Atsushi Fujimura

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

Larvae of intertidal species develop at sea and must return to adult habitats to replenish populations. Similarly, nutrients, detritus and plankton provide important subsidies spurring growth and reproduction of macroalgae and filter-feeding invertebrates that form the foundation of intertidal communities. Together, these factors determine the density and intensity of interactions among community members. We hypothesized that spatial variation in surfzone hydrodynamics affects the delivery of plankton subsidies. We compared entire zooplankton communities inside and outside the surf zone daily while monitoring physical conditions for one month each at two shores with different surfzone characteristics. Opposite cross-shore distributions of larvae and other zooplankters occurred at the two sites: zooplankton was much more abundant inside the mildly sloping dissipative surf zone (DSZ) with rip currents and was more abundant outside the steep reflective surf zone (RSZ). Biophysical numerical simulations demonstrated that zooplankters were concentrated in rip channels of the DSZ and were mostly unable to enter the RSZ, indicating the hydrodynamic processes behind the observed spatial variation of zooplankters in the surf zone. Differences in the concentration of larvae and other zooplankters between the inner shelf and surf zone may be an underappreciated, key determinant of spatial variation in inshore communities.

Numerical Simulation of the Wind-Stress Effect on SAR Imagery of Far Wakes of Ships

Centerline wakes of ships in synthetic aperture radar (SAR) images were modeled in 2-D with the computational fluid dynamics (CFD) software Fluent and a radar-imaging algorithm. We initialized the model with a pair of vortices generated by a ship hull and applied wind stress perpendicular to the ship wake. Results of the CFD simulation using a nonhydrostatic model have demonstrated ship-wake asymmetry with respect to the wind-stress direction relative to the ship course. Due to the wind stress, flow convergence increased on the upwind side of the centerline wake and reduced on the downwind side of the wake. The radar-imaging algorithm processed with the surface velocity field produced by the CFD model revealed ship-wake asymmetry relative to the wind direction. These results are qualitatively consistent with SAR images from the TerraSAR-X satellite and representative statistics of photographic images of the ship wake collected from a volunteer observing ship.

Variation in the abundance of Pseudo-nitzschia and domoic acid with surf zone type

Most harmful algal blooms (HAB) originate away from the shore and, for them to endanger human health, they must be first transported to shore after which they must enter the surf zone where they can be feed upon by filter feeders. The last step in this sequence, entrance into the surf zone, depends on surf zone hydrodynamics. During two 30-day periods, we sampled Pseudo-nitzschia and particulate domoic acid (pDA) in and offshore of a more dissipative surf zone at Sand City, California (2010) and sampled Pseudo-nitzschia in and out of reflective surf zones at a beach and rocky shores at Carmel River State Beach, California (2011). At Sand City, we measured domoic acid in sand crabs, Emerita analoga. In the more dissipative surf zone, concentrations of Pseudo-nitzschia and pDA were an order of magnitude higher in samples from a rip current than in samples collected just seaward of the surf zone and were 1000 times more abundant than in samples from the shoals separating rip currents. Domoic acid was present in all the Emerita samples and varied directly with the concentration of pDA and Pseudo-nitzschia in the rip current. In the more reflective surf zones, Pseudo-nitzschia concentrations were 1-2 orders of magnitude lower than in samples from 125 and 20m from shore. Surf zone hydrodynamics affects the ingress of Pseudo-nitzschia into surf zones and the exposure of intertidal organisms to HABs on the inner shelf.

The impact of thermal stratification and wind stress on sea surface features in SAR imagery

We have conducted high-resolution numerical experiments with the computational fluid dynamics (CFD) software ANSYS FLUENT on the dynamics of centerline ship wakes and rain-formed plumes in the presence of wind stress. The ship wake model is initialized with the velocity field from numerical simulations using a model with a ship hull and propellers. We then apply wind stress perpendicular to the centerline wake. We simulate SAR images with a radar imaging algorithm using the surface velocity field produced by the CFD model. Results show ship wake asymmetry since the wind stress enhances flow convergence upwind of the centerline wake and reduces it on the downwind side. The results are qualitatively consistent with available SAR images. We added a near-surface thermal stratification (e.g., diurnal thermocline) to the model during initialization and investigated the impact on the ship wake hydrodynamics. Stratification appears to influence the spreading of the wake, while the circulation in the wake can bring colder water to the surface. For the plume simulation, we initialize the model with a low-density plume imitating randomly distributed rainfall. The plume structure also shows asymmetry relative to the wind direction in simulated radar images.

