Patch Of Seafloor Research Articles

Hidden in plain sight: The invasive macroalga Caulerpa prolifera evades detection by environmental DNA methods

AbstractEnvironmental managers need a rapid and cost‐effective monitoring tool for tracking the spread of invasive species, particularly at the onset of introduction. The macroalgae Caulerpa prolifera is considered an invasive species outside its native range, colonizing large patches of seafloor, reducing native species, and altering ecosystem functioning. Here, we developed a droplet digital PCR assay for detection of C. prolifera from environmental DNA seawater samples using the internal transcribed spacer (ITS) region. While the assay itself was confirmed to be highly efficient, we discovered concentrations of C. prolifera eDNA were present below detectable levels in the water column surrounding an outbreak. To understand why, we conducted tank‐based experiments for two California invasive algae species, Caulerpa prolifera and Sargassum horneri. The steady‐state eDNA concentration (eDNA copies/ gram of biomass detected) of C. prolifera was found to be two orders of magnitude lower than S. horneri. A meta‐analysis of steady‐state concentrations reported in the literature showed a remarkable range from ~104–1011 (copies/g), revealing C. prolifera to have the lowest recorded steady‐state concentrations of eDNA of any known species. We attribute C. prolifera's low steady‐state eDNA concentration to its unique biology as a unicellular macroscopic algae which reduces the possible modes of eDNA release compared to similarly sized multicellular organisms. Critically our results demonstrate the potential limits of eDNA approaches, the influence of shedding rates in the reliability of species detections, and the vital importance of benchmarking and validating eDNA assays in both field and laboratory settings.

Design and field testing of a non-linear single-beam echosounder for multi-frequency seabed characterization

Seabed mapping and characterization are best performed using several frequencies and several angles of incidence. This is often an issue because of the need to employ different sonars, with distinct frequencies but co-located as much as possible to image the same patch of seafloor. This article presents the design, calibration and field testing of a multiple-frequency single-beam echosounder (SBES), mounted on a mechanical pan-and-tilt head. It uses very high transmitting levels to produce non-linear effects and generate harmonics of a 100 kHz fundamental frequency. PZT transducers are used to transmit high acoustic powers and PDVF transducers enable the reception of scattering levels over a very broad frequency band (for the different harmonics). Tank experiments are used to verify effective harmonic generation. The shock distance (at which harmonics are at their maximum level) is measured as 2 m from the transmitter and recommended as the minimum far-field range. Non-linear transmission losses (distinct from linear losses) are calibrated using a full metal sphere 38.1 mm in diameter and of known frequency response, up to ranges commensurate with the depths expected in the field (⩽30 m). The −3 dB beamwidth varies from 5.8° at 100 kHz to 2.8° at 300 kHz. Harmonics are used to resolve phase ambiguities in detecting seabed depths. Backscattering strengths BS are matched to the Generic Seafloor Acoustic Backscatter (GSAB) model to derive the best-fitting parameters. Field validation took place in the Bay of Brest (France) in May 2016, over three different types of seafloor (namely: sandy mud; gravel; gravelly coarse sand with maerl). Additional in situ calibration was used. The echosounder was pointed at angles from 0° (nadir) to 60° by 5° steps. One of the areas surveyed (“Carré Renard”), commonly used for instrument calibration and comparison with other measurements, showed differences <1 dB at 200 kHz. Videos and photographs of the seafloor were used to ground truth interpretations of the BS curves. The results show that these BS curves measured with the echosounder are relevant for seabed classification and characterization. The different shapes and levels of BS when compared to ground truth are coherent with the Jackson model. The main limit of this prototype of echosounder is the signal to noise ratio, in particular for high frequency harmonics (⩾400 kHz). The in situ calibration is unavoidable because of the non-linear parameter variations with water characteristics (temperature, salinity…). Calibrated BS curves from 100 kHz to 300 kHz can be directly compared to other measurements, for example to calibrate other instruments.

Segment‐scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, M id‐ A tlantic R idge Rainbow region (35°45′–36°35′N)

Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow-spreading Mid-Atlantic Ridge (35o45'-36o35'N) centered on the Rainbow non-transform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multi-directional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif. This article is protected by copyright. All rights reserved.

