlorophyll‐a concentrations are located onshore of the coastal jet, while the largest mean zooplankton concentrations are found farther offshore, typically on the offshore side of the jet, except on Heceta Bank. During intermittent downwelling periods the largest concentrations of mean zooplankton and phytoplankton, however, are both inshore of the jet. The 20 day oscillations in the modeled nutrients and plankton, the time lags between the wind stress and these ecosystem components, and the decorrelation timescales of phytoplankton agree with those obtained from mooring and satellite measurements. A detailed analysis of the balance between biological and physical forcing helps to clarify the temporal and spatial response of the ecosystem.

Modeling upwelling circulation off the Oregon coast

Time‐dependent, three‐dimensional, upwelling circulation on the continental shelf off the Oregon coast is studied using a primitive equation numerical model. A limited area domain with a high‐resolution curvilinear grid is utilized. The response of the coastal ocean to forcing by observed wind stress and heat flux during the summer 2001 time period of the Coastal Ocean Advances in Shelf Transport (COAST) field experiment is investigated. Model results are compared to COAST current and hydrographic measurements. The shelf velocity and density fields are generally characterized by the presence of a southward alongshore coastal jet with an upwelling density front on the shoreward side of the jet. The large variability in shelf topography associated with Heceta Bank exerts a major influence on the shelf velocity and density fields. Over the bank the alongshore coastal jet is displaced offshore, and colder upwelled water extends farther from the coast. Northward mean flow and upward motion are found inshore of the jet. Three‐dimensional flow structures in response to variable shelf bottom topography are presented, and an analysis of time‐ and space‐dependent alongshore momentum balances is applied to clarify the associated dynamics. In general, northward pressure gradients, set up over the bank during southward upwelling winds, accelerate currents on the inshore side of the jet northward when the winds relax. Analysis of term balances in the depth‐averaged equation for potential temperature shows that during upwelling, across‐shore advection makes the major contribution to cooling over most of the region, except inshore over Heceta Bank, where alongshore advection also plays a significant role.

Time‐dependent, wind‐driven flow over a shallow midshelf submarine bank

During summer 2001, high‐resolution hydrographic, velocity, and bio‐optical surveys were conducted over Heceta Bank off central Oregon. North of the bank, upwelling over simple bottom topography exhibited a classic response with a midshelf, baroclinic coastal jet and upwelled isopycnals. The coastal upwelling jet follows the bank topography as it widens offshore before reaching the southern end of the bank, where the shelf break turns almost 90° back toward the coast. The ensuing adjustment involves the offshelf transport of coastal water and the material it contains. The jet meanders anticyclonically in the “lee” of the bank before resuming its equatorward path seaward of the shelf break. A pool of high‐chlorophyll (>15 mg m−3) near‐surface water is found in the low‐velocity “lee” region created by the bank. A low‐temperature, high‐salinity bottom water pool, supplied primarily from the south, lies over the inshore region of the bank. This bottom pool is up to 40 m thick and contains elevated levels of suspended material. By creating an east‐west perturbation in the coastal upwelling front, the flow‐topography interaction introduces an alongshore pressure gradient. When southward upwelling‐favorable winds relax, this alongshore pressure gradient can drive flow back to the north. This leads to northward flow on the inshore side of the bank, ultimately setting up the potential for recirculation around the entire bank complex on a timescale of about 10 days. Strong northward winds during summer lead to downwelling within 15 km of the coast, accompanied by significant (>0.2 m s−1) wind‐driven northward currents.

Spatial variations in phytoplankton pigment ratios, optical properties, and environmental gradients in Oregon coast surface waters

