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2016
Callaghan, AH, Deane GB, Stokes MD.  2016.  Laboratory air-entraining breaking waves: Imaging visible foam signatures to estimate energy dissipation. Geophysical Research Letters. 43:11320-11328.   10.1002/2016gl071226   AbstractWebsite

Oceanic air-entraining breaking waves fundamentally influence weather and climate through bubble-mediated ocean-atmosphere exchanges, and influence marine engineering design by impacting statistics of wave heights, crest heights, and wave loading. However, estimating individual breaking wave energy dissipation in the field remains a fundamental problem. Using laboratory experiments, we introduce a new method to estimate energy dissipation by individual breaking waves using above-water images of evolving foam. The data show the volume of the breaking wave two-phase flow integrated in time during active breaking scales linearly with wave energy dissipated. To determine the volume time-integral, above-water images of surface foam provide the breaking wave timescale and horizontal extent of the submerged bubble plume, and the foam decay time provides an estimate of the bubble plume penetration depth. We anticipate that this novel remote sensing method will improve predictions of air-sea exchanges, validate models of wave energy dissipation, and inform ocean engineering design.

Stokes, MD, Deane G, Collins DB, Cappa C, Bertram T, Dommer A, Schill S, Forestieri S, Survilo M.  2016.  A miniature Marine Aerosol Reference Tank (miniMART) as a compact breaking wave analogue. Atmospheric Measurement Techniques. 9:4257-4267.   10.5194/amt-9-4257-2016   AbstractWebsite

In order to understand the processes governing the production of marine aerosols, repeatable, controlled methods for their generation are required. A new system, the miniature Marine Aerosol Reference Tank (miniMART), has been designed after the success of the original MART system, to approximate a small oceanic spilling breaker by producing an evolving bubble plume and surface foam patch. The smaller tank utilizes an intermittently plunging jet of water produced by a rotating water wheel, into an approximately 6 L reservoir to simulate bubble plume and foam formation and generate aerosols. This system produces bubble plumes characteristic of small whitecaps without the large external pump inherent in the original MART design. Without the pump it is possible to easily culture delicate planktonic and microbial communities in the bulk water during experiments while continuously producing aerosols for study. However, due to the reduced volume and smaller plunging jet, the absolute numbers of particles generated are approximately an order of magnitude less than in the original MART design.

DeMott, PJ, Hill TCJ, McCluskey CS, Prather KA, Collins DB, Sullivan RC, Ruppel MJ, Mason RH, Irish VE, Lee T, Hwang CY, Rhee TS, Snider JR, McMeeking GR, Dhaniyala S, Lewis ER, Wentzell JJB, Abbatt J, Lee C, Sultana CM, Ault AP, Axson JL, Martinez MD, Venero I, Santos-Figueroa G, Stokes MD, Deane GB, Mayol-Bracero OL, Grassian VH, Bertram TH, Bertram AK, Moffett BF, Franc GD.  2016.  Sea spray aerosol as a unique source of ice nucleating particles. Proceedings of the National Academy of Sciences of the United States of America. 113:5797-5803.   10.1073/pnas.1514034112   AbstractWebsite

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 degrees C, averaging an order of magnitude increase per 5 degrees C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using "dry" geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.

Deane, GB, Stokes MD, Callaghan AH.  2016.  The saturation of fluid turbulence in breaking laboratory waves and implications for whitecaps. Journal of Physical Oceanography. 46:975-992.   10.1175/jpo-d-14-0187.1   AbstractWebsite

Measurements of energy dissipated in breaking laboratory waves, averaged over time and space and directly visualized with a bioluminescent technique, are presented. These data show that the energy dissipated in the crest of the breaking waves is constrained: average turbulence intensity within the crest saturates at around 0.5-1.2 W kg(-1), whereas breaking crest volume scales with wave energy lost. These results are consistent with laboratory and field observations of the Hinze scale, which is the radius of the largest bubble entrained within a breaking crest that is stabilized against turbulent fragmentation. The Hinze scale depends on turbulence intensity but lies in the restricted range 0.7-1.7 mm over more than two orders of magnitude variation in underlying unbroken wave energy. The results have important implications for understanding the energetics of breaking waves in the field, the injection of turbulence into the upper ocean, and air-sea exchange processes in wind-driven seas.