Coupled Model Simulation of Wind Stress Effect on Far Wakes of Ships in SAR Images

A high-resolution 3-D hydrodynamic model capable of simulating far wakes of ships has been implemented using computational fluid dynamics software. We feed the surface velocity field produced by the hydrodynamic model into a numerical radar imaging model to simulate synthetic aperture radar (SAR) signatures of the wake. Potential capabilities of this modeling method are demonstrated for an example of wind stress effects on the centerline (turbulent) ship wake. The numerical simulations show that an interaction of the wind-induced surface current with circulations in the ship wake results in a convergence zone on the upwind side of the centerline wake and a divergence zone on the downwind side. In the simulated radar image, the convergence zone appears to be bright because of enhanced surface roughness and radar backscattering. The divergence zone looks dark due to an attenuation of short gravity capillary waves and a corresponding reduction of the backscattered power. This combined hydrodynamic and radar imaging model predicts an asymmetry of the centerline wake with respect to the wind direction, which is consistent with observed ship wake signatures in high-resolution satellite SAR images. The approach developed in this work could be also useful for simulations of other natural and artificial fine-scale features on the sea surface (sharp frontal interfaces, freshwater plumes, etc.) and their interpretation in high-resolution SAR imagery.

Mechanisms of cross-shore transport and spatial variability of phytoplankton on a rip-channeled beach

We investigated whether cross-shore distributions of coastal phytoplankton to the surf zone are controlled by hydrodynamics and their biological characteristics. Data from a rip-channeled beach indicate that concentrations of phytoplankton are higher in the surf zone than offshore. To examine how phytoplankton is transported toward the shore, we used a coupled biophysical model, comprised of a 3D physical model of coastal dynamics and an individual-based model (IBM) for tracking phytoplankton on the rip-channeled beach. Waves and wind in the biophysical model were parameterized by the conditions during the sampling period. Previous studies indicated that growth rates of phytoplankton can be enhanced by high turbulence, which might contribute to high phytoplankton concentration in the surf zone. Some numerical and laboratory works showed that turbulence can also increase the downward velocity of phytoplankton, which could be carried by onshore bottom currents and remain in the surf zone. Furthermore, we adapted the IBM with the theoretical model of diurnal vertical migration (DVM) for phytoplankton. The theoretical DVM works as follows: in the morning, phytoplankton cells adhere to air bubbles and stay at the surface and close to the shore in the daytime because onshore wind and surface current direction is usually onshore; in the late afternoon, the cells switch their attachment from air bubbles to sand grains and sink to the bottom where the water flow is normally onshore at night. Finally, depth-varying growth of phytoplankton was also incorporated into the DVM module. Simulations using neutral passive particles do not give the expected results of observed patterns. All tested mechanisms, i.e., wind- and wave-driven currents, rip-current circulation, turbulence-driven growth and sinking, DVM, and depth-varying growth, enhanced onshore phytoplankton migration and cell concentrations in the surf zone, indicating that both biological traits and physical factors can be essential to phytoplankton cross-shore transport and spatial variability. Our model is open to be modified and re-parameterized, followed by further analysis and validation, so that it can be more adequate for ecological assessment of coastal areas.

Critical Information Gaps Impeding Understanding of the Role of Larval Connectivity Among Coral Reef Islands in an Era of Global Change

Populations of marine organisms on coral reef islands (CRI) are connected in space and time by seawater that transports propagules of plants, animals, and algae. Yet, despite this reality, it is often assumed that routine replenishment of populations of marine organisms on CRI is supported by locally-sourced propagules (hereafter, larvae). Following large disturbances, however, distantly-sourced larvae from less disturbed CRI within a regional meta-population are likely to be important for local population recovery, but evaluating the roles of locally- versus distantly- sourced larvae remains difficult. While larval sources are relatively well known for many fishes, they remain virtually unknown for most taxa, particularly those associated with the benthos, including hermatypic corals. We make the case that CRI provide natural laboratories in which studies of connectivity can enhance understanding of community dynamics under future disturbance regimes, especially where ongoing changes have created novel systems that are functioning in ways differing from the recent past. However, this potential cannot be realized due to the limited breadth, detail, and spatio-temporal concordance of exiting research. Targeted research on the role of connectivity in mediating ecosystem resilience of CRI is required to understand how populations of marine organisms will change in a future affected by large-scale disturbances of anthropogenic origin. Using the coral reefs of Mo’orea (French Polynesia), Okinawa (Japan), and St. John (US Virgin Islands) as examples, we describe the data required to achieve this objective, and discuss why provision of these data will require new modes of multidisciplinary and collaborative research.