Integrated measurements of acoustical and optical thin layers I: Vertical scales of association

This study combined measurements from multiple platforms with acoustic instruments on moorings and on a ship and optics on a profiler and an autonomous underwater vehicle (AUV) to examine the relationships between fluorescent, bioluminescent, and acoustically scattering layers in Monterey Bay during nighttime hours in July and August of 2006 and May of 2008. We identified thin bioluminescent layers that were strongly correlated with acoustic scattering at the same depth but were part of vertically broad acoustic features, suggesting layers of unique composition inside larger biomass features. These compositional thin layers nested inside larger biomass features may be a common ecosystem component and are likely to have significant ecological impacts but are extremely difficult to identify as most approaches capable of the vertical scales of measurement necessary for the identification of sub-meter scale patterns assess bulk properties rather than specific layer composition. Measurements of multiple types of thin layers showed that the depth offset between thin phytoplankton and zooplankton layers was highly variable with some layers found at the same depth but others found up to 16 m apart. The vertical offset between phytoplankton and zooplankton thin layers was strongly predicted by the fraction of the water column fluorescence contained within a thin phytoplankton layer. Thin zooplankton layers were only vertically associated with thin phytoplankton layers when the phytoplankton in a layer accounted for more than about 18–20% of the water column chlorophyll. Trophic interactions were likely occurring between phytoplankton and zooplankton thin layers but phytoplankton thin layers were exploited by zooplankton only when they represented a large fraction of the available phytoplankton, suggesting zooplankton have some knowledge of the available food over the entire water column. The horizontal extent of phytoplankton layers, discussed in the second paper in this series, is likely an important factor contributing to this selective exploitation by zooplankton. The pattern of vertical offset between phytoplankton and zooplankton layers was consistent between studies in different years and using different combinations of platforms, indicating the importance of the relationship between zooplankton layers and the fraction of phytoplankton within a layer at night within Monterey Bay. These results highlight the value of integrating measurements of various types of organisms to understand thin layers processes and the importance of assessing ecological interactions in plankton thin layers within the context of the properties of the entire water column, like the animals themselves do

Ecological insights into abyssal bentho-pelagic fish at 4000 m depth using a multi-beam echosounder on a remotely operated vehicle

Ecological and behavioral data on mobile, low density, benthopelagic animals is difficult to collect in the abyssal environment. However, these species occupy an important position in the abyssal food chain. At-depth ROV-mounted echosounder studies provide a powerful tool to gather in-situ information on abyssal benthopelagic assemblages and discern their distribution, behavior and habitat associations. This study presents a new perspective on mobile benthopelagic assemblages at the long-term study site, Station M (∼4000 m), using a Seabat T20-S MBES mounted on the ROV Doc Ricketts. The targets (∼45 m off the seafloor) are believed to be the abyssal grenadier of the species Coryphaenoides armatus or C. yaquinae, species known to dominate the mobile benthopelagic fauna at Station M. The swimming behavior of the targets indicated little evidence of avoidance or attraction to the slowly moving ROV and demonstrates the effectiveness of this platform to collect data on benthopelagic fish. The information on targets in close (<1 m) association with the seafloor from the MBES corresponded well to target densities recorded by the video transects. However, in addition the MBES resolved the distribution of targets up to 45 m above the seafloor. Target density had a small peak close to the seafloor (<1 m) but increased in density with height above the seafloor, exceeding the maximum near-bottom density by ∼50 times. ROV-mounted MBES surveys can effectively provide data on the distribution and behavior of benthopelagic fish and further understanding of the pelagic-benthic links in the abyssal deep-sea.acceptedVersio

Zooplankton Avoidance of a Profiled Open-Path Fluorometer

Significant avoidance of acoustically detected zooplankton was observed in response to a profiling instrument package. Avoidance decreased acoustic scattering from zooplankton averaged over the entire profile by more than a factor of 2, while the maximum avoidance decreased zooplankton acoustic scattering by a factor of 15 over the depth of some discrete scattering layers. Experimental manipulation of the profiler and its instruments revealed that an open-path fluorometer was triggering the avoidance. Avoidance occurred at an average of 8 m below the profiler with a range between 2 and 13 m. Effect range was positively correlated with the average attenuation coefficient of light over the effect range and consistently resulted in avoidance when light levels of approximately 0.013 µmol photons m−2 s−1 were received by the zooplankton. These results have important implications for the analysis of zooplankton data collected from platforms carrying open-path fluorometers and may also warrant careful interpretation of optical measurements from these packages