Deane, GB, Stokes DM, Latz MI.  2016.  Bubble stimulation efficiency of dinoflagellate bioluminescence. Luminescence. 31:270-280.   10.1002/bio.2957   Abstract

Dinoflagellate bioluminescence, a common source of bioluminescence in coastal waters, is stimulated by flow agitation. Although bubbles are anecdotally known to be stimulatory, the process has never been experimentally investigated. This study quantified the flash response of the bioluminescent dinoflagellate Lingulodinium polyedrum to stimulation by bubbles rising through still seawater. Cells were stimulated by isolated bubbles of 0.3–3 mm radii rising at their terminal velocity, and also by bubble clouds containing bubbles of 0.06–10 mm radii for different air flow rates. Stimulation efficiency, the proportion of cells producing a flash within the volume of water swept out by a rising bubble, decreased with decreasing bubble radius for radii less than approximately 1 mm. Bubbles smaller than a critical radius in the range 0.275–0.325 mm did not stimulate a flash response. The fraction of cells stimulated by bubble clouds was proportional to the volume of air in the bubble cloud, with lower stimulation levels observed for clouds with smaller bubbles. An empirical model for bubble cloud stimulation based on the isolated bubble observations successfully reproduced the observed stimulation by bubble clouds for low air flow rates. High air flow rates stimulated more light emission than expected, presumably because of additional fluid shear stress associated with collective buoyancy effects generated by the high air fraction bubble cloud. These results are relevant to bioluminescence stimulation by bubbles in two-phase flows, such as in ship wakes, breaking waves, and sparged bioreactors. Copyright © 2015 John Wiley & Sons, Ltd.

Brady, JM, Stokes MD, Bonnardel J, Bertram TH.  2016.  Characterization of a quadrotor unmanned aircraft system for aerosol-particle-concentration measurements. Environmental Science & Technology. 50:1376-1383.   10.1021/acs.est.5b05320   AbstractWebsite

High-spatial-resolution, near-surface vertical profiling of atmospheric chemical composition is currently limited by the availability of experimental platforms that can sample in constrained environments. As a result, measurements of near surface gradients in trace gas and aerosol particle concentrations have been limited to studies conducted from fixed location towers or tethered balloons. Here, we explore the utility of a quadrotor unmanned aircraft system (UAS) as a sampling platform to measure vertical and horizontal concentration gradients of trace gases and aerosol particles at high spatial resolution (1 m) within the mixed layer (0-100 m). A 3D Robotics Iris+ autonomous quadrotor UAS was outfitted with a sensor package consisting of a two-channel aerosol optical particle counter and a CO2 sensor. The UAS demonstrated high precision in both vertical (+/- 0.5 m) and horizontal positions (+/- 1 m), highlighting the potential utility of quadrotor UAS drones for aerosol- and trace-gas measurements within complex terrain, such as the urban environment, forest canopies, and above difficult-to-access areas such as breaking surf. Vertical profiles of aerosol particle number concentrations, acquired from flights conducted along the California coastline, were used to constrain sea-spray aerosol-emission rates from coastal wave breaking.

2015
Jean, CB, Kyser TK, James NP, Stokes MD.  2015.  The antarctic brachiopod Liothyrella uva as a proxy for ambient oceanographic conditions at McMurdo Sound. Journal of Sedimentary Research. 85:1492-1509.   10.2110/jsr.2015.94   AbstractWebsite

Brachiopods have been widely used as proxies to reconstruct ancient oceanographic conditions based on the assumption that their shell calcite is precipitated in near isotopic equilibrium with ambient seawater. Few studies, however, have tested the presumption of equilibrium precipitation for specimens from extreme polar environments. Furthermore, reported isotopic values for polar specimens are largely indicative of disequilibrium precipitation, leading to the conclusion that brachiopods living in extreme environments may be poor recorders of ambient oceanography. The results of shell chemistry of the Antarctic brachiopod Liothyrella uva (L. uva) are compared to the local oceanographic data at Cape Armitage, McMurdo Sound, to assess the suitability of extreme environment brachiopods as environmental proxies. Results reveal that significant kinetic fractionation occurs during primary-layer shell secretion, resulting in whole-shell isotopic compositions that do not reflect equilibrium with ambient seawater. Secondary-layer shell calcite, however, is less affected by biological fractionation and exhibits isotopic compositions that fall within the range of predicted equilibrium values. Additionally, whole-shell concentrations of elements including Ba, Cd, Cr, Fe, Mn, Na, Sr, and Zn exhibit trends that are interpreted to reflect their relative concentrations in ambient seawater. Concentrations of Mg and B, however, are found to be largely controlled by physiological processes related to brachiopod growth rate. While this study concludes that the shell chemistry of L. uva does reflect the local oceanographic conditions at McMurdo Sound, the reliability of extreme-environment brachiopods should be assessed on a species basis as differences in brachiopod physiology and microstructure can significantly influence the degree of equilibrium reflected in shell calcite.

