Physics-informed laboratory estimation of Sargassum windage

A recent Maxey--Riley theory for \sarg raft motion, which models a raft as a network of elastically interacting finite-size, buoyant particles, predicts the carrying flow velocity to be given by the weighted sum of the water and air velocities $(1-\alpha)\mathbf{v} + \alpha \mathbf w$. The theory provides a closed formula for parameter $\alpha$, referred to as \emph{windage}, depending on water-to-particle-density ratio or buoyancy ($\delta$). From a series of laboratory experiments in an air--water stream flume facility under controlled conditions, we estimate $\alpha$ ranging from 0.02 to 0.96\pct. On average, our windage estimates can be up to 9 times smaller than considered in conventional \emph{Sargassum} raft transport modeling, wherein it is customary to add a fraction of $\mathbf w$ to $\mathbf{v}$ chosen in an ad-hoc piecemeal manner. Using the formula provided by the Maxey--Riley theory, we estimate $\delta$ ranging from 1.00 to 1.49. This is consistent with direct $\delta$ measurements, ranging from 0.9 to 1.25, which provide support for our $\alpha$ estimation

Larval fishes utilize Batesian mimicry as a survival strategy in the plankton

Marine teleost fishes often experience over 99% mortality in the early life stages (eggs and larvae), yet larval survival is essential to population sustainability. Marine fish larvae from a wide range of families display elaborate, delicate features that bear little resemblance to adult forms and hinder their swimming escape ability by increasing drag. Here, we systematically examine the criteria needed for Batesian mimicry to evolve as a survival strategy and present new evidence from in situ imaging technology and simulation modelling to support the hypothesis that many larval morphological features (particularly long, delicate fin rays) and behaviors evolved at least in part through Batesian mimicry of less palatable or noxious gelatinous zooplankton. Many of these organisms (e.g. hydromedusae, ctenophores, and siphonophores) are much more abundant than previously recognized. The high predation mortality during the larval phase provides strong potential for selection in favor of maintaining complex and metabolically costly features that mimic gelatinous zooplankton, provided that larger fishes, as selective visual predators, can occasionally be fooled. We conclude that recent advances in our understanding of mimicry combined with information obtained from plankton imaging supports the hypothesis that Batesian mimicry is a widespread survival strategy for larval fishes, which could have broad implications for fish population dynamics. However, further research is needed in the areas of predator cognition and larval fish behavior in the presence of different predators and models to elucidate the circumstances in which the larval fish mimicry hypothesis may apply.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Inter-Research. The published article can be found at: http://www.int-res.com/journals/meps/meps-home/Keywords: Survival, Plankton, Larval fish, Batesian mimicry, Recruitment, Gelatinous zooplankto

Ocean convergence and the dispersion of flotsam

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material

Ocean convergence and the dispersion of flotsam

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km2 converged into a 60 × 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material

Image Analysis Techniques to Accompany a new In Situ Ichthyoplankton Imaging System

We have built a high resolution towed digital imaging system (ISIIS) capable of imaging water volumes sufficient to accurately quantify even rare plankton (e.g. larval fish) in situ. This imaging system produces very high resolution imagery at very high data rates necessitating automated image analysis. As we are interested in the identification and quantification of a large number of organisms, some of which are relatively similar to each other, we are developing an automated system for detection and recognition of organisms of interest using computer vision tools. Our method aims at (i) the detection of multiple regions (organisms) of interest automatically, while filtering out noise and out-of-focus organisms, and (ii) the classification of the detected organisms into pre-defined categories using shape and texture information. For the organisms detection, we use a probabilistic scheme based on image statistics to locate the regions of interest and -based on size and shape constraints - we filter out the noise, i.e., regions that are detected but do not correspond to organisms. For the classification of the detected organisms, we use the Scale Invariant Feature Transform (SIFT) for matching between the detected regions and the organism images in our database ("dictionary")

On the transport and landfall of marine oil spills, laboratory and field observations

The dynamics of crude oil and different surface ocean drifters were compared to study the physical processes that govern the transport and landfall of marine oil spills. In a wave-tank experiment, drifters with drogue did not follow oil slicks. However, patches of undrogued drifters and thin bamboo plates did spread at the same rate and in the same direction as the crude oil slicks. Then, the trajectories of the Deepwater Horizon oil spill and 1300 drifters released near the spill source were investigated. Undrogued drifters were transported twice as fast as drogued drifters across the isobaths. 25% of the undrogued drifters landed, versus about 5% of the drogued ones, for the most part, on the same coastline locations where oil was found after Deepwater Horizon. Results highlight the importance of near surface gradients in controlling the cross-shelf transport and landing of surface material on the Gulf of Mexico's northern shores. •Undrogued drifters and crude oil slicks advect and disperse similarly in waves.•Undrogued drifters make landfall 5 times more than near-surface drogued drifters.•Near-surface vertical shear sets the cross-shelf transport of surface material

Larval Fishes Utilize Batesian Mimicry as a Survival Strategy in the Plankton

Marine teleost fishes often experience over 99% mortality in the early life stages (eggs and larvae), yet larval survival is essential to population sustainability. Marine fish larvae from a wide range of families display elaborate, delicate features that bear little resemblance to adult forms and hinder their swimming escape ability by increasing drag. Here, we systematically examine the criteria needed for Batesian mimicry to evolve as a survival strategy and present new evidence from in situ imaging technology and simulation modelling to support the hypothesis that many larval morphological features (particularly long, delicate fin rays) and behaviors evolved at least in part through Batesian mimicry of less palatable or noxious gelatinous zooplankton. Many of these organisms (e.g. hydromedusae, ctenophores, and siphonophores) are much more abundant than previously recognized. The high predation mortality during the larval phase provides strong potential for selection in favor of maintaining complex and metabolically costly features that mimic gelatinous zooplankton, provided that larger fishes, as selective visual predators, can occasionally be fooled. We conclude that recent advances in our understanding of mimicry combined with information obtained from plankton imaging supports the hypothesis that Batesian mimicry is a widespread survival strategy for larval fishes, which could have broad implications for fish population dynamics. However, further research is needed in the areas of predator cognition and larval fish behavior in the presence of different predators and models to elucidate the circumstances in which the larval fish mimicry hypothesis may apply