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Dispersal, retention, and population connectivity are impacted by current regime and the behaviors that drive larval distribution, so understanding both is key to informing restoration of native species like the Olympia oyster (Ostrea lurida) across its range in western North America. This study explores the relationships between several factors (temperature, [chl a], larval size, tidal stage, and estimated current speed) and Olympia oyster larval vertical distributions in Fidalgo Bay (48.4828, − 122.5811), a shallow, tidally flushed bay in the Salish Sea. Olympia oyster larvae collected from four depths over the tidal cycle from July 11–14, 2017, were ~ 20% deeper near slack tide and shallower during the faster parts of both ebb and flood, with a threshold for this transition around an estimated 25 cm s−1. This pattern does not suggest tidally timed migrations as has been shown in another population of Olympia oysters, nor can this pattern be totally explained by passive processes. Larvae did not cluster at depths with specific temperatures or [chl a] but there was a difference in larval size between surface and bottom waters, with older, larger larvae more common at the bottom. Fidalgo Bay does not exhibit two-way flow or strong vertical shear, so vertical distribution of larvae likely has little effect on transport in this system but might in other similarly shallow habitat areas with higher stratification that are target restoration sites in the Salish Sea. These results add to the growing number of studies that show location-specific differences in larval vertical distribution and behavior within taxa and underscore the importance of integrating local hydrodynamics into predictions of bivalve larval transport.

As the base of almost all marine food webs, phytoplankton play a dominant role in determining the productivity of marine ecosystems. Recent studies have highlighted the dynamic variability of phytoplankton abundance in nearshore ecosystems over synoptic time scales. Therefore, a greater understanding of the physical mechanisms that contribute to this variability is required to assess impacts of current as well as future weather patterns on these ecosystems. In this study, chlorophyll fluorescence data from a nearshore location in southern Monterey Bay was used to identify the timing and duration of increases in phytoplankton concentrations. Regional physical parameters, including wind stress, wave height and water temperature were analyzed to determine what, if any, physical processes are associated with observed blooms. A significant negative correlation between water temperature and chlorophyll (high chlorophyll associated with cold water) was found for the two summer seasons studied (2012, 2013). The timing of high chlorophyll events coincided with relaxations of upwelling favorable wind conditions. Analysis of surface current data suggests that the incoming cold water masses are the result of poleward currents from the south of Monterey Bay. A conceptual model is proposed in which phytoplankton are advected into southern Monterey Bay during wind relaxation events preceded by upwelling events of great enough duration to establish well defined circulation patterns. The reversal of circulation patterns transports cold, phytoplankton-rich water into the bay from south of the Monterey Peninsula. This study demonstrates that wind relaxation events can be an important driver of localized high phytoplankton biomass events on the inner shelf by advecting recently upwelled water into coastal embayments that are relatively sheltered from strong upwelling favorable winds.

Closely spaced CTD stations showed elevated oxygen within Monterey Submarine Canyon. Anomalously high (2 to 5 umol kg -1) dissolved oxygen was found between 600-1100m in the O2

Work done by the present authors and collaborators in the first year of a 2-year field study established the frequent presence of layers of suspended particulate matter detached from the seafloor over the southern shelf of Monterey Bay, California. In this contribution, we document similar findings over a 1-month period in Fall 2012 and investigate physical processes leading to vertical convergence of particles to the observed layers, both from resuspended sediment originating below the layer depth and from phytoplankton originating above the layer depth. Physical and optical vertical structure was measured by an autonomous vertical profiler, thermistor chain, and acoustic Doppler current profiler, and optical sensors fixed to a bottom-mounted frame measured beam attenuation and particle size distribution within the benthic boundary layer (BBL). These data support a conceptual model for layer formation in which (1) bottom material was mobilized into the BBL by semidiurnal internal tidal currents; (2) brief “updraft” events regularly injected particles into the interior water column during the downslope phase of the semidiurnal internal tide; and (3) particles converged at the detached layer due to a measured, but previously unreported, long time scale mean vertical convergence in flow. Subduction and vertical mixing of phytoplankton to the subeuphotic layer were also observed and are presented in two case studies in connection with horizontal convergences of surface water masses.