Lee, C, Sultana CM, Collins DB, Santander MV, Axson JL, Malfatti F, Cornwell GC, Grandquist JR, Deane GB, Stokes MD, Azam F, Grassian VH, Prather KA.  2015.  Advancing model systems for fundamental laboratory studies of sea spray aerosol using the microbial loop. Journal of Physical Chemistry A. 119:8860-8870.   10.1021/acs.jpca.5b03488   AbstractWebsite

Sea spray aerosol (SSA) particles represent one of the most abundant surfaces available for heterogeneous reactions to occur upon and thus profoundly alter the composition of the troposphere. In an effort to better understand tropospheric heterogeneous reaction processes, fundamental laboratory studies must be able to accurately reproduce the chemical complexity of SSA. Here we describe a new approach that uses microbial processes to control the composition of seawater and SSA particle composition. By inducing a phytoplankton bloom, we are able to create dynamic ecosystem interactions between marine microorganisms, which serve to alter the organic mixtures present in seawater. Using this controlled approach, changes in seawater composition become reflected in the chemical composition of SSA particles 4 to 10 d after the peak in chlorophyll-a. This approach for producing and varying the chemical complexity of a dominant tropospheric aerosol provides the foundation for further investigations of the physical and chemical properties of realistic SSA particles under controlled conditions.

2014
Callaghan, AH, Stokes MD, Deane GB.  2014.  The effect of water temperature on air entrainment, bubble plumes, and surface foam in a laboratory breaking-wave analog. Journal of Geophysical Research-Oceans. 119:7463-7482.   10.1002/2014jc010351   AbstractWebsite

Air-entraining breaking waves form oceanic whitecaps and play a key role in climate regulation through air-sea bubble-mediated gas transfer, and sea spray aerosol production. The effect of varying sea surface temperature on air entrainment, subsurface bubble plume dynamics, and surface foam evolution intrinsic to oceanic whitecaps has not been well studied. By using a breaking wave analog in the laboratory over a range of water temperatures (T-w=5 degrees C to T-w=30 degrees C) and different source waters, we have examined changes in air entrainment, subsurface bubble plumes, and surface foam evolution over the course of a breaking event. For filtered seawater, air entrainment was estimated to increase by 6% between T-w=6 degrees C and T-w=30 degrees C, driven by increases of about 43% in the measured surface roughness of the plunging water sheet. After active air entrainment, the rate of loss of air through bubble degassing was more rapid at colder water temperatures within the first 0.5 s of plume evolution. Thereafter, the trend reversed and bubbles degassed more quickly in warmer water. The largest observed temperature-dependent differences in subsurface bubble distributions occurred at radii greater than about 700 m. Temperature-dependent trends observed in the subsurface bubble plume were mirrored in the temporal evolution of the surface whitecap foam area demonstrating the intrinsic link between surface whitecap foam and the subsurface bubble plume. Differences in foam and plume characteristics due to different water sources were greater than the temperature dependencies for the filtered seawater examined. Key Points Entrainment increases with increasing water temperature Integrated foam area increases with increasing water temperature Subsurface bubble population evolution exhibits a temperature dependence

Collins, DB, Zhao DF, Ruppel MJ, Laskina O, Grandquist JR, Modini RL, Stokes MD, Russell LM, Bertram TH, Grassian VH, Deane GB, Prather KA.  2014.  Direct aerosol chemical composition measurements to evaluate the physicochemical differences between controlled sea spray aerosol generation schemes. Atmospheric Measurement Techniques. 7:3667-3683.   10.5194/amt-7-3667-2014   AbstractWebsite

Controlled laboratory studies of the physical and chemical properties of sea spray aerosol (SSA) must be underpinned by a physically and chemically accurate representation of the bubble-mediated production of nascent SSA particles. Bubble bursting is sensitive to the physicochemical properties of seawater. For a sample of seawater, any important differences in the SSA production mechanism are projected into the composition of the aerosol particles produced. Using direct chemical measurements of SSA at the single-particle level, this study presents an intercomparison of three laboratory-based, bubble-mediated SSA production schemes: gas forced through submerged sintered glass filters ("frits"), a pulsed plunging-waterfall apparatus, and breaking waves in a wave channel filled with natural seawater. The size-resolved chemical composition of SSA particles produced by breaking waves is more similar to particles produced by the plunging waterfall than those produced by sintered glass filters. Aerosol generated by disintegrating foam produced by sintered glass filters contained a larger fraction of organic-enriched particles and a different size-resolved elemental composition, especially in the 0.8-2 mu m dry diameter range. Interestingly, chemical differences between the methods only emerged when the particles were chemically analyzed at the single-particle level as a function of size; averaging the elemental composition of all particles across all sizes masked the differences between the SSA samples. When dried, SSA generated by the sintered glass filters had the highest fraction of particles with spherical morphology compared to the more cubic structure expected for pure NaCl particles produced when the particle contains relatively little organic carbon. In addition to an intercomparison of three SSA production methods, the role of the episodic or "pulsed" nature of the waterfall method on SSA composition was undertaken. In organic-enriched seawater, the continuous operation of the plunging waterfall resulted in the accumulation of surface foam and an over-expression of organic matter in SSA particles compared to those produced by a pulsed plunging waterfall. Throughout this set of experiments, comparative differences in the SSA number size distribution were coincident with differences in aerosol particle composition, indicating that the production mechanism of SSA exerts important controls on both the physical and chemical properties of the resulting aerosol with respect to both the internal and external mixing state of particles. This study provides insight into the inextricable physicochemical differences between each of the bubble-mediated SSA generation mechanisms tested and the aerosol particles that they produce, and also serves as a guideline for future laboratory studies of SSA particles.

