M. Dale Stokes

Scale dependence of bubble creation mechanisms in breaking waves

Breaking ocean waves entrain air bubbles that enhance air–sea gas flux, produce aerosols, generate ambient noise and scavenge biological surfactants. The size distribution of the entrained bubbles is the most important factor in controlling these processes, but little is known about bubble properties and formation mechanisms inside whitecaps. We have measured bubble size distributions inside breaking waves in the laboratory and in the open ocean, and provide a quantitative description of bubble formation mechanisms in the laboratory. We find two distinct mechanisms controlling the size distribution, depending on bubble size. For bubbles larger than about 1 mm, turbulent fragmentation determines bubble size distribution, resulting in a bubble density proportional to the bubble radius to the power of -10/3. Smaller bubbles are created by jet and drop impact on the wave face, with a -3/2 power-law scaling. The length scale separating these processes is the scale where turbulent fragmentation ceases, also known as the Hinze scale. Our results will have important implications for the study of air–sea gas transfer.

Bringing the ocean into the laboratory to probe the chemical complexity of sea spray aerosol

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.

Sea spray aerosol as a unique source of ice nucleating particles

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 °C, averaging an order of magnitude increase per 5 °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.

Air Entrainment Processes and Bubble Size Distributions in the Surf Zone

Abstract A new optical instrument was deployed in the surf zone in a trial experiment to measure bubble size distributions and visualize air entrainment and bubble formation mechanisms within breaking surf. Images of bubbles and the evolving air–water mixture inside and beneath breaking wave crests are presented. The images resolve features of the air–water mixture to length scales of hundreds of microns across a 3.7-cm field of view. Two qualitatively different large-scale air entrainment processes are observed. First, intrusions of air and water, thought to be created by jets penetrating the water’s surface, fragment into plumes of bubbles. Second, an air cavity trapped by the overturning wave crest is observed to disintegrate into bubbles. The timescale for the evolution from a compacted air–water mass to individual bubbles was on the order of 90 ms or less for both of these processes. In addition, small-scale air filaments hundreds of microns wide and millimeters long have been discovered beneath wave...

Multivariate analysis of behavioural response experiments in humpback whales (Megaptera novaeangliae)

SUMMARY 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.

Two Regimes of Laboratory Whitecap Foam Decay: Bubble-Plume Controlled and Surfactant Stabilized

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 infilteredseawaterpumpedfromLaJollaShoresBeachwithandwithouttheadditionofthesurfactantTriton X-100. Concentrations of Triton X-100 (204 m gL 21 ) were chosen to correspond to ocean conditions of mediumproductivity.Whitecapfoamandsubsurfacebubble-plumedecaytimesweredeterminedfromdigital images for a range of wave scales and wave slopes. The experiment showed that foam lifetime is variable and controlledbysubsurfacebubble-plume-degassingtimes,whichareafunctionofwavescaleandbreakingwave slope. This is true whether or not surfactants are present. However, in the presence of surfactants, whitecap foam is stabilizedand persists for roughly a factorof 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.

Hypolithic Colonization of Opaque Rocks in the Arctic and Antarctic Polar Desert

Abstract The colonization of the underside of rocks normally requires that the material is sufficiently translucent to allow the penetration of photosynthetically active radiation. We examined the underside of 950 opaque rocks in sixteen locations in the Arctic for hypolithic colonization by photosynthetic microorganisms. Greater than 90% of rocks were colonized. The mean width of the bands of colonization was 3.1 ± 1.9 cm on Devon Island, and 3.0 ± 1.6 cm on Cornwallis Island. The width of the bands of colonization was less in the interior of frost-sorted polygons compared to their edges (in the arctic location, 0.7 ± 0.8 and 3.6 ± 1.4 cm in the interior and at the edges, respectively), suggesting the importance of frost sorting in enhancing the penetration of light around the edges of rocks to their undersides, and thus allowing colonization by photosynthetic organisms. We observed a similar pattern of colonization in antarctic polygons. The hypolithic habitat provides protection from environmental extremes. We show that within the hypolithic habitat organisms are protected from UV radiation. From radiolabeled carbon uptake measurements we estimate the productivity of the arctic communities to be approximately 0.8 g m−2 a−1, potentially as high as above-ground productivity. We discuss the potential implications of climate change for these communities.