The article of record as published may be found at http://dx.doi.org/10.1002/2014GL061897 The USGS data sets presented herein can be obtained by sending a written request to the corresponding author. Several sequential upwelling events were observed in fall 2012, using measurements from the outer half of the continental shelf in Monterey Bay, during which the infiltration of dense water onto the shelf created a secondary, near-bottom pycnocline. This deep pycnocline existed in concert with the near-surface pycnocline and enabled the propagation of near-bottom, cold, semidiurnal internal tidal bores, as well as energetic, high-frequency, nonlinear internal waves of elevation (IWOE). The IWOE occurred within 20m of the bottom, had amplitudes of 8–24 m, periods of 6–45 min, and depth-integrated energy fluxes up to 200Wm 1. Iribarren numbers (

[1] Physical and optical measurements taken over the mud belt on the southern continental shelf of Monterey Bay, California documented the frequent occurrence of suspended particulate matter features, the majority of which were detached from the seafloor, centered 9–33 m above the bed. In fall 2011, an automated profiling mooring and fixed instrumentation, including a thermistor chain and upward-looking acoustic Doppler current profiler, were deployed at 70 m depth for 5 weeks, and from 12 to 16 October a long-range autonomous underwater vehicle performed across-shelf transects. Individual SPM events were uncorrelated with local bed shear stress caused by surface waves and bottom currents. Nearly half of all observed SPM layers occurred during 1 week of the study, 9–16 October 2011, and were advected past the fixed profiling mooring by the onshore phase of semidiurnal internal tide bottom currents. At the start of the 9–16 October period, we observed intense near-bed vertical velocities capable of lifting particulates into the middle of the water column. This “updraft” event appears to have been associated with nonlinear adjustment of high-amplitude internal tides over the mid and outer shelf. These findings suggest that nonlinear internal tidal motions can erode material over the outer shelf and that, once suspended, this SPM can then be transported shoreward to the middle and shallow sections of the mud belt. This represents a fundamental broadening of our understanding of how shelf mud belts may be built up and sustained.

Wind-driven upwelling is weak and intermittent in the Southern California Bight, a region sheltered from the strong, prevailing equatorward winds typical of most of the California Current System. Thus, other physical transport processes than wind-driven upwelling supply subsidies of nutrients, biogenic particles, and other water-borne materials to support the highly productive temperate reef ecosystems of the Southern California Bight, including forests of giant kelp (Macrocystis pyrifera) and a broad diversity of reef organisms. This article focuses on results from the Santa Barbara Coastal Long Term Ecological Research project related to the various transport processes important for maintaining nearshore ecosystems in the California Current System. It reviews along-shelf and cross-shelf flow mechanisms that deliver these subsidies, discusses how these mechanisms interact, and examines some of the biological patterns that result from these transport processes.

To quantify the effect of basin morphometry on internal wave dynamics, we installed thermistor arrays around the perimeter of Mono Lake, California. The lake's complex bathymetry includes alongshore bottom slopes ranging from < 1% up to 9% and a steep-sided peninsula-like island. Under the influence of the dominating winds from the south, isopycnals downwelled along the long axis of the lake as well as along an axis parallel to the peninsula, generating vertical mode 2 horizontal mode 2 internal waves. On relaxation of the wind, a horizontal mode 2 Kelvin wave evolved with a period of ∼ 22 h in each of the two sub-basins. The response of the internal wave field to high winds was spatially variable, with higher vertical excursions of isopycnals and higher rates of strain, indicative of non-linearity and turbulence, at both sides of the peninsula and near steep sloping boundaries to the south. Rate of strain squared was enhanced up to three orders of magnitude relative to background rates depending on bottom slope. Model comparisons in which the island was moved to the center of the lake or removed showed a dominant horizontal mode 1 wave and stronger influence of rotation. Increased complexity of basin morphometry modified the horizontal and vertical phase structure of dominant basin-scale waves, redistributed the potential energy in the internal wave field over space, and shifted the positions of the basins in which the waves rotated. Whether turbulence increased with these shifts depended on the incidence of steep topography.