Structure, temporal evolution, and heat flux estimates from the Lucky Strike deep‐sea hydrothermal field derived from seafloor image mosaics

Here we demonstrate with a study of the Lucky Strike hydrothermal field that image mosaicing over large seafloor areas is feasible with new image processing techniques, and that repeated surveys allow temporal studies of active processes. Lucky Strike mosaics, generated from &gt;56,000 images acquired in 1996, 2006, 2008 and 2009, reveal the distribution and types of diffuse outflow throughout the field, and their association with high‐temperature vents. In detail, the zones of outflow are largely controlled by faults, and we suggest that the spatial clustering of active zones likely reflects the geometry of the underlying plumbing system. Imagery also provides constraints on temporal variability at two time‐scales. First, based upon changes in individual outflow features identified in mosaics acquired in different years, we document a general decline of diffuse outflow throughout the vent field over time‐scales up to 13 years. Second, the image mosaics reveal broad patches of seafloor that we interpret as fossil outflow zones, owing to their association with extinct chimneys and hydrothermal deposits. These areas encompass the entire region of present‐day hydrothermal activity, suggesting that the plumbing system has persisted over long periods of time, loosely constrained to hundreds to thousands of years. The coupling of mosaic interpretation and available field measurements allow us to independently estimate the heat flux of the Lucky Strike system at ∼200 to 1000 MW, with 75% to &gt;90% of this flux taken up by diffuse hydrothermal outflow. Based on these heat flux estimates, we propose that the temporal decline of the system at short and long time scales may be explained by the progressive cooling of the AMC, without replenishment. The results at Lucky Strike demonstrate that repeated image surveys can be routinely performed to characterize and study the temporal variability of a broad range of vent sites hosting active processes (e.g., cold seeps, hydrothermal fields, gas outflows, etc.), allowing a better understanding of fluid flow dynamics from the sub‐seafloor, and a quantification of fluxes.

3D spatial sampling with a cylindrical multibeam sonar array

Various beam pair combinations can be formed with cylindrical multibeam sonar arrays to obtain a 3D spatial sampling of a patch of seafloor for each ping. This capability is explored with a 286 deg sector cylindrical array transmitting a stepped FM pulse over a 243 deg vertical fan beam centered on nadir and receiving with twenty-seven beam pairs, symmetrically steered about nadir in the fore-aft direction and spaced at 7.16 deg intervals across track. Conventional conjugate product techniques yield two across-track profiles of 26 soundings each (52 phasors total) per ping. However, by combining one or two phasors along-track and one, two or three phasors across-track, soundings can be obtained at the spatial bisectors of the angles between the 52 phasors. This yields three profiles consisting of 51 soundings each, for a total of 153 soundings. This effectively creates a 3D patch of detected seafloor for each ping, which may overlap with the patches from adjacent pings depending on the sonars attitude changes and altitude above the seafloor. [Work supported by NRL-SSC grant N00173-00-1-G912 and NOAA grant NA170G228.]

Acoustic determination of seafloor roughness with multibeam echo sounders.

Multibeam echo sounders have been used extensively to gather swath bathymetry in the world’s oceans with co-registered depth samples a few percent of the water depth apart in the athwartships direction. The resulting maps provide seafloor roughness information down to scales of a few hundred meters. A much finer roughness resolution (cm scale) can be achieved when analyzing the beamformed seafloor echoes received by such sonar systems. One method consists of processing the corresponding acoustic data to remove system dependencies and to correct for bottom slopes and for geometric effects due to the area insonified by the transmitted pulse within the confines of the cross-fan beam geometry, so as to derive an angular dependence function of the average power in the returns for a given patch of seafloor. By fitting this function with a Helmholtz–Kirchhoff model for acoustic backscatter from a rough surface [D. R. Jackson etal., J. Acoust. Soc. Am. 79, 1410–1422 (1986)], a bottom roughness estimation is obtained. The model assumes Gaussian statistics for the roughness with a spectrum expressed as a power law of the wave number k, W(k)=βk−γ, where β is a roughness parameter which enters in the fitting process along with the exponent γ. Results for various seafloor areas are presented and implications for automatic seafloor classification schemes are discussed. [Work supported by ONR.]

Exotic Middle Miocene Sediment from Cape Verde Rise and Its Relation to Piercement Structures: GEOLOGIC NOTES

A clast of early middle Miocene sediment was recovered in a piston core on the lower Cape Verde Rise. The core was taken in one of a series of slightly elevated patches of seafloor which appear to be the surface expression of piercement structures. The mechanism which emplaced the exotic Miocene inclusion within 3 m of the seafloor probably is related to the intrusion tectonics of the underlying piercement structures although the possibility of its arrival at the present site by a mass flow from upslope cannot be eliminated as an alternative.

Side-Looking Sonar Navigation

Navigation and mapping techniques developed for surface ships are not optimum for accurate sub-surface navigation of submersible vehicles. The long-range acoustic transponder techniques which have been developed for use by towed vehicles in deep water are also not suitable for use in relatively shallow depths (200 fathoms or less). For these reasons, the Marine Physical Laboratory has undertaken a series of experiments to develop techniques using equipment compatible with that which could be installed on submersible vehicles having limited payload capability. These techniques allow the use of side-looking sonar to map a patch of sea floor (about 5 sq mi) without reliance on surface-acquired information, and with confidence that there is continuous coverage of the area. Mapping techniques can be selected to cope with the two extreme cases of a continuous distribution of targets, and an essential absence of targets.