An automatic and quantitative approach to the detection and tracking of acoustic scattering layers

Acoustic scattering layers are ubiquitous, horizontally extensive aggregations of both vertebrate and invertebrate organisms that play key roles in oceanic ecosystems. However, currently there are no conventions or widely adaptable automatic methods for identifying these often dynamic, spatially complex features, so it is difficult to consistently and efficiently describe and compare results. We developed an automatic scattering layer detection method that can be used to monitor changes in layer depth, width, and internal structure over time. Extensive, contiguous regions of the water column that have echo strengths above a threshold were identified as “background layers.” They correspond to regions of the water column that contain scattering from diffusely distributed organisms. Often, background layers contained contiguous, horizontally extensive features of concentrated acoustic scattering we identified as “strata.” These features were identified by fitting Gaussian curves to the echo envelope of each vertical profile of scattering, and their boundaries were identified as the endpoints of the region containing 95% of the area under the fitted curves. These endpoints were linked horizontally to make continuous tracks. Bottom and top tracks were paired to identify features that sometimes extended horizontally for tens of kilometers. This approach was effective in three disparate ecosystems (the Gulf of California, Monterey Bay, and the Bering Sea), and a sensitivity analysis showed its robustness to changes in input parameters. By allowing a comparable, automated approach to be used across environments, this method promotes the improved classification and characterization of acoustic scattering layers necessary for examining their role in oceanic ecosystems.This is the publisher’s final pdf. The published article is copyrighted by the American Society of Limnology and Oceanography, Inc. and can be found at: http://www.aslo.org/lomethods/

A Critical Time Window for Organismal Interactions in a Pelagic Ecosystem

To measure organismal coherence in a pelagic ecosystem, we used moored sensors to describe the vertical dynamics of each step in the food chain in shelf waters off the west shore of Oahu, Hawaii. Horizontally extensive, intense aggregations of phytoplankton, zooplankton, and micronekton exhibited strong diel patterns in abundance and vertical distribution, resulting in a highly variable potential for interaction amongst trophic levels. Only around dusk did zooplankton layers overlap with phytoplankton layers. Shortly after sunset, micronekton ascended from the deep, aggregating on the island’s shelf. Short-lived departures in migration patterns were detected in depth, vertical distribution, density, and total abundance of micronekton when zooplankton layers were present with typical patterns resuming within one hour. Layers of zooplankton began to disappear within 20 minutes of the arrival of micronekton with no layers present after 50 minutes. The effects of zooplankton layers cascaded even further up the food chain, affecting many behaviors of dolphins observed at dusk including their depth, group size, and inter-individual spacing. As a result of these changes in behavior, during a 30-minute window just after dusk, the number of feeding events observed for each dolphin and consequently the feeding time for each individual more than doubled when zooplankton layers were present. Dusk is a critical period for interactions amongst species in this system from phytoplankton to top predators. Our observations that short time windows can drive the structure and function of a complex suite of organisms highlight the importance of explicitly adding a temporal dimension at a scale relevant to individual organisms to our descriptions of heterogeneity in ocean ecosystems

Automatic active acoustic target detection in turbulent aquatic environments

This work is funded by the Environment and Food Security theme Ph.D. studentship from the University of Aberdeen, the Natural Environment Research Council (NERC) and Department for Environment, Food, and Rural Affairs (Defra grant NE/J004308/1), and the Marine Collaboration Research Forum (MarCRF). We would like to gratefully acknowledge the support from colleagues at Marine Scotland Science.Peer reviewedPublisher PD