In situ optics and hydrographic measurements, along with discrete samples for calibration, were used to assess spatial variations in phytoplankton characteristics and taxonomic composition (pigment ratios, relative particle size distribution, and chlorophyll a (chl a) concentration) in Oregon coast surface waters during August 2001. The relationships between environmental parameters (nutrients, light, temperature), photoprotective:photosynthetic carotenoid (PPC:PSC) ratios, and in situ spectral absorption and beam attenuation from ac‐9 measurements were also evaluated. We computed a “slope” index to evaluate changes in shapes of the phytoplankton absorption (aph) spectra: aph slope = (aph488 − aph532)/[aph676 · (488–532 nm)]. Significant linear relationships were seen between PPC:PSC ratios and the aph slope parameter for surface samples grouped by location, date, and water mass characteristics. We observed high spatial variability in the phytoplankton photophysiological and taxonomic indicators derived from our optical measurements. Lower PPC:PSC ratios and larger particle size distributions were seen near the coast in colder upwelled water compared to locations farther offshore with warmer, more nutrient deplete water. Temperature and photosynthetic available radiation explained 42% of the variability in PPC:PSC ratios for all surface samples and 61% for surface samples with <2 μM dissolved inorganic nitrogen. Temperature alone explained 87% of the variability in PPC:PSC ratios (estimated from ac‐9 absorption data) and 72% of the variability in relative particle size distribution (estimated from ac‐9 beam attenuation data) for Heceta Bank samples collected over 4 days, inshore and offshore of an upwelling front.

Coastal ocean response to summer upwelling favorable winds in a region of alongshore bottom topography variations off Oregon

Repeated mesoscale surveys of waters over the shelf and slope off Oregon were conducted during spring and summer of 2001 to study the spatial structure of the velocity and hydrographic fields. The ocean response to wind forcing is compared between a region of relatively simple topography with alongshore uniformity and a region of complex topography including a shallow submarine bank (Heceta Bank). In the simple topography region the upper water column is influenced by upwelling and fresh water from the Columbia River during spring, with the low‐salinity water located farther offshore during summer. Variability in the fields is mostly confined within 30 km from the coast. Over Heceta Bank the region of higher variability is broader, spanning most of the shelf. The coastal upwelling jet is located inshore of the pinnacle of the bank during spring, moving offshore during summer. The region inshore of the bank is characterized by low velocities and flow recirculation. Near‐surface fields show that the circulation evolves considerably between seasons. In both regions the area influenced by upwelled water is much broader during summer. Dense waters found over midshelf off Newport are upwelled to the north and advected south. Dense waters inshore of Heceta Bank are substantially influenced by water from the north during spring, but their source is mostly from the south during summer. South of the bank, the separation of the jet significantly increases the cross‐isobath transport, constituting an efficient mechanism for transport of material from the shelf into deeper waters.

Distant effect of assimilation of moored currents into a model of coastal wind‐driven circulation off Oregon

An optimal interpolation (OI) sequential algorithm is implemented for a three‐dimensional primitive equation model to assimilate current measurements from acoustic Doppler profilers moored on the Oregon shelf as a part of the Coastal Ocean Advances in Shelf Transport (COAST) upwelling experiment (May–August 2001). A stationary estimate of the forecast error covariance required by the OI is computed based on the error covariance in the model solution not constrained by data assimilation. Lagged model error covariances are used to account for the effect of previously assimilated data. The forecast error covariance has a shorter alongshore spatial scale than the model error covariance unconstrained by the data, as an effect of propagating dynamical modes. Assimilation of currents from one or two of the moorings located on the path of the upwelling jet helps to improve the model data rms error and correlation at the mooring sites located at an alongshore distance of 90 km, south or north from the assimilation sites. The coastal jet is deflected offshore over Heceta Bank, and assimilation of data from an inner‐shelf mooring in the jet separation zone does not help to improve prediction in the far field. Larger improvements are obtained for the first part of the study period (yeardays 146–190). In the second part (days 191–237) the geometry of our limited area model possibly limits prediction accuracy. In numerical experiments involving assimilation of data from only one mooring the actual and expected rms error improvements are compared, providing a consistency test for the forecast error covariance.

Cetacean distributions relative to ocean processes in the northern California Current System