A decade-long time series recorded in Central California demonstrates that a shallow, near-shore environment (17 m depth) is regularly inundated with pulses of cold, hypoxic and low-pH water. During these episodes, oxygen can drop to physiologically stressful levels, and pH can reach values that potentially result in dissolution of calcium carbonate. Pulses of the greatest intensity arose at the onset of the spring upwelling season, and fluctuations were strongly semidiurnal and diurnal. Arrival of cold, hypoxic water on the inner shelf appears to be driven by tidal-frequency internal waves pushing deep, upwelled water into nearshore habitats. We found no relationship between the timing of low-oxygen events and the diel solar cycle. These observations are consistent with the interpretation that hypoxic water is advected shoreward from the deep, offshore environment where water masses experience a general decline of temperature, oxygen and pH with depth. Analysis of the durations of exposure to low oxygen concentrations establishes a framework for assessing the ecological relevance of these events, but physiological tolerance limits to such hypoxic events are not well documented for most near-shore organisms expected to be impacted.

Microstructure measurements along the axes of Monterey and Soquel Submarine Canyons reveal 200–300-m-thick well-stratified turbulent near-bottom layers with average turbulent kinetic energy dissipation rates 〈ɛ〉 = 4 × 10−8 W kg−1 and eddy diffusivities K = 16 × 10−4 m2 s−1 (assuming mixing efficiency γ = 0.2) to at least thalweg depths of 1200 m. Turbulent dissipation rates are an order of magnitude lower in overlying waters, whereas buoyancy frequencies are only 25% higher. Well-mixed bottom boundary layer thicknesses hN are an order of magnitude thinner than the stratified turbulent layer (hN ≪ hɛ). Because well-stratified turbulent layers are commonly observed above slopes, arguments that mixing efficiency should be reduced on sloping boundaries do not hold in cases of energetic internal-wave generation or interaction with topography. An advective–diffusive balance is used to infer velocities and transports, predicting horizontal upslope flows of 10–50 m day−1. Extrapolating this estimate globally suggests that canyon turbulence may contribute 2–3 times as much diapycnal transport to the World Ocean as interior mixing. The upcanyon turbulence-driven transports are not uniform, and the resulting upslope convergences will drive exchange between the turbulent layer and more quiescent interior. Predicted density surfaces of detrainment and entrainment are consistent with observed isopycnals of intermediate nepheloid and clear layers. These data demonstrate that turbulent mixing dynamics on sloping topography are fundamentally 2D or 3D in the ocean, so they cannot be accurately described by 1D models.

The article of record as published may be located at http://dx.doi.org/10.4319/lom.2011.9.571

Species density of macrofaunal invertebrates living in marine soft sediments was highest at the shelf-slope break (100‐150 m) in Monterey Bay (449 m )2 ). There were 337 species m )2 in the mid-shelf mud zone (80 m). There were fewer species along the slope: 205 m )2 from the lower slope (950-2000 m) and 335 m )2 on the upper slope (250-750 m). Species density was highest inside the bay (328-446 m )2 ) compared to outside (336-339 m )2 ), when examining samples at selected water depths (60-1000 m). There was little difference in local species density from 1 km of shoreline compared to regional species density along 1000 km of shoreline at both shelf and slope depths. The highest species densities worldwide in the literature are recorded along the Carolina slope in the Atlantic Ocean, where peak species density (436/0.81 m 2 ) at 800 m and values at the largest sample areas are similar to those on the Monterey Bay shelf. We speculate that the highest species densities occur where ocean water exchanges energy with shoaling topography at the continental margin, bringing more food to the benthos ‐ areas such as the very productive waters in the upwelling system of Monterey Bay.

Diurnal-period internal waves were observed near Point Conception California, using an array of moorings extending 120 km along the inner shelf. The waves have an along-shelf coherence scale of at least 50 km, and appear to propagate nearly straight onshore. Wave amplitudes vary over time, depending on thermal stratification and the amplitude of the diurnal sea breeze oscillation. Barotropic tides and vorticity over the mid-shelf are not correlated with internal wave amplitude. Large amplitude internal waves, with supercritical Froude numbers, are observed in mid-summer. Although such waves may drive vertical mixing and cross-shelf transport of passive particles, there is no significant correlation between wave amplitude and invertebrate settlement in the Santa Barbara Channel.