Cooperative prey herding by the pelagic dolphin, Stenella longirostris

Sonar techniques were used to quantitatively observe foraging predators and their prey simultaneously in three dimensions. Spinner dolphins foraged at night in highly coordinated groups of 16–28 individuals using strict four-dimensional patterns to increase prey density by up to 200 times. Herding exploited the prey’s own avoidance behavior to achieve food densities not observed otherwise. Pairs of dolphins then took turns feeding within the aggregation that was created. Using a proxy estimate of feeding success, it is estimated that each dolphin working in concert has more access to prey than it would if feeding individually, despite the costs of participating in the group maneuvers, supporting the cooperation hypothesis. Evidence of a prey density threshold for feeding suggests that feedback from the environment may be enough to favor the evolution of cooperation. The remarkable degree of coordination shown by foraging spinner dolphins, the very strict geometry, tight timing, and orderly turn taking, indicates the advantage conferred by this strategy and the constraints placed upon it. The consistent appearance of this behavior suggests that it may be a critical strategy for energy acquisition by spinner dolphins in energy poor featureless environments in the tropical Pacific Ocean

Energy : converting from acoustic to biological resource units

Acoustic backscattering strength is often used as an index of biomass; however, the relationship between these variables has not been directly validated. Relationships were investigated between acoustic cross section at 200 kHz, measured as part of a previous study, and measured values of length, biovolume, dry weight, ash-free dry weight, and caloric content of the same individual specimens. Animals were part of the Hawaiian mesopelagic boundary community and included shrimps, squids, and myctophid fishes. The strong relationships found between all the variables measured make it possible to approximate any one variable from the measured values of others within a class of animals. The data show that for these midwater animals, acoustic scattering can be used as an index of biomass. Dorsal-aspect acoustic cross section at 200 kHz predicted dry weight and ash-free dry weight at least as well as did body length, a standard predictor. Dorsal-aspect acoustic cross section at 200 kHz was also a strong predictor of total caloric content. The relationship between dorsal-aspect acoustic cross section and caloric content of Hawaiian mesopelagic animals was linear and additive. Consequently, it is possible to directly convert acoustic energy from these animals to organic resource units without having knowledge of the size distribution of the populations being studied.Copyright 2002 Acoustical Society of America. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Acoustical Society of America. The following article appeared in J. Acoust. Soc. Am. 111(5): 2070-2075 (2002), and may be found at http://link.aip.org/link/?JAS/111/2070. Permalink at http://dx.doi.org/10.1121/1.1382620.Keywords: Acoustical detection of marine life; passive and active, Acoustics, Underwater soundKeywords: Acoustical detection of marine life; passive and active, Acoustics, Underwater soun

Target strength measurements of Hawaiian mesopelagic boundary community animals

A 200-kHz echosounder modified to digitize the envelope of the received echoes directly into a computer was used to measure the ex situ target strength (TS) of live animals from the Hawaiian mesopelagic boundary community as a function of animal size, tilt and roll angle, and biological classification. Dorsal aspect TS (in dB//1 m) at 200 kHz was related to the animal's length: myctophid fish TS = 20 log (standard length in cm)–58.8, r2 = 0.91, squid TS = 18.8 log (mantle length in cm)–61.7, r2 = 0.81, shrimp TS = 19.4 log (length in cm)–74.1, r2 = 0.83. Tilting the fish 5° and 10° changed the measured TS by up to 3.0 dB, decreasing TS as the fish was tilted forward and increasing TS as the fish was tilted backwards. In shrimp, forward tilt increased TS while backward tilt decreased TS by up to 3.3 dB. No consistent trend in squid TS change was observed with tilt angle. Roll angles of 5° and 10° increased the TS of all groups by up to 3.0 dB. Myctophid lateral aspect TS was consistently about 6 dB higher than the dorsal TS. Physiological analysis of the fishes' swimbladders revealed that the swimbladder is not the dominant scattering mechanism in the myctophid fishes studied.Keywords: bioacoustics, underwater sound, acoustic wave scattering, oceanographic region

Taking movement data to new depths : Inferring prey availability and patch profitability from seabird foraging behavior

Funded byNatural Environment Research Council. Grant Number: NE/K007440/1 and Marine Scotland Science and Seabird Tracking and Research (STAR) Project led by the Royal Society for the Protection of Birds (RSPB)Peer reviewedPublisher PD