Abstract Associations between cetacean distributions, oceanographic features, and bioacoustic backscatter were examined during two process cruises in the northern California Current System (CCS) during late spring and summer 2000. Line-transect surveys of cetaceans were conducted across the shelf and slope, out to 150 km offshore from Newport, Oregon (44.6°N) to Crescent City, California (41.9°N), in conjunction with multidisciplinary mesoscale and fine-scale surveys of ocean and ecosystem structure. Occurrence patterns (presence/absence) of cetaceans were compared with hydrographic and ecological variables (e.g., sea surface salinity, sea surface temperature, thermocline depth, halocline depth, chlorophyll maximum, distance to the center of the equatorward jet, distance to the shoreward edge of the upwelling front, and acoustic backscatter at 38, 120, 200 and 420 kHz) derived from a towed, undulating array and a bioacoustic system. Using a multiple logistic regression model, 60.2% and 94.4% of the variation in occurrence patterns of humpback whales Megaptera novaeangliae during late spring and summer, respectively, were explained. Sea surface temperature, depth, and distance to the alongshore upwelling front were the most important environmental variables during June, when humpbacks occurred over the slope (200–2000 m). During August, when humpbacks concentrated over a submarine bank (Heceta Bank) and off Cape Blanco, sea surface salinity was the most important variable, followed by latitude and depth. Humpbacks did not occur in the lowest salinity water of the Columbia River plume. For harbor porpoise Phocoena phocoena, the model explained 79.2% and 70.1% of the variation in their occurrence patterns during June and August, respectively. During spring, latitude, sea surface salinity, and thermocline gradient were the most important predictors. During summer, latitude and distance to the inshore edge of the upwelling front were the most important variables. Typically a coastal species, harbor porpoises extended their distribution farther offshore at Heceta Bank and at Cape Blanco, where they were associated with the higher chlorophyll concentrations in these regions. Pacific white-sided dolphin Lagenorhynchus obliquidens was the most numerous small cetacean in early June, but was rare during August. The model explained 44.5% of the variation in their occurrence pattern, which was best described by distance to the upwelling front and acoustic backscatter at 38 kHz. The model of the occurrence pattern of Dall's porpoise Phocoenoides dalli was more successful when mesoscale variability in the CCS was higher during summer. Thus, the responses of cetaceans to biophysical features and upwelling processes in the northern CCS were both seasonally and spatially specific. Heceta Bank and associated flow-topography interactions were very important to a cascade of trophic dynamics that ultimately influenced the distribution of foraging cetaceans. The higher productivity associated with upwelling near Cape Blanco also had a strong influence on the distribution of cetaceans.

Mesoscale structure and its seasonal evolution in the northern California Current System

Abstract During spring and summer 2000, two mesoscale mapping cruises were carried out in the northern California Current System between 41 . 9 ∘ N and 44 . 6 ∘ N and between the shallow, inner continental shelf and up to 150 km offshore. Measurements were made using a towed undulating vehicle equipped with a conductivity-temperature-depth instrument and a chlorophyll fluorometer. A shipboard acoustic Doppler current profiler (ADCP) measured water velocities, and surface drifter trajectories and satellite sea-surface temperature imagery provide context for the mesoscale maps. Nearly identical upwelling favorable wind stresses of up to 0.2 N m - 2 existed during both the spring and summer surveys. Early in the season (late May, early June) the upwelling front and jet followed the continental shelf bottom topography. There was cold water inshore of the shelfbreak all along the coast with pockets of elevated phytoplankton biomass (chlorophyll (chl) up to 4 mg m - 3 ) near the coast. Mesoscale activity was minimal. During late-summer (August), the upwelling front and jet were much more convoluted, including significant meanders offshore associated with a major submarine bank (Heceta Bank, 44.0– 44 . 6 ∘ N ) and a large coastal promontory (Cape Blanco, 42 . 8 ∘ N ). High levels of phytoplankton biomass were found over Heceta Bank (chl ∼ 20 mg m - 3 ) and near the coast south of Cape Blanco (chl ∼ 10 mg m - 3 ). Low velocities inshore of Heceta Bank, measured by both shipboard ADCP and surface drifters, indicate the potential for retention of water over the Bank, leading to favorable conditions for phytoplankton biomass accumulation. The large offshore meander near Cape Blanco carried cold, nutrient-rich, high phytoplankton biomass (chl of 2– 5 mg m - 3 ) water over 100 km offshore. This flow–topography interaction feature was generated in mid-June and remained an important part of the regional circulation for about 2.5 months.