Oceanic sources of nutrients to the kelp forests of the Santa Barbara Channel were diagnosed using time series from three moorings in 12- to 17-m water depth. An in situ nitrate autoanalyzer on the moorings provided the first high-resolution time series of nitrate + nitrite (dissolved inorganic nitrogen, DIN) concentrations for this environment. Measurements between February 2001 and May 2003 show that the major mechanisms that supply DIN to the inner shelf of the Santa Barbara Channel are upwelling, diurnal internal motions, and storm runoff. These supply mechanisms vary in importance seasonally. Upwelling dominates increases of inner-shelf DIN concentration between March and May and accounts for more than half of the annual advective DIN transport to shelf reefs. In summer, baroclinic motions akin to internal waves are an important source of DIN because they occur when surface nutrient concentrations are depleted and other supply mechanisms are inactive. Brief episodes of upwelling become important in late autumn and early winter. DIN inputs from storm runoff, detected as salinity dilution at the moorings and estimated from measurements of stream discharge and nutrient concentration, are significant during winter runoff events.

Near-boundary mixing affects the dispersal of seawater constituents and may have important consequences for ecological and geological processes since continental boundaries are a fundamental source of lithogenic sediments, nutrients, iron and carbon. This paper examines the idea that gravitational collapse after near-boundary vertical mixing events leads to enhanced dispersal in the horizontal (along-isopycnal) direction. Dye studies from the continental shelf and laboratory investigations of intrusions generated by internal-wave breaking suggest that this is a viable mechanism for offshore dispersal of boundary-layer fluid. However, there have been few attempts to examine this process in the ocean or to quantify it in a form amenable to parameterization. Here this process is considered primarily for continental slopes and conditions of relatively uniform stratification, rather than for shelves where the boundary can intersect a front or seasonal thermocline. This paper reviews a selection of studies examining this concept in the laboratory and ocean, and reviews studies linking internal-wave reflection and mixing to the offshore dispersal of suspended sediment from continental margins.

Conductivity–temperature–depth and transmissometer surveys were undertaken to investigate the characteristics and seasonal nature of intermediate nepheloid layers (INLs) over the outer shelf and upper slope of the northern California margin, near Eureka, CA. Observed INLs could generally be grouped into one of two categories: INLs that formed and spread seaward from the continental shelf, and INLs generated at continental slope depths greater than 150 m. Shelf INLs, forming between 70 and 150-m depth on the outer shelf and extending seaward 15–20 km past the shelf break, were prevalent during winter and early spring. Continental slope INLs were found during all seasons of the year. Their suspended particulate matter concentration was lower than that of the shelf INLs, and their horizontal extent was 3–7 km seaward from the slope. Mooring data from 450-m depth show strong bottom-trapped semidiurnal internal tidal energy and asymmetry between upslope and downslope semidiurnal motions, indicative of bore-like behavior. These analyses suggest that critical reflection of internal tides is a common feature of the outer continental slope in this region. The location of slope-depth INLs was often observed to coincide with regions where the topographic slope angle was critical for internal tide reflection. In addition, seasonal variability in the span of critical topography over the slope matches seasonal variability in the density of observed INLs over the continental slope. Slope-depth INLs appear to be associated with energetic tidal motions and interaction between internal tides and bathymetry, whereas shelf-depth INLs are likely formed by resuspension events on the outer shelf and subsequent transport offshore by mesoscale circulation processes.

Benthic currents, temperature, salinity, and suspended sediment concentration were measured in 1993–1994 on the Bear Island Fan, at water depths of approximately 1700 m, near the wreckage of the Russian nuclear submarine “Komsomolets”. A maximum current speed of 51 cm/s and a mean speed of approximately 10 cm/s was recorded. There was a marked seasonal pattern in flow strength, and the net transport was northward along the slope contours. The strongest flow event of the year, however, was directed to the southwest. The smooth (apart from burrows created by benthic fauna), unscoured appearance of the sediment surface in bottom photographs suggest that the deep currents on the continental slope are weak. Our measurements, however, show that the currents frequently exceeded the erosion threshold speed. Peak concentration values during the energetic winter period were within the range expected for an erosion event. The data indicate a high potential for sediment erosion and transport on the Bear Island Fan, with implications for the possible transport of contaminants from the submarine.