Reproduction of the Florida lancelet (Branchiostoma floridae) : spawning patterns and fluctuations in gonad indexes and nutritional reserves

The reproductive biology of the Florida lancelet (Branchiostoma floridae) is described from over 21,000 animals collected from early summer 1991 through late summer 1995. Collections were made daily during summer and less frequently during other seasons. Minimal length at sexual maturity was about 23 mm for both sexes. The ratio of males to females was 1:1 throughout the year, and no hermaphrodites were found. During the breeding season (late spring through late summer), the population spawned just after sunset at intervals of 1-15 days, with an average interval of roughly a week. The proportion of lancelets spawning on a given date ranged from a small fraction to about 90% of the adult population. Spawning dates were not significantly correlated with lunar phase or fluctuations in salinity or water temperature. Lancelets could be induced to spawn by electric shock in the laboratory, but only on dates when they spawned in the field. A daily determination of gonad indexes suggested that individual males and females could spawn and then ripen and spawn again in the same breeding season, possibly in as little as 1-2 weeks. Fin-ray indexes (assumed to be proportional to somatic nutritional reserves) were highest in winter, when the lancelets were apparently well nourished but not breeding, and consistently low during the summer breeding season, when the animals were repeatedly spawning and presumably draining their nutritional reserves. Additional key words: amphioxus, cephalochordate The subphylum Cephalochordata of phylum Chordata comprises about 25 species of lancelets (=amphioxus) living in shallow marine or estuarine habitats. Because of their phylogenetic ties to vertebrate chordates, lancelets have been extensively studied anatomically and embryologically for over a century. In contrast, much less is known of their ecology in general and reproductive biology in particular, partly due to the relative inaccessibility of large populations of lancelets. Although most lancelet species have a wide geographic distribution, they are typically rare through much of their range and occur at high densities in only a few scattered localities (e.g., Lagos Lagoon, Nigeria; Xiamen and Qingdao, China; Kingston Harbour, Jamaica; and Tampa Bay, Florida). Much of the work on lancelet reproduction (reviewed in Wickstead 1975) concerns temperate populations of Branchiostoma lanceolatum in Europe and B. belcheri in China. The reproduction of one tropical lancelet, B. nigeriense, has also been studied extenaCurrent address: Hopkins Marine Station, Stanford University, Pacific Grove CA 93950, USA. sively, although for an estuarine population exterminated once a year by fresh water runoff and replenished after each rainy season by animals from the open sea. These studies may give a distorted picture of reproduction in cephalochordates as a whole, because most lancelets have sub-tropical and tropical distributions and maximal lifetimes of several years. The purpose of the present study is to describe the reproductive biology of Branchiostoma floridae HUBBS 1922. In contrast to lancelets previously studied in detail, B. floridae is sub-tropical and has a maximal lifetime of several years (Stokes 1996); therefore, the reproductive biology of this species may more fairly represent that of cephalochordates generally. The population studied lives in Tampa Bay, Florida, and, from a previous description of larval settlement patterns (Stokes 1996), its breeding season is known to last from late spring through late summer. Our results also help clarify the role of lancelet finrays, which lack any homology with vertebrate finrays (Azariah 1965) and comprise retroperitoneal accumulations of glycoprotein and lipid (Holland & Holland 1991 a). Here we assume that fin-ray volumes This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 05:51:42 UTC All use subject to http://about.jstor.org/terms

Microbiology and Vegetation of Micro-oases and Polar Desert, Haughton Impact Crater, Devon Island, Nunavut, Canada

The input of nutrients into arctic polar deserts, aided by some physical processes, can result in localized areas of high biological productivity—“micro-oases.” We examined the vegetation cover, and microbial and nematode abundance in the polar desert and in 38 micro-oases at the Haughton impact crater, Devon Island, Arctic Canada. Our sites were split between the alluvial terraces along the banks of the Haughton River and the breccia deposits resulting from the asteroid or comet impact 22 Myr ago that flank the alluvial terraces. The alluvial terraces have a vegetation cover that ranges from 2 to 11% depending on substrate and water availability with a species richness of 5 in most locations. The vegetation cover on the breccia is much lower, between 0.02 and 3% depending on water availability. The micro-oases on both substrates support between 2 and 98% cover, but they are smaller and more sparsely distributed than similar features found in the Truelove Lowland, Devon Island, and on Bathurst Island. Microbial and nematode numbers were an order of magnitude greater inside the micro-oases compared to outside. Micro-oases are often dominated by a particular species, resulting in well-defined groups of micro-oases that were separated by TWINSPAN analysis. The micro-oases at Haughton Crater provide insights into the process of colonization of a substrate resulting from an asteroid or comet impact and the unique biological characteristics of such substrates.

Deterministic forward scatter from surface gravity waves

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 Marthas 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.