The spatial distribution of euphausiid aggregations in the Northern California Current during August 2000

Abstract The location and size of euphausiid aggregations (patches) were identified using acoustic volume backscattering strength at 38 and 120 kHz and vertically stratified MOCNESS sampling during a US GLOBEC cruise off southern Oregon and northern California in August 2000. Euphausiid patches were clustered inshore of Heceta Bank (44.0°N) and off Cape Blanco (42.8°N), but were not common elsewhere. The distribution of euphausiid patches throughout the study area was correlated with that of near-surface chlorophyll. We suggest that Heceta Bank and Cape Blanco were zones of plankton retention and concentration along the shelf and slope, and also that large offshore meanders in the equatorward upwelling jet carried upwelled water, chlorophyll, and euphausiids offshore. In particular, the complex interaction of mesoscale physical features, shoaling bottom topography, and diel vertical migration created large patches of euphausiids. These euphausiid patches, probably persisting on timescales of days to weeks, could function as relatively large and persistent resources for euphausiid predators.

Physical and biological variables affecting seabird distributions during the upwelling season of the northern California Current

Abstract As a part of the GLOBEC-Northeast Pacific project, we investigated variation in the abundance of marine birds in the context of biological and physical habitat conditions in the northern portion of the California Current System (CCS) during cruises during the upwelling season 2000. Continuous surveys of seabirds were conducted simultaneously in June (onset of upwelling) and August (mature phase of upwelling) with ocean properties quantified using a towed, undulating vehicle and a multi-frequency bioacoustic instrument (38–420 kHz). Twelve species of seabirds contributed 99% of the total community density and biomass. Species composition and densities were similar to those recorded elsewhere in the CCS during earlier studies of the upwelling season. At a scale of 2–4 km, physical and biological oceanographic variables explained an average of 25% of the variation in the distributions and abundance of the 12 species. The most important explanatory variables (among 14 initially included in each multiple regression model) were distance to upwelling-derived frontal features (center and edge of coastal jet, and an abrupt, inshore temperature gradient), sea-surface salinity, acoustic backscatter representing various sizes of prey (smaller seabird species were associated with smaller prey and the reverse for larger seabird species), and chlorophyll concentration. We discuss the importance of these variables in the context of what factors seabirds may use to find food. The high seabird density in the Heceta Bank and Cape Blanco areas indicates them to be refuges contrasting the low seabird densities currently found in most other parts of the CCS, following decline during the recent warm regime of the Pacific Decadal Oscillation.

Quantitative benthic habitat characterization at Heceta Bank, Oregon

Recent studies have highlighted the shortcomings of regional trawl surveys for quantifying abundance of demersal fishes in rugged habitats. Many species show strong affinities to areas of high topographic relief and rugosity, therefore precluding sampling by bottom trawl gears. Consequently, advanced survey technologies such as submersibles and camera sleds have been utilized to survey demersal fishes in situ. One prerequisite in extrapolating densities of demersal fishes observed in situ is the mapping of benthic habitats beyond the observational extent of submersibles. Habitat characteristics observed from submersible and ROV dives in the late 1980s, 2000, and 2001 were used to establish habitat classes that have been statistically shown to correlate with demersal fish distributions. Those habitat characteristics were then extrapolated over the extent of a multibeam sonar survey conducted in 1998 using quantitative parameters derived from high-resolution bathymetric and acoustic backscatter data. The resultant map predicts the coverage of four lithologic habitat classes: mud (unconsolidated), sand (unconsolidated), boulder/cobble (high acoustic reflectivity), and rock outcrop (high vertical relief). Those four habitat classes will facilitate the extrapolation of fish densities to the larger spatial scales at which resident fish populations and physiographic provinces occur.

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.

The GLOBEC Northeast Pacific California Current System Program

Summary and Conclusions The field observations of the NEP CCS are now about half completed. We have been fortunate to sam- ple during a period when climate signals have been strong, providing an opportunity for the program to witness contrasting responses of the coastal ocean ecosystem. We have learned that some of the shelf zoo- plankton that are swept offshore in the squirts and jets can survive, while other shelf species either avoid being swept offshore or die quickly once offshore, since they are rarely encountered in the offshore jet waters. Juvenile coho and chinook salmon were never found in the offshore, deep (water >200 m) region during exten- sive sampling conducted in 2000. The swimming abili- ty of the juvenile salmon may be sufficient to prevent them from being transported offshore as the jet encounters the influence of Heceta Bank and Cape Blanco. We have been fortunate