Leichter, JJ, Stokes MD, Vilchis LI, Fiechter J.  2014.  Regional synchrony of temperature variation and internal wave forcing along the Florida Keys reef tract. Journal of Geophysical Research-Oceans. 119:548-558.   10.1002/2013jc009371   AbstractWebsite

Analysis of 10 year temperature records collected along the Florida Keys reef tract (FLKRT) reveals strong, regional-scale synchrony in high-frequency temperature variation suggestive of internal wave forcing at predominately semidiurnal frequencies. In each year and at all sites, the amplitude of semidiurnal temperature variation was greatest from March to September, and markedly lower from October to February. Comparisons of the semidiurnal component of the temperature variation among sites suggest complex patterns in the arrival of internal waves, with highest cross correlation among closely spaced sites and synchrony in periods of enhanced internal wave activity across the length of the FLKRT, particularly in summer. The periods of enhanced semidiurnal temperature variation at the 20 and 30 m isobaths on the reef slopes appear to be associated with the dynamics of the Florida Current and the onshore movement of warm fronts preceding the passage of Florida Current frontal eddies. Regional-scale satellite altimetry observations suggest temporal linkages to sea surface height anomalies in the Loop Current (upstream of the Florida Current) and setup of the Tortugas Gyre. The synchronized forcing of cool water onto the reef slope sites across the FLKRT is likely to affect physiological responses to temperature variation in corals and other ectothermic organisms, as well as larval transport and nutrient dynamics with the potential for regionally coherent pulses of larvae and nutrients arriving on reef slopes across the FLKRT. Key Points Temperature variation from internal waves is modulated regionally in the FL Keys High-frequency temperature variation changes synchronously across sites Thermocline dynamics are associated with Florida Current frontal eddies

2013
Dunlop, RA, Noad MJ, Cato DH, Kniest E, Miller PJO, Smith JN, Stokes MD.  2013.  Multivariate analysis of behavioural response experiments in humpback whales (Megaptera novaeangliae). Journal of Experimental Biology. 216:759-770.   10.1242/jeb.071498   AbstractWebsite

The behavioural response study (BRS) is an experimental design used by field biologists to determine the function and/or behavioural effects of conspecific, heterospecific or anthropogenic stimuli. When carrying out these studies in marine mammals it is difficult to make basic observations and achieve sufficient samples sizes because of the high cost and logistical difficulties. Rarely are other factors such as social context or the physical environment considered in the analysis because of these difficulties. This paper presents results of a BRS carried out in humpback whales to test the response of groups to one recording of conspecific social sounds and an artificially generated tone stimulus. Experiments were carried out in September/October 2004 and 2008 during the humpback whale southward migration along the east coast of Australia. In total, 13 'tone' experiments, 15 'social sound' experiments (using one recording of social sounds) and three silent controls were carried out over two field seasons. The results (using a mixed model statistical analysis) suggested that humpback whales responded differently to the two stimuli, measured by changes in course travelled and dive behaviour. Although the response to 'tones' was consistent, in that groups moved offshore and surfaced more often (suggesting an aversion to the stimulus), the response to. social sounds' was highly variable and dependent upon the composition of the social group. The change in course and dive behaviour in response to 'tones' was found to be related to proximity to the source, the received signal level and signal-to-noise ratio (SNR). This study demonstrates that the behavioural responses of marine mammals to acoustic stimuli are complex. In order to tease out such multifaceted interactions, the number of replicates and factors measured must be sufficient for multivariate analysis.