[1] Continuous records of temperature and chlorophyll-a [chl-a] fluorescence were used to characterize phytoplankton variability at two shallow intertidal stations on the northern California coast. Chl-a records from spring and summer 2007 and 2008 were characterized by distinct peaks persisting for 1.5 to five days. These peaks represent bloom events, which often coincided between the two sites even though they are separated by ∼150 km. Blooms did not appear to be directly forced by individual upwelling episodes. While some events were associated with reversals of upwelling-favorable winds, there was a stronger relationship between chl-a peaks and peaks in surface swell height. This relationship was dominant in spring and early summer, but no longer evident by July. We suggest that these nearshore chl-a peaks are an accumulation of phytoplankton caused by convergence of onshore wave transport against the impermeable coastal boundary. Scaling arguments show this mechanism to be consistent with observed chl-a increase rates. This mechanism has not been previously considered as a forcing for blooms at rocky coasts, and it may have significant implications for understanding coastal productivity, larval dispersal, and nearshore water quality.

[1] A complete year of hourly 3 km resolution high-frequency radar measured surface currents covering the Gulf of the Farallones were analyzed with the following three primary objectives: (1) describe the seasonal surface circulation, (2) identify tidal currents, and (3) determine the influence of wind forcing. Three predominant seasonal circulation regimes were identified: relaxation, storm, and upwelling. The relaxation period exhibited mean poleward flow over the slope, variable equatorward flow over the shelf, and cyclonic and anticyclonic eddies south of Pt. Reyes. The storm period mean flow was variable and exhibited evidence of coastally trapped buoyancy flow from the mouth of the San Francisco Bay. The upwelling period exhibited equatorward flow throughout the gulf with regions of intensified flow at the northern and southern regions over the slope. The tidal variance ranged from 4 to 60%, was highest around the mouth of the Bay, decreased past the shelf, and its spatial pattern reflected the combined influence of the K1 and M2 tidal current amplitudes. K1 ellipses typically rotated clockwise throughout the gulf and decreased in amplitude past the shelf. M2 ellipses were comparably more variable in orientation and magnitude. Harmonic “tidal” analyses of buoy wind data over 4 month time periods showed significant K1 amplitudes which did not appear when the analysis was done for the entire year. This indicates that harmonic tidal analyses on surface currents are probably more effective at disassociating diurnal sea breeze driven currents when performed over long periods of time such as a year.

[1] Sub-mesoscale eddies are described along the mainland coast of the Santa Barbara Channel based on observations from a network of high frequency (HF), current-measuring radars and near-shore moorings. The eddies are 4–15 km in diameter and typically last about 2 days, although some last up to 6 days. Most eddies within the radar coverage area are anti-cyclonic with relative vorticities of −0.4 f to −0.8 f where f is the Coriolis parameter, but cyclonic eddies are also observed. Moored observations over the inner shelf (12 m water depth) of a sequence of two eddies in December 2001 show an increase in nitrate plus nitrite from the background levels of 1–2 μM to a maximum of 10–12 μM when the eddies are present. We speculate that these eddies are an important transport mechanism for nutrients and biogenic particles to inner shelf ecosystems of the Southern California Bight.

[1] Laboratory experiments were used to investigate the growth of intrusions due to internal-wave reflection from a sloping boundary. When normalized by the incident energy density flux, the average intrusion spreading velocity was found to be a linear function of the frequency ratio ω/ωc, where ω is the frequency of the incident wave and ωc is the critical frequency, at which the wave characteristic has the same angle as the bottom slope. Evenly spaced layers, indicating thin perturbations in the background density gradient, developed within the mixing region and spread into the tank interior. The vertical spacing of these layers also bore a linear relationship to ω/ωc. A linear model of internal-wave reflection suggests that these layers may be related to an isopycnal displacement, or overturn, scale. Intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions. A balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived. Fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux. This balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal-wave mixing at oceanic margins.

This was a two-part study to investigate mechanisms for generating intermediate nepheloid layers (INLs) on continental slopes. The first part was a laboratory experiment that generated internal waves on a sloping bed in a stratified fluid which produced INLs. The second part applied the physical understanding gained from the laboratory results to analyze field data collected from the Northern California continental slope as part of the STRATAFORM project. Field data showed that INLs were often associated with regions of critical slope topography where internal wave reflection is common, suggesting that internal waves may be responsible for the generation of slope INLs.