Multistatic active system performance modeling in shallow waters

The Bistatic KRAKEN Reverberation (BiKR) model is an active system performance prediction model for bistatic source/receiver geometries based on adiabatic normal mode propagation [W. A. Kuperman, M. B. Porter, J. S. Perkins, and R. B. Evans, J. Acoust. Soc. Am. 89, 125–133 (1991)]. By exploiting ray-mode analogies, it can provide detailed predictions of reverberation, target echo, and signal excess for low-frequency (<1 kHz) multistatic systems in range-dependent environments. This paper presents an overview of the models underlying principles and analysis capabilities such as the estimation of signal spreading loss. Data/model comparisons that required the inclusion of losses due to shear will be presented for a recent sea test on the Heceta Bank off the Oregon coast. [Research sponsored by ONR.]

Crustal structure beneath the central Oregon convergent margin from potential‐field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop

Models of magnetic and gravity anomalies along two E‐W transects offshore central Oregon, one of which is coincident with a detailed velocity model, provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop. The models indicate that the backstop‐forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America ∼50 million years ago, has a seaward dip of less than 60°3. Seismic, magnetic, and gravity data are compatible with no more than 2 km of subducted sediments between the Siletz terrane and the underlying crystalline crust of the Juan de Fuca plate. The data also suggest the presence of a N‐S trending, 200‐km‐long basaltic ridge buried beneath the accretionary complex from about 43°N to 45°N. Although the height and width of this ridge probably vary along strike, it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the Siletz terrane beneath Heceta Bank. Several models for the origin of this ridge are discussed. These include: a sliver of Siletz terrane detached from the main Siletz terrane during a late Eocene episode of strike‐slip faulting; imbrication and thickening of subducted oceanic crust in place; an aseismic ridge rafted in on the subducting oceanic crust during the past 1.2 million years; and a series of ridges and/or seamounts rafted in over a longer period of time and transferred from the subducting plate to the overlying plate. The last model is the most consistent with the complicated history of local uplift, subsidence, and slope instability recorded in the ridges, basins, and banks of this part of the margin. We speculate that the massive seaward dipping western edge of the Siletz terrane in this region inhibits subduction of seamounts and sediments, resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction. This process may also be responsible for thickening of lower accretionary complex material, oversteepening of slopes leading to massive slumping, and north‐south extension through strike‐slip faulting in the accretionary complex to the west of the buried ridge. Regardless of its origin, the ridge may currently be acting as an asperity inhibiting subduction.

Low frequency edge waves over a trench-ridge topography adjoining a straight coastline

Abstract The possible interaction of trapped midoceanic boundary waves with a nearby coastline is examined by considering a step trench-ridge topography adjoining a semi-infinite straight coastline. The full dispersion equation, including the effect of the earth's rotation, is derived for long waves over this topography. It is shown that the presence of the coastline begins to have a significant effect on the behaviour of quasigeostrophic ridge waves whenever the wave length is greater than three times the ridge coastline separation. As an example, the dispersion curves are presented for the topography of the Heceta Bank off the coast of Oregon and it is conjectured that the presence of this off-shore ridge may provide an explanation for the anomalous direction of propagation of the 0.1 c.p.d. shelf wave reported by Mooers and Smith (1968).

Pacific northwest siliceous phytoplankton

Submarine exposures of silica-rich claystones from widespread areas on Heceta Bank and offshore from Cape Arago on the Orego coast have yielded a diverse flora of diatoms and silicoflagellates. A similar and apparently synchronous flora was recovered from a small exposure of diatomite on the coast south of Bandon, Oregon. The floral diversities from these claystone are comparable to those from many Neogene diatomites in California, and a temperate shallow marine environment is indicated by the presence of many middle latitude, littoral and neritic benthonic forms. Several species of diatoms and silicoflagellates present in the floras are unreported elsewhere from sediments either older or younger than Late Miocene. The Oregon floras are correlative with several lithologically comparable diatomaceous sediments in the northeast Pacific area. The closest of these adjacent exposures is the Wildcat group in Humbolt County, California, of which the floras of the Delmontian Pullen formation display the greatest similarities to the Oregon material.