Modini, RL, Russell LM, Deane GB, Stokes MD.  2013.  Effect of soluble surfactant on bubble persistence and bubble-produced aerosol particles. Journal of Geophysical Research-Atmospheres. 118:1388-1400.   10.1002/jgrd.50186   AbstractWebsite

The effect of soluble surfactant on the persistence of salt-water bubbles and their ability to produce aerosol particles upon bursting was investigated. Ensembles of individual, millimetric bubbles were produced in NaCl solutions of varying surfactant concentration. Aerosol production efficiency-a fundamental property of single bubbles defined as the number of particles produced per bubble film cap area-decreased by 79% to 98% following the addition of surfactant and increase in solution film pressure from 1-2 to 7-27 mN m(-1). The generated particle size distributions (0.01-10 mu m dry diameter) contained up to three modes and did not change much for film pressures up to 13.8 mN m(-1). The persistence of the bubbles at the water surface and the thickness of their film caps were investigated with high-speed videography. Addition of soluble surfactant increased average bubble persistence providing more time for the bubbles to drain and thin out with the aid of marginal regeneration flows. Bubble film cap thicknesses ranged from around 1 mu m for relatively clean, short-lived bubbles to less than 0.1 mu m for surfactant-stabilized, persistent bubbles. The suppression of aerosol production from the surfactant-stabilized bubbles may have resulted from the dramatic thinning of their caps or reduced surface forces at high film pressure. Previously reported Sea Spray Aerosol source functions were compared to measured aerosol production efficiencies and found to be significantly greater in magnitude, suggesting that there is a source of particles from whitecaps that was not captured in these single-bubble experiments.

Dunlop, RA, Cato DH, Noad MJ, Stokes DM.  2013.  Source levels of social sounds in migrating humpback whales (Megaptera novaeangliae). Journal of the Acoustical Society of America. 134:706-714.   10.1121/1.4807828   AbstractWebsite

The source level of an animal sound is important in communication, since it affects the distance over which the sound is audible. Several measurements of source levels of whale sounds have been reported, but the accuracy of many is limited because the distance to the source and the acoustic transmission loss were estimated rather than measured. This paper presents measurements of source levels of social sounds (surface-generated and vocal sounds) of humpback whales from a sample of 998 sounds recorded from 49 migrating humpback whale groups. Sources were localized using a wide baseline five hydrophone array and transmission loss was measured for the site. Social vocalization source levels were found to range from 123 to 183 dB re 1 mu Pa @ 1 m with a median of 158 dB re 1 mu Pa @ 1 m. Source levels of surface-generated social sounds ("breaches" and "slaps") were narrower in range (133 to 171 dB re 1 mu Pa @ 1 m) but slightly higher in level (median of 162 dB re 1 mu Pa @ 1 m) compared to vocalizations. The data suggest that group composition has an effect on group vocalization source levels in that singletons and mother-calf-singing escort groups tend to vocalize at higher levels compared to other group compositions. VC 2013 Acoustical Society of America.

Jin, K, Klima JC, Deane G, Dale Stokes M, Latz MI.  2013.  Pharmacological investigation of the bioluminescence signaling pathway of the dinoflagellate Lingulodinium polyedrum: evidence for the role of stretch-activated ion channels. Journal of Phycology. 49:733-745.   10.1111/jpy.12084   AbstractWebsite

Dinoflagellate bioluminescence serves as a whole-cell reporter of mechanical stress, which activates a signaling pathway that appears to involve the opening of voltage-sensitive ion channels and release of calcium from intracellular stores. However, little else is known about the initial signaling events that facilitate the transduction of mechanical stimuli. In the present study using the red tide dinoflagellate Lingulodinium polyedrum (Stein) Dodge, two forms of dinoflagellate bioluminescence, mechanically stimulated and spontaneous flashes, were used as reporter systems to pharmacological treatments that targeted various predicted signaling events at the plasma membrane level of the signaling pathway. Pretreatment with 200 μM Gadolinium III (Gd3+), a nonspecific blocker of stretch-activated and some voltage-gated ion channels, resulted in strong inhibition of both forms of bioluminescence. Pretreatment with 50 μM nifedipine, an inhibitor of L-type voltage-gated Ca2+ channels that inhibits mechanically stimulated bioluminescence, did not inhibit spontaneous bioluminescence. Treatment with 1 mM benzyl alcohol, a membrane fluidizer, was very effective in stimulating bioluminescence. Benzyl alcohol-stimulated bioluminescence was inhibited by Gd3+ but not by nifedipine, suggesting that its role is through stretch activation via a change in plasma membrane fluidity. These results are consistent with the presence of stretch-activated and voltage-gated ion channels in the bioluminescence mechanotransduction signaling pathway, with spontaneous flashing associated with a stretch-activated component at the plasma membrane.

Callaghan, AH, Deane GB, Stokes MD.  2013.  Two regimes of laboratory whitecap foam decay: Bubble-plume controlled and surfactant stabilized. Journal of Physical Oceanography. 43:1114-1126.   10.1175/jpo-d-12-0148.1   AbstractWebsite

A laboratory experiment to quantify whitecap foam decay time in the presence or absence of surface active material is presented. The investigation was carried out in the glass seawater channel at the Hydraulics Facility of Scripps Institution of Oceanography. Whitecaps were generated with focused, breaking wave packets in filtered seawater pumped from La Jolla Shores Beach with and without the addition of the surfactant Triton X-100. Concentrations of Triton X-100 (204 mu g L-1) were chosen to correspond to ocean conditions of medium productivity. Whitecap foam and subsurface bubble-plume decay times were determined from digital images for a range of wave scales and wave slopes. The experiment showed that foam lifetime is variable and controlled by subsurface bubble-plume-degassing times, which are a function of wave scale and breaking wave slope. This is true whether or not surfactants are present. However, in the presence of surfactants, whitecap foam is stabilized and persists for roughly a factor of 3 times its clean seawater value. The range of foam decay times observed in the laboratory study lie within the range of values observed in an oceanic dataset obtained off Martha's Vineyard in 2008.

Prather, KA, Bertram TH, Grassian VH, Deane GB, Stokes MD, DeMott PJ, Aluwihare LI, Palenik BP, Azam F, Seinfeld JH, Moffet RC, Molina MJ, Cappa CD, Geiger FM, Roberts GC, Russell LM, Ault AP, Baltrusaitis J, Collins DB, Corrigan CE, Cuadra-Rodriguez LA, Ebben CJ, Forestieri SD, Guasco TL, Hersey SP, Kim MJ, Lambert WF, Modini RL, Mui W, Pedler BE, Ruppel MJ, Ryder OS, Schoepp NG, Sullivan RC, Zhao DF.  2013.  Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol. Proceedings of the National Academy of Sciences of the United States of America. 110:7550-7555.   10.1073/pnas.1300262110   AbstractWebsite

The production, size, and chemical composition of sea spray aerosol (SSA) particles strongly depend on seawater chemistry, which is controlled by physical, chemical, and biological processes. Despite decades of studies in marine environments, a direct relationship has yet to be established between ocean biology and the physicochemical properties of SSA. The ability to establish such relationships is hindered by the fact that SSA measurements are typically dominated by overwhelming background aerosol concentrations even in remote marine environments. Herein, we describe a newly developed approach for reproducing the chemical complexity of SSA in a laboratory setting, comprising a unique ocean-atmosphere facility equipped with actual breaking waves. A mesocosm experiment was performed in natural seawater, using controlled phytoplankton and heterotrophic bacteria concentrations, which showed SSA size and chemical mixing state are acutely sensitive to the aerosol production mechanism, as well as to the type of biological species present. The largest reduction in the hygroscopicity of SSA occurred as heterotrophic bacteria concentrations increased, whereas phytoplankton and chlorophyll-a concentrations decreased, directly corresponding to a change in mixing state in the smallest (60-180 nm) size range. Using this newly developed approach to generate realistic SSA, systematic studies can now be performed to advance our fundamental understanding of the impact of ocean biology on SSA chemical mixing state, heterogeneous reactivity, and the resulting climate-relevant properties.

Stokes, MD, Deane GB, Prather K, Bertram TH, Ruppel MJ, Ryder OS, Brady JM, Zhao D.  2013.  A Marine Aerosol Reference Tank system as a breaking wave analogue for the production of foam and sea-spray aerosols. Atmospheric Measurement Techniques. 6:1085-1094.   10.5194/amt-6-1085-2013   AbstractWebsite

In order to better understand the processes governing the production of marine aerosols a repeatable, controlled method for their generation is required. The Marine Aerosol Reference Tank (MART) has been designed to closely approximate oceanic conditions by producing an evolving bubble plume and surface foam patch. The tank utilizes an intermittently plunging sheet of water and large volume tank reservoir to simulate turbulence, plume and foam formation, and the water flow is monitored volumetrically and acoustically to ensure the repeatability of conditions.

2012
Callaghan, AH, Deane GB, Stokes MD, Ward B.  2012.  Observed variation in the decay time of oceanic whitecap foam. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc008147   AbstractWebsite

Whitecap foam decay times for 552 individual breaking waves determined from digital images of the sea surface are reported. The images had sub-centimeter pixel resolution and were acquired at frame rates between 3 and 6 frames per second at the Martha's Vineyard Coastal Observatory over a 10-day period in 2008, subdivided into 4 observation periods. Whitecap foam decay times for individual events varied between 0.2 s to 10.4 s across the entire data set. A systematic positive correlation between whitecap foam decay time and maximum whitecap foam patch area was found for each observation period. For a given whitecap size within each observation period, the decay times varied between a factor of 2 and 5, with the largest variation occurring during unsteady environmental forcing conditions. Within observation periods, bin-averaged decay times varied by up to a factor of 4 across the range of foam patch areas. Between observation periods, the effective whitecap foam decay time, which we define as the area-weighted mean decay time, varied by a factor of 3.4 between 1.4 s and 4.8 s. We found a weak correlation between decay times and individual event-averaged breaking wave speeds. The variation in the active breaking area across all 4 observation periods was small, indicating relatively uniform surface whitecap area generating potential. We speculate that the variation in the foam decay times may be due to (i) the effect of surfactants on bubble and foam stability, and (ii) differences between bubble plume characteristics caused by a variation in breaking wave type.

Deane, GB, Preisig JC, Tindle CT, Lavery A, Stokes DM.  2012.  Deterministic forward scatter from surface gravity waves. The Journal of the Acoustical Society of America. 132:3673-3686.   10.1121/1.4763996   AbstractWebsite

Deterministic structures in sound reflected by gravity waves, such as focused arrivals and Doppler shifts, have implications for underwater acoustics and sonar, and the performance of underwater acoustic communications systems. A stationary phase analysis of the Helmholtz–Kirchhoff scattering integral yields the trajectory of focused arrivals and their relationship to the curvature of the surface wave field. Deterministic effects along paths up to 70 water depths long are observed in shallow water measurements of surface-scattered sound at the Martha's Vineyard Coastal Observatory. The arrival time and amplitude of surface-scattered pulses are reconciled with model calculations using measurements of surface waves made with an upward-looking sonar mounted mid-way along the propagation path. The root mean square difference between the modeled and observed pulse arrival amplitude and delay, respectively, normalized by the maximum range of amplitudes and delays, is found to be 0.2 or less for the observation periods analyzed. Cross-correlation coefficients for modeled and observed pulse arrival delays varied from 0.83 to 0.16 depending on surface conditions. Cross-correlation coefficients for normalized pulse energy for the same conditions were small and varied from 0.16 to 0.06. In contrast, the modeled and observed pulse arrival delay and amplitude statistics were in good agreement.

Leichter, JJ, Stokes MD, Hench JL, Witting J, Washburn L.  2012.  The island-scale internal wave climate of Moorea, French Polynesia. Journal of Geophysical Research-Oceans. 117   10.1029/2012jc007949   AbstractWebsite

Analysis of five-year records of temperatures and currents collected at Moorea reveal strong internal wave activity at predominantly semi-diurnal frequencies impacting reef slopes at depths >= 30 m around the entire island. Temperature changes of 1.5 degrees C to 3 degrees C are accompanied by surges of upward and onshore flow and vertical shear in onshore currents. Superimposed on annual temperature changes of approximately 3 degrees C, internal wave activity is high from Oct-May and markedly lower from Jun-Sep. The offshore pycnocline is broadly distributed with continuous stratification to at least 500 m depth, and a subsurface fluorescence maximum above the strong nutricline at approximately 200 m. Minimum buoyancy periods range from 4.8 to 6 min, with the maximum density gradient occurring at 50 to 60 m depth in summer and deepening to approximately 150 to 200 m in winter. The bottom slope angle around all of Moorea is super-critical relative to the vertical stratification angle suggesting that energy propagating into shallow water is only a portion of total incident internal wave energy. Vertical gradient Richardson numbers indicate dominance by density stability relative to current shear with relatively limited diapycnal mixing. Coherence and lagged cross-correlation of semi-diurnal temperature variation indicate complex patterns of inter-site arrival of internal waves and no clear coherence or lagged correlation relationships among island sides. Semi-diurnal and high frequency internal wave packets likely arrive on Moorea from a combination of local and distant sources and may have important impacts for nutrient and particle fluxes in deep reef environments.

2011
Stokes, MD, Leichter JJ, Wing S, Frew R.  2011.  Temperature variability and algal isotopic heterogeneity on a Floridian coral reef. Marine Ecology-an Evolutionary Perspective. 32:364-379.   10.1111/j.1439-0485.2011.00469.x   AbstractWebsite

On Conch Reef, Florida Keys, USA, we examine relationships between temperature variability measured with a unique, high-resolution sensor network, and the abundance of benthic reef organisms and carbon, nitrogen and oxygen stable isotopic composition of two abundant macroalgal species. Visualization of spatial temperature anomalies mapped onto measured reef bathymetry reveals episodic thermal patchiness and persistent thermal features likely indicating meter scale variability in nutrient supply. These patterns result from the relatively shallow depths of the offshore thermocline and an active internal wave field creating a hydrographic structure conducive to interactions of an offshore, subthermocline nutrient pool with reef topography. The distribution of major benthic reef organism taxa, as well as the stable isotopic composition of C, N and O, from the macroalgae Dictyota menstrualis and Halimeda tuna are all highly variable on scales of less than 10 m and are not necessarily depth-dependent. Covariability between spatial patterns of the temperature minima, depth and delta(13)C of D. menstrualis and H. tuna suggest a role of oceanographic forcing in inorganic carbon acquisition in these species. Comparisons of the measured temperature with the predicted temperature based on delta(18)O from H. tuna aragonite suggest a possible influence of low salinity water in the system and non-linear interactions between ambient conditions and calcification at fine temporal and spatial scales. These observations have implications for the interpretation of both contemporary and paleontologic reef environments where oceanographic forcing interacts with complex reef topography.

Slattery, M, Lesser MP, Brazeau D, Stokes MD, Leichter JJ.  2011.  Connectivity and stability of mesophotic coral reefs. Journal of Experimental Marine Biology and Ecology. 408:32-41.   10.1016/j.jembe.2011.07.024   AbstractWebsite

Mesophotic coral ecosystems (MCEs), occurring at depths of 30 to over 150 m, represents the deep continuum of adjacent shallow coral reefs about which little is known. These reefs are most developed, and most frequently studied, in areas where light penetration is high and photosymbiotic reef-builders (corals) can persist as common members of the benthic community. The upper mesophotic zone typically extends to 60 m and comprises communities that are generally similar to those found in shallow reef systems. Below 60 m, the lower mesophotic zone is dominated by sponges and algae that are uncommon or absent from shallower areas, and a fish fauna that is largely specialized to these intermediate depths. It is likely that these ecosystems are much more widespread and important than was previously recognized. MCEs have the potential to function as refugia for shallow reefs because many disturbances in the upper 30 m may leave MCEs largely intact. Ontogenetic movement and spawning migrations by reef fishes provide an important ecological component to connectivity between shallow and mesophotic reefs and may represent the effective range of larval dispersal. However, there is currently limited information on genetic connectivity between deep and shallow coral reef populations, and this is a critical gap in understanding the role of MCEs in coral reef resilience. Here we review the connectivity and trophic relationships between the shallow and mesophotic coral reefs, the potential role of MCEs as refugia, and the effects of a new biological invader that exposes critical gaps in our understanding of the stability of these ecosystems. (C) 2011 Elsevier B.V. All rights reserved.

2010
Deane, GB, Stokes MD.  2010.  Model calculations of the underwater noise of breaking waves and comparison with experiment. Journal of the Acoustical Society of America. 127:3394-3410.   10.1121/1.3419774   AbstractWebsite

A model for the underwater noise of whitecaps is presented and compared with the noise measured beneath plunging seawater laboratory waves. The noise from a few hundred hertz up to at least 80 kHz is assumed to be due to the pulses of sound radiated by bubbles formed within a breaking wave crest. The total noise level and its dependence on frequency are a function of bubble creation rate, bubble damping factor and an 'acoustical skin depth' associated with scattering and absorption by the bubble plume formed within the crest. Calculation of breaking wave noise is made using estimates of these factors, which are made independently of the noise itself. The results are in good agreement with wave noise measured in a laboratory flume when compensated for reverberation. A closed-form, analytical expression for the wave noise is presented, which shows a -11/6 power-law dependence of noise level on frequency, in good agreement with the -10/6 scaling law commonly observed in the open ocean. (C) 2010 Acoustical Society of America. [DOI: 10.1121/1.3419774]

Stokes, MD, Leichter JJ, Genovese SJ.  2010.  Long-term declines in coral cover at Bonaire, Netherlands Antilles. Atoll Research Bulletin. 582:1-22. Abstract

The reefs surrounding Bonaire have long been renowned for their high abundance and diversity of scleractinian corals and for their long-time status as a protected marine park. As part of a comprehensive study of the current status of Bonaire’s reefs, a quantitative benthic community survey was conducted at 7 locations across depths from 10 to 30 m in early 2008. This showed mean coral cover ranging from 23.7% ± 3.8 (SE) to 38.4% ± 4.4 (SE) on the island’s leeward shore and 1.6% ± 0.4 (SE) to 22.4% ± 3.6 (SE) on the windward shore. Percent cover of macrophytes and turf algae ranged from 41.7% ± 2.0 (SE) to 51.8% ± 3.0 (SE) on the leeward shore and 60.8% ± 4.1 (SE) to 82.7% ± 2.5 (SE) on the windward shore. Comparison of these results to earlier work from 1982 on the leeward shore and 1988 on the windward shore points to a significant, system-wide decrease in corals, increase in macroalgae and increase in the ratio of algae to corals. These observations point to significant degradation of reefs at a site often described as relatively ‘pristine’ in the Caribbean and highlight the sensitivity of coral reefs to anthropogenic and natural stresses even on well-protected islands.